04 700913 ch01 - wiley...john p. mccarthy, pe, se, smith group architecture, engineering, interiors,...

CHAPTER 1Element A: Substructure

CHAPTER 2Element B: Shell

CHAPTER 3Element C: Interiors

CHAPTER 4Element D: Services

CHAPTER 5Element E: Equipment and Furnishings

CHAPTER 6Element F: Special Construction and Demolition

CHAPTER 7Element G: Building Sitework

3

63

275

375

499

589

675

SECTION 1

BUILDING ELEMENTS

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COPYRIG

HTED M

ATERIAL

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ELEMENT A: SUBSTRUCTURE 1

INTRODUCTIONWell-designed foundations are a necessity to building design. A basic understanding of factors that influence facil-ity substructure design — bearing strata, settlement, and the effects of adjacent structures, slopes, and buildingmodification that either physically expand the structure or change the use of a structure—is essential for goodbuilding design.

This chapter provides a basic vocabulary for the design team to use to communicate when assisting in the designof the optimum foundation system to satisfy cost, schedule, and building constraints. This chapter also focuses onthe work of the geotechnical engineer, who, together with the structural engineer, creates solutions to complexdesign constraints. When engineering insight is combined with practical construction methods to produce struc-tures that support increasingly larger loads in more efficient way, it reduces the risk that the effectiveness of struc-tures above grade will be diminished by a misunderstanding or lack of attention to an important detail below grade.

In addition to exploring soils and geotechnical investigation, this chapter examines climatic and seismic consider-ations relating to the facility substructure and provides a review of standard and special foundations, includingbasement construction. The topic of basement construction addresses basement excavation, soil support, shoringstrategies, and basement wall construction, using both concrete and masonry, and methods of waterproofing ordampproofing these elements.

4 Soils and Soils Explorations

18 Foundations

40 Basement Construction

55 Case Studies

Contributor:John P. McCarthy, PE, SE, Smith Group Architecture, Engineering, Interiors, Planning, Detroit, Michigan.

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ELEMENT A: SUBSTRUCTURE SOILS AND SOILS EXPLORATIONS4

Bringing together project team design professionals, includinggeotechnical engineers, structural engineers, and architects to dis-cuss the matter of soils and foundations is fundamental to ensurethat the foundation selected satisfies the constraints of the projectbudget as well as the functionality of the structure.

Understanding the vocabulary of geotechnical science (for exam-ple, the difference between “cohesive” and “cohesionless” soils)is the first step toward fostering collaborative communication,which becomes increasingly important as the process continues.What should be tested, what the test should be, why it is important,and what the limitations of the test are must be addressed.Likewise, identifying foundation and ground modification alterna-tives (as well as their pros and cons) will aid in the preliminarydesign phase, when the building foundations are being developed.

Understanding the geotechnical investigation report and geo-graphical variations such as climate and seismic conditions willassist the design professionals in discussing important foundationissues.

SOILS DEFINITIONS: TERMS ANDCLASSIFICATIONS

It is critical that geotechnical and structural engineering informa-tion be understood properly; to that end, the following definitionsof common soils and other terms are included for reference:

• Clay: Determined by the size of particles and composition, claysare chemically different from their parent materials as a resultof weathering. Clays are typically inorganic and have grain sizesless then 0.0002 in. in diameter. This material contains chargedparticles and has an affinity for water. Because of their size andchemical composition, clays exhibit cohesion and plasticity.Clays can be classified as stiff, medium, or soft, depending onthe moisture content, with drier clays typically being stiffer.Clays make a satisfactory bearing material under some condi-tions. Long-term settlement can sometimes control the allow-able bearing pressure. Because of the cohesive nature of clay,excavations can have steep slopes for short periods of time.

• Silt: Silt consists of inorganic particles between 0.003 in. and0.0002 in. in diameter. These fine-grained particles are similar incomposition to the rocks from which they are derived, and arenot plastic in nature. Organic silt is found on the bottom of lakesand river deltas.

• Sand: Classifications of sand vary from fine to coarse, these rockparticles range in size from 0.003 in. to 0.079 in. in diameter.Adequately compacted, sand makes an ideal bearing material.The coarser the sand, the higher the allowable bearing pres-sures. Fine sands are susceptible to becoming quick when sub-jected to unbalanced hydrostatic pressures, and may liquefywhen they are loose, saturated, or subjected to seismic forces.Settlement is usually immediate, with little long-term settlement.

• Gravel: Classifications of gravel vary from fine to coarse, andthese unconsolidated rock fragments range from .75 in. to about3 in. Except for gravels composed of shale, this material makesa good foundation material. Depending on the compactness andthe underling material, very high bearing pressures are allowedby some building codes.

• Cobbles: Ranging in size from about 3 in. to about 10 in., theserock fragments can make reliable foundation-bearing materials,but can be difficult to properly compact when used for fill.Cobble-sized materials can interfere with pile driving anddrilled-pier construction causing significant problems.

• Boulders: Typically classified as rock fragments greater than 10in., boulders can be used as part of a fill mass if the voidsbetween the boulders are filled with finer-grained sands andsilts. These materials are generally not considered suitable fordirect foundation support because of their size and shape, andthe difficulty in excavating the material to desired shapes. Aswith cobbles, boulders can cause significant problems duringconstruction.

• Bedrock: Unbroken hard rock that is not over any other materi-al is considered bedrock. Depending upon its composition, it canbe capable of withstanding extremely high bearing pressure,and is desirable for foundations supporting high loads. If therock has been weathered or is cracked, its bearing capacity maybe compromised. Settlement of buildings on bedrock is primari-ly limited to the elastic settlement of the foundation.

• Residuum: Residuum consists of soil derived from the in-placedecomposition of bedrock materials. In general, these soils aremore weathered near the surface, and gradually transition to amore rocklike material with depth. Where residual soils revealevidence of the stratification and structure of the parent rock,they are known as saprolitic materials.

• Alluvial soils: Because materials are eroded, transported, anddeposited through the action of flowing water, these soils aretypically loose and saturated, hence often are unsuitable forsupport of structures or pavements.

• Colluvial soils: Because materials are transported by gravity,typically associated with landslides, these soils are generallyirregular in composition and loose. They require improvementprior to being used to support buildings and pavements.

• Aeolian soils: These soils are transported and deposited by the

wind. Typically, they consist of silt or sand-sized soils. Loess, oneof the more common types of aeolian soils, is composed of finecemented silt. While this material may be competent in place, itloses much of its strength when disturbed or recompacted.

• Till: Till is a mixture of clay, silt, sand, gravel, and bouldersdeposited by glaciers. Consolidated tills that are well graded(indicated by a uniform distribution of particle size) are excep-tionally strong and make excellent foundation strata. Loose tillscan cause differential settlements if used as a bearing material.

• Loam: This organic material, made up of humus and sand, silt, orclay, provides excellent material for agriculture but should notbe used for foundations. Organic materials will settle a greatdeal over time, and even lightly loaded slabs on grade will settleif bearing on loam.

• Cohesionless soils: These types of soils consist of cobbles, grav-els, sands, and nonplastic silts. They are generally formed fromthe mechanical weathering of bedrock brought about by water,ice, heat, and cold. They are typically composed of the same min-erals as the parent rock. The strength of cohesionless materialsis derived primarily from interparticle friction.

• Cohesive soils: These types of soils contain clay minerals with anunbalanced chemical charge. As a result, they tend to attract

SOILS AND SOILS EXPLORATIONS

VALUE AS ASYMBOLS FOUNDATION FROSTDIVISION LETTER HATCHING COLOR SOIL DESCRIPTION MATERIAL ACTION DRAINAGE

Gravel and GW Red Well-graded gravel, or gravel sand mixture; Excellent None Excellentgravelly soils little or no fines

GP Red Poorly graded gravel, or gravel-sand mixtures; Good None Excellentlittle or no fines

GM Yellow Silty gravels, gravel sand-silt mixtures Good Slight Poor

GC Yellow Clayey gravels, gravel clay-sand mixtures Good Slight Poor

Sand and SW Red Well-graded sands, or gravelly sands; Good None Excellentsandy soils little or no fines

SP Red Poorly graded sands, or gravelly sands; Fair None Excellentlittle or no fines

SM Yellow Silty sands, sand-silt mixtures Fair Slight Fair

SC Yellow Clayey sands, sand-clay mixtures Fair Medium Poor

Silts and clays ML Green Inorganic silt, rock flour, silty or clayey fine Fair Very high PoorLiquid Limit sands, or clayey silts with slight plasticity

CL Green Inorganic clays of low to medium plasticity, Fair Medium Imperviousgravelly clays, silty clays, lean clays

OL Green Organic silt-clays of low plasticity Poor High Impervious

Silts and MH Blue Inorganic silts, micaceous ordiatamaceous clays Liquid fine sandy or silty soils, elastic silts Poor Very high PoorLimit >50

CH Blue Inorganic clays of high plasticity, fat clays Very poor Medium Impervious

OH Blue Organic clays of medium to high plasticity, Very poor Medium Imperviousorganic silts

Highly organic Pt Orange Peat and other highly organic soil Not suitable Slight Poorsoils

SOIL TYPES AND THEIR PROPERTIES1.1

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SOILS AND SOILS EXPLORATIONS ELEMENT A: SUBSTRUCTURE 5

water and bond together. The strength of cohesive materials isderived from a combination of these chemical bonds and frominterparticle friction.

• Consolidation: When soils are subjected to loads, water withinthe void spaces initially supports the change in stress throughan increase in pressure. Excess pressures gradually dissipate inproportion to the permeability of the soil. Coarse-grained mate-rials drain rapidly, while finer-grained silts and clays drain moreslowly. As the excess pore pressures dissipate, the void spacescompress and transfer the loads to the soil grains. The resultingreduction in volume over time is known as consolidation.

• Underconsolidated soils: Soils that have built up in river deltasand other water bodies are deposited in a very loose state.These soils are often underconsolidated, in that they have neverexperienced stresses equal to or greater than current overbur-den stresses. These materials tend to consolidate under theirown weight over time, until all excess pore pressures have beendissipated and the soils become “normally consolidated.”Foundations bearing on underconsolidated soils can typicallyexpect large short- and long-term settlement.

• Overconsolidated soils: Unlike many other types of materials,soils are not elastic. When stresses are applied to soils, theycompress. However, when the same stress is removed, they donot rebound to the same height. When reloaded, the soils“remember” previously loaded conditions and compress to theirhistorical level of stress. Soils that have previously been loadedto stresses above those created by the current soil overburdenare considered to be overconsolidated. Foundations bearing onoverconsolidated soils can typically expect less short- and long-term settlements.

• Desiccation: All soils typically contain some moisture within thevoids between soil particles. When soils are dried, capillary ten-sion tends to pull the soil grains together, causing the soil toshrink and lose volume. This action can cause the soil to becomeoverconsolidated, as the capillary tension results in stress.

SOIL CLASSIFICATIONSThe Unified Soil Classification system, as defined in ASTM D2487,is the classification system most commonly used by geotechnicalengineers as a basis for defining the soils found in their tests. Thissystem allows for reproducible and reliable understanding of soilsfound from the testing performed at the site. Because some of thetesting is qualitative, it also provides a mechanism to correlate thesoils.

SOIL STUDIES AND REPORTS

For a structure to perform properly, it is essential that it bear on asuitable foundation tailored to the specific subsurface conditionswithin the project site. While geologic conditions can be generallyconsistent within broad areas of the country, subsurface conditionscan vary considerably within short horizontal distances caused byboth natural and human-made factors. Consequently, geotechnicalinvestigation must be performed for each new development tocharacterize site specific conditions and to evaluate cost-effectiveand practical site preparation and foundation recommendations.

Two types of geotechnical investigations are often conducted. In apreliminary study, several sites may be considered for construc-tion, and only a broad understanding of conditions may be requiredto find the parcel with the least cost of development. In a final,design-level study, more detailed information is required toaddress a number of issues, including, but not limited to, the fol-lowing:

• Geologic hazards: (1) Is the site in an area prone to sinkholes,subsidence, expansive soils? (2) Are landslides prevalent? (3)Will soils in the area liquefy or rupture during a seismic event?

• Historical issues: (1) Was the site developed in the past? (2) Isexisting fill present? (3) Was mining conducted within orbeneath the site?

• Excavation issues: (1) What types of materials will need to beexcavated as part of development? (2) Can excavations besloped, or will they require shoring? (3) Will groundwater beencountered in excavations, and how may it be dealt with? (4)

Can excavated materials be reused for fill? (5) What is the depthand type of topsoil, and can it be stripped, stockpiled, andreused?

• Applicable foundations: (1) Is the site compatible with shallowfoundations, or can subsurface conditions be improved to sup-port shallow foundations? (2) What types of deep foundationsare most cost-effective? (3) What parameters should be used inthe design of foundations? (4) What factors should be consid-ered in foundation construction, given the nature of subsurfaceconditions? (5) What is the expected performance of the foun-dation?

• Seismic Issues: (1) Is the site near a fault? (2) How will the sitereact to a seismic event? (3) What seismic building code param-eters are applicable to the site? (4) Are there any local regula-tory restrictions on construction?

Subsurface studies employ a broad array of investigative tech-niques that are often specific to an area of the country. Certainly,specific needs should be delineated by the project design team, butbroad latitude should be given to the geotechnical engineer todefine the scope of study required to properly assess subsurfaceconditions and to develop design recommendations for site prepa-ration and substructures.

The following multistep process will help to ensure that the properinformation is provided to the geotechnical engineer who will beperforming and documenting the investigations and making rec-ommendations.

REQUEST FOR PROPOSAL FORGEOTECHNICAL INVESTIGATIONProper foundation design based on accurate and complete geot-echnical data cannot be overemphasized. The first step, is thedevelopment of a request for proposal (RFP) for a geotechnicalinvestigation, prepared by the design team, including the projectstructural engineer, civil engineer, and other specialists.

1. Gather specific information about the proposed structure:• Column loads• Wall loads• Lateral loads at braces, moment frames, and shear walls• Uplift, if anticipated• Locations of basements and pits• Settlement limitations, if applicable• Vibration issues, if applicable

2. Identify the information required from the geotechnical report:• Foundation recommendations• Foundation depths• Over excavation, if anticipated• Expected settlements• Expected differential settlements• Lateral load capacity of foundations• Minimum size of foundations• Maximum size of foundations• Spacing limitations of spread and pile foundations• Group effects of pile foundations• Seismic coefficients• Lateral earth pressure: active, static, and passive• Pavement design• Water table depth• Perimeter and underslab drainage design• Excavation slopes• Backfill and compaction requirements• Expansive soil characteristics

3. Identify the format of the RFP; either prescriptive or perform-ance-based.• Prescriptive RFPs are used when the design team knows the

requirements for designing the substructure and is familiarwith the geological aspects of site. The advantage of a pre-scriptive RFP is that it is easier to evaluate multiple propos-als, since all proposals are based on the same scope. The dis-advantage of a prescriptive RFP is that the result may not bethe most effective geotechnical study available. Because theRFP is based on scope, its content is limited to items request-ed in the RFP; if the RFP does not include the appropriatenumber of borings or depths, for example, then the reportmay contain more or less information than is necessary to

adequately design the substructure. In addition, geotechnicaloffices familiar with the area may use previous reports andstudies to reduce the amount of drilling required.

• Performance RFPs are used when the geological aspects ofthe site are unknown and specific building code parametersmay not be established. The geotechnical engineer deter-mines the appropriate number, depth, and spacing of borings,as well as required testing to evaluate the site. The advantageof using performance RFPs is that they utilize the geotechni-cal engineers professional judgment to evaluate the site andrecommend the extent of soil and substructure investigationnecessary for the proper design of the substructure. The dis-advantage of using performance RFPs is that the scope ofservices and format of the proposals will be inconsistent,making it difficult to evaluate multiple RFPs.

4. Determine the amount of professional liability insurancerequired of the geotechnical engineer. Most geotechnical engi-neers will have disclaimers that limit liability to either theamount of the study, or $50,000. If coverage for $1 million ormore from the geotechnical engineer is required, include this inthe RFP; the geotechnical engineer may need to purchase thiscoverage for the particular project.

5. Indicate the format for the geotechnical investigation report andthe requirements for submitting the report. Consider items suchas: requirements for report draft copies, in case you need addi-tional information to be included in the final report. State in theRFP the required number of report copies, recipient, and deliv-ery method.

6. Determine any additional professional services the geotechnicalengineer will or may be required to perform (for example,attend project meetings, review drawings or specifications, per-form additional tests). Clearly state these requirements in theRFP. It may be appropriate to request fees for additional servic-es that may not be included in the scope and for reimbursableexpenses.

7. Evaluate the proposed based on set evaluation criteria. Respondto all submissions.

READING A SOILS REPORTA geotechnical report helps the design team understand the site onwhich the structure is to be built. Most geotechnical reports con-tain the following information, based on the previously definedscope of exploration:

• Report summary• Project information• Exploration methods• Description of soil and groundwater conditions• Design recommendations• Construction considerations• Appendix• Location diagram• Soil-boring or test pit logs• Soil profiles• Laboratory test results

The report summary is generally one to two pages long, and pro-vides the most salient information and recommendations of thereport. Use the summary as quick reference, but read the entirereport for details and qualifications/limitations. Most reports canbe read within 30 minutes. Check and verify the project informationand criteria (i.e., building height, structural loads, floor/basementlevels, and so on). The scope of the evaluation and recommenda-tions are based on this information. Also included in the reportwould be project information describing the building and site char-acteristics such as number of stories, building construction mate-rials, foundation loadings, basement data if applicable, and grades.The exploration section defines how the geotechnical engineerobtained the soil information required to describe the foundationthis would include number, location and depth of soil borings andtest pits, and laboratory and field testing to be performed.

The general soil and groundwater conditions include a generaloverview of the results of the geotechnical engineer’s tests. Moredetailed information is contained in the soil-boring and test pitlogs, which can be reviewed when required.

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The design recommendations section is of greatest interest to theproject design team, as it makes specific recommendations con-cerning the design of foundations, grade slab, walls, drainagerequirements, and other key building components. It should beread together with the section on construction considerations,which identifies potential problems during construction that can beavoided or minimized by both the design team and contractor wheneveryone understands the challenges for the project.

Often reports will provide a transverse section of the soil profile,combining the soil-boring information in a convenient picture. Thiswill enable the reader to better understand approximately how thesoil properties vary across the site.

SITE SOIL PROFILE1.2

WOH

118

5

18

32

55

68

5B – 25 B – 26

WATER

B – 27

0

–5

–10

–15

–20

–25

–30

5

0

–5

–10

–15

–20

–25

–30LEGEND

GRAY, VE RY SOFT ORGANIC SILT; VERY LOOSE,

O RGANIC, SILTY SAND AND VERY SOFT PEAT

T

LIGHT GRAY, LOOSE TO MEDIUM DENSE, SILTY SAND

DARK GRAY, MEDIUM DENSE TO DENSE SAND,

TRACE SILT

REDDISH-BROWN HARD SANDY CLAY, CLAYEY

SAND AND SILTY CLAY

ELEV

ATIO

N IN

MET

ERS

ELEV

ATIO

N IN

MET

ERS

GRAY, MEDIUM HARD TO HARD, FINE TO MEDIUM

GRAINED SANDSTONE; CLOSELY TO MODE RATELY

JOINTED; MODERATELY BEDDED

NBLOWS/FT

NBLOWS/FT

SAND

SILT

ORGANIC SILT

CLAY

PEAT

SANDSTONE

A = RUN NO.

B = % RECOVERY

57

56

65

637699

123

4WOR

28

182221333836344940476155463944

43

61

50

63

837993

123

WOR

123

15

18

20

38

45

52

50

38

45

60

617592

123

BA

11

1

NBLOWS/FTOWS/FTNBLOWS/FT

READING A BORING LOGA soil-boring log, which is prepared by the geotechnical engineer,identifies the layers of soil found at specific depths beneath thesurface and several measured characteristics of this soil, whichcould impact the design of any structure built upon it.

Source: Foundation Engineering Handbook, 2nd Edition (Van Nostrand Reinhold, New York, 1991).

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SOILS AND SOILS EXPLORATIONS ELEMENT A: SUBSTRUCTURE 7

SOIL BORING LOG1.3

Source: Foundation Engineering Handbook, 2nd Edition (Van Nostrand Reinhold, New York, 1991).

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ELEMENT A: SUBSTRUCTURE SOILS AND SOILS EXPLORATIONS

Contributors:James W. Niehoff, PE, PSI, Wheat Ridge, Colorado; Timothy H. Bedenis,PE, Soil and Materials Engineers, Inc., Plymouth, Michigan; John P.McCarthy, PE, SE, SmithGroup, Architecture, Engineering, Interiors,Planning, Detroit, Michigan.

8

BLOW COUNTSBlow counts (also known as N-values or standard penetration testvalues) reflect a number that has been correlated with density insands and unconfined compressive strength in clays and settle-ments. Developed by Karl von Terzaghi and Ralph B. Peck in the late1940s, blow counts are been widely used as an indicator of thestrength of the soil many feet below the surface.

Blow counts are not infallible, however, and therefore can only beused as a guide. For sandy soils, there is better correlationbetween blow count and relative density and settlements, except invery loose and very dense sands, which sometimes are of the mostinterest. For clays, the results are more unreliable and should beused to correlate actual samples taken from the sampler and test-ed in the laboratory. The influence of sand types and the amount ofwater during the boring can result in greatly differing readings.

Blow counts are calculated through standard penetration testing,developed to provide a method to test cohesionless soils continu-ously along the length of a bore hole, without taking samples. Thistest has now become routine for all types of soils. The test utilizesthe following procedure:

1. Drive a standard split-tube sampler a distance of 18 in. into thesoil.

2. Drive the sampler with a 140-lb weight dropping 30 in.3. Count the number of blows to drive the device the last 12 in.

The number of blows required to drive the sampler the last 12 in.is the blow count.

There are many benefits to standard penetration tests:

• The cost of obtaining test information through this method isless than with other available methods.

• Samples can be examined, even if they are disturbed.• Data from the test can be collected along the length of the bore

hole.• Numerous geotechnical engineers at many sites have this test

data.• Equipment, though expensive, has a long service life.

That said, there are several factors that also make the standardpenetration test unreliable:

• The 30-in. drop of the weight is often done by eye, and so is notexact.

• The rope and guide for the weight could become fouled or pro-duce interference, thus reducing the effect of the drop.

• The bottom of the hole might not be cleaned properly beforestarting the test.

• There could be a stone being driven by the sampler, disruptingthe test.

EXPANSIVE SOILS

Expansive soils contain clay minerals derived from volcanic ash andmarine deposits that attract and readily absorb water. The waterchemically bonds with the clay mineral and causes the soil toexpand. It is not unusual for such materials to swell 5 to 10 per-cent upon wetting and to exert pressures of 10,000 psf or more.These soils are prevalent in the southwestern and western parts ofthe United States, but are present in limited areas of the South and

Midwest. These soils need to be removed, treated, or bypassedthrough the use of deep foundations to prevent damage to struc-tures.

CLIMATIC FOUNDATION ISSUES

When designing for climatic conditions, the following should beconsidered:

• Cold and permafrost climates• Cold and underheated climates• Hot, arid climates• Humid, overheated climates

DESIGNING FOR COLD ANDPERMAFROST CLIMATESCold climates in North America are generally north of the 40th par-allel. “Very cold” is identified by the southern boundary of the 32°Fmean annual temperature, and includes most of Canada andAlaska, except along the Pacific Coast. Permafrost extends frombelow the Hudson Bay and just north of the southern coast ofAlaska to the Arctic Ocean. The Arctic Circle designates the south-ernmost point where continuous daylight in summer and continu-ous darkness in winter exists. Foundations in these climates areextremely important, and technically challenging to design.

The danger for foundations in these climates is that the soil aroundand under the foundation might thaw and lose strength. There areseveral strategies for designing in these areas. Both self-con-tained convection (passive) and mechanically refrigerated (active)

ABUNDANT EXPANSIVE SOILS LESS ABUNDANT EXPANSIVE SOILS

EXPANSIVE SOILS ACROSS THE UNITED STATES1.4

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SOILS AND SOILS EXPLORATIONS ELEMENT A: SUBSTRUCTURE

Contributors:Edwin Crittenden, FAIA, and John Crittenden, AIA, Anchorage, Alaska;G. H. Johnston, Permafrost Engineering, Design and Construction,National Research Council of Canada (John Wiley & Sons, Hoboken,New Jersey, 1981); Arvind Phukan, Frozen Ground Engineering(Prentice Hall, Upper Saddle River, New Jersey); Eb Rice, Building in theNorth, Geophysical Institute, University of Alaska, Fairbanks, Alaska.

9

systems are used for new construction and stabilization of existingfoundations, either directly as pipe piles or in smaller pipes(probes) that can be placed beside a pile or under slab or founda-tion. Passive systems rely on natural convection of a liquid or gasmedium to remove heat from the ground to keep it frozen; activesystems use pumps and refrigeration technology. Recent concernsregarding global warming have caused renewed interest in thedesign parameters for passive systems.

PERMAFROST, ICE WEDGES AND LENSES, ANDFROST HEAVEIn the scope of climate issues, the following are important terms:

• Permafrost: Ground of any kind that stays colder than the freez-ing temperature of water throughout several years. Depth canextend to 2,000 ft. below the active layer.

• Active layer: Top layer of ground subject to annual freezing andthawing. Can be up to 10 ft or only 18 in. over some permafrost.

• Frost heaving: Lifting or heaving of soil surface created by thefreezing of subsurface frost-susceptible material.

• Frost-susceptible soil: Soil that has enough permeability andcapillary action (wickability) to allow ice lenses to form andexpand upon freezing.

• Ice lense (taber ice): Subsurface pocket of ice in soil.• Ice wedge: Wedge-shaped mass of ice within thaw zone.

Wedges range up to 3 or 4 ft wide and 10 ft deep.• Pereletok: Frozen layer at the base of the active layer that

remains unthawed during some cold summers.• Residual thaw zone: Layer of unfrozen ground between the per-

mafrost and active layer. This layer does not exist when theannual frost extends to the permafrost but is present duringsome warm winters.

CONDITION OF BUILDINGS ON PERMAFROST• Condition 1: Buildings elevated on piles allow the dispersion of

building heat to prevent ground thaw and allow the wind toremove snow. Wooden piles, with low thermal conductivity,induce minimal heat into the frozen ground, while thermopilescan remove heat to retain frozen state.

• Condition 2: Buildings elevated on non-frost-susceptible gravelpads to provide insulation in addition to existing ground cover.Rigid insulation adds to the protection from thaw of the per-mafrost. Thermopiles are used to refreeze fill and keep per-mafrost frozen.

NORTHERN HEMISPHERE PERMAFROST LIMITS1.5

ARCTIC SUB-ARCTIC COLD WET

ARCTIC CIRCLE 32 F MEAN TEMPERATURE LIMIT OF CONTINUOUS PERMAFROST

LIMIT OF DISCONTINUOUS OR SPORADIC PERMAFROST NORTHERN BOUNDARY OF TREES

COLD

IRKUTSK

BRATSK

NOVOSIBIRSK

OMSK

YAKUTSK

MAGADAN

MOSCOW SUB-ARCTIC

REGION NORILSK ARCHANGEL LENINGRAD

STOCKHOLM

OSLO

HELSINKI

KIRUNA

MURMANSK

LONDON

REYKJAVIK

HAY RIVER

ARCTIC REGION

PETROPAVLOSK

BARROW NOME

PRINCE RUPERT

ANCHORAGE

EDMONTON

MONTREAL

WINNIPEG

CALGARY

FROBISHER BAY

GODTHAB

FAIRBANKS

INUVIK THULE

CHURCHILL

SEATTLE

CHICAGO

HALIFAX

50

60

70

80

40

VLADIVOSTOK

SOIL CROSS SECTION AT PERMAFROST ZONE1.6

CONDITION 1: ELEVATED BUILDING ON PILES1.7

CONDITION 2: BUILDING ON GRAVEL PAD1.8

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DESIGNING FOR COLD ANDUNDERHEATED CLIMATESCold and underheated climate conditions occur over the northernhalf of the United States and in mountainous regions. These condi-tions can be generally quantified as where the frost depth is 12 in.

SLAB-ON-GRADE CONSTRUCTION IN COLD CLIMATES1.9

BASEMENT CONSTRUCTION IN COLD CLIMATES1.10

ENERGY-EFFICIENT WALL SECTIONS FORUNDERHEATED CLIMATES1.11

or greater. Designing foundations for these conditions is treated ina more typical manner, such as: providing a foundation below thefrost depth, including a basement, and providing insulation on theexterior to reduce the chances of cold ground temperatures reach-ing the structure.

FROST ISSUESDetrimental frost action in soils is obviously limited to those areasof the United States where subfreezing temperatures occur on aregular basis and for extended periods of time. “Frost action,” asused in this context, is the lateral or vertical movement of struc-tures supported on or in the soil. Frozen soil is, in itself, not nec-essarily detrimental to the supported structures. It becomes detri-mental when, through the growth of ice lenses, the soil, and what-ever is resting on the soil above the ice lenses, heaves upward.This causes foundations and the structures supported by the foun-dations to distort and suffer distress. Other common problems arethe heaving of sidewalks, pavements, steps, retaining walls, fencepoles, and architectural features.

The depth of frost penetration is directly related to the intensityand duration of the freezing conditions, a measure that is termedthe freezing degree day index. In milder climates in the UnitedStates, the local building codes might stipulate a frost protectiondepth for foundations of 12 in. In the northern portions of theUnited States, the frost protection depth might be 42 to 60 in. asrequired by local building codes. These guidelines are usually con-servative, but there are situations where deeper frost protectiondepths are warranted. For instance, if the emergency entrance toa hospital is on the north side of the hospital, where the sun neverwarms the pavement adjacent to the building, and the pavement iskept 100 percent snow-free for safety reasons, then the frost pen-etration can easily exceed the code requirements.

Carefully evaluate exposure conditions to see if a special conditionexists. Grass and snow are very effective insulators for the groundbelow. Avoid the use of sloping exterior faces on grade beams orfoundations that give the freezing forces something to pushagainst when the frost heave situation develops.

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Contributors:Richard O. Anderson, PE, Somat Engineering, Taylor, Michigan; Eric K.Beach, Rippeteau Architects, PC, Washington DC; Stephen N. Flandersand Wayne Tobiasson, U.S. Army Corps of Engineers, Hanover, NewHampshire; Donald Watson, FAIA, Rensselaer Polytechnic Institute,Troy, New York; Kenneth Labs, New Haven, Connecticut; Jeffrey Cook,Arizona State University, Tempe, Arizona; K. Clark and P. Paylore, DesertHousing: Balancing Experience and Technology for Dwelling in Hot AridZones, Office of Arid Land Studies, University of Arizona, Tucson,Arizona, 1980. J. Cook, Cool Houses for Desert Suburbs, Arizona SolarEnergy Commission, Phoenix, Arizona, 1984.

11

DESIGNING FOR HOT, ARID CLIMATES

CLIMATE IMPLICATIONSThough classified as arid and overheated, severe desert climatesin the United States typically have four distinct periods for deter-mining comfort strategies:

• The hot dry season, occurring in late spring, early summer, andearly fall, has dry, clear atmospheres that provide high insula-tion levels, high daytime air temperatures, very high sol-air tem-peratures, and large thermal radiation losses at night, produc-ing a 30° to 40°F daily range. Nighttime temperatures may fallbelow the comfort limits and are useful for cooling. Low humid-ity allows effective evaporative cooling.

• The hot humid season occurs in July and August. In addition tohigh insulation, it is characterized by high dew point tempera-tures (above 55°F), reducing the usefulness of evaporative cool-ing for comfort conditioning. Cloudiness and haze prevent night-time thermal reradiation, resulting in only a 20°F or less dailyrange. Lowest nighttime temperatures are frequently higherthan the comfort limits. Thus, refrigeration or dehumidificationmay be needed to meet comfort standards.

• The winter season typically has clear skies, cold nights, very lowdew point temperatures, a daily range of nearly 40°F, and theopportunity for passively meeting all heating requirements fromisolation.

• The transitional or thermal sailing season occurs before andafter the winter season and requires no intervention by envi-ronmental control systems. This season can be extended by thepassive features of the building. Other desert climates have sim-ilar seasons but in different proportions and at cooler scales.

CONSTRUCTION DETAILSCapitalize on conditions climatic conditions by incorporating con-struction practices that respond in beneficial ways to the environ-ment, including:

• Insulate coolant and refrigerant pipes from remote evaporativetowers and condensers for their entire length.

• In hot locations, use roof construction similar to the cold climateroof detail.

• Do not use exposed wood (especially in small cross sections)and many plastics, as they deteriorate from excessive heat andhigh ultraviolet exposure.

• Although vapor retarders may not be critical to control conden-sation, implement them as a building wrap or wind shield, bothto control dust penetration and to avoid convective leaks fromhigh temperature differentials.

• Avoid thermal bridges such as extensive cantilevered slabs.• Radiant barriers and details appropriate to humid overheated

climates are at least as effective as vapor retarders, but avoidholes in assembly where convection would leak their thermaladvantage.

• Ventilate building skin (attic or roof, walls) to relieve sol-air heattransfer.

DESIGNING FOR HUMID, OVERHEATEDCLIMATESHumid, overheated conditions are most severe along the GulfCoast, but occur across the entire southeastern United States.Atmospheric moisture limits radiation exchange, resulting in dailytemperature ranges less than 20°F. High insulation gives first pri-ority to shading. Much of the overheated period is only a fewdegrees above comfort limits, so air movement can cool the body.Ground temperatures are generally too high for the Earth to beuseful as a heat sink, although slab-on-grade floor mass is useful.The strategies are to resist solar and conductive heat gains and totake best advantage of ventilation.

AVERAGE DEPTH OF FROST PENETRATION (IN.)1.12

TYPICAL WALL SECTIONS FOR HOT, ARID CLIMATES1.13

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Contributors:Donald Watson, FAIA, Rensselaer Polytechnic Institute, Troy, New York;Kenneth Labs, New Haven, Connecticut; Subrato Chandra, Philip W.Fairey, Michael M. Houston, and Florida Solar Energy Center, Coolingwith Ventilation, Solar Energy Research Institute, Golden, Colorado.1982.; K. E. Wilkes, Radiant Barrier Fact Sheet, CAREIRS, Silver Spring,Maryland; P. Fairey, S. Chandra, A. Kerestecioglu, Ventilative Cooling inSouthern Residences: A Parametric Analysis, PF-108-86, Florida, SolarEnergy Center, Cape Canaveral, Florida 1986; William W. Stewart, FAIA,Stewart-Schaberg Architects, Clayton, Missouri.

12

SEISMIC FOUNDATION ISSUES

INTRODUCTION TO SEISMIC DESIGNAccording to the theory of plate tectonics, the Earth’s crust is divid-ed into constantly moving plates. Earthquakes occur when, as aresult of slowly accumulating pressure, the ground slips abruptlyalong a geological fault plane on or near a plate boundary. Theresulting waves of vibration within the Earth create groundmotions at the surface, which, in turn, induce movement withinbuildings. The frequency, magnitude, and duration of the groundmotion; physical characteristics of the building; and geology of asite determine how these forces affect a building.

DESIGN JUDGMENTDuring a seismic event, buildings designed to the minimum levelsrequired by model codes often sustain damage, even significantstructural damage. Early discussions with an owner should explorethe need to limit property loss in an earthquake, and the desirabil-ity of attempting to ensure continued building operation immedi-ately afterward. To achieve these results, it may be necessary tomake design decisions that are more carefully tuned to the seismicconditions of a site than the code requires.

The relationship between the period of ground motion and the peri-od of building motion is of great importance to building design.Fundamental periods of motion in structures range from 0.1 sec-ond for a one-story building to 4.0 seconds or more for a high-risebuilding. Ground generally vibrates for a period of between 0.5 and1.0 second. If the period of ground motion and the natural periodof motion in a building coincide, the building may resonate, and theloads will be increased. Theoretically, one part of the seismicdesign solution is to “tune” the building so that its own period ofmotion falls outside the estimated range of ground motion fre-quency. In practice, this tuning is very seldom carried out. Rather,design professionals rely on increased load effects required by theapplicable code to take care of the problem.

SEISMIC CODESThe building code adopted in most jurisdictions in the United Statesis International Building Code (IBC). There are some significantchanges to the seismic forces determined by this code comparedto seismic forces determined by previous building codes. The IBC2006 code seismic provisions are designed around a level of earth-quake that is expected to be exceeded only 2 percent of the time inthe next 50 years. The level of seismic design for most structures,per the IBC, is based on a “collapse protection” strategy (com-monly referred to as a “life safety” level), which assumes thatthere may be significant damage to the structure up to the point ofcollapse but that the structure does not collapse.

The structural engineer will design a lateral force-resisting struc-tural assembly to resist a design-level earthquake. These designsare developed from detailed maps that indicate the ground spec-tral accelerations of buildings, which are based upon known pastseismic events, in combination with probability studies. Thesemaps typically include known fault locations, which help to deter-mine the distance of the building from any known fault. The groundaccelerations can typically be found down to the county level in theUnited States. The geotechnical engineer works with the designteam to develop the site coefficient, which is dependent on thelocal soils layers and depths.

The following information is based on the requirements in the IBC2006 Building Code, which in turn is based on the 2000 NationalEarthquake Hazards Reduction Program (NEHRP). Detached one-and two-family dwellings are exempt from seismic regulations inareas other than those with high seismicity. (Note: Seismic codesare constantly evolving, so consult the applicable code beforebeginning a project.)

ENERGY-EFFICIENT WALL SECTION: VENTED SKIN MASONRY WALL WITH INSIDE INSULATION FOR HUMID,OVERHEATED CLIMATES1.14

ENERGY-EFFICIENT WALL SECTION: VENTED SKIN WALL WITH RADIANT BARRIER FOR HUMID, OVERHEATEDCLIMATES1.15

MAIN CAUSES OF FOUNDATION FAILURE1.16

FUNDAMENTAL PERIODS1.17

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Contributors:William W. Stewart, FAIA, Stewart-Schaberg Architects, Clayton,Missouri; Scott Maxwell, PE, SE, Adrian, Michigan.

13

• Dynamic analysis: A structural analysis based on the vibrationmotion of a building. Dynamic analysis is time-consuming, andnormally reserved for complex projects.

• Forces, in-plane: Forces exerted parallel to a wall or frame.• Forces, out-of-plane: Forces exerted perpendicular to a wall or

frame.

different types of lateral force-resisting systems used). This forceis applied at the base of the structure then is distributed verticallythroughout the building according to the mass, and horizontallythroughout the building according to the stiffness of the lateral ele-ments of the structure (for a “rigid” diaphragm), or according totributary width of the lateral elements of the structure (for a “flex-ible” diaphragm).

DESIGN FOR RESISTING SEISMICFORCES AND FOUNDATION ISSUESA design that resists seismic forces for a structure makes use ofthe lateral systems’ ductility. Such ductile lateral systems aredesigned to deflect more under seismic loading than what would beexpected from something such as wind loading. This allows for theuse of smaller effective seismic design forces and more reason-ably sized members. It is important, however, that the overalldesign still be capable of handling the expected deflections. Storydrifts that are too large can result in secondary forces and stress-es for which the structure was not designed, as well as increasethe damage to the interior and exterior building components, andhinder the means of egress from the building.

Typical means of resisting these forces include the use of momentframes, shear walls, and braced frames. Each of these types of lat-eral systems can be made up of one of the main structural materi-als (such as steel or reinforced concrete moment frames; mason-ry, wood, or reinforced concrete shear walls; or steel or reinforcedconcrete-braced frames). The building configuration and designparameters will have a major effect on which system to chose and,subsequently, the lateral system chosen will have a major impacton the foundations required to resist the loads.

Moment frames typically are distributed more evenly over thebuilding footprint and have little or no uplift; they also generallyhave large base moments that can be difficult to resist. In addition,moment frames will tend to have greater lateral deflections thanother stiffer systems (such as shear walls or braced frames).Concrete shear walls and steel-braced frames are more localized,not only concentrating lateral shear at the base but also having ahigh potential for net uplift forces to be resisted. These forces aredifficult to resist with some foundation systems and should bereviewed extensively before selecting the lateral load-resistingsystem.

Tall, narrow structures tend to have overturning issues before theywill face sliding issues, whereas short structures face sliding prob-lems rather than overturning problems. Seismic motion rocks thebuilding, increasing overturning loads, and can act in any direction.Thus, resistance to overturning is best achieved at a building’sperimeter, rather than at its core.

Building foundations must be designed to resist the lateral forcestransmitted through the earth and the forces transmitted from thelateral load-resisting system to the earth. In general, softer soilsamplify seismic motion.

SEISMIC ACCELERATION FOR LOW BUILDINGS EXPRESSED AS A PERCENTAGE OF GRAVITY1.18

Source: Map courtesy of the U.S. Geological Survey, National Seismic Hazard Mapping Project (June 1996)

BASE SHEAR AND DRIFT1.19

FORCE DIAGRAMS1.20

SHEAR WALLS AND DIAPHRAGMS1.21

• Design earthquake: Earthquake ground motion for which abuilding is designed. This is typically about two-thirds of themaximum considered earthquake (MCE) (defined below) whendesigning per the IBC codes.

• Drift and story drift: Lateral deflection of a building or structure.Story drift is the relative movement between adjacent floors.

• Ductility: The ability of a structural frame to bend, but not break.Ductility is a major factor in establishing the ability of a buildingto withstand large earthquakes. Ductile materials (steel, in par-ticular) fail only after permanent deformation has taken place.Good ductility requires special detailing of the joints.

TERMSThe seismic community has an extensive set of terms that describecommon conditions in the field. Here is a short list of these termsand their definitions:

• Base shear (static analysis): Calculated total shear force actingat the base of a structure, used in codes as a static representa-tion of lateral earthquake forces. Also referred to as equivalentlateral force.

• Maximum considered earthquake (MCE): The greatest ground-shaking expected to occur during an earthquake at a site. Thesevalues are somewhat higher than those of the design earth-quake, particularly in areas where seismic events are very infre-quent. The code maps are based on earthquakes of this magni-tude.

• Reentrant corner: The inside building corner of an L-, H-, X-, orT-shaped plan.

ESTABLISHING SEISMIC FORCESThe equivalent lateral force procedure is the most common methodused to determine seismic design forces. In it, the seismic load, V(base shear), is determined by multiplying the weight of the build-ing by a factor of Cs (V=CsW). The value of Cs depends on the sizeof the design earthquake, the type of soil, the period of the build-ing, the importance of the building, and the response-modificationfactor (a variable that accounts for different levels of ductility for

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NOTE

1.23 Land uses should reflect the relative risk of the location. High-riskactivities should be located on low-risk sites to reduce the potential forproperty damage and loss of life.

Contributors:William W. Stewart, FAIA, Stewart-Schaberg Architects, Clayton,Missouri; Scott Maxwell, PE, SE, Adrian, Michigan; Richard Eisner, FAIA,Governor’s Office of Emergency Services, Oakland, California.

14

SITE DESIGN FOR SEISMIC AREASEach building and site lies within a broader context of regionalseismicity, localized geology, community vulnerability, and adjacentstructures and land uses. Project site decisions, therefore, canhave a marked impact on the overall seismic performance of astructure. Consider the following when selecting a site:

• Avoid unstable sites.• Avoid nonengineered fill.• Avoid or design for sites that can subside or liquefy.• Avoid building over surface faulting.• Avoid adjacent hazardous buildings.• Prevent battering from adjacent buildings.• Create safe areas of refuge when redeveloping older buildings.

Decisions critical to performance include: appropriate land usesfor a specific site, separation from active ground faulting, site sta-bility, and separation from adjacent buildings. Although many ofthese factors have traditionally been considered city planningissues, the design professional must also incorporate them intothe development of a seismically resistant building.

Within a fault zone, a trench is required to determine the exactlocation of the fault trace. Development within a fault zone shouldbe restricted to low-density land uses, open space, and other low-occupancy activities.

OVERTURNING AND SLIDING1.22

UNSTABLE SITES1.24

SURFACE FAULTING1.25

RELATIVERISK OF SITE LAND USE

Low High-density commercial/retail

High occupancy and assembly

Essential services (fire stations, hospitals, emergencyemergency operations centers, etc.)

Hazardous industrial processes

Medium Medium- and low-density residential

Low-rise commercial/retail

Industrial uses

High Very low-density residential

Nonhazardous industrial

Recreation

Public open space

Public rights-of-way

SEISMIC ZONATION TO REDUCE RISK1.23

On sloping sites, earthquakes can trigger landslides; also, alluviumand unconsolidated soils can increase the violence and duration ofground-shaking. Therefore, in areas of young soil deposits, designfor greater ground-shaking. For example, during the 1989 LomaPrieta earthquake in northern California, ground-shaking in SanFrancisco’s Marina District, on nonengineered fill, was more thantwice as violent and lasted more than twice as long as ground-shaking on adjacent bedrock sites.

SUBSIDENCE OR LIQUEFACTION1.26

Avoid sites subject to liquefaction (water-saturated sandy soils),design foundation systems to withstand ground failure, drainwater from the site, and change the composition of the soil andcompact the site.

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BUILDING CONFIGURATION FORSEISMIC AREASA load path is the path seismic forces take from the roof to thefoundation of a structure. Typically, the load travels from thediaphragms through connections to the vertical lateral force-resisting elements and on to the foundation by way of additionalconnections. This path should be direct and uninterrupted. Seismicdesign begins with, and codes require, the establishment of a con-tinuous load path.

The seismic-resistant framing system selected for a structure mustmeet both architectural and seismic design requirements.Although most buildings can be made seismic-resistant, somearchitectural configurations interrupt the load path, or otherwiseinterfere with the seismic design process. Inappropriate designchoices increase construction cost and make the seismic restraintsystem less effective.

The examples show configurations with potential problems inareas with high levels of seismicity, and include variations thatcould be used to avoid these problems.


NOTES

1.30 a. The base should not be much larger than the tower above.b. It is best to have uniform stiffness, but some variation is acceptable.1.31 Horizontal diaphragms (floors and roofs) can more readily transferearthquake loads to the vertical force-resisting system when the sizeand number of holes in the diaphragm are limited.

Contributor:Richard Eisner, FAIA, Governor’s Office of Emergency Services, Oakland,California.

15

ADJACENT HAZARDOUS BUILDINGS1.27

BATTERING FROM ADJACENT BUILDINGS1.28

VERTICAL GEOMETRY IRREGULARITY1.30

SAFE AREAS OF REFUGE IN OLDER BUILDINGS1.29

DIAPHRAGM DISCONTINUITIES1.31

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NOTES

1.33 The lateral force-resisting system for a symmetrical building ismuch easier to design than that for an asymmetrical building. Becausethe source of an earthquake cannot be known, symmetry in both direc-tions should be considered.1.34 This is a variation of the symmetry issue. When the notch gets toobig, the building tends to tear at the inside corner.1.35 Not all floors have to be the same; nevertheless, it is importantthat no floor has much more mass than those adjacent.1.36 When a taller (inherently softer) first floor is desired, anticipate

using much heavier first-floor framing to equalize the stiffness with thatof the floors above.1.37 Although both drawings illustrate shear walls in the same plane,one arrangement is discouraged because the load path is not direct.

Contributor:William W. Stewart, FAIA, Stewart-Schaberg Architects, Clayton,Missouri.

16

FOUNDATION STRATEGIES FOR HIGH-SEISMIC LOADSUPLIFTBraced buildings typically end up with high-tension loads at thefoundations. Shallow foundations are difficult to design with high-tension loads. Some strategies are available to resist these upliftforces:

• Increase the dead load by removing adjacent columns, increas-ing the tributary area.

• Deepen the footing, increasing the soil load.• Increase the footing dimensions, to increase the soil and con-

crete loads.• Decrease column spacing, to decrease the brace forces.• Change the foundation type (typically, to a deep foundation that

can resist the uplift more effectively).

SHEARBraces and shear walls tend to collect the lateral forces and con-centrate the loads in a few locations. Shallow and deep founda-tions have limited lateral load-resisting capability. By combiningseveral foundations together, it is possible to effectively increasethe lateral load resistance. The concrete tie beams are typicallydesigned to distribute the lateral loads through tension or com-pression of the beam.

OUT-OF-PLANE VERTICAL OFFSETS1.32

TORSION IN PLAN1.33

REENTRANT CORNERS1.34 IN-PLANE DISCONTINUITY

1.37

MASS IRREGULARITY1.35

SOFT STORY1.36

BRACE WITH UPLIFT1.38

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NOTES

1.40 a. Increase the dead load by removing adjacent columns, increas-ing the tributary area.b. Combine several footings into a mat foundation.

17

CONCRETE TIE BEAMS1.39

OVERTURNINGShear walls have a tendency to produce high-overturning momentsat the base. If the foundation under the shear wall is isolated,these overturning moments can result in tension at one edge of thefoundation. Since soil cannot resist tension, modifications to thefoundation system or framing are required.

SHEAR WALL OVERTURNING1.40

PERIMETER WALL WITH CONTINUOUS FOOTINGS1.41

PERIMETER WALLSAt the perimeter walls of a building, especially one with a base-ment, there are several methods of designing the foundations. Inmany cases, the foundations are designed individually, without uti-lizing the effect of the rigid basement wall. If the forces in the lat-eral system become very great, it is sometimes advantageous tocombine the foundations along the wall and treat the foundation asone big footing. This will spread out the load more effectively. Thefoundation wall would need to be checked for strength and stiff-ness in distributing the forces. Basement walls would typicallyhave sufficient capacity; foundations walls sometimes requireadditional thickness and reinforcement.

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ELEMENT A: SUBSTRUCTURE FOUNDATIONS

Contributor:James W. Niehoff, PE, Chief Engineer, PSI, Wheat Ridge, Colorado.

18

GENERAL

Foundations, because they are hidden below the surface, are oftenoverlooked and their importance minimized by the design team. Agreat deal of scientifically guided creativity is often necessary toproduce a foundation that supports the loads of the structure insuch a way as to economically maintain the aesthetics and functionof a facility. The wide variety of soil types and conditions across theUnited States—from the bedrock near the surface in New York tothe sink holes and coral in Florida to the deep soft clays in theMidwest and South, and expansive soils in the Southwest to theseismically active areas of the country such as the West Coast—pose a challenge to the design team. The most popular and eco-nomical foundation solution is the spread footing. Spread footingsare typically shallow, simple to design and construct, and performwell under many conditions. When properly designed, load isspread from a column to the soil at a bearing pressure that caus-es neither excessive settlement nor failure of the soil.

Should the soil conditions near the surface be weak, poorly com-pacted, filled with debris or organic material, or too compressible,deep foundations are warranted. The effect is to extend the foun-dation through the weak strata to a soil type that can withstand theloadings with tolerable settlement. Deep foundations come in sev-eral types and, depending on the soil conditions, may include driv-en piles, bored piles (bored, augered or drilled) or caissons. Deepfoundations resist imposed loads either by end bearing or side fric-tion or some combination. It is not necessary to drive or drill adeep foundation to rock, only to the depth required to reach a suit-able stratum.

A spread footing is not always appropriate, such as when theproperty line limits the extent of the foundation in one direction, orwhen the soil conditions are very weak and suitable soil strata toodeep to reach with a deep foundation. In these cases, other typesof special foundations (such as combined footings, strap footings,and raft foundations) are sometimes required.

Two basic criteria should be met for all foundations:

• Soil strength (bearing capacity): The ability of a soil to support aload without experiencing failure is known as the bearing capac-ity and is a function of the foundation size as well as the inher-ent strength properties of the soil. If the pressures exerted by afoundation exceed the strength of a soil, the soil mass experi-ences a shear failure leading to gross movements of both thesoil and the supporting foundation element.

• Limitations of settlement: Settlement can happen either imme-diately (foundations on sands), or over period of time, short orlong term (foundations on clays). Some settlement is expected,over various parts of the country, typical and acceptable settle-ment is usually less then 1 in. Settlement is not as importantwith a solitary structure, but becomes more important when: (1)buildings adjacent to an existing structure need to be intercon-nected, (2) long utility runs need to be connected to the struc-ture, or (3) there is sensitive equipment in the building. Uniformsettlement is somewhat better tolerated than differential settle-ment that is uneven across several columns. Differential settle-ment distorts the structure and causes cracking of the exteriorskin and interior partitions, broken windows, and doors thatdon’t open. Allowable differential settlement may be dependanton the material of the skin and structure; for example, brick andconcrete masonry buildings tolerate less differential settlementthan curtain wall buildings. Differential settlement of 1/4 in. istypically considered tolerable for most building types.

The importance of proper foundation design and detailing cannotbe overemphasized. Working with the geotechnical engineer, famil-iar with the soil conditions in the area, and a structural engineer,familiar with the proposed design and detailing of the foundation,will help ensure the building functions as intended for its life cycle.

SETTLEMENT AND DIFFERENTIALSETTLEMENT

Often, settlement governs the allowable bearing pressure, which isset at an intensity that will yield a settlement within tolerable lev-els for the building type. Allowable settlement is typically building

and use-specific. Total and differential settlement, as well as thetime rate of the occurrence of the settlement, must be consideredwhen evaluating whether the settlement is tolerable. For example,in the case of a conventional steel frame structure, in typical prac-tice a total maximum settlement of 1 in. is usually acceptable, anddifferential settlement of one-half of the total settlement is alsousually tolerable.

ANGULAR DISTORTIONSettlement tolerance is commonly referred to in terms of angulardistortion in the building or settlement between columns. Typically,an angular distortion of 1:480 is used for conventional structures.This equates to 1 in. in 480 in., or 1 in. in 40 ft. Depending on thetype of structure, the allowable angular distortion might vary from1:240 for a flexible structure (such as a wood frame, single-storystructure) to 1:1000 for a more “brittle” or sensitive structure.

EFFECTS OF SOIL TYPESWhen load is applied to granular soils, the grains of soil are able torespond almost immediately, and they will densify as the packing ofthe grains becomes tighter.

Clay soils exhibit a time-dependent relationship associated withthe consolidation of the clay soil. In order for the clay to consoli-date, and the overlying soil or structure to settle, the excess pres-sures that are induced in the water in the clay must dissipate, andthis takes time because of the low permeability of the clay.Depending on the drainage characteristics of the clay, the timerequired for 90 percent of the consolidation (and settlement) tooccur may vary from a few months to several years. If there is ahigh frequency of sand layers or seams within the clay mass, thenthe consolidation will be quicker, because the excess pore waterpressure can be dissipated faster.

Both sand and clay soils have a built-in “memory” that, in effect,remember the maximum load that was applied to the soil at sometime in the past. This memory is referred to as the preconsolidationpressure. If 10 ft of soil has been removed (by excavation or ero-sion) from a soil profile then the equivalent weight of that 10 ft ofsoil (approximately 1250 lbs per square foot) could be reapplied to

PERIMETER WALL WITH COMBINED FOOTING1.42

FOUNDATIONS

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NOTES

1.43 a. Lines defining areas are approximate only. Local areas can bemore or less severe than indicated by the region classification.b. A “severe” classification refers to weather conditions that encourageor require the use of deicing chemicals or where there is potential forcontinuous presence of moisture during frequent cycles of freezing andthawing. A “moderate” classification refers to weather conditions thatoccasionally expose concrete, in the presence of moisture, to freezingor thawing, but where deicing chemicals are not generally used. A “neg-

ligible” classification refers to weather conditions that rarely exposeconcrete, in the presence of moisture, to freezing and thawing.c. Alaska and Hawaii are classified as severe and negligible, respec-tively.

ContributorsTimothy H. Bedenis, PE, SME, Plymouth, Michigan; Richard O. Anderson,PE, Somat Engineering, Taylor, Michigan; James W. Niehoff, PE, PSI,Wheat Ridge, Colorado; Sidney Freedman, Precast/PrestressedConcrete Institute, Chicago, Illinois; William W. Stewart, FAIA, Stewart-

Schaberg Architects, Clayton, Missouri; James Kellogg, AIA, HOK, SanFrancisco, California; John P. McCarthy, PE, SE, SmithGroup,Architecture, Engineering, Interiors, Planning, Detroit, Michigan;American Concrete Institute, www.concrete.org.

19

the soil profile without the soil below sensing any difference.Depending on the process that deposited the soil, weatheringprocesses, past climatological changes, or human activities, thepreconsolidation pressure of the soil may be far in excess of thepressures induced by the current soil profile. When that is the case,settlement of conventional structures is rarely a significant con-cern. But when the soil has not been preconsolidated, the additionof any new load may result in excessive settlement.

SPECIAL TYPES OF SETTLEMENTThree special types of settlement are sometimes of concern:

• Elastic compression: Soil is a visco-elastic material, which meansthat if a pressure is applied, the soil will deform elastically to acertain point. For instance, in excavating for a large basement,the weight of the soil removed will relieve pressure on the under-lying soil, and the soil will immediately expand vertically upwardin an elastic manner because of the stress relief. If the excava-tion is filled back in with soil or a building, the underlying soil willexperience elastic recompression. Normally, the elastic heaveand recompression are relatively minor, and are usually takeninto consideration by the geotechnical engineer.

• Lowering of the water table: The lowering of a groundwatertable can have significant settlement consequences if proper

precautions are not taken. The lowering of the groundwater tablecan occur naturally through drought conditions or vegetation orfrom human activities. If a temporary dewatering system isinstalled in conjunction with a construction project, or if a per-manent perimeter drain is constructed around a basement belowthe original groundwater table, then settlement of nearby struc-tures can occur. The reason is that the clay soil that was previ-ously below the groundwater table and is now above the ground-water table after dewatering has lost its buoyancy condition. Thisincreases the effective stress and weight on the underlying soil.While this is a rather complicated process, the design profes-sional should be aware that any significant lowering of thegroundwater table could adversely affect adjoining property.

• Vibration: This type of special settlement problem occurs whengranular soils (primarily sand and gravel) are subjected to vibra-tions. The vibrations can be from machinery installations, traffic,pile driving, other construction operations, or seismic events.When the vibrations encounter a granular soil deposit that is ina less than optimum dense condition, the vibration energy caus-es the particles to rearrange themselves. The soil settles into adenser condition, and settlement results. Clay soils are notnearly as susceptible to densification by vibrations as are gran-ular soils.

STANDARD FOUNDATIONS

GENERALFootings lie under the basement, crawl space, or foundation walls,and transfer structural loads from the walls of the building to thesupporting soil. Footings are typically cast-in-place concrete,extending beneath the frost depth to prevent damage and heavingcaused by freezing of water in the soil.

The bottom surface of the footing should not exceed a slope of 1 in10. The footing should rest on undisturbed native soil, unless thissoil is unsuitable. in which case, the unsuitable soil should beremoved and replaced with compacted fill material. Similarly, treeroots, construction debris, and ice should be removed prior to plac-ing footings.

Footings should be carefully aligned so that the supported wall willbe near the centerline of the footing. The top surface of cast-in-place concrete footings should be relatively level and generallyshould not be troweled smooth, as a slightly roughened surfaceenhances the bond between the wall and footing.

Building Code Requirements for Structural Concrete andCommentary, ACI 318, and Requirements for Residential ConcreteConstruction and Commentary, ACI 332 establish requirements forfooting design, which take into account the weathering probability,as indicated in Figure 1.43.

SEVERE MODERATE NEGLIGIBLE

Source: Based on ACI 332 Requirements for Residential Concrete Construction and Commentary, Figure 4.1, reprinted with permission of the American Concrete Institute.

WEATHERING PROBABILITY MAP FOR CONCRETE1.43

FOUNDATIONS ELEMENT A: SUBSTRUCTURE

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ELEMENT A: SUBSTRUCTURE FOUNDATIONS20

SHALLOW FOUNDATIONS

Shallow foundations are typically the most economical foundationsto construct where soil and loading conditions permit. Coordinationwith local codes for frost depth and with the underground utilitiesis required. The thickness of the footing has to be coordinated withanchor bolt and dowel embedment. Typically, one layer of steel inthe bottom of the footing is required to resist the bending of thefooting caused by soil-bearing pressures.

SPREAD FOOTING RESISTING MOMENT, SHEAR, ANDAXIAL LOADS1.49

WEATHERING PROBABILITY

NEGLIGIBLE MODERATE SEVERE MAXIMUMTYPE OF LOCATION OF CONCRETE CONSTRUCTION f�c PSI f�c PSI f�c PSI SLUMP (IN.)

Type 1: Walls and foundations not exposed to weather; interior 2500 2500 2500 6slabs-on-ground, not including garage floor slabs.

Type 2: Walls, foundations, and other concrete work exposed 2500 3000 3000 6to weather, except as noted in Type 3.

Type 3: Driveways, curbs, walk-ways, ramps, patios, porches, 2500 3500 4500 5steps, and stairs exposed to weather and garage floors, slabs.

MINIMUM SPECIFIED COMPRESSIVE STRENGTH AT 28 DAYS F’C, AND MAXIMUM SLUMP OF CONCRETE1.44

Source: Based on ACI 332 Requirements for Residential Concrete Construction and Commentary, Table 4.1, Reprinted with permission of theAmerican Concrete Institute.

NOMINAL MAXIMUM AIR CONTENT (TOLERANCE ±0.015)

AGGREGATE SIZE (IN.) MODERATE SEVERE

3/8 0.06 0.075

1/2 0.055 0.07

3/4 0.05 0.07

1 0.045 0.06

1-1/2 0.045 0.055

AIR CONTENT FOR TYPE 2 AND TYPE 3 CONCRETEUNDER MODERATE OR SEVERE WEATHERINGPROBABILITY1.45

Source: Based on ACI 332 Requirements for Residential ConcreteConstruction and Commentary, Table 4.2, Reprinted with permission ofthe American Concrete Institute.

FOOTING MINIMUM DIMENSIONAL REQUIREMENTS1.46

AXIALLY LOADED SPREAD FOOTING1.48

DOWEL AND KEYWAY REQUIREMENTS FOR FOOTINGS1.47

Source: Based on ACI 332 Requirements for Residential Concrete Construction and Commentary, Figures R6.5 and R 6.6. Reprinted with permissionof the American Concrete Institute.

Axial loads are distributed in a uniform manner under the footing.The allowable bearing pressure necessary to resist the load deter-mines footing size.

SPREAD FOOTING SIZE LIMITATIONS1.50

Minimum sizes of spread footings are specified by the geotechni-cal engineer, to reduce the possibility of local soil failures by punch-ing shear of an overall movement of soil mass. Maximum sizes ofspread footings keep the nonuniform bearing pressure frombecoming extreme and overstressing the soil.

Axial loads, combined with shear and overturning forces can beresisted by spread footings. The combination of axial load andmoment forces on the foundation need to be balanced to keep thecalculated loads on the footing less than the allowable bearingpressure of the soil as determined by the geotechnical engineer.

NOTES

1.44 Maximum slump refers to the characteristics of the specified mix-ture proportion based on water cement ratio only. Midrange and high-range water-reducing admixtures can be used to increase the slumpbeyond these maximums.1.45 American Concrete Institute (ACI) and International Building Code(IBC) have requirements for the minimum footing dimensions.

Contributor:American Concrete Institute, www.concrete.org.

04_700913_ch01.qxd 2/2/07 11:33 AM

FOUNDATIONS AT GRADE1.54


NOTES

1.51 a. Soil below footing should not be disturbed after excavation.b. Footing size based on allowable bearing pressure.c. Thickness based on bending and shear requirements.d. Reinforcing steel based on bending and minimum steel requirements.e. Dowels as required to transfer load.1.52 a. Provide concrete fill after all dead load has been applied to column.b. Thickness of nonshrink grout to accommodate unevenness of footingsurface and leveling nuts.c. Anchor bolts designed to resist moments and shears from axial loads,as well as lateral loads.

Contributors:Anthony L. Felder, Concrete Reinforcing Steel Institute, Schaumburg,Illinois; Kenneth D. Franch, AIA, PE, Phillips Swager Associates, Inc,Dallas, Texas; Donald Neubauer, PE, Neubauer Consulting Engineers,Potomac, Maryland; Mueser Rutledge Consulting Engineers, New YorkCity, New York; SmithGroup, Architecture, Engineering, Interiors,Planning, Detroit, Michigan.

21

SPREAD FOOTING—CONCRETE COLUMN1.51

SPREAD FOOTING—STEEL COLUMN1.52

TYPICAL FOUNDATION WALL AND SPREAD FOOTING DETAIL1.53

FOUNDATIONS AT BASEMENT1.55

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Contributors:Anthony L. Felder, Concrete Reinforcing Steel Institute, Schaumburg,Illinois; Kenneth D. Franch, AIA, PE, Phillips Swager Associates, Inc,Dallas, Texas; Donald Neubauer, PE, Neubauer Consulting Engineers,Potomac, Maryland; Mueser Rutledge Consulting Engineers, New YorkCity, New York; SmithGroup, Architecture, Engineering, Interiors,Planning, Detroit, Michigan; National Concrete Masonry Association,Herndon, Virginia.

22

GRADE BEAMSGrade beams are a structural component spanning between stan-dard and special foundations that transfer building loads to spreadfootings, piles, or caissons. The depth of the grade beam is deter-mined by the frost depth, the geotechnical engineers recommen-dation, or the depth required to support the loads over the span —whichever is deeper.

Reinforcing is critical in the grade beam, and, typically, there areheavy bars (top and bottom) to resist the bending moments, andstirrups to resist the shearing forces. These stirrups should beclosed if there is any torsional loading (such as unsymmetrical wallloading, the grade beam is on a curve, or there is a load reversalfrom wind or seismic forces).

FOUNDATION WALLSFoundation walls are used where basements are not required andthe need to support a limited load on firm soil exists. Typically, theexcavation is to below frost depth or as required by the geotechni-cal engineer. The wall thickness is typically 8 in. but may be thickerif the wall it is supporting is thicker. Minimal amounts of reinforc-ing steel are required to limit cracking. Reinforcing requirementsmay increase as the height of the building wall increases.

In colder climates, insulation is sometimes provided on the insideface of the foundation and under the slab to minimize the cold pen-etration to the interior.

When required, brick ledges need to be coordinated betweengrade elevation and desired brick coursing. Generally, an isolationjoint is provided at the slab-wall interface to allow the slab andgrade wall to settle independently.

Standard foundation walls may be constructed using concrete,masonry units or wood.

Concrete unit masonry used in foundation wall construction shouldbe laid on a clean footing. Mud, oil, dirt, ice, or other materials thatreduce the bond should be removed prior to wall construction. Toaccommodate surface irregularities in the footing, set the first unitmasonry course on a mortar bed ranging in thickness from 1/4 to3/4 in. Fully bed the first course of unit masonry including webs,mortar should not protrude excessively into masonry cells that willbe grouted. Many building codes require Type M or Type S mortarfor use in foundation wall construction.

For reinforced construction, reinforcing steel must be properlylocated. In most cases, vertical reinforcing is positioned towardthe interior face of below-grade walls to provide the greatestresistance to soil pressure.

A solid top course on the below-grade concrete masonry wallspreads loads from the building above. Where only the top courseis to be grouted, wire mesh or other equivalent grout stop materi-al can be used to contain the grout.

For residential construction, anchor bolts are typically embedded 7in. into the masonry, are 1/2 in. in diameter, and are spaced at amaximum of 6 ft. on center, to attach the home to the foundation.Wood in direct contact with masonry materials should be pressure-treated or naturally decay-resistant.

GRADE BEAM 1.56

CONCRETE FOUNDATION WALL1.57

CMU FOUNDATION WALL AT BASEMENT1.58

Source: Based on NCMA Annotated Design and Construction Details for Concrete Masonry Figure 3E.1. Courtesy of National Concrete MasonryAssociation.

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NOTES

1.59, 1.60, 1.61, 1.62, 1.63: Refer to Figure 1.108 for flowchart of loca-tion logic of vapor retarders under slabs.

Contributor:National Concrete Masonry Association, Herndon, Virginia.

23

CMU FOUNDATION WALL AT CRAWL SPACE1.59

CMU FOUNDATION WALL WITH BRICK LEDGE1.61

INTERIOR LOAD-BEARING CMU FOUNDATION WALL1.63

Source: Based on NCMA Annotated Design and Construction Details forConcrete Masonry Figure 3E.3. Courtesy of National Concrete MasonryAssociation.



CMU FOUNDATION WALL AT CRAWL SPACE WITHABOVE-GRADE MASONRY1.60


FOUNDATION WALL DETAILS1.62

Source: Based on NCMA Annotated Design and Construction Details for Concrete Masonry Figure 3E.7. Courtesy of National Concrete MasonryAssociation.

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NOTES

1.65 a. Slab joints should be located away from thickened slab.b. This detail is for relatively lightly loaded walls. Refer to Figure 1.63for load-bearing interior foundation walls1.66 These are highly loaded, closely spaced columns.1.67 Use if the load on the exterior is great, and the distance between thefootings too large for economic use of combined or trapezoidal footing.1.68 Large load along wall footing shifts the load centroid to the left.Trapezoidal footings shift the footing centroid to the left.

Contributor:National Concrete Masonry Association, Herndon, Virginia.

24

EXTERIOR LOAD-BEARING CMU FOUNDATION WALL1.64

COMBINED FOOTING1.66

FOOTING AT PROPERTY LINE1.69

Source: Based on NCMA Annotated Design and Construction Details forConcrete Masonry Figure 8B.1. Courtesy of National Concrete MasonryAssociation.

INTERIOR NON-LOAD-BEARING WALL1.65

STRAP FOOTING1.67

Source: Based on NCMA Annotated Design and Construction Details forConcrete Masonry Figure 8B.2. Courtesy of National Concrete MasonryAssociation.

SPECIAL FOUNDATIONS

There are times when shallow footings are acceptable, but geom-etry limits the effective use of isolated spread footings. Other pos-sibilities include combined, trapezoidal, and strap footings. Thebasic premise of these types of foundations is to modify the foun-dation such that the center of gravity of the foundation aligns withthe center of the applied loads. Depending on whether the heavierload is along the exterior, due to the exterior wall, or the heavierload is on the interior column due to the larger tributary area,either the combined footing or the trapezoidal footing may beappropriate.

If the distance between the footings is large, it might become cost-prohibitive to incorporate a continuous footing from column to col-umn. In such cases, the strap footing provides an alternative thatrequires a stiff grade beam spanning between the footings to dis-tribute the loads and to help make them act as one. Typically, thesoil beneath the grade beam is disturbed, so there is no soil pres-sure transfer below the grade beam.

These are but a few of the options available when faced with atroublesome foundation constraint such as property lines in closeproximity to the edge of the building.

TRAPEZODIAL FOOTING1.68

DEEP FOUNDATIONSWhen the soil near the surface is not suitable to support the build-ing loads, it is sometimes necessary to support the structure ondeep foundations. There are several scenarios when a deep foun-dation makes more sense than a shallow foundation, including thefollowing:

• If a suitable and reliable bearing stratum is at a much lower ele-vation.

• If scour from flowing water can occur at the surface.• If the structure transmits great loads to the foundation.• If the structure transmits tension loads to the foundation.• If there are large horizontal loads transmitted to the foundation.• If reducing settlement or differential settlement is crucial.

Deep foundations come in several types and, depending on the soilconditions, may include driven piles, bored piles (bored, augered ordrilled) or caissons.

DRIVEN PILESDriven piles concrete piles, steel piles, timber piles and compositepiles. The resistance to vertical compressive load typically resultsfrom friction along the sides and end bearing as these types offoundations are driven into the ground with a mechanical hammer.The geotechnical conditions that make driven piles advantageousinclude cohesionless soils with few boulders. Driven piles areselected over bored piles and caissons when groundwater ormethane gas is present above the suitable bearing soil layer. Theseconditions make bored piles difficult and more costly than drivenpiles.

BORED PILESBored piles include auger cast grout piles, bored and socketedpiled, bored concrete piles, drilled caissons, drilled concrete piersand shafts, and drilled micro-piles. In a similar manner to drivenpiles, resistance to vertical loads typically results from a combina-tion of end bearing and side friction. The geological conditions thatmake bored piles advantageous include: cohesive soils, obstruc-tions that can be tolerated with the right size of drill, and a densereliable bearing surface.

Bored piles are preferred over driven piles when:

• The loads are exceptionally high and the bearing surface reli-able.

• Work is in close proximity to existing structures where vibrationfrom driven piles may damage existing structures.

• Inspecting the bearing surface is important, and limited inspec-tion of the bearing material is possible.

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NOTES

1.71 a. Applicable material specifications: Concrete ACI 318; TimberASTM D 25; Structural Sections ASTM A 36, A 572, and A 690. For selec-tion of type of pile, consult a geotechnical engineer.b. A mandrel is a member inserted into a hollow pile to reinforce the pileshell while it is driven into the ground.c. Timber piles must be treated with wood preservative when any por-tion is above the groundwater table.1.72 a. Test soils to determine their allowable bearing capacity.b. “H” (depth of shaft reinforcing, below concrete cap) is the function of

the passive resistance of the soil, generated by the moment applied tothe pier cap.c. Piers may be used under grade beams or concrete walls. For veryheavy loads, pier foundations may be more economical than piles.

Contributors:Mueser Rutledge Consulting Engineers, New York City, New York; John P.McCarthy, PE, SE, SmithGroup, Architects, Engineers, Interiors, Planners,Detroit, Michigan; AB Chance, Centralia, Missouri.

25

PILE TYPES1.70

MAXIMUM OPTIMUM MAXIMUM OPTIMUM CAPACITY LOAD RANGE USUAL

PILE TYPE LENGTH (FT) LENGTH (FT) SIZE (IN.) (TONS) (TONS) SPACING

TIMBERTimber 110 45–65 5–10 tip; 40 15–25 2�–6� to 3�–0�

12–20 butt

STEELSteel H pile 250 40–150 8–14 200 50–200 2�–6� to 3�–6�

Pipe—open end, concrete-filled 200 40–120 7–36 250 50–200 3�–0� to 4�–0�

Pipe—closed end, concrete-filled 200 30–80 10–30 200 50–70 3�–0� to 4�–0�

Shell—mandrel, concrete-filled; 100 40–80 8–18 75 40–60 3�–0� to 3�–6�straight or taper

Shell—no mandrel, concrete-filled 150 30–80 8–18 80 30–60 3�–0� to 3�–6�

Drilled caisson, concrete-filled 250 60–120 24–48 3500 1000–2000 6�–0� to 8�–0�

CONCRETEPrecast concrete 100 40–50 10–24 100 40–60 3�–0�

Prestressed concrete 270 60–80 10–24 200 100–150 3�–0� to 3�–6�

Cylinder pile 220 60–80 36–54 500 250–400 6�–0� to 9�–0�

Drilled pier with socket 120 10–50 30–120 500 30–300 3�–0� to 8�–0�

Drilled pier with bell 120 25–50 30–120 500 30–200 6�–0�

Auger cast grout or CFA 120 40–80 12 - 40 500 75–150 3�-0�(Continuous Flight Auger) pile

Minipiles 200 25–70 2.5–7 100 5–40 2�–0� to 4�–0�

COMPOSITEHelical pier 120 20–70 1-1/2" sq. to 100 15–60 4�- 0� to 15�–0�

4-1/2 dia.

Helical pulldown micropile 100 20–70 4" dia. to 7" dia. 150 20–80 4�- 0� to 15�–0�

Concrete—pipe 180 60–120 10–23 150 40–80 3�–0� to 4�–0�

Steel H pile and prestressed concrete 200 100–150 20–24 200 120–150 3�–6� to 4�–0�

Pile stem with precast concrete tip 80 40 13–35 tip; 180 30–150 4�–6�19–41 butt

GENERAL PILE DATA1.71

DRILLED PIER WITH BELL1.72

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NOTES

1.73 a. Set pier into a socket in rock to transmit high compression ortension lads into rock by side friction and end bearing.b. Pier shaft should be poured in dry conditions if possible, but tremiepours can be used.

26

DRILLED PIER WITH SOCKET1.73

REINFORCING AXIALLY LOADED DRILLED PIERS1.75

PILE SUPPORTED FOUNDATIONS1.76

CONSTRUCTION ISSUES WITH DRILLED PIERS1.74

PILE CAPSTypically, more than one deep foundation element is required toresist the gravity and lateral loads; in order to distribute the loadsfrom the single point column to the multiple foundation elements,a pile cap is required. Pile caps are thick, reinforced concreteblocks that distribute the load from the column to the foundationsthrough a combination of flexure and shear.

Other applications of the pile caps include providing a method ofconnecting the columns to the foundations, and easing the con-struction tolerance issues that occur when installing deep founda-tions. These pile caps are designed and detailed to encase a smallportion of the deep foundation, and transition to the column sup-port elevation, thus providing a convenient location to positionanchor bolts and column dowels.

04_700913_ch01.qxd 2/2/07 11:33 AM

FOUNDATIONS ELEMENT A: SUBSTRUCTURE 27

PILE CAP1.77

VIBRATORY HAMMER1.79

BORED PILE SEQUENCING—STEP ONE1.80

DEEP FOUNDATION MEANS ANDMETHODSDRIVEN PILESRams, called hammers are used to drive piles to impart a verticalforce to the top of the pile. Most hammers use a weight that isrepeatedly lifted and dropped on top of the pile.

Several methods are used to lift the hammer or ram, includingmechanical, hydraulic, and pneumatic, driven devices. Vibratoryhammers consist of eccentric weights that oscillate at a very highfrequency, thus imparting a downward vibratory force to the pile.These are especially effective in granular soils. Vertical steel sup-ports or leads are used to support the hammer and position thepile before and during driving. Crawler-type cranes are generallyused to position the hammer leads over the pile locations.

BORED PILESBored piles use various types of boring equipment and tools toremove a cylindrical area of soil and replace it with a grout or con-crete. The boring equipment is generally attached to a tracked orrubber-tired vehicle to allow easy access to the pile location. Thedrilling equipment uses a power source (gas engine, diesel engine,or electric motor) to turn either a continuous flight of augers or ashort length of auger on the end of the steel shaft. The augerstransport the soil to the ground surface, where the spoil is dis-posed and removed.

Cohesive soils generally allow an open excavation of a large drilledshaft without any additional support. In contrast, granular soils

PILE-DRIVING HAMMERS1.78

and some fill soils require the use of steel casings, or a drillingslurry for large shaft construction. Concrete or grout is placed inthe excavated hole either by direct fall methods or by pumping ortremie methods.

BORED PILE SEQUENCING—STEP TWO1.81

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TYPICAL MAT FOUNDATIONS1.83


ContributorsTimothy H. Bedenis, PE, SME, Plymouth, Michigan; Richard O. Anderson,PE, Somat Engineering, Taylor, Michigan; James W. Niehoff, PE, PSI,Wheat Ridge, Colorado; AB Chance, Centralia, Missouri; DonaldNeubauer, PE, Neubauer Consulting Engineers, Potomac, Maryland;Mueser Rutledge Consulting Engineers, New York City, New York;SmithGroup, Architecture, Engineering, Interiors, Planning, Detroit,Michigan.

28

DEEP FOUNDATION TESTINGTesting of deep foundations is conducted (and is often required) toverify the load capacity and structural integrity of deep foundationelements. Load testing of deep foundations shafts is sometimesdone to provide greater design capacity than can be obtained withconventional analytical methods or with presumptive values com-monly used in a particular region. Two common types of testingare:

• Static load test: This is the most common type of load test for adeep foundation. It consists of incrementally loading a pile witha static load at a specified rate, and measuring the movement ofthe pile. The pile is normally loaded to twice the design load ofthe pile, which will result in a minimum safety factor of 2.Compressive loading is the most common static load test,although tension and lateral load testing is sometimes per-formed if such loads are deemed critical to the structure.

• Dynamic load test: Piles can also be tested using dynamic loads.The pile dynamic analyzer (PDA) is a device that measures the

BORED PILE SEQUENCING—STEP THREE1.82

down to reach with deep foundations. This condition can be reme-died or improved by excavating out the amount of soil equal to theweight of the building, placing a mat or raft foundation at this level,and building up from there. As a rough approximation, each foot ofsoil removed is equal to one story of the building to be construct-ed. In other words, for a building with 10 supported levels, anexcavation of about 10 ft deep would be needed for an approximatebalance. Although there are increased costs involved with the deepbasement and foundation walls, these are sometimes offset by thesavings in deep foundation costs.

SURCHARGED FOUNDATIONSIf the soil within a site is moderately to highly compressible and itis not practical to excavate and recompact the soil, surchargingmay be an option. Surcharging involves the placement of a mass offill over the building limits to preload the underlying soil and inducesettlement prior to building construction. Depending upon thenature of subsurface conditions, surcharging may require a fewweeks to many months. The amount of the surcharge is typically 1.5to 2.0 times the weight of the building to be constructed.

During the period of time when the surcharge is in place, the set-tlement of the original ground surface is carefully monitored. Whenthe total settlement under the surcharge weight reaches the esti-mated settlement of the building, without the surcharge, then thesurcharge is removed. Either a mat or spread footings can typical-ly be used after the soil is removed, and the resulting settlementof the building should be tolerable.

response of a pile under a dynamic impact load such as the driv-ing hammer. The results from the PDA are used to determine themost probable static load capacity for each strike of the ham-mer. A common analytical tool used to predict driven pile capac-ities is the wave equation. The hammer-pile-soil system is math-ematically modeled in the computer program to simulate thedriving of the pile. The output is the estimated ultimate capacityof the pile at various driving resistances, and the inducedstresses in the pile at those resistances. This analytical tool isvery useful for determining what size of pile, as well as what sizeand type of hammer, are required to achieve a desired capacityin the given soil profile.

RAFT FOUNDATIONSMAT FOUNDATIONMat foundations combine all the column loads and distribute themto the soil on a single large foundation. Mat foundations are usedwhere the allowable soil-bearing pressures are low and the size ofa single spread footing becomes excessive or overlaps adjacentfootings. For structures that cannot tolerate differential settlementbetween columns, mat foundations become ideal. Structures thatuse mat foundations include chimneys, silos, and large pieces ofequipment.

COMPENSATED FOUNDATIONSCompensated, or floating, foundations are useful when there aresoft soils susceptible to settlement and the dense soils are too far

COMPENSATED FOUNDATION1.84

04_700913_ch01.qxd 2/2/07 11:33 AM


Contributors:Timothy H. Bedenis, PE, SME, Plymouth, Michigan; Richard O. Anderson,PE, Somat Engineering, Taylor, Michigan; James W. Niehoff, PE, PSI,Wheat Ridge, Colorado; Donald Neubauer, PE, Neubauer ConsultingEngineers, Potomac, Maryland; Mueser Rutledge Consulting Engineers,New York City, New York; SmithGroup, Architecture, Engineering,Interiors, Planning, Detroit, Michigan.

29

SURCHARGE FOUNDATION1.85

MINIPILE CONSTRUCTION1.86

MINIPILESMinipiles, micro-piles, root piles, and pin piles are small diameterpiles that vary in size from 2 to 10 in. They are conventionally drilledby using a casing to the required depth. Depending on the loadrequirement, a high-capacity steel bar is grouted into place withinthe pile. Capacities of these piles can range from 20 kips to 400kips, depending on the bearing conditions.

Minipiles are commonly used to reinforce foundations or to createnew foundations when conventional methods of deep foundationscannot be used because of headroom limitations, vibration, andnoise restrictions. These piles can be used in tension as well ascompression. Pile connections to existing foundations vary, but areeasily accomplished using reinforcing dowels.

HELICAL ANCHORSHelical anchors provide an economical solution to many geotechni-cal problems. Relying on the bearing capacity of the helical plates,this foundation is screwed into the earth until the resistance reach-es the value determined by the structural engineer and the helicalanchor manufacturer. Applications include new construction, pro-viding additional structural or column support, and underpinning.

These anchors provide a large tension capacity, which is useful fortieback systems in retaining and seawalls, tower applications, andsoil screw systems for vertical soil excavations. Helical anchorscan be part of an overall plan to reinforce and retrofit an existingstructure that has a weekend foundation or settlement.

HELICAL ANCHOR APPLICATIONS1.87

Source: AB Chance, Centralia, Missouri

04_700913_ch01.qxd 2/2/07 11:33 AM

NOTES

1.88 Vertical post-tensioning can be used to resist uplift forces; momentresistance is achieved.1.91 Two connections per panel are typical.


FOUNDATIONS DETAILSBuildings, other than cast-in-place concrete or steel framingrequire special foundation details to support the loads and ensureeconomical construction. Several common building type founda-tions have been illustrated including precast concrete and tilt-upconcrete, including the unique connection details.

POST-TENSIONED CONCRETE WALL-TO-FOUNDATIONCONNECTION1.88

GROUTED WALL-TO-FOUNDATION CONNECTION1.89

WELDED PLATE TO FOUNDATION CONNECTION1.92

PIER CONNECTION (SECTION)1.96

BOLTED WALL-TO-FOUNDATION CONNECTION1.90

OVERSIZED BASE PLATE AT COLUMN BASECONNECTION1.94

COLUMN BASE CONNECTION1.93

WELDED WALL-TO-FOUNDATION CONNECTION1.91

TILT-UP CONCRETE WALL FOUNDATION TYPES1.95

SEISMIC BASE ISOLATION

Base isolation of structures refers to the concept of detaching thestructure from the foundation for the purpose of reducing the levelof seismic forces imparted to the structure through horizontalground-shaking by changing the dynamic characteristics of thestructure. The separation of foundation and structure is accom-plished by providing a flexible connection between the two thatallows for horizontal movement of the foundation at a different fre-quency of that from the structure above. Energy-dissipatingdevices are used in conjunction with the isolators to control struc-tural displacements.

Reduced demands on the lateral force-resisting structural ele-ments, including foundations, allows for a more economical designof these elements. By limiting the amount of force that is deliveredto the structure, and by also limiting the amount of movement,damage to the structure, facade, and building contents is signifi-cantly lessened.

Base isolation of structures in high-seismic regions is most appro-priate for low- to midrise buildings, essential service buildings(such as hospitals and police and fire stations), and buildings withvaluable contents (such as museums and scientific research facili-ties). Base isolation has been successfully used for both new andexisting structures.

When using base isolation, it is important to ensure that the isola-tors are the only places where the building touches the surround-ing earth. This is normally accomplished by positioning the building

04_700913_ch01.qxd 2/2/07 11:33 AM


ContributorsSidney Freedman, Precast/Prestressed Concrete Institute, Chicago,Illinois; William W. Stewart, FAIA, Stewart-Schaberg Architects,Clayton, Missouri; James Kellogg, AIA, HOK, San Francisco, California;SmithGroup, Architecture, Engineering, Interiors, Planning, Detroit,Michigan.

31

in a large, excavated area, and connecting the building to the sur-rounding ground with flexible “bridges.” The base isolators areusually located in a subbasement dedicated to their use.

A recent variation of base isolation is offered by a family of devicesthat absorb or dissipate energy and change the response of astructure to seismic activity. These devices are most useful forretro existing structures without the need for an entirely newstructural system.

BASE ISOLATION1.97

UNDERPINNING WALL ELEVATION1.98

UNDERPINNING AND FOUNDATIONREINFORCING

Underpinning is a process of transferring the weight and support-ed loads of an existing structure from their original foundation to anew and lower stratum. Underpinning may be required to deepenthe foundation because of new work in the immediate vicinity ofthe structure. It may also be required if the building itself requiresnew foundations at a lower level; to furnish additional foundationcapacity to an inadequately supported structure that may be set-tling or in distress; or when additional loads may be added to exist-ing structures, requiring reinforcement and/or enlargement oftheir foundations.

Underpinning requires great ingenuity and good engineering judg-ment, as it is a specialized form of construction, one that must beproperly planned and performed by experienced field personneland engineers. The work requires careful consideration of thesoils, the water table, the character of new work, the details of theexisting structure, the maintenance and use of structures duringthe work, and magnitude of the loads. Before designing underpin-ning, the structure to be underpinned must be analyzed to deter-mine the loads to be supported. If original drawings are available,these loads can be determined based on the original design; ifdrawings are not available, loads must be calculated based onthese factors: the type of framing, the height of the building, thecolumn spacing, the wall thickness, the window spacing, wind, theusage of the structure, and seismic design factors.

Likewise, the extent of underpinning has to be carefully deter-mined. Usually, adjacent excavation influences a foundation that iswithin a one-to-one slope from the bottom of the foundations.Softer material (such as silt or clay) may require a flatter slope. Ifinterior columns are outside of the one-to-one slope, they may notrequire underpinning.

UNDERPINNING WALLSOne method of underpinning brick, masonry, and concrete walls is to:

1. Dig an intermittent approach pit approximately 3 ft wide and 4 ftlong, at approximately 15- to 20-ft intervals.

2. Extend an approach pit under the footing.3. When the pit has been excavated to the required depth, build a

form across the approach pit, then concrete the pit to within 3in. of the underside of the footing.

4. After the concrete is set, dry-pack the joint with a stiff grout.

UNDERPINNING WALL CONSTRUCTION1.99

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UNDERPINNING WALLS OR FOOTINGS USINGNEEDLE BEAMSNeedles are long horizontal beams inserted through a foundationor pier, which transfers the loads to adjacent soil. A series ofholes—some 5 to 10 ft apart—are made in the wall. Steel beamsare placed through them and supported on mud sills or blocking.The spaces between walls or footings and steel beams are filledwith concrete or grout. The excavation, concreting, and dry-packingcan then be done. The needle beam and their supports are removedand the wall is patched. It is also possible to support needles ascantilevers.

NEEDLE BEAM1.100

UNDERPINNING COLUMNS1.102

CANTILEVER NEEDLE BEAMS1.101

Where an isolated column and footing is to be underpinned, twobeams or channels are connected to the existing column by weld-ing or by drilling the column and installing bolts. These needlebeams are supported upon timber posts or screw jacks grillage ora mud sill. Once the column is supported on these needle beams,the footing can be underpinned.

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FOUNDATIONS ELEMENT A: SUBSTRUCTURE 33

SHORING TO CORRECT SETTLEMENT1.103

SOIL TREATMENT USING MICROFINE CEMENT AND CHEMICAL GROUTS1.105

A way of placing new permanent foundations under a building thatis settling is to add greater loads to it. Pits are excavated andsheathed in a series. Capped steel pipes are jacked down in shortpieces to form piles that are filled with concrete. Jacks are placedon the top of pile section and under the footing.

REPLACING FOUNDATIONS WITH PILING1.104

If space is available, an outside pile is driven or jacked down, andthe inside pile is jacked down. A beam is inserted one at a time andthen shimmed tight to the beam; after completion, the beam isencased with concrete, for protection.

FOUNDATION REINFORCINGThree methods may be used for foundation reinforcing: structuralchemical grouting, jet grouting, and compaction grouting.

STRUCTURAL CHEMICAL GROUTINGStructural chemical grouting permeates sands with grouts andproduces a stronger sandstone-like strata that carries the loads.Water-control chemical grouting permeates the sands with groutmixtures to fill the voids and control the flow of water. Soil treat-ment using microfine cements and chemical grouts are becomingmore common when the existing soil conditions are relatively per-meable such as granular soils with silt contents of less than 10percent.

The use of microfine cement is a permanent form of treatment.Upon saturation of the soil, bearing capacity will greatly improve.Strengths may vary from 50 psi to 1000 psi, depending on soil type,water-to-cement ratio, and volumes injected. There are many appli-

cations, including water cutoff, earth retention, and for underpin-ning. This system eliminates the need to conventionally underpinfoundations when accessibility is difficult to obtain in areas such ashospitals, offices, schools, and industrial sites.

JET GROUTINGJet grouting is a form of hydraulic erosion using cement, water,and air. The three basic types of jet grouting are:

• Single system: using cement• Double system: using cement and air• Triple system: using cement, water, and air

The process involves eroding the soil using high-pressure (4500 to8000 PSI) through horizontal nozzles at the end of a drill rod. Basedon pressure, volumes, and rotation speeds, uniform soil andcement columns can be achieved. With strengths ranging from 300PSI to 2000 PSI, this type of soil treatment can provide earthretention, water cutoff, and foundation support. It is not uncom-mon to have 65 ft vertical cuts alongside building foundations, toaccommodate new construction.

Some advantages of jet grouting over other types of underpinningstrategies include speed, no maintenance, very low vibration influ-ence, the ability to apply in specific locations and a wide variety ofshapes, the ability to use in areas with limited workspace, highlevel of safety, and applicability in a wide variety of soil types.

JET GROUTING1.106

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NOTES

1.107 a. Slump is assumed to be achieved using a Type A water-reduc-ing admixture.b. Compressive strength at 28.c. FF :Floor Flatness index.

Contributors:Ganpat M. Singhvi, PhD, PE, SE, Senior Engineer, SmithGroup, Detroit,Michigan; Steve Maranowski, Spartan Specialties, Sterling Heights,Michigan; American Concrete Institute, www.concrete.org.

34

COMPACTION GROUTINGCompaction grouting is the process of injecting a stiff mortar groutinto the soil. Sands are the most applicable soil for this treatment.The purpose of the material is to displace the existing soil, whichwill densify the surrounding soil. This process is used to improvebearing capacity, as well as to lift structures if required. It is notuncommon to lift multistory buildings with compaction grouting, orto restore bearing capacity to the soil.

ANTICIPATED TYPECLASS SLUMPa CONCRETE STRENGTHb OF TRAFFIC USE SPECIAL CONSIDERATIONS FINAL FINISH

1. Single course 5� 3000 psi Exposed surface—foot traffic Offices, churches, commercial, Uniform finish, nonslip aggregate Normal steel-troweled finish, institutional, multiunit residential in specific areas, curing nonslip finish where required

Decorative Colored mineral aggregate, color As requiredpigment or exposed aggregate, stamped or inlaid patterns, artistic joint layout, curing

2. Single course 5� 3000 psi Covered surface—foot traffic Offices, churches, commercial, Flat and level slabs suitable for Light steel-troweled finishmultiunit residential, institutional applied coverings, curing. with floor coverings Coordinate joints with applied

coverings.

3. Two-course 5� 3000 psi Exposed or—covered surface Unbonded or bonded topping over Base slab—good uniform level Base slab—troweled finish foot traffic base slab for commercial or surface tolerance; curing under unbonded topping; clean,

nonindustrial buildings where textured surface under bonded construction type or schedule topping dictates

Unbonded topping—bond breaker Topping—for exposed surface, on base slab, minimum thickness normal steel-troweled finish; 3", reinforced; curing for covered surface, light

steel-troweled finish

Bonded topping—properly sized aggregate, 3/4� minimum thickness; curing

4. Single course 5� 3500 psi Exposed or covered surface— Institutional or commercial Level and flat slab suitable for Normal steel-troweled finishfoot and light vehicular traffic applied coverings; nonslip

aggregate for specific areas; curing. Coordinate joints with applied coverings.

5. Single course 5� 3500 psi Exposed surface—industrial Industrial floors for manufacturing, Good uniform subgrade, joint Hard steel-troweled finishvehicular traffic (i.e., pneumatic processing, and warehousing layout, abrasion resistance, wheels and moderately soft solid curingwheels)

6. Single course 5� 3500 psi Exposed surface—heavy-duty Industrial floors subject to heavy Good uniform subgrade, joint Special metallic or mineral industrial vehicular traffic (i.e., traffic; may be subject to impact layout, load transfer, abrasion aggregate surface hardener; hard wheels and heavy wheel loads resistance, curing repeated hard steel trowelingloads)

7. Two-course 5� 3500 psi Exposed surface—heavy-duty Bonded two-course floors subject Base slab—good uniform Clean, textured base slab surfaceindustrial vehicular traffic (i.e., to heavy traffic and impact subgrade, reinforcement, joint suitable for subsequent bonded hard wheels and heavy wheel layout, level surface, curing topping. Special power floats forloads) topping are optional; hard steel-

troweled finish

Topping—composed of well-graded all-mineral or all-metallic aggregate; minimum thickness 3/4". Mineral or metallic aggregate surface hardener applied to high-strength plain topping to toughen; curing

8. Two-course 3� 4000 psi As in Classes 4, 5, or 6 Unbonded topping—on new or Bond breaker on base slab, As in Classes 4, 5, or 6old floors where construction minimum thickness 4", abrasion sequence or schedule dictates resistance, curing

9. Single course or topping 5� 4000 psi Exposed surface—superflat or Narrow-aisle, high-bay Varying concrete quality Exposed surface—superflat critical surface tolerance required. warehouses; television studios, requirements. Special application or critical surface tolerance maySpecial materials-handling vehicles ice rinks, or gymnasiums. Refer procedures and strict attention to be required. Refer to ACI 302.1R or robotics requiring specific to ACI 36OR for design guidance. detail are recommended when Guide for concrete floor and slabtolerances shake-on hardeners are used. construction Section 8.9.

FF 50 to FF 125 (“superflat” floor); curing.

CLASSES OF FLOORS ON THE BASIS OF INTENDED USE AND SUGGESTED FINAL FINISH TECHNIQUE1.107

• Applications of compaction grouting include: improving thecapacity of loosened soil, loosely placed fill, rubble fill, and soilssusceptible to liquefaction; and in tunneling operations to com-pensate for ground loss.

• Advantages of compaction grouting include: pinpoint treatmentapplications, speed of installation, very low space and headroomconditions, no spoil disposal, and no connection to the existingfooting required for this application.

CAST-IN-PLACE CONCRETE SLAB

GENERALFactors to be considered in the design and construction of cast-in-place concrete slab, often called slab-on-grade, include the intend-ed use, base and subbase materials, concrete thickness, concretecompressive or flexural strength, concrete mix design, joint loca-tions, reinforcement, surface finish and treatment, curing, andjoint-filling materials and installation.

Source: Based on ACI 302.1R Guide for Concrete Floor Construction and Slab Construction, Table 2.1, reprinted with permission of the American Concrete Institute.

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NOTES

1.108 a. If granular material is subject to future moisture infitration,use Figure 2.b. If Figure 2 is used, a reduced joint spacing, alow-shrinkage mixdesign, or other measures to minimize slab curl will likely be required.


35

SUBGRADESlab-on-grade design should be based on adequate geotechnicalinformation. The capability of a slab to support loads depends onthe integrity of both slab and soil support. The common classifica-tions encountered in slab-on-grade design delineate soil accordingto grain size, moisture content, plastic, and liquid limit. Atterbergplastic and liquid limits indicate the percentage of moisture wheresoil acts as a semisolid, or acts as a liquid, and will affect capacityand performance of the slab. Unified soil classifications are themost recognized and referred by geotechnical firms. The basematerial should be compactable and well drained and provide uni-form load-bearing support; coarse-grained materials are mostlikely to be specified for this application. Soil materials designatedas fine-grained are generally associated with either a high degreeof compressibility or instability resulting from volume changes, andmay allow the floor slab to settle or sink over a long period. Whenfine-grained soils are encountered, input from the geotechnicalengineer is advised.

CONCRETE MIXThe performance of concrete slabs depends on the specific char-acteristics of ingredients; therefore, the concrete mix design mustbe reviewed for strength, shrinkage, finishability, and durability.The portland cement content and the content of other cementitiousproducts, if used, should be sufficient for finishability and durabili-ty, per ACI 301. The setting characteristics of concrete should berelatively predictable, and concrete should not experience exces-sive retardation and surface crusting. Concrete floors exposed tofreezing and thawing should have water/cementitious ratios of nomore than 0.5, and if subjected to deicing chemicals, of no more

than 0.45. Concrete in these exposure conditions should also haveair contents between 4 and 7 percent.

SLAB REINFORCEMENTReinforcement in slabs, including reinforcing bars, welded wirefabric, steel or synthetic fibers, is provided primarily to control thewidth of cracks caused by shrinkage. Only when reinforcing steelis properly sized and placed can the effect of shrinkage cracks becontrolled. The best way to reduce or prevent shrinkage cracking isto pay close attention to the smoothness of the base, the quality ofthe concrete, the curing process, and the joint spacing and timingof the joint cutting.

VAPOR RETARDERSACI 302 recommends a minimum of 10-mil thickness for vaporretarders. This material will limit vapor transmission from the soilsupport system through concrete slabs, which can adversely affectmoisture-impermeable or moisture-sensitive floor finishes. ACI302 recommendations regarding where to place the vapor retarderare governed by local conditions and construction sequences.

JOINTSJoints in the concrete slab-on-grade are provided to reduce thefrequency and width of random cracks caused by concrete shrink-age, or to allow for the practical limits of concrete placement.These types of joints are classified as one of the following:

• Isolation joints: Allow complete freedom of horizontal and verti-cal movement between the floor and adjoining walls not requir-ing lateral restraint from the slab, columns, equipment founda-tions, or other points of restraint (such as drains, manholes,sumps, and so on).

• Construction joints: Define the extent of the individual place-ment of concrete, generally in conformity with a predeterminedjoint layout. These can also function as contraction or isolationjoints.

• Contraction (control) joints: Induce cracking at preselected loca-tions, usually at column lines, with intermediate joints located atequal spaces between column lines. Spacing of contractionjoints depends on the method of design, slab thickness, type andamount of reinforcement, base friction, layout of foundations,and curing, among other items. Saw-cut contraction jointsshould be made as early as practical after finishing the slab, andin areas with wet conditions, hygienic and dust control require-ments, or where the floor is subjected to traffic by hard-wheeledvehicles such as forklifts, they should be filled. A semirigid fillerwith a Shore Hardness “A” of at least 80 should be used in jointssupporting forklift traffic.

DECISION FLOWCHART FOR DETERMINING NEED FOR VAPOR RETARDER AND WHERE TO PLACE IT1.108

Source: Based on ACI 302.1R Guide for Concrete Floor Construction and Slab Construction, Figure 3.1, reprinted with permission of the AmericanConcrete Institute.

APPROPRIATE LOCATIONS FOR JOINTS1.109

Source: Based on ACI 302.1R Guide for Concrete Floor Construction andSlab Construction, Figure 3.2, reprinted with permission of theAmerican Concrete Institute.

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NOTE

1.111 Alternate to round or square dowels.


36

TYPICAL JOINT DETAILS1.110

Source: Armored Construction Joint Detail based on ACI 302.1R Guidefor Concrete Floor Construction and Slab Construction, Figure 3.15,reprinted with permission of the American Concrete Institute.

DIAMOND-SHAPED LOAD PLATES AT SLAB CORNER1.111


DOWELED JOINT DETAIL FOR MOVEMENT PARALLELAND PERPENDICULAR TO THE JOINT1.112


TYPICAL ISOLATION JOINTS AT TUBE COLUMNS1.113


FINISH AND FLOOR FLATNESSIn general, concrete floor slabs are monolithically finished by float-ing and troweling, to achieve a smooth and dense surface finish.ACI 302 provides guidance for appropriate finishing procedures tocontrol achievable floor flatness. ACI 302, ACI 360, and ACI 117provide guidance for flatness selection, as well as techniques bywhich flatness and levelness are produced and measured.

Floor finish tolerance is measured by placing a freestanding 10-ftstraightedge on the slab surface, or by F-Numbers. The preferredmethod of measuring flatness and levelness is the F-NumberSystem. Special finishes are available to improve appearance, aswell as surface properties. These include sprinkled (shake) finish-es or high-strength toppings, either as monolithic or separate sur-faces.

PROTECTIVE AND DECORATIVE COATINGSConcrete surfaces may require a sealer or coating for the following:

• To protect against severe weather, chemicals, or abrasions.• To prevent dusting of the surface layer.• To harden the surface layer.• To add a decorative finish.

Sealers are usually clear and are expected to penetrate the sur-face without leaving a visible film. Coatings are clear or opaque,and, though they may have some penetration, they leave a visiblefilm on the surface. Sealers and coatings should allow vapor emis-sion from the concrete but, at the same time, prevent moisturefrom penetrating after curing.

Decorative coatings usually protect as well, and are formulated in awide selection of colors. Decorative coatings include the following:

• Water-based acrylic emulsion• Elastomeric acrylic resin• Liquid polymer stain• Solvent-based acrylic stain• Portland cement-based finish coating• Water-based acidic stain (a solution of metallic salts)

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NOTES

1.114 a. Floor-hardening agents are applied to reduce dusting andincrease hardness slightly at the surface.b. Consult a qualified specialist to determine the correct coating or seal-er for a particular application.c. There may be restrictions on the use of solvent-based coatings andsealers in some areas due to the presence of volatile organic com-pounds (VOCs).1.115 Expect some cracking in concrete construction. Generally, crack-ing is controlled with joints and reinforcement. However, not all cracks

indicate errors or performance problems, and not all cracks need to berepaired.


37

TYPICAL DOWELED JOINT DETAIL FOR POST-TENSIONED SLAB1.116

FINISH USE

Cementitious acrylic Aesthetic treatmentpolymeric coating

Two-component epoxy coating Protects damp or underwater surfaces

Solvent-based aliphatic Resists graffiti, chemicals, abrasionurethane coating

Epoxy coal-tar-based coating Waterproof; resists corrosion

Coal-tar-modified epoxy Nonskid waterproof resin coating surface membrane

Water-based epoxy coating Chemical and abrasion resistance for interiors

Vinyl ester–based coating High chemical resistance

Aliphatic urethane coating Chemical and abrasion resistance

Solvent-based acrylic Reduces water penetrationmethacrylate copolymer sealer

Silane/siloxane sealer-penetrating Protects from deicers andwater repellent freeze/thaw damage

PROTECTIVE COATINGS AND SEALERS1.114

EARLY CONCRETE SURFACE CRAZING PLASTIC SHRINKAGE VOLUME CHANGES OTHER CRACKING

Cause Shrinkage of cement paste at Water at the concrete surface As concrete cools and hardens, Subgrade Prematureexposed concrete surfaces because evaporates too rapidly because concrete volume shrinks; settlement excessive of concrete mix, too-wet excessive of job-site conditions (such as cracking will occur if slab is loading onbleeding, overtroweling surface, low humidity, high wind speeds, restrained at any point. slabrapid drying of surface high concrete temperatures, high

to moderate air temperatures).

Effect Unsightly cracking of surface layer, Parallel cracking, fairly wide at Random or regularly spaced Slab will bend Punch-throughalthough surface is probably sound the exposed surface, but shallow; cracks, usually passing and crack at edge by

doesn’t typically extend to slab completely through slab; during heavy edge; crack spacing and length saw cutting of joints, crack equipment,vary greatly. may jump ahead of saw cut. and so on.

Preventive Reduce amount and rate of Reduce rate at which surface Not always preventable, though Compact Generally, measures shrinkage at concrete surface by moisture evaporates by erecting careful joint design or subgrade well curing periods

avoiding wet mixes, limiting windbreaks or building walls reinforcement may help. Other of 4 to 7 bleeding by increasing sand or air before slab, avoiding wet mixes, measures include: tool or saw- days, followed content, limiting troweling/not dampening subgrade before cut joints 1/4 of slab thickness, by 1 to 2 days troweling too early, curing as soon concrete pour, curing as soon as min.; time saw cut according to of dryingas possible. possible, avoiding vapor barrier concrete curing rate; locate

under slab unless necessary. contraction joints at column lines, min.; for unreinforced slabs, space joints at 24 to 36 times slab thickness, max.; post-tension at slab; isolate slabs from adjoining structures with preformed joint filler, or if continuity is required, increase slab reinforcement.

CRACKING IN CONCRETE SLABS-ON-GRADE CONSTRUCTION1.115

TYPES OF SLABS-ON-GRADESThere are six identifiable slab types that are commonly used perACI 360:

• Type 1, Plain concrete slab: This type contains no reinforcementor fibers. Joint spacings are 24 to 36 times the slab thickness.Up to a maximum of 18 ft is recommended to reduce shrinkagecracking.

• Type 2, Slab-reinforced for shrinkage: Shrinkage cracking iscontrolled by a nominal amount of distributed reinforcement orsteel fibers to hold tightly and prevent cracks that may developbetween joints. Joint spacing is usually the same or slightly larg-er than Type 1 concrete slabs.

• Type 3, Shrinkage-compensating concrete slab: The shrinkage-compensating concrete used in this slab contains an expansiveadmixture or Type K concrete with reinforcing, which is placed inthe upper half of the slab to limit the initial slab expansion andto restrain the slab’s subsequent drying shrinkage. Joints can bespaced farther apart than in Type 1 or 2 slabs.

• Type 4, Slab post-tensioned to offset shrinkage: These slabscontain post-tensioning tendons intended for crack control. Theprestressing force allows a wide spacing of construction jointswith no intermediate contraction joints.

• Type 5: Slab post-tensioned and/or reinforced with active pre-stress: These slabs are designed following the Post-TensionInstitute’s procedures using active prestress, which permitsthinner slabs. They are reinforced with post-tensioning tendonsand mild steel reinforcement, and incorporate monolithic beamsto increase the rigidity of the section. In addition, these slabsare designed to accept structural edge loading from buildingsuperstructures, as well as to resist the forces caused byswelling or shrinking of unsuitable soils.

• Type 6: Structurally reinforced slabs: These slabs are designedas reinforced concrete slabs to allow cracking at some prede-termined level of loading.

POST-TENSIONED CONCRETE SLAB-ON-GRADEPost-tensioned concrete slabs-on-grade were introduced in theUnited States in the early 1960s to use steel tendons as reinforce-ments, in lieu of conventional temperature and shrinkage rein-forcement, to introduce a relatively high compressive stress inconcrete by means of post-tensioning. This compressive stressprovides a balance for crack-producing tensile stress as concreteshrinks during curing process. The uncracked section enhancesstiffness and flexural stress, which are important factors associat-ed with slab-on-grade.

Post-tensioned concrete slabs-on-grade are used in large industri-al or commercial buildings to decrease slab thickness, to eliminatemost slab contraction joints, and to provide “superflat” floors.

Source: Based on ACI 302.1R Guide for Concrete Floor Construction and Slab Construction, Figure 3.16, reprinted with permission of the AmericanConcrete Institute.

Post-tensioned concrete slabs-on-grade are concentrated in a fewstates, with 50 percent of the residential applications constructedin Texas; another 25 percent are constructed in California. Nevada,Louisiana, Arizona, Florida, Georgia, and Colorado. account for theremaining 25 percent. Slabs built in Texas are sometimes castdirectly on the vapor retarder, while California slabs are typicallycast on a thin sand base placed over the vapor retarder.

Hydraulic jacks stress steel tendons after the concrete has beenplaced and has achieved 75 percent of its 28-day compressivestrength (usually four to seven days after placement). The tendons

are typically 1/2 in. diameter, 270 ksi steel strands with a factory-applied coating of lubricant that is corrosion-resistant and encasedin a plastic sheath. This results in an “unbonded tendon,” whichleaves the prestressing steel free to move inside the sheath.

In the areas where expansive and compressible soils occur, use ofstiffened (tee beam) post-tensioned concrete slab-on-grade forresidential or commercial floors can provide sufficient strengthand rigidity to allow for swelling and contraction of soil caused bychanges in the moisture contents.

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Contributors:Ganpat M. Singhvi, PhD, PE, SE, SmithGroup, Detroit, Michigan; GrantHalvorsen, SE, PE, Wheaton, Illinois; American Concrete Institute,www.concrete.org; Post-Tensioning Institute, Phoenix, Arizona; WireReinforcement Institute, Hartford, Connecticut.

38

TYPICAL STIFFENED POST-TENSIONED CONCRETE SLAB-ON-GRADE1.117

elevation. Acceptable means of water disposal should be deter-mined prior to implementation. Following are descriptions some ofthe more common types of dewatering methods.

DITCHINGWhen excavations are to extend only a short distance below thegroundwater level, and subsurface soils contain a significant quan-tity of silts and clays, often groundwater may be controlled byditching. In this technique, narrow trenches are excavated into thesoil to a depth several feet below the groundwater level. Thetrenches are sloped such that the intercepted water drains to a lowpoint, from which it is removed by gravity or pumped to a point off-site. Ditches are typically excavated along the upslope portion ofthe site or around the perimeter, to intercept water before it reach-es the site. On larger sites, temporary ditches may also berequired within the construction area. As this approach is typicallyonly applicable to sites containing slow-draining, finer-grainedsoils, ditching must often be performed well in advance of con-struction.

WELLPOINTSWellpoints may be utilized for site dewatering where granularmaterials such as sands and gravels are present and where exca-vations are to extend to only a moderate depth below grade.Wellpoints consist of a series of slotted pipes inserted verticallyinto the ground and connected in series to a horizontal headerpipe. The header pipe, in turn, is connected to a vacuum pump. Thesystem essentially creates negative pressure in the wells toextract water from the soil through suction. With a normal atmos-

pheric pressure of about 14 psi and normal pressure losses in thesystem, the maximum depth to which this technique is effective istypically about 15 ft.

DEEP WELLSWhere groundwater must be lowered to a significant depth, orwhere subsurface materials contain significant quantities of siltand/or clay, wellpoints may not be adequate to lower the ground-water to the desired level. In such instances, deep wells are oftenused. A deep well consists of one or more boreholes drilled wellbelow the groundwater level and the depth of the proposed cut,then fitted with a filtered pipe and a positive displacement pump.The pump, which is situated in the base of the well, is not depend-ent on suction to extract water from the surrounding soil. Waterpasses into the bottom of the well by gravity, and the pump dis-charges the collected fluid to the surface. Unlike wellpoints, deepwells can lower the water table 100 ft or more, depending on thedepth and spacing of the wells and the capacity of the pump.

CONSTRUCTION DEWATERING CONSIDERATIONSWhen groundwater is pumped from the ground, the effectiveweight of the soil within the dewatered portion of the soil profile isincreased roughly twofold. As a result, dewatering causes anincrease in pressure on subsurface materials. For example, lower-ing of the groundwater level by 10 ft will cause an increase instress of more than 600 psf to the materials below the newgroundwater level. Such stress increases will result in some con-solidation of the soils, and resulting in settlement of the groundsurface both within the immediate project site and within a larger

The Post-Tensioning Institute (PTI) and Wire ReinforcementInstitute (WRI) provide recommendations for establishing thedesign requirement for slabs on either stable, expansive, or com-pressible soils. Soil properties needed for design (such as allow-able bearing pressure, edge moisture variation, distance, differen-tial soil movement, and subgrade friction) are determined by thegeotechnical consultant.

A post-tensioned concrete slab-on-grade does provide superiorstructural behavior, thanks to its uncracked sections; there are,nevertheless, several areas of caution to keep in mind:

• Anchors must be of sufficient size and holding capacity.• Tendons must be properly placed, stressed, and anchored.• Slab penetrations made after construction must be properly

located to avoid severing tendons.• Construction loads on concrete must be limited until slabs are

fully stressed.

Contact an engineering firm that is thoroughly experienced in thedesign and installation details for guidelines regarding the designand construction of post-tensioned slabs.

DEWATERINGThe construction of buildings, tunnels, utilities, and roadways oftenrequires excavations that extend below the groundwater level,potentially resulting in detrimental effects. Soils above thegroundwater level provide confinement to hydrostatic pressures.Once these overburden soils are removed, upward fluid pressuresact on the remaining soil particles, loosening them and causing amarked reduction in strength. In the case of granular soils (includ-ing sands), excavation below the groundwater can cause soils toact like a viscous fluid and lose virtually all strength. For finer-grained soils (including materials containing silt and clay), hydro-static pressures can cause a significant increase in moisture con-tent, with a corresponding decrease in strength.

It is important to note that groundwater can cause adverse effectson overburden soils even when excavations do not extend belowthe static groundwater level. The use of vibratory equipment orheavy construction vehicles working in close proximity to the watertable can cause water to migrate upward and cause a similar lossin strength.

To prevent loss of density and strength to soils that will supportstructural elements, it is critical that groundwater be properlyassessed during the geotechnical investigation and that appropri-ate measures be taken to control water during the constructionphase. The need for construction or permanent dewatering, andthe means of dealing with it, should be addressed in the soilsreport for the project.

CONSTRUCTION DEWATERINGA number of methods can be used to control groundwater duringthe construction phase of a project. The selection of an appropriatemethod, or combination of methods, should be based on the natureof the subsurface materials through which the water flows and thedepth to which the excavation must extend below the groundwater

DITCHING1.118

WELLPOINTS1.119

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NOTES

1.122 a. Subdrainage is laid out to meet the needs of a specific site. Agrid, parallel lines, or random pattern at low points in the topographymay be used to collect subsurface water.b. Depth and spacing of subdrainage pipes depend on soil conditions.

Contributor:James W. Niehoff, PE, PSI, Wheat Ridge, Colorado; American ConcreteInstitute, www.concrete.org.

39

zone of influence extending some distance beyond the excavationlimits. This factor is of particular concern when an excavation is tobe made immediately adjacent to developed properties.

Dewatering can also change the direction of local groundwaterflow. Should there be a contaminant plume in the vicinity of the site,it may be directed toward the dewatering system. Water collectedunder such conditions would need to be treated prior to discharge.

Alternatives to dewatering may include:

• Installation of sheet piles, or slurry walls into a low permeabili-ty layer below the depth of the proposed excavation, to signifi-cantly reduce groundwater inflow without affecting the level ofthe groundwater beyond the excavation limits.

• Freezing of the ground by employing injections of liquid nitrogen.This method causes the soil to act as an impermeable solid andallows the excavation to be made without removing water oraffecting water beyond the zone of freezing.

PERMANENT DEWATERINGIn general, structures constructed or installed below the perma-nent groundwater table should be designed to be watertight, sothat permanent dewatering is unnecessary. Where this is cost-pro-hibitive, and where a permanent lowering of the groundwater isdesirable, several methods of permanent dewatering may be con-sidered.

FRENCH DRAINSThe most common form of permanent dewatering involves the con-struction of a ditch filled with gravel and a slotted pipe, to collectand discharge water to a lower area or to a sump that is pumpeddry. Typically, to prevent erosion of fine soils into the gravel withinthe trench, the sides of the trench are covered by a nonwoven geo-textile. Similar methods are available for collecting and dischargingwater from beneath floor slabs and from behind basement walls.

FOUNDATION DRAINAGE1.121

FRENCH DRAIN1.120

SUBDRAINAGE PIPING1.122

UNDERSLAB DRAINAGE1.123

DEEP WELLSAs noted previously, deep wells consist of boreholes fitted withslotted pipe and a positive displacement pump. This type of systemmay be used as part of permanent dewatering, as well as for tem-porary construction applications. However, this method is not pas-sive in nature, and requires periodic maintenance and upkeep, aswell as a constant source of power, accessible cleanouts, and pres-sure-relief plugs.

SUBDRAINAGESubdrainage is very different engineering designs from surfacedrainage. Surface drainage intercepts and collects stormwaterrunoff and conveys it away from a building and site with the use oflarge inlets and storm drains. Subsurface drainage typically issmaller in size and capacity, designed to intercept the slowerunderground flows of a natural groundwater table, undergroundstream, or infiltration of soils from surface sources. Surface andsubsurface drainage typically requires discharge either through apumping station or by gravity.

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NOTES

1.124 a. The depth of a drain determines how much subsurface waterlevels will be reduced.b. When a slotted drain is used, install it with the holes facing down.c. When used to intercept hillside seepage, the bottom of a trenchshould be cut a minimum of 6 in. into underlying impervious material.

Contributors:Joseph P. Mensch, PE, Wiles Mensch Corporation, Reston, Virginia; KurtN. Pronske, PE, Reston, Virginia; Harold C. Munger, FAIA, MungerMunger + Associates Architects, Toledo, Ohio.

40

TYPICAL SUBDRAINAGE1.124

• No surface or groundwater issues with the excavation.• No loads near the top of the excavation, including: vehicles,

existing buildings, soil loads, or construction equipment.

Slope stability determines the slope of the sides of the excavation,which typically run at approximately 1.5 horizontal to 1.0 verticalfor cohesionless soils under normal circumstances.

SHORED EXCAVATIONMost excavations that cannot be safely sloped back are supportedwith some type of temporary wall, called shoring, that can be con-structed from the ground surface. Types of temporary walls includeinterlocking steel sheet piles or steel soldier piles with wood lag-ging, secant walls, slurry walls, cofferdams, and trenching.

SHEET PILES AND TIMBER SHORINGThe most common types of shoring for earth support are inter-locking steel sheet piling and timber shoring. Sheet piling is readi-ly available in many shapes, sizes, and lengths, and is relativelywatertight. Timber shoring uses less steel and is generally lessexpensive than sheet piling; however, it cannot be used as a barri-er against groundwater flow, and could prove to be challengingwhere running sands are retained behind the walls.

Sheet piling is installed from the surface with a pile hammer, whichwill generate noise and vibration. Timber shoring can also beinstalled with a pile hammer, or by drilling and setting the soldierpiles, thereby significantly reducing the noise and vibration fromthe pile-driving operation.

ELEMENT A: SUBSTRUCTURE BASEMENT CONSTRUCTION

DRY WELL1.125

Dry wells provide a method for removal of surface runoff, but theireffectiveness is in direct proportion to the porosity of surroundingsoils, and they are efficient only for draining small areas. High rain-fall runoff rates cannot be absorbed at the rather low percolationrates of most soils, so the difference is stored temporarily in a drywell. Efficiency is reduced during extended periods of wet weather,when receiving soils are saturated and the well is refilled before itdrains completely. In addition, the wells can become clogged withsediment and require periodic cleaning to maintain their effective-ness.

BASEMENT CONSTRUCTION

In residential construction, particularly in the colder climates,basements are inexpensive spaces, because the frost depthsrequire excavations deep enough that a little more excavation canresult in a habitable area, providing space for the utilities andwarmth for the first floor.

In commercial construction, basements are sometimes providedfor the mechanical and electrical facility services. Basements canalso be economical if a relatively thin layer of poor soil is encoun-tered. The cost of removing and replacing the poor soil can be off-set by the excavation and installation of a basement.

BASEMENT EXCAVATION

Nearly all new structures require some excavation to constructfoundations or basements. The removal of soil by excavationresults in changes of the soil stresses below and around the exca-vation. Nature reacts to a vacuum, and gravity will cause theground—and sometimes the groundwater surrounding the exca-vation—to move toward the excavated area. The deeper the exca-vation and steeper the excavation slope, the greater the unbal-anced forces.

Most soils have sufficient inherent strength to resist at least someof these unbalanced forces, at least for a short period of time. Forshallow sloped excavations, the strength or resistance of mostsoils is sufficient to prevent excessive movements during the timerequired for construction. However, for deep excavations and/orareas that do not have sufficient room for a sloped cut, a tempo-rary excavation support system is required to prevent the earthfrom caving in on the excavation. The Occupational Safety andHealth Administration (OSHA) stipulates numerous regulationscontrolling soil excavations. These regulations should be consultedprior to excavation work, particularly if personnel will be workingwithin the excavated area.

OPEN-CUT EXCAVATIONSOpen-cut excavations are generally excavations of large areas withgradual sloping sidewalls that do not require shoring. This methodcan be acceptable when the following conditions have been con-sidered:

• Sufficient room for the slope on either side of the excavation.• Adequate room for placing the spoils away from the excavation.

OPEN EXCAVATION1.126

SHORING1.128

SOIL TYPES

1 2 3 L/H REMARKSFill Rock >1.5 Check sliding of Soil 1

Soft clay Hard clay Rock >1.0 Check sliding of Soil 1

Sand Soft clay Hard clay >1.5 Check lateral displacement of Soil 2

Sand Sand Hard clay >1.5

Hard clay Soft clay Sand >1.0 Check lateral displacement of Soil 2

EMBANKMENT STABILITY FOR OPEN EXCAVATION1.127

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BASEMENT CONSTRUCTION ELEMENT A: SUBSTRUCTURE 41

SECANT WALLSOther less common (but effective) walls are known as secant-drilled shafts. These walls typically consist of steel-reinforced con-crete or cement-based grout. These walls tend to be thicker thansheet piling or soldier piling with lagging, so are several timesstiffer. This is a significant advantage when wall deflection is criti-cal (such as when an excavation is located adjacent to an existingbuilding). The secant shafts can be installed with conventionaldrilled shaft equipment or augured cast-in-place (i.e., augercast)pile equipment. If properly constructed, secant shaft walls are rel-atively watertight.

Although the cost of the secant walls is significantly higher thansheet piling or soldier piling with lagging, the concrete walls areoften incorporated into the final design as permanent walls for thestructure, saving both time and money.

As with secant walls, the cost of the slurry diaphragm foundationwalls is significantly higher than sheet piling or soldier pile and lag-ging walls; but because the concrete walls are often incorporatedinto the final design as permanent walls for the structure, bothtime and money is saved.

COFFERDAMSCofferdams are temporary structures used to prevent both earthand water from entering excavations. They are typically used in theconstruction of bridge foundations, dams, locks, and berthingstructures in and near rivers, ports, and coastlines. Cofferdamsmust be strong enough to resist large lateral hydrostatic and earthpressures, and be sufficiently resistant to water infiltration toallow construction “in the dry.” Sheet piling is most often used inthe construction of cofferdams, but precast concrete and timbermay also be used. For short cofferdams earth berms or sandbagsare sometimes used.

Single-wall cofferdams with one or more levels of bracing are gen-erally suitable for moderate depths (10 to 30 ft) of excavationbelow the adjacent water level. For deeper excavations, the largelateral pressures require more massive structures. Cellular struc-tures consisting of interlocking flat steel sheet piles placed in var-ious patterns are used as temporary cofferdams and for perma-nent construction. The cells are filled with sand and gravel. Thecombination of the earth within the cells and sheet pile creates agravity structure, which is very stable for tall conditions. Engineerscan choose from several patterns of cells, depending on the geo-logic conditions and the constraints of the site.

SINGLE-CELL DIAPHRAGM TYPE COFFERDAM1.131

SECANT WALLS1.129

SLURRY WALLS1.130

SLURRY DIAPHRAGM FOUNDATION WALLSConcrete slurry diaphragm foundation walls, which are stiffer andeven more resistant to groundwater flow and hydrostatic pres-sures, are constructed using special equipment. They are con-structed by excavating deep trenches while simultaneously inject-ing a mixture of water and bentonite clay to replace the soil. Thehydrostatic pressure from the slurry counteracts the forces fromthe weight of the soil and groundwater. Once a section of thetrench is completed, steel reinforcing (consisting of steel cages ormembers) is placed into the excavated trench, and concrete ispumped through a tremie pipe extending to near the bottom of thetrench. The concrete forces the slurry out of the excavation, whichis reused on new portions of the trench.

The resulting reinforced concrete wall provides a very stiff andnearly watertight structure for both temporary and long-term earthsupport. The walls can also support light to moderate foundationloads. Lateral support of the walls is provided either by externaltieback anchors or with internal support from basement floors.

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ELEMENT A: SUBSTRUCTURE BASEMENT CONSTRUCTION42

MULTIPLE-CELL CLOVERLEAF TYPE COFFERDAM1.132

LATERAL SUPPORTWalls that extend to depths of 10 to 15 ft can generally be self-sup-ported or cantilevered and do not require additional lateral sup-port. This is accomplished by extending the shoring sufficient depthbelow the excavation. These “cantilevered” walls resist the lateralpressures from the soil and groundwater, although some deflectionat the top of the wall should be expected. However, walls thatextend deeper or have significant surcharge loads, or wheredeflection is a concern, will require some type of additional lateralsupport (known as “braced walls”).

INTERNAL SUPPORTThe lateral support can be internal to the excavation, in the form ofwales, struts, or rakers. Internal bracing is very effective but inter-feres with the construction of the permanent structure.

TRENCHINGTrenches are narrow vertical cuts in soil used to place utilities andto construct continuous foundations. For short periods of time,many trenches may appear to be stable, but then collapse sudden-ly. Unsupported trenches can be dangerous to workers, as theearly-warning signs of a trench collapse cannot be seen from thebottom of the trench. Moreover, in a narrow trench, there isnowhere to escape, if a sudden collapse of the trench wall occurs.And, because 1 to 2 cu ft of soil weighs as much as 100 to 250 lbs.,even a relatively small collapse can severely injure or kill a work-er. For safety purposes, therefore, OHSA requires trenches morethan 5 ft deep be supported with shoring, or protected with atrench box, before allowing worker access.

TIMBER-BRACED TRENCH1.133

HYRAULICALLY BRACED TRENCH1.134

TRENCH BOX1.135

INTERNALLY BRACED EXCAVATIONS1.136

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INTERNALLY BRACED EXCAVATION SCHEMES1.137

EXTERNAL SUPPORTExternal bracing of the excavation is usually in the form of drilledtieback (ground) anchors. The use of tieback anchors to supportthe wall enables a clear and unencumbered excavation, butrequires access to areas beyond the building, which may extendoutside the property line. For this reason, tieback anchors arealmost always the most desirable method of lateral support if theowner controls the property around the excavation, or if temporaryeasements can be obtained from private owners or public munici-palities.

EXTERNALLY BRACED EXCAVATION DETAILS1.138

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EXTERNALLY BRACED EXCAVATION1.139

SOIL STABILIZATIONSoil stabilization is sometimes called soil modification or soil con-ditioning. For deep excavations in very soft soils, the constructionof earth support systems can be quite massive and expensivebecause of the higher lateral soil and hydrostatic pressures.Furthermore, in some situations, wall installation can be difficult toaccomplish because of space requirements.

Various techniques have been developed to improve the effectivestrength or resistance of the soils around and below the excava-tions. Improving the in-situ soil strength or resistance reduces oreliminates the net pressure at the face of the excavation. Theunderground construction industry has developed the followingtechniques for improving or reinforcing soil.

SOIL-MIXING STABILIZATIONSoil mixing stabilization is the mixing the soils in the ground withasphalt, cement, lime, fly ash, or lime–fly ash to stabilize andstrengthen the soil. Often, only a selected portion of soil massmust be treated to provide a significant increase in overall soilstrength or resistance.

PRESSURE GROUTING SOIL STABILIZATIONPressure grouting soil stabilization is the injection of cement orchemicals into the pores of the soil. Soil particles are then “glued”together to form a hardened solid mass of grouted soil. The spac-ing of the injection points and the grout mixture is varied to achievea specific pattern and grout coverage, depending on the require-ments of the project.

GROUND FREEZINGGround freezing uses the natural moisture in the soil and artificialcooling methods to freeze and harden the soil. The ambient tem-perature of nearly all soil is above freezing (except for permafrostregions). Lowering the soil temperature to below freezing causesthe water in the pores of the soil to freeze and “cement” the soilparticles together. This results in a very hard, impermeable condi-tion. However, soil has a relatively low thermal conductivity. LiquidNitrogen may be used to freeze the soil in a timely manner. A con-tinual flow of a brine solution through a series of embedded refrig-eration pipes must be used to maintain the soil in a frozen state.

SOIL NAILINGSoil nailing is a method used for both temporary excavation brac-ing and for permanent retaining walls. This technique employsclosely spaced, high-strength steel anchors grouted into the soil,and may include a reinforced shotcrete facing. The major advan-tages of soil nail wall construction over more traditional excavationbracing methods include relatively low cost and the ability to con-

struct the wall system from the top down, as the excavation pro-ceeds. A typical installation sequence for grouted soil nailing is asfollows:

1. Excavation begins by exposing a cut about 3 to 5 ft in depth.2. A borehole (typically 4 to 6 in. in diameter) is drilled into the face

of the excavation at a downward angle of approximately 15° tothe horizontal. The length of the borehole is dependent on theheight of the cut and the nature of the material exposed in theexcavation. Typical lengths range from 60 to 70 percent of thewall height.

3. A high-strength, threaded reinforcing bar is inserted into theborehole, then the borehole is grouted to the excavation face.

4. After the grout has cured, a wire mesh is placed over theexposed face of the cut, and reinforcement is placed to spanover the borehole and reinforcing bar. When groundwater is aconcern, a geosynthetic drainage mat is typically placed againstthe soil face to intercept water and direct it to the base of thewall.

5. The exposed excavation face is sprayed with shotcrete, typical-ly 8 to 10 in. thick.

6. A bearing plate is fitted over the reinforcing bar, and a nut isscrewed into place to tension the soil nail.

7. If the method is to be used as a permanent wall, a secondapplication of shotcrete is used to cover the soil nail head andbearing plate.

8. After completion of the first level, the excavation extends down-ward an additional 4 to 6 ft, and the process is repeated.

TEMPORARY TIEBACK SECTION1.140

PERMANENT TIEBACK AT FOUNDATION WALL1.141

SLOPE STABILITY WITH SOIL NAILS1.142

EXCAVATION SUPPORT WITH SOIL NAILS1.143

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BASEMENT CONSTRUCTION ELEMENT A: SUBSTRUCTURE

NOTE

1.145 Reinforcing is based on unbraced backfill height, soil pressure,and groundwater conditions.

Contributors:Donald Neubauer, PE, Neubauer Consulting Engineers, Potomac,Maryland; Mueser Rutledge Consulting Engineers, New York City, NewYork; James W. Niehoff, PE, PSI, Wheat Ridge, Colorado; Timothy H.Bedenis, PE, Soil and Materials Engineers, Inc., Plymouth, Michigan;American Concrete Institute; Grace S. Lee, Rippeteau Architects, PC,

Washington DC; Stephen S. Szoke, PE, National Concrete MasonryAssociation, Herndon, Virginia; Daniel Zechmeister, PE, Masonry Instituteof Michigan, Livonia, Michigan; Paul Johnson, AIA, Senior Architect,SmithGroup, Detroit, Michigan.

45

FIELD DENSITY TESTSField density tests are performed on compacted soil to verify thata specific level of compaction has been achieved. There are sever-al methods for determining the in-place density of soil. Today, themost commonly used method involves the nuclear density gauge, adevice that measures the reflection of atomic particles from a tinyradioactive source material to determine the soil density and mois-ture content. The test is performed at the surface without anyexcavation, and results can be obtained faster than with other testmethods.

MOISTURE CONTROLMoisture content of the fill and backfill should be near the optimummoisture content. Otherwise, the minimum required field density isvery difficult (or impossible) to obtain, no matter how much energyis used for compaction. If the soil is too wet, the water in the porescannot be expelled fast enough to allow for a sufficient decrease involume. If the soil is too dry, the capillary forces around the soilparticles are too large to be broken down by the compactive ener-gy. Therefore, controlling the moisture of the fill and backfill towithin specific limits near the optimum moisture content is neces-sary to achieve the required level of compaction.

SOIL TYPE AND COMPACTION EQUIPMENTTo be most effective, the compaction equipment must match thetype of soil to be compacted. In general, compaction equipmentcan be divided into two basic groups: rollers or plates. Rollerscome in large variations in size, but all use a weighted wheel ordrum to impart energy to the soil. In addition, some rollers use anelectric rotor to vibrate the drum, thereby increasing the energy tothe soil. Other drums have protrusions called sheepsfoots, whichimpart a kneading action to the soils. Some plate compactors alsouse vibratory energy to compact the soils, while other plate com-pactors, called tampers, move up and down, imparting a verticaldynamic load to the soil.

In general, coarse-grained granular soils such as sands and grav-els are more easily compacted than fine-grained soils such as claysand clayey silts. Vibratory energy is very effective in densifyingsands and gravels, since the interparticle bonds are relativelyweak. When the granular soils are vibrated at the correct frequen-cy, the soil particles rearrange themselves into a denser stateunder their own weight and the weight of the compactor. Vibratorysteel drum rollers and vibratory plate compacters are consideredthe most effective compaction equipment for granular soils.

Fine-grained soils hold more moisture and have higher internalinterparticle forces. Vibratory energy is much less effective forthese soils. Clayey soils require more mechanical energy to breakdown the internal forces during compaction. The kneading action ofa sheepsfoot roller is very effective in this regard.

Fine-grained soils generally have a narrower range of moisturecontents for optimum compaction. Often, the clayey soils are toowet to compact and the moisture content must be reduced.Reducing the moisture content in clay is typically done by allowingwater to evaporate from the surface of the clay. The rate of evapo-ration is dependent on the ambient air temperature and wind con-ditions. However, the drying process can be enhanced by using aprocess called aeration in which steel discs are used to periodical-ly turn the soil, thus exposing more of the soil to the atmosphere.

BASEMENT WALLS

Basement walls may be constructed of various materials, includ-ing, concrete, masonry, and wood.

BASEMENT WALL CONSTRUCTIONCONCRETE BASEMENT WALLSConcrete basement walls may be either cast-in-place or precast.Cast-in-place concrete basement walls provide a cost-effectivemeans of supporting a floor and resisting soil pressures.Commercial and residential applications of cast-in-place concretebasement walls are prevalent. Forms are easily placed in the exca-vation on the footings. Reinforcing steel may be tied on or off-site,and is placed within the wall formwork. Depending on the soil andgroundwater conditions, dampproofing should typically be used onfoundations walls and waterproofing is generally required on base-ment walls prior to backfilling. Unless lateral bracing is utilized, thetop of the basement wall must be supported by the first floor andthe base of the wall by the footing or slab-on-grade before back-filling against the wall can begin. Keeping the wall heights uniform,as well as reducing the number of penetrations and maintaining asimple plan configuration, will help reduce the final cost of the wall.

Precast concrete basement walls enable basement construction inless time than conventional cast-in-place concrete. In addition totime and construction methods other advantages of precast con-crete include the ability of the precast supplier to utilize concreteadmixtures that focus on ultimate strength, rather than cure timeand temperature. Precast concrete manufacturers are able to pro-duce mixes that cure to 5000 psi, which is stronger than concreteunit masonry or cast-in-place concrete walls. Additionally, bettercontrol of the concrete mixture and curing environment allows theuse of low water/cement ratios, which results in a dense materialthat reduces water penetration.

SOIL NAIL WALL DETAIL1.144

MOISTURE-DENSITY RELATIONSHIP1.145

BACKFILL AND COMPACTIONFILL AND BACKFILLFill is typically used to raise or level site grades. Backfill is used to fillin spaces around below-grade structural elements, such as aroundbasement walls, The fill must have sufficient strength or resistanceand low compressibility to support its own weight and any other over-lying structures pavements, floor slabs, foundations, etc.) withoutexcessive settlement. When soils are excavated, they become loos-ened and disturbed. If they are suitable for reuse as structural fill orbackfill, the soils must be placed in thin layers and compacted toachieve the required strength, resistance and stability.

COMPACTIONCompaction is the process by which mechanical energy is appliedto a soil to increase its density. The degree to which soil can bedensified depends on the amount and type of compactive effort,type of soil, and moisture content. Soil is made up of solids and thevoid spaces between the solid particles. The void space almostalways contains some water. If the water completely fills the void,the soil is considered to be totally saturated. During compaction,the total volume of the soil is decreased by reducing the volume ofvoids, while the volume of solids remains essentially unchanged. Ifthe soil is saturated or nearly saturated during compaction, watermust be expelled to decrease the void space.

MOISTURE-DENSITY RELATIONSHIPNearly all soil exhibits a defined moisture-density relationship fora specific level of compactive effort.

These relationships can be graphed in a nearly bell-shaped curve,with the maximum density at the apex, corresponding to the opti-mum moisture content.

Standard laboratory tests, such as the Standard Proctor (ASTM D698) and Modified Proctor (ASTM D 1557), use a standard-sizemold and a specific level of compactive energy to develop the mois-ture density curve for a specific soil. The maximum density fromthese curves defines the 100 percent level of compaction for agiven soil. Compaction requirements for fill and backfill are gener-ally specified as a percentage of the maximum density, typicallybetween 90 to 95 percent, as determined using one of the standardlaboratory tests mentioned above. The low and high moisture con-tents are usually represented as a horizontal line connecting oppo-site sides of the Proctor curve for a given density. This representsthe range of moisture content within which the soil can be com-pacted most readily.

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NOTES

1.148 a. Drainage must be provided on surface and below grade toremove groundwater from the basement wall. The backfill must begranular, and the soil conditions nonexpansive.b. Backfill pressure on wall is assumed to be 30 psf/ft of depth of wall.Soil pressures may be higher, and greater thicknesses required at agiven location. Consult with local code officials or geotechnical engi-neers.

46

MASONRY BASEMENT WALLSMasonry walls have long served as foundations for structures. Today,most masonry basement walls consist of a single wythe, or hollow,solid concrete masonry units, depending on the required bearingcapacity. The walls are reinforced as necessary to resist lateral loads.Generally, such reinforcing should be held as close to the interior faceshell as possible, to provide the maximum tensile strength.

Basement walls should protect against heat and cold, insect infes-tation (particularly termites), fire, and penetration of water andsoil gases.

If radon is a major concern, the top course of the masonry and thecourse of masonry at or below the slab should be constructed ofsolid units or fully grouted hollow units using foundation drain tocollect and drain condensation moisture from basements, shouldbe avoided in areas where soil-gas entry is a concern.

Architectural masonry units may be used to improve the appear-ance of the wall. Masonry units with architectural finishes facingthe interior can be used for economical construction of finishedbasement space.

Masonry easily accommodates any floor plan, and returns and cor-ners increase the structural performance of the wall for lateralload resistance.

Source: Based on ACI 332 Requirements for Residential ConcreteConstruction and Commentary, Figure R7.1. Reprinted with permissionof the American Concrete Institute.

MAXIMUM DEPTHBASEMENT WALL NOMINAL OF UNBALANCEDCONSTRUCTION THICKNESS (IN.) FILL (FT)

CMU – hollow units, 8 5ungrouted 10 6

12 7

CMU – solid units 8 5

10 7

12 7

CMU – hollow or solid 8 7units, fully grouted 10 8

12 8

THICKNESS OF CMU BASEMENT WALLS1.148

TYPICAL MASONRY BASEMENT WALL1.147

CONCRETE BASEMENT WALLS1.146

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VERTICAL REINFORCEMENT SPACING1.149


NOTES

1.151 a. The empirical design method of the Building Code Requirementsfor Masonry Structures, ACI 530/ASCE 5, Chapter 9, allows up to 5 ft ofbackfill on an 8-in., nonreinforced concrete masonry wall.b. As an alternate, W1.7 joint reinforcement placed in joints numbers 3,4, 5, 7, 8, and 11 may be used.c. Use of vapor retarders should be verified by proper analysis.d. Backfill pressure on wall is assumed to be 30 psf/ft of depth of wall.Soil pressures may be higher, and greater thicknesses required at agiven location. Consult with local code officials or geotechnical engi-neers.

47

S = spacing of vertical reinforcing barsB = bar sizeH = height of backfill

HEIGHT OF BACKFILL, H

8� 7� 6� 5� 4�

Bar size, B #6 #6 #5 #5 #4

Spacing, S 64� 56� 64� 72� 72�

CMU BASEMENT WALL REINFORCEMENT (BAR SIZEAND MAXIMUM BAR SPACING)1.150

HEIGHT OF BACKFILL, H

MORTAR JOINT 8� 7� 6� 5� 4�

13 — — — — —

12 — — — — —

11 W2.1 W1.7 W1.7 W1.7 W1.7

10 — — — — —

9 W2.1 W1.7 W1.7 W1.7 —

8 — — — — W1.7

7 W2.1 W1.7 W1.7 W1.7 —

6 — — — — —

5 W2.1 W1.7 W1.7 W1.7 —

4 — W1.7 W1.7 W1.7 W1.7

3 W2.1 W1.7 W1.7 W1.7 W1.7

2 — — — — —

1 — — — — —

CMU BASEMENT WALL: HORIZONTAL JOINTREINFORCEMENT1.151

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4

4

1

2

3

3

3

3

5

1 = LOW2 = MODERATE3 = INTERMEDIATE4 = HIGH5 = SEVERE

53

1

2

TREATED WOOD BASEMENT WALLSThe construction of treated wood foundations and basements issimilar to the construction of standard wood light-frame wallsexcept for two factors:

• The wood used is pressure-treated with wood preservatives.• The extra loading and stress requirements caused by below-

grade conditions must be accommodated in the design anddetailing of the fasteners, connections, blocking, and wall cor-ners.

As with standard masonry or concrete foundations, treated woodfoundations require good drainage to maintain dry basements andcrawl spaces. However, the drainage system typically used withtreated wood foundations is different from that used with mason-ry or concrete systems. The components of a drainage system suit-able for use with a treated wood foundation include:

• A highly porous backfill material, which directs water down to agranular drainage layer.

• A porous granular drainage layer under the entire foundationand floor to collect and discharge water.

• Positive discharge of water by means of a sump designed for thesoil type. This drainage system (developed for treated woodfoundations) takes the place of a typical porous backfill over aperimeter drainage pipe.

Benefits of a treated wood foundation include system:

• All framing is standard 2 by construction.• Treated wood foundations can be erected in any weather and

when site access for other methods is difficult.• Deep wall cavities allow use of high R-value thermal insulation

without loss of interior space.• Wiring and finishing are easily achieved.

Considerations when working with treated wood foundations:

• Treated wood foundations are not appropriate for all sites.Selection of the proper foundation for a project depends on siteconditions, including soil types, drainage conditions, groundwa-ter, and other factors. Wet sites in low areas (especially areaswith coarse-grained soil) should be avoided if a full basement isdesired, although a crawl space type foundation can be used inthese cases. Consult a geotechnical engineer to determine theviability of any foundation system. Also, refer to the wood dete-rioration zones indicated in Figure 1.150. Lumber and plywoodused in treated wood foundations must be grade-stamped forfoundation use. These are typically pressure treated with chro-mated copper arsenate. Treated wood products used in founda-tion construction are required to contain more preservativesthan treated wood used in applications such as fencing anddecking. Codes generally call for hot-dipped, galvanized fasten-ers above grade and stainless steel fasteners below grade.

• Avoid skin contact and prolonged or frequent inhalation of saw-dust when handling or working with any pressure-treated woodproduct.

• Consult applicable building codes and the American Forest &Paper Association’s Permanent Wood Foundation System—Design, Fabrication, Installation Manual for requirements anddesign guidelines. In the early stages of a project, consult withthe building code officials for the area or jurisdiction to assesstheir familiarity with and willingness to approve this type of con-struction.

• The vertical and horizontal edge-to-edge joints of all plywoodpanels used in these systems should be sealed with a suitablesealant.

• Correct materials and details of construction are very importantfor treated wood foundations. If the contractor to be used forthe installation is unfamiliar with this foundation type, thedesign should include the use of shop-fabricated foundationpanels. Most problems with treated wood foundations can betraced to improper installation by inexperienced workers.

• This type of foundation depends especially on the first-floor deckto absorb and distribute any backfill loads; therefore, backfillingcannot occur until the first floor deck is complete unless lateralbracing is utilized.

WOOD DETERIORATION ZONES1.152

Source: Based on AWPA Book of Standards 1997.

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NOTES

1.153 a. Geotextile material may be used under and around drainagelayers and backfill, if soil conditions warrant.b. Stud size and spacing vary with material grade and backfill depth. Ingeneral, 42-in. backfill requires 2 by 4 at 12 in. o.c., 64-in. requires 2 by6 at 16 in. o.c., and 84-in. requires 2 by 6 at 12 in. o.c.1.155 a. Joists to be butted end to end over pressure-treated woodsleepers.b. Floor stiffness will be increased by blocking between each joist aboveeach sleeper.c. Check with applicable code for underfloor ventilation requirements.

49

TYPICAL TREATED WOOD BASEMENT WALL1.153

TREATED WOOD BASEMENT WALL WITH EXTERIORKNEE WALL1.154

WOOD SLEEPER FLOOR SYSTEM1.155

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Contributor:Partnership for Advancing Technology in Housing (PATH), Washington,DC.

50

BACKFILLING AGAINST BASEMENTWALLSBackfilling against a basement wall is not appropriate, unlessresistance to the overturning load has been provided. The mostcommon approach is to construct the first-floor slab before back-filling. Alternate approaches include designing the wall as a free-

standing retaining wall or providing temporary lateral bracing forthe basement wall. A retaining wall is either a freestanding or alaterally braced wall that bears against soil or other fill materialthat resists lateral forces from the material in contact with the faceof the wall. Below grade portions of basement walls are consid-ered retaining walls.

By constructing a freestanding retaining wall or providing lateralbracing on the perimeter, backfilling operations can progress with-out the first floor being complete. An additional cost is involved inconstructing the basement wall as a freestanding retaining wall,because more concrete, reinforcing, and excavation are required.

BACKFILLING FORCES ON BASEMENT WALLS1.156

BACKFILLING AGAINST BASEMENT WALLS1.157

SOIL PRESSURE ON BASEMENT WALLSSoil, similarly to water, exerts pressure on the back face of base-ment walls. The pressure exerted by water is equal to the densityof water times the depth. Soil also exerts pressure in proportion tothe density of the soil times the depth of the wall. This proportionis based on an earth pressure coefficient, which is dependent onthe type and magnitude of soil movement and flexibility of the wall.It is important to understand these influences before designingretaining walls.

Four types of soil pressure need to be understood and resisted byretainings walls: active, at rest, passive, and surcharge.

ACTIVE SOIL PRESSUREActive soil pressure exists when a wall moves inward and the soilfails (shears) in the zone right behind the wall. The active soil pres-sure is dependent on the shear resistance (i.e., strength) of thesoil. The higher the strength of the soil, the lower the active pres-sure on the wall. The retaining wall must be flexible to allowenough movement to occur so the active pressure can develop.Unfortunately, most concrete basement walls do not behave flexi-bly enough to allow for the full active condition to develop; howev-er, freestanding retaining walls rotate about the base to allow foractive pressure to be used for design. An active earth pressurecoefficient of typically around 0.30 to 0.35 is used for compactedsand wall backfill.

AT-REST SOIL PRESSUREAt-rest soil pressures occur when there is no wall movement(inward or outward) as a result of placement of soil against thewall. This condition most often occurs when backfilling against rigidor stiff walls which deflect very little under the lateral soil pres-sures. Since there is no soil movement, the soils do not shear andthe pressures are transmitted elastically to the walls. Typical base-ment walls are considered rigid enough to resist the soil in an at-rest condition. The coefficient of at-rest soil pressure is typicallyaround 0.50 to 0.55 for compacted sand wall backfill.

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Contributors:James Kellogg AIA, HOK, San Francisco, California; Andrea Reynolds,PE, SmithGroup, Detroit, Michigan.

51

PASSIVE SOIL PRESSUREPassive soil pressures occur when a foundation wall actuallymoves into the soil away from the wall due to an external force.This typically happens when the lateral loads on the structure areresisted by the foundation. Here, the soil is being pushed awayfrom the wall with the pressure increasing with the amount of lat-eral wall movement until a ultimate (maximum) pressure occurs.The ultimate passive pressure is also based on the shearing resist-ance of the soil. Care must be taken to determine how much move-ment is anticipated for the level of passive resistance needed toresist the lateral loads from the structure. Typical values of ulti-mate passive soil coefficients can range around 2.0 to 5.0 for com-pacted sand wall backfill, but the movement required to obtainthese values is greater than most structures can tolerate.Therefore, a lower “allowable” passive soil coefficient of around1.0 to 1.5 is used for design, with the lateral movement required toachieve this passive resistance limited to between 1/4 and 1/2 in.

ACTIVE SOIL PRESSURE1.158

AT-REST SOIL PRESSURE1.159

SURCHARGE SOIL LOADING1.161

PASSIVE SOIL PRESSURE1.160

SURCHARGE SOIL LOADINGSurcharge loading is an often-overlooked load on a foundationwall, especially for construction surcharge loadings. Surchargeloads can be due to a number of sources: car, truck, or pedestriantraffic; storage of materials; landscaping berms; snow piles, etc.Additional lateral pressures on the walls occur due to the verticalsurcharge loads being applied at or near the surface. Uniform sur-charge pressures are typically given in feet-of-soil equivalents(such as 1, 2, or 3 ft of soil). This typically equates to 100, 200, or300 psf of surcharge loading. The effective lateral load from theuniform surcharge is computed with the same earth pressure coef-ficient used in calculating the lateral load on the foundation wall:either the active or the at-rest coefficient.

Various types of surcharge loadings should be considered whendesigning the foundation walls. Construction loads are typicallypoint loads, and transient. The lateral load distribution of concen-trated-point loads is a bulb-type loading, and affects only a shortlength of the wall. The uniform load of storage material can existover a great length of the wall and can have a significant effect onthe design of the wall. Because emergency equipment often musthave close access to a building, foundation walls should bedesigned to resist the large concentrated loads of fire and emer-gency equipment.

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NOTE

1.162 Place 12-in. neoprene strips over joints in sheet piling.

52

BASEMENT—WATERPROOFING/DAMPPROOFING/INSULATIONConsult a geotechnical engineer to determine soil types andgroundwater levels, as well as their effect on drainage and water-proofing methods. Consult a waterproofing specialist to determinea specific design approach for problem soils and conditions. Sitesmay have groundwater contamination that will degrade the dura-bility of the waterproofing materials. Generally, waterproofing willbe necessary if a head of water is expected against the basementwall or under the slab. Because groundwater levels can vary withseasons, it is important to understand these seasonal fluctuationsand design for the maximum expected head.

Foundation drainage is recommended when the groundwater levelmay rise above the top of the floor slab or when the foundation issubject to hydrostatic pressure after heavy rain. Geosyntheticdrainage material conveys water to the drainage piping, thusreducing hydrostatic pressure. It is important to understand thehydrostatic pressures exerted on the floor slab and wall systems ifthe drainage system is not adequate to remove all the water.

Special negative-side coatings on interior face of foundation wall,such as metallic oxide, are recommended only when the exterior isnot accessible (such as pits and trenches, and in particular, eleva-tor pits).

BASEMENT WALL VERTICAL WATERPROOFINGThe grading around the building is an important part of the overallwater management plan. The backfilling operation usually results ina more porous material than the adjacent undisturbed soil, whichmakes it easier for water to collect next to the building. The finishedgrade should slope away from the building, and an impervious layerof soil placed on top of the backfill against the building. Drainagefrom downspouts should be diverted away from the foundations.

Types of waterproofing include built-up bituminous, sheet, fluid-applied, cementitious and reactive, and bentonite.

• Built-up bituminous: Composed of alternating layers of bitumi-nous sheets and viscous bituminous coatings. Bituminouswaterproofing includes built-up asphalt and cold-tar water-proofing systems.

• Sheet waterproofing: Formed with sheets of elastomeric, bitu-minous, modified bituminous or thermoplastic materials. Sheetwaterproofing may be either mechanically attached or self-adhered. Sheet waterproofing provides an impermeable surfaceto water penetration.

• Fluid-applied: Applied in a hot or cold viscous state. Includes hotfluid-applied rubberized asphalt. As with sheet waterproofing,fluid-applied waterproofing will bridge minor cracks in a con-crete surface.

• Cementitious and reactive: Types of waterproofing that achievewaterproof qualities through chemical reaction and include poly-mer modified cement, crystalline, and metal-oxide waterproofingsystems. Metal oxide is recommended for use when the exteriorsurface is not accessible, as in the case of an elevator pit.

• Bentonite: Formed from clay into panels and composite sheets.When moistened, the clay swells and takes on a gel-like consis-tency, forming an impermeable retarder when confined.Bentonite clay works well only when moistened. For applicationswhere the water table fluctuates, there may be a time lagbetween the rising water table and when the bentonite takeseffect, during which time there is the possibility of water infil-tration. Therefore, when the water table varies, caution is inorder when relying on bentonite clay for waterproofing. Propercoordination between the wall construction details and thewaterproofing termination is required.

At the interface of the foundation wall and slab, waterstops areplaced on top of the footing, at vertical concrete keyed wall joints.

Most waterproofing materials require a stable, rigid, and level sub-strate. Generally, a mud slab (subslab that is nonreinforced andnonstructural) is used when the waterproofing material is placedbelow the structural slab and/or when a solid working surface isneeded on unstable soils. When waterproofing materials areplaced on top of the structural slab, a protective cover, such asanother concrete slab, is required.

WATERPROOFING APPLICATIONS AT BASEMENT CONDITIONS1.162

WATERPROOFING AT FOOTINGS1.163

WATERPROOFING UNDERSLAB1.164

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Protection of waterproofing or coatings during construction andbackfilling is essential. Protection materials should be selectedaccording to soil, climate, and cost requirements. These materialsinclude the following:

• Composite drainage board: Recommended when water is fre-quently present in soils surrounding foundations. Usually, this ismade up of a rigid open-weave material, approximately 3/4 in.thick, covered on both sides by a geotextile filter fabric that pre-vents small stones or other materials from clogging thedrainage route of water inside. Typically, the drainage materialis terminated at drain tiles at the bottom of the foundation. Thissystem has a higher in-place cost than other protection boardmaterials.

• Board insulation: Used above the frost line or if ground temper-atures are low. Rigid insulation boards are usually made ofexpanded or extruded polystyrene, with a minimum thickness of1/4 in. when used as protection board only, and 1 in. or thick ifthermal insulation properties are desired.

• Protection board: Used only to protect waterproofing; it doesnot drain or insulate. Protection board is usually made of 1/8 in.asphalt-impregnated fiberboard or 1/4 in. extruded polystyrene.This is the least expensive material when protection of thewaterproofing is the only requirement.

• Grout: Packed around pipes penetrating the foundation andother types of waterproofing, grout should contain a mixture ofiron oxide, which chemically alters the grout to be more water-resistant.

steel framing). Note that this may not only be a problem at exteri-or walls but can also be problematic at foundations and piers(especially crawl spaces) and within the building interior if thereare unfavorable water conditions.

Another appropriate use of dampproofing is at site-retaining wallsthat may have special finishes such as architectural precast con-crete applied to a structural retaining wall. In this case, it would behighly desirable to prohibit excessive water penetration from theback side to be transmitted to the decorative finish. Waterproofingis sometimes not appropriate in these conditions; however, damp-proofing often provides the correct level of protection in thesecases.

Dampproofing is most often a spray-, roller-, or brush-applied bitu-minous material (asphalts), but may include cementitious andsheet materials. Fluid-applied and cementitious applications havelittle capability to bridge across cracks or discontinuities of theconcrete or masonry wall to which they are applied, and have verylimited capability to accept movement.

THERMAL INSULATIONInsulation requirements are proportional to heating loads. Thefoundation is often underinsulated and can be a major source ofheat loss. The desirable insulation level depends on the use of thebasement space, basement temperature, and insulation levels inthe rest of the building. An approximate thermal optimum is:

(Tbsmt � T0) R ins = � Rref � Rwall(T1 � T0)

R ins = R-value to be added to basement wall above grade

Rref = R-value of superstructure wall

Rwall = R-value of uninsulated basement foundation wall

Tbsmt = Average seasonal temperature of basement

T1 = Average seasonal temperature of living space

T0 = Average seasonal outdoor temperature

The added foundation insulation above grade is R ins. It shoulddecrease with depth by R�2 per foot in ordinary soils and R �1.5in wet soils. A horizontal skirt can be used to reduce floor perime-ter losses. Exterior insulation keeps the wall warm and eliminatescondensation and thermal bridges. As seasonal basement temper-ature decreases, losses to it from the superstructure increase, andbasement ceiling R-values should increase. As a very rough rule,the basement ceiling R-value should be greater than (Rref�Rins).

There are basically two insulation types used for foundations:

• Expanded polystyrene: Also known as “bead board,” this is gen-erally a low-density, low-compressive strength material that issupplied in sheet form, usually 2 to 4 ft wide and 8 ft long (2 in.is a typical thickness of insulation).

• Extruded polystyrene: This is a sheet material that is also usedas masonry cavity wall insulation. This material can come incompressive strengths of 20 to 100 psi in sheets of 2 to 4 ft wideand 8 ft in length. (2 in. is a fairly typical basement insulationthickness.)

Both the extruded and expanded polystyrene insulations are sus-ceptible to chemical reaction to direct contact with petroleum-based chemicals in the soil, or driven by percolation from the sur-face.

Expanded or cellular glass (also known as “stink board” becauseof the sulphur that is trapped within the closed cell-glass structure)is inert and may be the best insulation product in these situations.However, if cellular glass products are used in areas prone tofreezing temperatures, special precautions are necessary, to pre-vent deterioration and damage to the insulation caused by freezingof water that gets into the open cells at the insulation edges. Theedges of the board are cut and, therefore, have open cells alongthe edges. The edges of these sheets must be sealed whenexposed to freezing temperatures.

This insulation also comes in sheet form, 2 by 4 ft; thicknesses canvary according to need. Cell-glass insulation may be appropriatefor projects requiring foundation insulation adjoining vehicle main-

tenance and large-scale traffic circulation. The design should takeinto account control and filtering of storm and drainage water, toavoid percolation and potential contamination of adjoining soils.

It should be noted that the insulation sheets can also be used as avery effective protection board for waterproofing. Some water-proofing manufacturers have strong recommendations regardingthe appropriate type of protection board.

REFERENCESThe National Association of Home Builders Research Center, awholly-owned subsidiary of the National Association of HomeBuilders, is the research and development leader in the home-building industry. Government agencies, manufacturers, builders,and remodelers rely on the expertise and objectivity that are at thevery heart of the research center and its activities.

The National Institute of Building Sciences (NIBS) is a nonprofit,nongovernmental organization that brings together representa-tives of government, the professions, industry, labor, and consumerinterests to focus on the identification and resolution of problemsand potential problems that hamper the construction of safe,affordable structures for housing, commerce and industry through-out the United States.

The National Roofing Contractors Association (NRCA) is one of theconstruction industry’s oldest trade associations and the voice ofprofessional roofing contractors worldwide.

PIPE PENETRATION AT WALL1.165

BASEMENT WALL DAMPPROOFINGDampproofing is generally provided to reduce or prohibit theabsorption of condensation and high-humidty into below-gradeconcrete or masonry and to reduce the likelihood of water notunder a head of pressure from moving through or up the construc-tion. Examples of applications requiring dampproofing include theback side of site retaining walls or basement walls where there isno head of water

Dampproofing is not “watertight” and will not perform to the samelevels as waterproofing, and so should not be used in applicationsthat require waterproofing.

In addition to the desire to resist water intrusion through a wall,dampproofing has historically been utilized to prevent water intru-sion into concrete or masonry below grade, as a means to limit oreliminate “rising damp”—that is, a condition in which moisture isabsorbed into the subgrade construction and travels upward bycapillary action to drier materials such as masonry or wood. Waterinfiltration of this type can lead to rotting of wood, efflorescence ofmasonry, or freeze-thaw masonry damage, as well as corrosion ofunprotected or improperly protected metals (such as light-gauge

BASEMENT FOUNDATION AND CEILING INSULATION1.166

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Contributors:Krommenhoek/McKeown & Associates, San Diego, California; RichardJ. Vitullo, AIA, Oak Leaf Studio, Crownsville, Maryland, in consultationwith James B. Thompson, San Marino, California; James Kellogg AIA,HOK, San Francisco, California; Paul Johnson, AIA, SmithGroup, Detroit,Michigan.

54

NISTIR: Envelope Design Guidelines for Federal Office Buildings—Thermal Integrity and Airtightness. These guidelines are organizedby envelope construction system and contain practical informationon the avoidance of thermal performance problems such as ther-mal bridging, insulation system defects, moisture migration, andenvelope air leakage.

The Partnership for Advancing Technology in Housing (PATH)serves builders as a reliable source of information on new prod-ucts or processes. It lists details on each technology and contactinformation for the manufacturers.

Sealant, Waterproofing and Restoration Institute (SWRI) providesa forum for those engaged in the application, design, and manu-facture of sealant, waterproofing, and restoration products.

Wall Systems is a guide intended to facilitate a better understand-ing of the basic principles behind heat, air, and moisture transfer(including bulk rainwater penetration and precipitation manage-ment) through the exterior walls of a building or structure.

The Whole Building Design Guide (WBDG) is a Web-based portalproviding government and industry practitioners with one-stopaccess to up-to-date information on a wide range of building-relat-ed guidance, criteria, and technology from a “whole buildings” per-spective.

SEE ALSOAuger Cast Grout PileBackfillBlanket InsulationBoard InsulationBored PilesCantilever Needle BeamsCast-In-Place ConcreteCofferdamsCompactionConcreteDewateringDrilled Concrete PierDriven PilesFillFoundation DrainageGeotechnical InvestigationGravel Drainage LayerGypsum BoardNeedle BeamsNon-Shrink Grout

Portland Cement StuccoPost-Tensioned ConcretePrecast ConcreteReinforcing SteelSeismic InvestigationSheathingSheet PileSlurry WallsSoil NailingSoil StabilizationSoil TreatmentSteel H PileSubdrainage Pipingthermal InsulationThermal InsulationTreated Wood FoundationTrenchingUnderpinningUnderslab Drainage PipingVapor RetarderWaterproofing


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RENOVATED OFFICE BUILDING AT

215 FREMONT STREET

The existing building was expanded by adding two floors to thestructure, raising the building from seven to eight stories totalingnearly 400,000 sq. ft. in size, and converting the existing basementstorage space into a useable parking area. The existing concretestructure required fitting with a dynamic new skin, a new core, andcomplete new building systems that embody sustainable designgoals of energy efficiency. Additionally, to engage public interactioninto the neighborhood and further animate the building, a retailarcade was added at the street level, and roof-top terraces wereintegrated into the design.

INTRODUCTION

The structural engineers for 215 Fremont Street project won theprestigious “Excellence in Engineering Award” from the StructuralEngineering Association of California for the use of conventionalstructural engineering technologies in a building retrofit. The reno-vation and expansion of 215 Fremont Street were completed inearly 2001. The transformation of the building had an immediate,positive impact on the neighborhood business culture, and quicklyattracted a leading national corporate enterprise, which occupiedthe entire building. In this project, innovative foundation design,seismic retrofit, and creative architectural expression resulted in ameasurable improvement in the financial value of the building anda desirability of the neighborhood as a corporate location.

The project at 215 Fremont Street was a renovation and expansionof an existing office building in downtown San Francisco, California.The project goal was to provide quality office space for tenants inthe growing digital market at a prime location in the city’s rapidlydeveloping “Media Gulch.”

One of the project’s major challenges was to transform the earlytwentieth century industrial building (which had been significantlydamaged in the 1989 Loma Prieta earthquake and vacant for morethan ten years) into a distinctive architectural statement that wouldcreate a focal point for the neighborhood, as well as an officespace signifying growth into the twenty-first century. Besides the

extensive seismic retrofit necessary to bring the structure up tocurrent standard building codes, other vital work was required.

The 215 Fremont Street building was built in 1927, and was origi-nally a seven-story, L-shaped office building consisting of large floorplates of about 46,000 sq. ft. The existing structure was a reinforcedconcrete flat slab floor system bounded by a column grid of 20 footo.c. The total building area was approximately 320,000 gsf. To meetcurrent building code requirements and achieve the goal of reno-vating the building and developing additional space, evaluation ofthe existing building, as well as analysis and design of the retrofit-ted structure and foundation, were necessary.

The project team began the design process by analyzing therequirements and determined the appropriate workplan to meetthe needs of this project. This included identification of consultants,specialists, and surveys necessary to evaluate existing conditions,and the gathering of data necessary for the design. The team col-laborated to share professional experience and develop solutionsthat would most successfully meet the project goals.

The first step was a review of the original design drawings. Thisreview provided an initial assessment of the building’s construc-tion. The design drawings supplied the following information:

• Column sizes and reinforcement• Slab reinforcing quantity and layout• Size and location of existing spread footings and grade beams• Thickness and depth of perimeter framing

METHODOLOGY

While the drawings proved useful, certain important informationwas either illegible or not included. To verify the information pro-vided and determine the unknown structural parameters, an engi-neering survey was required to perform a physical and visualinspection of the existing building, in addition to geotechnicalinvestigation of existing soil conditions.

The survey included a detailed evaluation of both superstructureand substructure data necessary for the design of the building. Theoriginal foundation system consisted of truncated pyramid-shapedspread footings at interior columns and grade-beam foundations,with strap beams along the building perimeter. Although some woodpiles were discovered during the building survey, the building had ashallow foundation system. The building’s underlying soil conditionsincluded different regions of dense, silty sands and stiff silts. Sincethe original construction, the building had experienced differential

settlements of up to 5 inches. Core samples and dynamic load testsof the existing floor slabs provided data necessary to evaluate theviability of components of the existing structure.

This report verified the findings of the field survey, noting the fol-lowing:

• The soils were subject to differential deflection. • The shallow sandy soils were susceptible to liquefaction and

would not support earthquake loading.

With this information in hand, the project team concluded that tomeet the goals of the project and provide a viable structuraldesign, this project would require an extensive effort to coordinatethe work of each discipline and evaluate the entire building using acomputer model.

SAN FRANCISCO, CALIFORNIA

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56

LATERAL BRACING DIAGRAM: Besides accommodating architec-tural facade considerations, attention was given to formulating astructural system that would use the full length and width of thebuilding to minimize seismic overturning forces on the foundation.The solution chosen was a combined system of concrete shearwalls and perimeter steel-braced frames, and steel framing for theadditional two floors of office space.

FOUNDATION PLAN RENOVATED: The innovative solution for thenew foundation system given the constraints and geotechnical con-ditions was to enhance the existing foundations, underpinningwhere needed, with a combination of “column load transferblocks” to grade beams with pin piles. The building loads would,therefore, be transferred from the columns/walls to the existingfootings by the transfer blocks, which then transfer the loads tothe pin piles through the grade beams spanning between footings.

TECHNICAL SYSTEMS

Based on the information acquired from the existing drawings andfield surveys, a computer model combining ETAB 1997 v6.2 forDynamic and Static Analysis and SAP 2000 for Three-dimensionalDesign of Structures was generated to determine the performanceof the existing structure under gravity and seismic loading. Fromthe computer model, it was determined that the existing structurewas inadequate for gravity support and for resisting seismicforces.

Since the existing structure would not be able to accommodate thehigh seismic force because of the large building weight and 1997UBC seismic-force coefficients, a new structural system was nec-essary. This system needed to be consistent with the architecturalconcept of transparent curtain wall skin on key facades, as well asthe long spans required for an effective office floor plan layout ofthe new additional floors.

New infill concrete slabs, dowelled into the existing slabs, werealso added at the new curtain wall elevation to create a unified

diaphragm to integrate the steel-braced frame into the existingstructure.

The challenges faced by the structural engineer working with thegeotechnical engineer included creating a foundation system thatcould support the existing concrete building with the additional twofloors, without exceeding differential settlement limits, whileresisting the large horizontal forces defined by UBC 1997 require-ments for seismic activities. Interpretation of the survey data indi-cated that the saturated, cohesion-less soil underneath the exist-ing shallow foundations could liquefy as it experienced strong seis-mic ground motion, and would experience temporary loss of shearstrength created by a transient rise in excess pore pressure.Therefore, the vertical forces from gravity loads, combined withoverturning forces generated from a seismic event, could not beresisted by shallow foundations.

A deep foundation system was required that would allow the loadsto pass through the liquefiable soil layers. Alternative shallow

foundation systems such as grade beams footings with combinedspread footings and mat foundations were not feasible solutions.The solution for deep foundations had to also account for the rela-tively close column spacing of the existing concrete structure, andthe limited vertical clearances within the existing basement.Therefore, standard driven piles were not feasible, and specialfoundations such as base isolation did not address the issue oftransferring seismic load to bearing soil layers.

The grade beams (which were 5 ft-10 in. wide by 3 ft-6 in. deep)were designed to resist the gravity loads and overturning loadsduring an earthquake from the existing columns and transfer themto the pin piles by shear and flexure. A minimum of four longitudi-nal reinforcing bars were drilled, and epoxy was grouted throughthe existing columns to transfer loads directly into the gradebeams with steel couplers. The balance of longitudinal reinforce-ment was placed symmetrically on either side of the existingcolumns to minimize penetrations through the existing elements.

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57

GRADE BEAM SECTION: At the east and west ends of the buildingfootprint, mat footings that are 4 ft-6 in. thick were used in lieu ofthe grade beams to support the high seismic overturning forces.Additional basement shear walls were also added to aid the eccen-tric load transfer from the walls to the pin piles.

COLUMN LOAD TRANSFER: The transfer blocks, typically 11 ft-0 in.by 11 ft-0 in., were constructed by chipping away the existing foot-ings around the columns to allow for the installation of verticaldowels embedded into the grade beams. The column loads aretransferred to the grade beams by these dowels, which tie allcolumns together.

SUMMARY

The architects and the structural engineers learned that to achievethe goals of the project, successful collaboration and rigorousapplication of a design process were required. This included iden-tification of key issues, as well as the use of specialists to evaluateexisting conditions and to gather data necessary for the design,and to arrive at solutions for a viable structural design that fit inte-grally into the architectural expression.

Given the damage the building suffered in the 1989 Loma Prietaearthquake and the high seismic forces determined from the 1997UBC, a new structural system needed to be developed for the proj-ect that would be sufficiently stiff to alleviate the induced internalforces in the existing floor slabs and punched exterior walls.

Additionally, the structural system needed to use the full length andwidth of the structure to minimize the seismic overturning forcesapplied to the foundation, while simultaneously being compatiblewith the architectural considerations for the building facade. Thisretrofit of an early twentieth century building led to the creation ofa unique connection between steel braces and concrete columns,as a combination structural system comprised of steel-brace,frame-and-concrete shear walls was developed to meet all criticalrequirements.

In conjunction with the new structural system, other importantchanges occurred. They included repairs and modification to theconcrete slabs, a redesign of the foundation system, the additionof two new floors, and the demolition and rebuilding of feature ele-vations to accommodate a new curtain wall.

Contributors:James Kellogg, AIA, HOK, San Francisco, California; Lynn Filar, HOK, SanFrancisco, California; Navinchandra R. Amin, SE, Middlebrook + Louie,Structural Engineers, San Francisco, California; Vivian L. K. Wan, PE,Middlebrook + Louie, Structural Engineers, San Francisco, California,Michael O’Callahan, Photography, San Francisco, California.

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PIN PILE SECTION AND DETAILS: Circular pin piles 7 in. in diame-ter were used to support the gravity and seismic loads. Pile loadtests were performed to determine ultimate pile capacity, length,and placement, and to limit differential settlement to 1/2 in. Thepin piles were confirmed to require an average length of 67 ft inlength below grade. They were installed in about 10-ft sections(determined by available working height) and reinforced with con-tinuous threaded reinforcing steel and couplers between each sec-tion. They were post-grouted directly to the noncased soil boringsat the bottom sections to achieve an average ultimate capacity of550 kips.

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VISTEON VILLAGE

IDEALIZED GEOLOGIC PROFILE SECTION: Idealized geologic profilesection through deep rubble area. The natural sand and gravel waspreviously mined and then replaced with rubble and loose soil fill.

SHEET PILING INSTALLATION ALONG SHORE OF LAKE: The individ-ual steel sheet piles are diven in stages to help maintain a straightalignment. The lower ones to the right have been driven to final ele-vation, while the higher ones to the left still need to be driven downto final elevation.

INTRODUCTION

Visteon Village is the new corporate headquarters in Van BurenTownship, MI, for the Visteon Corporation, a Tier 1 automobileparts supplier. Architecture design, engineering, and interiors wereprovided by SmithGroup, Inc. with geotechnical engineering pro-vided by SOMAT Engineering, Inc. Visteon Village is a collection ofdiscrete human-scaled buildings each slightly different from theothers in size, height, and finishes. In addition to administrativeoffices, technical offices, and laboratories, the corporate head-quarters will house amenities including a convenience store, abeauty salon, a barbershop, a restaurant, a bank, boardwalks, andnature trails accessible to employees, as well as the community.

The design consists of 11 buildings totaling 1,050,000 sq ft set ona scenic 265-acre site that includes a 37-acre lake. Nine of thebuildings have been constructed as part of Phase One totaling800,000 sq ft; the remaining two buildings will be constructed aspart of a Phase Two development of the site.

Each building is a simple rectilinear form based on the consistentuse of a 60-ft-wide building. The buildings are loft-like with openspaces and high exposed ceilings that provide maximum flexibility

for the future. The character of the village is developed through theuse of various materials, varied roof forms, and vertical elementsclipped to the rectilinear buildings. Facades are composed of brick,concrete masonry, glass, and metal panel. The four lakeside build-ings are four-story office buildings that include a lower level open-ing onto the lake. The three landside buildings are a combination ofone-story, high-bay laboratory buildings abutting two- and three-story office buildings. The Village Center is located in the heart ofthe campus, and is marked by its size and height, rising an addi-tional story above the rest of the buildings on campus. The centralplant is minimized by a simple one-story building with a gable roof.

Visteon Corporation (which became independent from Ford MotorCompany) was housed in as many as 30 buildings across the stateof Michigan. Communication and a shared sense of purpose wereincreasingly difficult with a workforce spread across so many dif-fering sites. The corporate leaders decided to consolidate many oftheir functions into a single site capable of housing a total work-force of 4000. The goal of the design team was to create this invit-ing environment at a cost that would be equivalent to the cost ofthe current lease space of its existing buildings.

The intent of the Visteon Village was to create an image that proj-ects a fresh, new expression of an environment that supports cre-ativity, human development, and work processes. It embodies acommunal spirit of innovation and expression—an environmentthat stretches its occupants to reach beyond themselves.

The characteristics of the buildings reinforce the “villageness” ofwhat otherwise would be an extremely large, extensive facility. Thecreation of many buildings, each human-scaled, each slightly differ-ent from any other, each associated with a unique function or uniquebranding opportunity, results in a rich, urban environment—a placewhere the Four Faces of Visteon (customers, employees, partners,and the community) will interact and innovate.

Sustainability was one of the goals for Visteon Village. For that rea-son, the LEED certification program was used as a tool for sus-tainable goal setting. The primary image of the complex is that of a“good neighbor” that respects the spirit and scale of the adjacentcommunities (by providing nature trails and environmental ameni-ties for the adjacent Van Buren community) while supportingVisteon’s image as responsible environmental steward.

A process of gathering information, testing, coordination of theinformation, and creativity were all used on the Visteon project.

To achieve the final substructure design, SmithGroup performed arigorous analysis of the structural loadings and configurations oflateral systems to determine the optimum column spacing and lat-eral load–resisting system consistent with the requirements of thearchitectural planning of the space. The geotechnical firm SOMATworked closely with The SmithGroup structural engineering team toensure that the requirements were met, that the information wasconsistent, and the foundation systems were economic and func-tional with the rest of the structure.

SOMAT Engineering began with a basic understanding of the build-ing configuration, heights, depths, loads, and overall dimensions.SOMAT reviewed existing information about the soils in the area,including existing soils reports. A site visit revealed topographicalconsiderations, existing building interferences, and utility clear-ances, as well as basic field observations of the area. SOMAT also

METHODOLOGY

used its local knowledge of the area concerning soils anticipated,bedrock considerations, and groundwater expectations.

A boring plan was submitted consistent with the information gath-ered. It was originally determined that 110 borings with a depth of15 to 50 ft would be required. SOMAT initiated the boring plan andreviewed the first several borings with the structural engineer toensure that no unusual conditions were uncovered.

After reviewing the preliminary site analysis report, and while onthe initial site visit, the SOMAT geotechnical engineer determinedthat there was something different about the site than was charac-terized by the preliminary site analysis. Lateral movement of theshoreline soil and areas of debris that dotted the site were appar-ent to the SOMAT geotechnical engineer.

After the initial borings, SOMAT immediately informed the engi-neers and contractors of new findings and possible impacts on theproject. SOMAT expressed great concern that a large portion of the

building site would be founded on an underlying unstable, uncom-pacted deep rubble field. Eventually, 207 borings with depths reach-ing 110 ft were performed using various types of drill rigs. Manydifferent drilling techniques were eventually conducted to evaluatethe property and to provide the geotechnical information requiredfor design of the structures, parking lots, and roadways.

An engineering evaluation process determined what soil remedia-tion schemes could be exercised and which foundation and slabsupport systems could be used. The contractor provided input as toconstruction cost and schedule impact for each of the combinationsproposed. Investigations of the man-made lake were started.Aerial photographs dating back to the 1940s helped determine theextent of the rubble field expected.

The SmithGroup structural engineers worked closely with theSOMAT geotechnical engineer to develop the multiple schemes. Thefinal outcome would stretch to the maximum the normal methodsof design and of detailing the foundations.

VAN BUREN TOWNSHIP, MICHIGAN

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MULTIPLE FOUNDATION SYSTEMS IN RELATION TO VARYING SUB-GRADE CONDITIONS: Because of the extreme variance in the geot-echnical conditions, different foundation systems were used tosupport the various structures. In four of the buildings (B, D, E, andG), different foundation systems were used within a building, apractice that is generally discouraged due to different settlementconcerns but required here.

TECHNICAL SYSTEMS

At first glance, the proposed site for the 11 buildings looked ideal.The overall parcel was approximately 265 acres with a 37-acreman-made lake in the central portion of the property. Approximatelythe northern half of the property was wooded wetlands that were tobe preserved. However, the first soil borings encountered concreterubble that could not be penetrated with conventional soil boringequipment. A large drill rig was brought in to determine the verticaland lateral extent of the concrete rubble. The lake was actually theremnants of a wet sand and gravel mining operation that was sub-sequently used as a disposal area for building demolition harddebris. The debris filling had taken place along the north shore ofthe lake where the client wanted to construct the office park.Because of various site constraints (such as the wetlands), the foot-print for the office park could not be shifted to any significantdegree. Therefore, the one- to five-story buildings would be con-structed on subsurface conditions that varied with up to 90 ft of con-crete rubble to various depths of loose-dumped clay and sand fill tonatural hard clay. The transitions between types of materials anddepth of fill were random, both vertically and horizontally. As aresult, many types and combinations of foundations and subgradepreparation were used to construct the village buildings.

The most challenging foundation condition was the presence of theconcrete rubble. This area covered about 10 acres in the central por-tion of the office park, including about half of the five-story VillageCenter building, and all or portions of five other buildings. The con-crete rubble varied from sidewalk pieces to massive concrete blocksof several cubic yards. The rubble had been end-dumped into thelake, and the lakeside face of the underwater rubble was almost ina vertical configuration, as judged by the soil boring results. Thelakeside face had been buttressed by end-dumping clay soil into thewater, but this fill had virtually no dependable strength. While drillingthrough this rubble, the pieces of concrete would sometimes shift,thus indicating the potential instability of the rubble mass. In fact,there were surface indications (such as cracks in the soil) that therubble mass was slowly sliding into the lake.

Several options were considered for dealing with the foundationsthat needed to be constructed in this area. These included removaland replacement with engineered fill, grouting the rubble mass,

densification, and deep foundations passing through the rubble.Costs and practical difficulties associated with performing therequired activities in 60 ft of water eliminated the removal andreplacement and grouting options. The design and constructionteam ultimately selected a combination of densification and steelH-piles for the foundations in this area.

The first step consisted of cutting the grade in this area down toabout 6 ft above the water level and then densifying the rubble withdeep dynamic compaction (DDC). This process consisted of repeat-edly dropping a 10-ton weight from a height of about 60 ft in a pat-tern across the affected area. This caused the ground surface tosubside about 4 to 5 ft. The purpose of this DDC was to densify therubble by shaking it down into a more stable condition. If the rub-ble were to fail and slide into the lake, engineers wanted it to hap-pen now rather than after the buildings were constructed. Thecratered surface was then filled and smoothed with clay engi-neered fill.

Prior to the DDC operation, a test pile program was constructed todetermine if piles could be driven through the concrete rubble.Twelve H-pile test piles were driven through the rubble and thenpulled out of the ground to determine their condition. It wasdetermined that an HP 12 by 84, with a cast steel protective point,was the lightest pile section that could be reliably driven throughthe rubble. The piles that were driven without the protective pointall suffered significant damage during driving.

The production pile driving through the rubble went exceptionallywell with only a handful of piles meeting refusal in the rubble.During pile driving, the rubble was stable with little to no shiftingof the rubble pieces, verifying the DDC was successful. Because ofa fear of sinkholes developing in the rubble area after constructionwas completed, the lowest level floor slab was constructed as asupported slab, independent of the underlying soil and rubble.

At the lateral extents of the rubble area, the transition to otherbearing conditions was abrupt. Most of the areas surrounding thedeep rubble had from 5 to about 15 ft of end-dumped clay fill at thesurface. The foundations in this area were constructed as short,straight shaft-drilled foundations bearing on the natural clay below

the clay fill. Prior to the construction of the foundations, the entirearea within the building footprints covered with clay fill was uni-formly compacted with light dynamic compaction. This permittedthe use of a slab-on-grade for these buildings, rather than using astructural slab. For this area, a 7-ton weight was dropped from aheight of about 20 to 40 ft, depending on the depth of the clay fillto be compacted. The compacted area subsided about 1 ft after thecompaction. Again, the compacted area was filled and smoothedwith engineered fill in preparation for the slab-on-grade.

In buildings that had basements, outside the rubble area, the ele-vations of the basements were such that the excavations wentthrough the existing clay fill to bear on the hard natural clay, andconventional spread foundations were used.

The boundaries between the various foundation types did not haveany relationship to the locations for the buildings, and, as a result,there was significant concern for differential settlement withinbuildings. This was taken into consideration in the structural designof the building. In addition, the structural engineer liberally usedexpansion joints within buildings to separate the various portionsof a structure that needed to move independently.

In addition to the foundation and subgrade challenges, the designand construction of the seawalls along the waterfront presentedother unusual challenges. The primary seawall along the front of thecorporate headquarters building needed to be constructed on theend-dumped clay fill. This fill had very little strength, as evidencedby the zero blow counts from the standard penetration tests to adepth of about 60 ft. The seawall would have about 5 ft of freeboardabove the lake level, which would mean about 5 ft of new fill wouldneed to be placed on this very low-strength soil. The existing fill soilwas not capable of supporting 5 ft of conventional fill, so instead ofusing soil fill, expanded polystyrene (EPS) blocks were used as thefill material behind the sheet piling. The sheeting was then stabilizedby using an upper and lower tieback that passed through the EPSblocks to anchorage in competent soil behind the EPS. The EPSblocks were covered with a petroleum resistant membrane, andfinally 1 ft of topsoil cover for the grass lawn.

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PILE CAP INSTALLATION: Installation of reinforced concrete pilecaps that serve to tie groups of steel H piles together so collec-tively they can support the heavy column loads.

SUMMARY

Visteon Village opened on schedule in late 2004, to rave reviews bythe employees of Visteon and the community. Comments from theleadership of Visteon about the new facility include: “VisteonVillage embodies the culture we’re striving to foster, one of open-ness, innovation, and collaboration that inspires our employees todevelop great products for our customers,” and “Visteon Villageenhances our emphasis on our customers, creates a sense of unityamong our employees and helps us share best practices.” The con-solidation of the multiple sites, the inviting surroundings, and theemployee-friendly amenities resulted in a very successful project.

The difficult soils conditions presented the design team with majorchallenges. Close collaboration, creativity, and teamwork created asolution that is remarkable in economy and engineering. While thebudget was increased to accommodate the changes in conditions,the final design resulted in a structure and foundation system thatmet the client’s budget expectations.

In addition to the structural and geotechnical challenges, themechanical engineer developed a means of using the lake water to

cool the building. Utilization of the lake water is projected to real-ize a 35 percent savings of water usage over standard use, andoverall project savings equal approximately 30 percent of energyuse versus standard systems.

One of the first items that would have helped the foundationswould have been a more rigorous preliminary site investigation.The preliminary site investigation should be commensurate withthe project scope, scale and location of structures.

The flexibility of the structural engineer, in listening to the geo-technical engineer’s ideas and working with the contractor todevelop alternate foundation scenarios, was one of the keys to thesuccessful completion of the Visteon Village project. Perseveranceis needed when challenged repeatedly with new problems of thesite and constraints of the architect. And finally, creativity to puttogether items that are not commonly associated with one anoth-er on one project and the courage to present these ideas to theteam are essential characteristics of successful teams working onchallenges of a lifetime.

Contributors:Andrea K. Reynolds, P.E., SmithGroup, Detroit, Michigan; Richard O.Anderson, PE, Hon. M. ASCE, SOMAT Engineering, Detroit, Michigan.

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ORIGIONAL PROJECT RENDERING: The Village concept for thisproject was planned form the beginning. Situated on the shore of aman-made lake, the setting on the surface was ideal.

CONSTRUCTION PHOTO: Overcoming tremendous geotechnicalproblems that were hidden from view required the ingenuity andcreativity of the entire design team. The result is a stunning groupof structures that opened to rave reviews of the client and Visteonemployees.

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04 700913 ch01 - wiley...john p. mccarthy, pe, se, smith group architecture, engineering, interiors,...

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