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5/60STRUCTURE magazine January 20145

CONTENTS

Publication of any article, image, or advertisement in STRUCTUREmagazine does not constitute endorsementby NCSEA, CASE, SEI, C3Ink, or the Editorial Board. Authors, contributors, and advertisers retain sole

responsibility for the content of their submissions.

FEATURES

COLUMNS

DEPARTMENTS

INEVERYISSUE

January 2014

e Pennoni PhiladelphiaStructural Division

investigated and developedrepair bid documents for anexisting, three-level, 1,200-

space precast concrete parking garage duringthe last quarter of 2012. Part 3 of the seriesof articles, page 20, conveys conclusionsregarding the feasibility of repairing the

garage in order to extend its service life.

28

30

46SpecialSection

Foundation Sector Grounded inOptimism for New YearBy Larry Kahaner

We have heard it before, but next year could be shapingup to be strong for the building business. What appearsto be driving the industry are a growing economy, newofferings that are attracting clients, and a general optimismamong those companies who have survived the lean yearsof the building slowdown.

Concrete Paint Arrests CofferdamCorrosion at Submarine PierBy Brian Robinson, P.E.

Often, steel sheet pile walls are incorporated as part ofcost-effective waterfront earth-retaining structures. Many of

these walls are subjected to a host of environmental factorsthat accelerate corrosion. Cellular sheet pile cofferdams areparticularly difficult to repair because of the lack of redundancyin the tensioned cell walls. This article highlights a uniquemethod to repair a cellular cofferdam using a reinforcedconcrete facing installed over the aging sheet piling.

Geopolymer Precast Floor PanelsBy Rod Bligh, B.Eng, MSc, CPEng, andTom Glasby, B.Eng (Civil), MBT, MIEAust, CPEng

Early in the design process, the structural engineering team

explored the potential for the incorporation of structuraltimber. Timber-Concrete Composite (TCC) fl oors wereof interest and TCC was proposed as a potential fl oorsystem that combined the benefi ts of timber framing withthe acoustic, fi re separation and wearing properties ofconcrete. It was at this stage that the strong potential for useof geopolymer concrete in the system was identifi ed.

7 Editorial

The Ins and Outs of theSoftware Black Box

By Andrew Rauch, CASE Chair

9 InFocusRisk and Virtue Ethics

By Jon A. Schmidt, P.E., SECB

10 Structural ForensicsUntreated Submerged TimberPile Foundations Part 2

By Giuliana Zelada-Tumialan, P.E.,William Konicki, P.E.,Philip Westover, P.E. and

Milan Vatovec, Ph.D., P.E.

16 Historic StructuresDeconstructing Bridge 92297

By Ryan Salmon, EIT andMeghan Elliott, P.E.

20 Structural RehabilitationPrescription for Repair Part 3

By D. Matthew Stuart, P.E., S.E.,SECB and Ross E. Stuart, P.E., S.E.

23 Construction IssuesWelding Reinforcing Steel

By John Hlinka, P.E.34 Professional Issues

Opposition to Structural LicensureBy Timothy M. Gilbert, P.E., S.E.,SECB

36 Structural TestingLateral Loads Generated byOccupants on Exterior Decks

By Brian J. Parsons,Donald A. Bender, P.E.,

J. Daniel Dolan, P.E.and Frank E. Woeste, P.E.

40 Engineers NotebookConcrete Column Design

By Jerod G. Johnson, Ph.D., S.E.

43 InSightsASTM A1085

By Kim Olson, P.E.

51 SpotlightTransforming the Fan Experience

By John M. Hann, P.E.

58 Structural ForumVelocity of Learning Revisited

By Tom Glardon, P.E. 8 Advertiser Index44 Resource Guide

(Anchor Updates)52 NCSEA News54 SEI Structural Columns

56 CASE in Point

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ONTHECOVER

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new trends, new techniques and current industry issuesEditorial The Ins and Outs ofthe Software Black BoxBy Andrew Rauch, CASE Chair

STRUCTURAL

ENGINEERING

INSTITUTE

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Last spring, I had the opportunity to attend one of theseveral terrific presentations that were part of the CASERisk Management Convocation at the 2013 StructuresCongress. is session was presented by James Parker and

Pedro Sifre of Simpson, Gumpertz, and Heger, and I thank themfor allowing me to present a portion of it to a wider audience.e use of design software is in integral part of the structural engi-

neering design process. None of us can imagine what our professionwould be like without it. At the same time, it presents challengesand concerns to those who are responsible for the operation of anengineering firm. e attendees at this session discussed some ofthose challenges, including staff skills and training, the black boxaspects of software, documentation of software results, and the

delegation of design and code interpretation to software companies.Are we training our staff to use software appropriately? Are wegiving up an opportunity for young engineers to develop skillsneeded to conceive and implement structural designs by allowingthem to extensively use software for design? As a young engineer,I learned design through repetition, reading code requirementsand applying them in preparing calculations. (How many of youeventually memorized the member properties of some of thecommon beam sizes?) I began to understand what the expectedresults should be prior to performing the calculations. Todaysengineers need and use very different skills. ey need to learnhow to use software effectively, to learn how to properly build astructural model, and to learn how to make their design model

interface with building information models. When and how dothey develop a feel for the structure and intuition about a designthat tells them if their design is reasonable?The situation may arise where an engineer is using software

to design a system they may not have previously designed. Thesoftware is able to provide design results for that structure, buthas the engineer developed the skills to determine if the designresults are correct or reasonable? Does the engineer have the skillsnecessary to approximate the design to verify the software results?

Obviously, in this situation,the engineer needs a significantamount of oversight.Structural engineering soft-

ware can also be a black box.How often have you heardthe explanation thats whatthe output said in responseto a question about a designresult? How does the programhandle design conditions suchas unbraced length, crackedmember stiffness, or the algo-rithm for selecting the numberof shear studs on a compos-ite beam? Often, the manual

provides little information to help the engineer determine whatprocess the software is using. Are we deferring code interpreta-tion and some of our quality assurance to the software provider?Documentation of design is another issue. Have you ever been

looking for design information to answer a question and found no

written calculations? You try to find a result from the software, onlyto find several versions of the model with no clear indication ofwhich one is the most current or what the different models signify.Young engineers will sometimes use the brute force method ofdesign, using the computer to run multiple iterations. When itcomes time to provide written documentation, suddenly there arepages and pages of calculation for a design problem that couldhave been designed much more simply. Are the requirements forcomputer analysis and design documentation procedures a partof your office policies and procedures?e final question posed at this session asked how the profession

should react. Should one (or all) of the structural engineering orga-nizations provide reviews and vetting of software? Should we leave

software verification to the purchasers and users, and let marketforces drive software quality? Should the structural engineeringorganizations work with authorities having jurisdiction to demandcertification or verification of software? While it would be niceto have third-party software verification, that is a Herculean taskfor structural engineering organizations that are run primarily byvolunteers. For now, the consensus of those in attendance was tolet market forces drive the quality.e question we must ask ourselves is how much of our design

skill and interpretation do we want to delegate to software com-panies? To our knowledge, there are no standards or requirementsfor software producers to check and verify their software. Writersof software codes are not required to be licensed to work underthe direction of a licensed engineer. Our experience has been thatevery software program we have purchased or licensedhas had some kind of error or bug that caused it notto work properly. How are we as individuals and as aprofession going to react?

Andrew Rauch is a principal with BKBM Engineers inMinneapolis, MN and is responsible for overseeing their qualityassurance and risk management programs. He is the currentchair of the CASE Executive Committee. He can be reached [email protected].

The question we must ask ourselvesis how much of our design skill

and interpretation do we want to

delegate to software companies?

8/11/2019 2014-01 Jan


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STRUCTURE (Volume 21, Number 1). ISSN 1536-42Publications Agreement No. 40675118. Owned byNational Council of Structural Engineers Associations published in cooperation with CASE and SEI monthly by CThe publication is distributed free of charge to membeNCSEA, CASE and SEI; the non-member subscription is $75/yr domestic; $40/yr student; $90/yr Canada; $6Canadian student; $135/yr foreign; $90/yr foreign studenchange of address or duplicate copies, contact your memorganization(s). Any opinions expressed in STRUCTURE magaare those of the author(s) and do not necessarily reflect the vof NCSEA, CASE, SEI, C3Ink, or the STRUCTURE Editorial Boa

STRUCTURE is a registered trademark of National CounStructural Engineers Associations (NCSEA). Articles may noreproduced in whole or in part without the written permisof the publisher.

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ADVERTISERINDEX PLEASESUPPORTTHESEADVERTISERS

Chair

Jon A. Schmidt, P.E., SECBBurns & McDonnell, Kansas City, MO

[email protected]

Craig E. Barnes, P.E., SECBCBI Consulting, Inc., Boston, MA

Mark W. Holmberg, P.E.Heath & Lineback Engineers, Inc., Marietta, GA

Dilip Khatri, Ph.D., S.E.Khatri International Inc., Pasadena, CA

Roger A. LaBoube, Ph.D., P.E.CCFSS, Rolla, MO

Brian J. Leshko, P.E.HDR Engineering, Inc., Pittsburgh, PA

John A. Mercer, P.E.Mercer Engineering, PC, Minot, ND

Editorial Board

Brian W. MillerDavis, CA

Evans Mountzouris, P.E.The DiSalvo Ericson Group, Ridgefield, CT

Greg Schindler, P.E., S.E.KPFF Consulting Engineers, Seattle, WA

Stephen P. Schneider, Ph.D., P.E., S.E.BergerABAM, Vancouver, WA

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new trends, new techniques and current industry issuesInFocus Risk and Virtue EthicsBy Jon A. Schmidt, P.E., SECB

Ihave previously (and repeatedly) cited a paper by philosophers

Allison Ross and Nafsika Athanassoulis that highlights the risk-taking nature of engineering practice and draws out some ofthe associated ethical implications. In two additional papers (A

Virtue Ethical Account of Making Decisions About Risk,Journalof Risk Research, Vol. 13, No. 2, March 2010, pp. 217-230; Riskand Virtue Ethics, chapter 33 in Handbook of Risk Teory, Springer,2012), the same authors discuss risk in a more general sense and argueconvincingly that virtue ethics provides the most adequate approachfor dealing with it.Ross and Athanassoulis concede that it would be convenient if there

was a formula for making good and right decisions about whether,when, and what to risk. Tis is essentially what the dominant modernethical theories purport to offer: definitive guidance derived from

universal principles, such as assumed duties and obligations (deontol-ogy) or assessment of anticipated outcomes (consequentialism). Bycontrast, virtue ethics recognizes that any truly substantive ethicalinquiry will lead to a complex, varied, and imprecise answer thatcannot be captured in an overriding rule.When it comes to risk, Ross and Athanassoulis raise specific objec-tions to consequentialism. Evaluations based only on what ultimatelyhappens ignore the contributions of luck; for example, avoiding theconsequences of ones recklessness does not make one any less respon-sible for it. Te alternative of assigning probabilities is problematicat best, and the corresponding utilitarian calculation often clashes

with our sense of fairness with respect to the equitable distributionsof the burdens of risk taking. Furthermore, it does not allow room

for differentiating between the bearers of risks and benefits, whomay not be the same parties.Virtue ethics shifts the focus from individualactions topatternsof

behavior choices that people make, those choices that are reaffirmedover time, and those choices that express their deeply held values andbeliefs. It is thus concerned primarily with someones long-termattitudetoward risk, especially with respect to the potential impactson the well-being of others. Te central concept is character, definedby Athanassoulis as the set of stable, permanent, and well-entrencheddispositions to act in particular ways. Tese dispositions qualify asvirtues when they enable and incline someone to respond well to

whatever situation is encountered.Te circumstances of greatest interest to Athanassoulis and Ross are

those in which a person say, an engineer must intentionally makechoices that involve risk to others; i.e., when all of the followingconditions hold:

Te person is deliberating whether to take a certain action. Te person cannot guarantee the outcome of that action; there are

multiple possibilities, one or more of which would affect others. Te person is able to estimate (at least roughly) the likelihood

of various outcomes. Some outcomes are desired, while others are unwanted (by the

person and/or others). Te person perceives the prospect of a positive outcome as

outweighing the danger of a negative one.

According to virtue ethics, the last item is crucial and cannot simply

be the product of a straightforward cost-benefit analysis. Instead,it requires a state in which the faculties of perception, motiva-tion, thought, and reason seamlessly interact to discern the relevantcontextual features and properly take them into account i.e., theexercise of practical judgment orphronesis. Since what subsequentlytranspires may not be entirely within the persons control, what mat-ters from an ethical standpoint is the quality of the decision at thetime when it is made.In other words, risk-taking is agooddecision whenever it is based on

defensible grounds, regardless of the actual results. Athanassoulis andRoss suggest that this criterion is usually satisfied whenever someonehas a clear and accurate view of the situation and produces a pro-portionate, rational response. Te underlying motives fear, desire

for pleasure, etc. are not necessarily good or bad in themselves; whatis important morally is how, when and why we are moved by them.Nevertheless, Ross and Athanassoulis acknowledge a prominent

place for emotions in the whole process: Decisions about risk thatproceed from a good character involve emotional responses, whichare integral to firm and stable dispositions to virtue Te personof practical wisdom is someone who has the appropriate emotions,to the right degree at the right time. Such sentiments may seem outof place in an engineering magazine; after all, engineers generallyview themselves and are widely viewed by others as paragons ofunbiased analysis and dispassionate design. But is this an accuratepicture? And if so, should it be?Te framework that I have proposed for applying virtue ethics to

engineering practice identifies not only objectivityand honestyas moralvirtues of engineering, but also care. Is it possible for engineers toexhibit genuine care for the people who will be affected by their work

while not experiencing any feelings toward them whatsoever? Can webe completely indifferent and still hold paramount the safety, health,and welfare of the public as stipulated by the most fundamentalcanon in our codes of ethics? Instead, perhaps emotions should playa more explicit role in our decision-making about risk.In summary, according to Athanassoulis and Ross, a decision to

risk is a complex decision which involves the bringing together ofpersonal reasons for acting, moral reasons for acting and a whole rangeof facts Good judgements require phronesisand sensitivity andthese are skills that are acquired and internalised through a process

of observation and emulation of good exemplars, practiceand reflection. As engineers, we are routinely confronted

with such decisions; are we going about them in the rightway and preparing ourselves accordingly?

Jon A. Schmidt, P.E., SECB ([email protected]), isan associate structural engineer at Burns &McDonnell in KansasCity, Missouri. He chairs the SRUCURE magazine EditorialBoard and the SEI Engineering Philosophy Committee, and sharesoccasional thoughts at twitter.com/JonAlanSchmidt.

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investigating structuresand their components

STRUCTURALFORENSICS

Giuliana Zelada-Tumialan, P.E.,is Senior Project Manager atSimpson, Gumpertz &Heger,Inc., Giuliana may be [email protected].

William Konicki, P.E., is SeniorPrincipal at Simpson, Gumpertz&Heger, Inc. William may bereached at [email protected].

Philip Westover, P.E., is a Staff

Consultant at Simpson, Gumpertz&Heger, Inc. Philip may bereached [email protected].

Milan Vatovec, Ph.D., P.E., is SeniorPrincipal at Simpson, Gumpertz &Heger, Inc. Milan may be reached [email protected].

By Giuliana Zelada-Tumialan, P.E.,William Konicki, P.E.,Philip Westover, P.E. and

Milan Vatovec, Ph.D., P.E.

Part 2: Estimating

Remaining Service Life

Untreated SubmergedTimber Pile Foundations

As discussed previously in UntreatedSubmerged Timber Pile Foundations Part 1 (STRUCTURE magazine,December 2013), deterioration of pile

tops exposed above groundwater levels is a well-known problem. It is less known that submergedportions of timber piles can also deteriorate withtime, albeit at a slower rate, due to bacterialattack. Tis may become critical when consider-

ing underpinning methods aimed at extendingservice life of structures supported on timber piles.Historically, timber-pile supported structures havebeen underpinned by the cut-and-post method,

where the top portion of the timber piles is cutand replaced with concrete posts or concrete-encased steel posts. Although the cut-and-postmethod appears to be relatively straight forwardand simple to execute, it remains an expensiveundertaking due to accessibility issues, requiredtemporary shoring and bracing, dewatering, andlabor costs. Klaassen (2008-1) reports that, in

the Netherlands, founda-

tion replacement or repairsometimes involves up to50% of the total renova-tion costs for a structure.Te authors experience inthe Boston, Massachusetts

area indicates cut-and-post underpinning of atypical downtown row house costs approximately$200,000 to $250,000. Bacterial attack in theremaining, submerged portion of the timberpiles, however, may limit the effectiveness of thecut-and-post method, as well as the estimatedremaining service life of the piles.

Remaining Service Life

Te aim of any foundation remediation/repairdesign is similar to that of new foundation

design: its design and execution must be ableto (1) safely sustain all likely applied loads with-out failure (i.e. without overloading beyond thestrength capacity of the foundation system),and (2) remain serviceable for the required useof the structure (e.g. without excessive settle-ment) during its intended service life. Hence,one of the greatest challenges in pile foundationremediation/repair design, and a key item for its

success, is performing a reliable assessment of thecurrent in-situ foundations material propertiesand loading history, after years in service andexposure to the surrounding natural environ-ment. is forms the basis for the estimationof the remaining service life of the foundationsystem, if it is to be re-used.Te estimated remaining service life of any foun-

dation system is governed by the determinedminimum structural capacity (dependent onmaterial properties and level of deterioration),the geotechnical capacity (dependent on soilproperties and soil-structure interaction), and

the magnitude of expected movements (e.g. settle-ment) compared to the allowable movements thata structure can sustain.

Determining Remaining

Structural Capacity

Based on the review of published literature andon relevant experience, the following approach isproposed to determine the remaining structuralcapacity of continuously submerged timber piles:Step 1 Estimate the applied compressive stress

acting on the timber pile cross-section versus

time, considering the reduction in availableload-bearing pile cross-section due to continuedbacterial decay penetration. For spruce and pinepiles, an estimated rate of advance of severe degra-dation of 0.0051 inch/year and 0.0098 inch/year,

Figure 1: Decrease in timber pile compressive strength with in-service age (Base figure from Van Kuilen, 2007).

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respectively, can be used (Klaassen 2009). Tefollowing considerations should be included:

Current measured penetration ofsevere bacterial deterioration (basedon probing to determine depth tosound wood) to be used as the startingpoint from which future reductions inpile cross-section will occur.

Determine the taper of the timber pile

for estimation of the pile tip diameterbased on pile top diameter. imberpiles in many cases tend to derive theircapacity by end bearing on a suitablesoil stratum; therefore, the criticalpile section is located at or nearthe pile tip. Te rate of severebacterial degradation should beapplied uniformly over the entirepile length.

Te rate of bacterial attackdecreases significantly beyond theheartwood-sapwood interface in

spruce and pine piles. Terefore,for these species at least, it isreasonable to assume that for themost typical required service life ofstructures (i.e. 100 years or less),only the sapwood will deterioratesignificantly and no furtherreduction in pile cross-section dueto bacterial decay is expected oncethe sapwood thickness has beenexpended. Te determination ofthe sapwood/heartwood boundaryrequires microscopic examination

for heartwood signs, and forbacterial invasion and deteriorationat different depths within the pile;this can be subjectively influencedby the examiners experience.

Without detailed microscopicobservations, the depth to theheartwood/sapwood boundary canonly be roughly estimated fromobvious color changes in the wood,or based on publications like eWood Handbook(USDA, 2010)or the Textbook of Wood Technology(McGraw-Hill, 1980).

Although the deteriorated sapwoodhas some measurable compressivestrength, it seems prudent to ignoreits contribution to the timber pilestrength capacity. Measured valuesof elastic modulus for specimensof deteriorated sapwood obtainedfrom piles (from previous projects)indicate that the ratio of elasticmoduli between deterioratedsapwood and sound heartwood is inthe range of 0.1 or less. Terefore, it

would be expected that more than 90%of the applied load is resisted by thestiffer and stronger heartwood.

Step 2 Estimate timber pile compressivestrength versus time by using the reduc-tion in compressive strength due to aging/duration of loading for heartwood shown inFigure 1. Tis assumes that all of the remain-ing pile section (based on probing to measure

the depth to sound wood), including anysmall amount of sapwood present, can berepresented by the reduced average compres-sive strength for heartwood.

Step 3 Determine estimated remainingstructural service life of submerged timberpiles by determining the time (from present)at which the demand-to-capacity (D/C) ratiofor the various timber pile diameters consid-ered (i.e. the ratio of applied compressivestress to the remaining compressive strength)is equal to the desired factor of safety level.

Alternatively, the designer may choose to use

a target minimum allowable percent loss inpile cross-section to determine the remain-ing structural service life of the submergedtimber pile.

continued on next page

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Figure 2shows a sample plot of the calculated reduced heartwoodcompressive strength with time (Step 2), superimposed on plots ofthe applied compressive stress curves developed for various timberpile diameters as bacterial decay penetrates to the heartwood/sapwoodboundary for a service load of 20 kips (Step 1). Te intersection of thetime under load dependent ultimate heartwood compressive strengthcurve and the applied compressive stress curves defines the expectedremaining pile service life for each pile diameter at the expected loadcombination, with no factor of safety included. Figure 2was developedfor spruce piles with an in-service age of 109 years, 2-inch sapwoodthickness, and measured severe bacterial attack penetration of about0.75 inches at time zero. Te plot of applied compressive stress is

shown in relation to the pile butt (pile top) diameter, rather than thecritical pile section (i.e. pile tip) used to calculate the applied stressesfor ease of use during investigations where, typically, just the pile topsare exposed. Figure 2also shows the time for 50% loss of original pilecross-section for reference.Figure 3shows plots of the estimated remaining service life for each

timber pile butt (top) diameter and different levels of applied serviceload calculated similarly to Figure 2, and considering a D/C ratio of1.0 (i.e. no factor of safety is included). Since Figure 2only considerstime up to 500 years, the curves in Figure 3level off at 500 years.

Evaluation of Remaining Geotechnical Capacity

Te Table(page 14) summarizes the results of load testing on five timber

piles from separate areas of a single project site in downtown Boston.wo of these piles were extracted after the load tests were performed.Results of the pile load tests indicate no apparent adverse impact

of timber pile deterioration on the geotechnical bearing capacityof the piles. Area 1-1, which showed a larger penetration depth ofsevere deterioration (with 75% or more loss of pile cross-section),performed stiffer and had a higher measured unit end bearing capac-ity than the Area 1-2 pile which showed less deterioration (about34% loss of pile cross-section). However, the data evaluated in thisanalysis is too limited to draw more in-depth conclusions regard-ing the impact of deterioration on the geotechnical capacity of thesubmerged timber piles.

Estimated Future Settlements

Once cut-and-post underpinning is performed(i.e. no inelastic settlement due to softening ofpile tops), and assuming no change in the level ofapplied loads or in soil or groundwater properties,the only viable mechanism for future settlement iselastic compression of the remaining submergedtimber pile sections. Tis can be due to potential

softening as a result of aging/creep, and decay(i.e. a decrease in the Youngs modulus, E) of thetimber piles. Te National Design Specification(NDS)for Wood Construction(2005) recom-mends a creep factor of 2 for wet wood, i.e. theE value should be decreased by 50% under long-term permanent loads. Current design standardsdo not provide recommendations for furtherreductions in E values due to decay.A review of limited data available from compres-sive-strength testing of timber pile samples fromvarious projects throughout northern U.S. (within-service ages ranging from about 100 years to

137 years) indicates that there may be an ongoingreduction in E values with time, similar to that of compressive strengthvalues. However, the data spread is too broad and the breadth of timeperiods too limited to be able to more accurately and reliably infer arate of degradation of the E value with time.Assuming a 50% loss of cross-section in the timber piles, a pile lengthof about 10.5 feet, and using an E value of 1.2 x 106psi for spruce(average published E value for fresh spruce from ASM D2555-06),the added submerged pile settlement under sustained loads varies fromless than 0.04 inches (for a service load of 50 kips combined with a

Figure 2: Sample plots of applied compressive stress and estimated heartwoodcompressive strength vs. time.

Figure 3: Sample plots of estimated remaining structural servicelife of submerged timber piles.

0

500

1000

1500

2000

2500

3000

0 100 200 300 400 500

)isp(ssertS

evisserpmoC

Time (in years from 2009)

Pile Service Load (P) = 20 kips

6-inch diam. pile butt







Estimated Average Heartwood

Ultimate Compressive Strength

Time to 50% Pile Tip Cross-Section Loss

Applied Compressive Stress:

2009

2109

2209

2309

2409

2509

2609

0

100

200

300

400

500

600

4 6 8 10 12

RemainingStructuralServiceLife(Year)

efiLecivreSlarutcurtSgniniameR

(Beyond2009,

inyears)

Pile Butt (Top) Diameter (in.)

P = 10 k P = 20 k P = 30 k P = 40 k P = 50 k

P = Applied Service Load (per pile):

continued on page 14

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12-inch pile diameter), to about 0.2 inches(for a service load of 50 kips and a 6-inchpile diameter). Even if it is assumed that,over time, the E value has degraded to about

50% of its original value throughout the pilelength, the added submerged pile settlementunder sustained loads for the same assumedconditions remains low, varying from about0.1 inches to 0.4 inches. If the applied loadsare sufficiently high, longer pile lengths couldresult in settlements greater than 0.5 inches.Based on the calculation case described

above, added settlement solely due to pileelastic compression will likely not exceed0.5 inches over the remaining service life oftimber piles. However, detailed settlementcalculations, taking into account actual pile

diameters, pile lengths, measured pile prop-erties and applied load magnitudes, mustalways be performed. Although most struc-tures can experience differential settlementson the order of 0.5 inches without resultingin much structural distress, the foundationremediation designer will have to take intoconsideration the present condition of thestructure. If the structure is fragile and hasalready undergone significant settlements,even small added settlements could have anadverse impact on its serviceability.

Conclusions Te remaining service life of in-service

timber piles appears to be controlledby the structural capacity of the timberpiles, rather than their geotechnicalcapacity. Evaluation of a moresignificant amount of data is necessaryfor confirmation of this postulate.

Measured rates of bacterialdeterioration indicate that, for piles

with 100 to 140 years of in-service age

and with diameters of 6 inches or less,bacterial decay may have advancedsufficiently that little to no remainingservice life is anticipated. For relatively

small applied service loads (around10 kips per pile and no factor ofsafety included), pile diameters of7 inches or greater are likely have aremaining service life of 100 years orgreater from present time. For largerapplied loads (on the order of 40 to50 kips per pile), pile diameters of 10inches or greater would be required toattain the same remaining service lifeexpectancy (100 years or more).

Once tops of piles are replaced andthe new pile cutoffs remain submerged

(e.g. cut-and-post underpinning isperformed), settlements due to pileelastic compression over the remainingservice life of the timber piles will likelynot exceed 0.5 inches.

The analysis used herein to estimate remain-ing service life of submerged timber pilesis based on average conditions (i.e. averagemeasured strength and/or modulus values).

Although measured strength and modulusdata is well distributed around the averagevalues used, there is still a 50% probabil-ity that the actual values may be lower or

higher than the ones used. In addition,other than limiting the depth of penetra-tion of bacterial decay to include only thethickness of the sapwood of submergedpiles, it is possible that local buildingcodes may require foundation remedia-tion/repairs be performed once a certainpercentage of the original pile capacity hasbeen lost. For the smaller-diameter timberpiles, this would likely result in smallerremaining service life expectancies thanthose indicated above.

Final Thoughts

Further research remains to be performedregarding the impact of bacterial attack onsubmerged timber pile structures, especially

any potential reduction of the piles geotech-nical capacity.Tere is also continued concern that soft-rot

deterioration could still occur even with ground-water levels maintained above the top of theuntreated pile cutoff. Recent research indicatesthat soft-rot attack may be supported even insubmerged conditions, if the dissolved oxygencontent in the groundwater is above a thresh-old value of 2 ppm (Klaassen 2005). Giventhat potable water is often used for recharginggroundwater levels near timber piles to maintainsubmersion, and this could lead to an increase

in dissolved oxygen levels in the groundwa-ter, further research is required to confirm thispotential deterioration mechanism.Development of a large database of U.S. his-

toric building stocks supported on untreatedtimber piles, similar to that currently in exis-tence for some European countries, wouldbe of significant value in evaluating currentconditions and required foundation remedia-tion/repair options. Based on the Europeanstudies on bacterial decay, existing untreatedsubmerged small-diameter timber piles withmore than about 100 years in service (which

represents a significant percentage of the exist-ing untreated timber pile stock in the U.S.)are likely to be reaching a level of bacterialattack at which there is little to no remainingservice life. For these structures, significantstructural settlement, with the consequentbuilding distress, may start developing withina relatively short-time from present.

Load TestPile Tip Diam

(inches)Max. Applied Load

(Kips)

Max. Measured PileTop, Total Settlement

(inches)

Inferred UltimateUnit End BearingCapacity (ksf (1))

eoretical UltimateUnit End BearingCapacity (ksf (2))

Area 1 1 7 (3) 70 0.56 224.5 220

Area 1 2 9 (4) 70 0.63 113.2 190

Area 2 1 6.5 (5) 40 0.57 130 130

Area 2 2 6 (5) 50 0.24 N/A 170

Area 2 3 8 (5) 60 0.71 114.5 110

(1) Based on Davissons Offset Criteria.(2) Based on Meyerhoffs method for driven piles in sand (Meyerhoff 1976).(3) Severe to moderate soft rot/bacteria deterioration penetrating about 1.5 to 2 inches into the pile at the pile tip.(4) Severe soft rot/bacteria deterioration penetrating about 0.75 inches into the pile at the pile tip.(5) Estimated based on pile taper. Condition of pile tip not known. Microscopy on upper pile sections indicate none to slight bacterial erosion in outer 0.5 inches.

Summary of pile load tests.

Te online version of this articlecontains detailed references. Please visit

www.STRUCTUREmag.org .

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16/60January 201416

significant structures of the past

HISTORICSTRUCTURES

Ryan Salmon, EIT ([email protected]), is a projectassociate and Meghan Elliott, P.E.,

Associate AIA ([email protected]), is the founder and owner atPreservation Design Works, LLC,a historic preservation consultingand project management firm in

Minneapolis, Minnesota.

By Ryan Salmon, EIT andMeghan Elliott, P.E., Associate AIA

From Destruction

Comes Knowledge

Deconstructing Bridge 92297

Two days of expected work turned intoa week; one equipment breakdowncascaded into another; a 30-minutedelay became 24 hours. A documen-

tation project that was scheduled to happen inJune did not begin

until September.e challenges ofkeeping a bridgedemolition proj-ect on schedule are

not unique, but the requirement for historicaldocumentation of a 1912 reinforced concretebridge by historians and engineers added anotherlayer of complexity to a highway wideningproject. However, this documentation effort ulti-mately provided interesting information aboutthe early development of reinforced concrete flatslab design.

e historians involvement was prompted bya routine set of circumstances. e structure inquestion, Bridge No. 92297 enumerated aspart of a statewide inventory of highway bridges was being demolished in order to facilitate a

joint Minnesota Department of Transportation(MnDOT) and Federal Highway Administration(FHWA) project to reconstruct and widen a sec-tion of the adjacent Interstate Highway I-35Ein St. Paul. e FHWA provided federal dollars,

which triggered the process known as a Section106 review. Passed in 1966, the National HistoricPreservation Act (NHPA) created the NationalRegister of Historic Places and requires all federalagencies to take historic resources into account

when funding, permitting, or licensing under-takings. Section 106 of the NHPA describes aprocess of planning for preservation in advanceof construction.For this project, MnDOT retained Summit

Envirosolutions, Inc. as the cultural resourceconsultant to complete the initial portion of theSection 106 review: identifying historic or poten-tially historic resources by researching propertiesand structures in the area that would be affectedby the highway expansion. rough this process,

the consultants determined that Bridge No.92297 was historically significant. In instances

when a federally funded project affects a historicresource, the project agency must work withthe State Historic Preservation Office (SHPO)to determine how best to mitigate the impact.

Options can range from major changes, such asre-routing a proposed road, to documenting thehistoric structure prior to demolition, as was thecase with Bridge No. 92297. e pending demoli-tion of the bridge presented a unique opportunityto investigate the steel reinforcement concealedwithin the structure. e team conductingthe sequenced research, documentation anddemolition included Summit Envirosolutions,Preservation Design Works (PVN), a photog-rapher, MnDOT engineers, and the contractor.Bridge No. 92297 was a monolithic, single-span,

reinforced concrete flat slab deck with vertical

abutments supported on reinforced concrete stripfootings, constructed in 1912 (Figure 1). It wasoriented on a 35-degree skew, measured 49 feet intotal length, and had a clear span of 41 feet witha 60-foot-wide deck. Without any backgroundabout its history, the bridge would have appearedrather unremarkable. However, research on thebridge revealed that it was an innovative designfor its time. Its documentation shed more light onthe work of the bridges designer, and also createda record available for future study.

C.A.P. Turner and the Flat Slab

Claude Allen Porter (C.A.P.) Turner, a Minneapolis-based structural engineer, was a pioneer in thedevelopment of the reinforced concrete flat slaband designed bridge No. 92297. According to sev-eral articles by Dario Gasparini, Turner was bornin Lincoln, Rhode Island in 1869, and graduatedfrom Lehigh University in 1890. He subsequently

worked for various bridge companies until 1901,when he began his own consulting firm withthe Minneapolis, St. Paul and Sault Ste. MarieRailroad (the Soo Line) as a principal client(Gasparini, 2002). As Turner progressed in

Figure 1: Bridge No. 92297 shortly before demolition. Photograph by Daniel R. Pratt, courtesy of MN HistoricalSociety Archives.

e online version of this articlecontains references. Please visitwww.STRUCTUREmag.org .

8/11/2019 2014-01 Jan


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his career, he expanded his practice to thedesign of buildings, including the first one inMinneapolis with reinforced concrete floorsand columns in 1904. His major break-through in concrete design would be realizedtwo years later: in 1906, Turner designed hisfirst building with the mushroom system offlat slab floors, the Johnson-Bovey buildingin Minneapolis (now demolished).

In the next few years, implementation ofurners proprietary flat slab floor systemgrew at a furious pace. His design consisted offloors with four-way reinforcement supporteddirectly on reinforced concrete columns, eachwith a distinctive flared capital. Between 1906and 1910, urner claimed that buildingsconstructed with his system were rapidlyapproaching a thousand acres of floor(urner, 1910; 7-12). Tis growth can beattributed in part to his extensive publicationof designs and load test results for his flooringsystem in nationally prominent engineering

journals, which proved their reliability andcost-effectiveness. However, a series of patentlawsuits and countersuits beginning in 1911resulted in a dramatic downturn in the useof Turners flat slab system. Nevertheless, hesubstantially contributed to the acceptanceof reinforced concrete flat slab technology

among practicing engineers (Gasparini, etal., 2001; 17-21).In addition to implementing his system in

buildings, Turner designed several reinforcedconcrete flat slab bridges, most as adaptationsof his mushroom floor system. o date, allknown flat slab bridges in the win Citiesdesigned by urner have been demolished. Tebridge decks were often designed with four-way

reinforcement similar to his floors, with longi-tudinal, transverse, and diagonal steel. With theexception of a tunnel originally located not farfrom the area studied for this project, Turnerspublished examples of flat slab bridges did notbear much resemblance to Bridge No. 92297(Gasparini, et al., 2001; 12-27). However,urner held a number of related patents forboth floor systems and bridges, one of whichbears a striking resemblance to Bridge No.92297, particularly the configuration of theabutment reinforcement (Figure 2).Copies of construction drawings and plans

dating to the erection of the bridge, as well ascorrespondence between the Soo Line railroadengineers and the city of Saint Paul engineers,revealed some insights into the bridges designand also raised questions. Although the discov-ery of original drawings was fortuitous andrare for a structure of this age the copies

Figure 2: Excerpt of C.A.P. Turners U.S. Patent

1,002,945: Short-Span Flat-Slab Bridge,filed October 1, 1909. Although the deckreinforcement of Bridge No. 92297 did notresemble the design in this patent, the profileof the deck, abutments, and footings, as wellas the abutment reinforcement bears a strikingresemblance. Digitized by Google Patents.

8/11/2019 2014-01 Jan


were of poor quality and onlypartially legible (Figure 3 ). Ofthe six sheets in the set, one

was stamped with CAP TurnerConsulting Engineer in thetitle block, while the ChiefEngineers Office of the railroadwas stamped on the remainingsheets. e date of the sheet

stamped with Turners firm wasillegible, but several of the sheetsstamped by the railroad engineers

were clear ly dated to 1912. ecorrespondence between engi-neers indicates that plans wereoriginally drawn for the bridgein 1908, and then were revisedin 1912 because the earlier plansdid not meet the standards ofthe 1907 city ordinance. SummitEnvirosolutions postulated that the drawingsheet stamped by Turner was part of the

original 1908 set, and the remaining sheetswere a revision of Turners design made bythe railroads engineers.Interpretation of the original drawings

was also hampered by their poor legibil-ity and a lack of corresponding notes orengineering calculations. This was com-pounded by the fact that changes hadobviously been made to the bridge afterits construction, such as the replacementof the railing and the installation of a newtopping slab, which complicated efforts todifferentiate original and more recently

added features. Despite these difficulties,comparison with observed conditions, theoriginal drawings, and Turners patent fora similar bridge design, led to the con-clusion that the structural design of thebridge can be substantially attributed toC.A.P Turner.e complications that the team expe-

rienced in reading the Bridge No. 92297drawings are actually typical obstacles tounderstanding historic engineering struc-tures. Any engineer asked to retrofit an olderbuilding can relate to the frustration of notbeing able to locate the original engineeringdesign drawings; while architectural draw-ings are often kept as much for their visualappeal as their content, engineering drawingsare often inadvertently lost, or even inten-tionally destroyed for insurance and liabilityreasons. Likewise, details of the constructionmethods and sequence may never have beenrecorded, but rather negotiated in the fieldby a contractor or builder. Finally, the struc-ture itself is often concealed, limiting theability to measure and record the structuralelements. While deconstruction is not often

considered an ideal method of research, theremoval of this 1912 bridge presented an

opportunity to gain additional knowledgeof early flat slab bridge design.

Deconstruction and

Documentation

Bridge No. 92297 was documented toMinnesota Historic Property Record(MHPR) standards. MHPR is a modifiedversion of the national standard HistoricAmerican Engineering Record (HAER)program. e HAER program documentsnationally significant historic mechanical

and engineering structures and sites; theextensive collection is digitized and availableto the public on the Library of Congress web-site (www.loc.gov/pictures/collection/hh/).Both programs maintain documentation ofhistoric resources, and have a target archivallife of 500 years. e MHPR materials forBridge No. 92297 included a report witha written description, large format photo-graphs, and measured drawings of selectedareas of the bridge highlighting its designand construction.Deconstructing and documenting a historic

bridge requires time, care and coordinationthat is not required with standard demoli-tion and removal (Figure 4). Determiningthe configuration of reinforcement for com-parison to the original construction drawingsrequired investigative openings in areas that

would expose representative samples of rein-forcement in the bridge deck, abutments andfootings. Maintaining stability of the bridgeto allow for safe access after its partial demo-lition, as well as to expose sections of theabutments and footings, required an extensiveamount of earthwork.

A two-stage demolition processaccommodated the documenta-tion process. Backhoes equippedwith hydraulic jackhammersremoved concrete in selected areasof the bridge to expose reinforce-ment. Fill placed below the bridgestabilized the abutment wallsduring the exposure and removal

of the deck. Two full-depth open-ings in the bridge deck one nearthe middle, and another along theedge and the adjoining transitioninto the top of the abutment facilitated its documentationbefore complete demolition.Next came excavating soil onboth sides of the abutment to thetop of the footing, then removingconcrete from the selected area to

expose the underlying reinforcement. einvestigation team took measurements and

photographs all along the way.is investigative process was hampered bypoor accessibility of the machinery, especiallyafter demolition of the bridge began to com-promise its ability to support heavy loads. ere

were several equipment breakdowns, and theexisting concrete was stronger than expected insome locations. ese issues created unforeseendelays that impacted the demolition schedule.Despite the slower than expected progress ofthe work, careful operation resulted in expo-sure of the majority of the reinforcement withminimal changes to its as-built configuration.

e destructive nature of the work resulted insome deformation or breakage of the reinforce-ment being recorded. In these cases, carefullyexposing adjacent sections made it possible todocument the typical configuration of rein-forcement as originally placed.e plan and profile of reinforcement was

generally congruent with the original construc-tion drawings from 1912, with the exceptionof minor details and extra reinforcement alongthe fillet corner in the deck-to-abutmenttransition. e skewed geometry of BridgeNo. 92297 was not well-suited to Turnerspatented short-span bridge design, but thetwo layers of slab reinforcement in the bridge

were similar to the configuration of diagonalreinforcement in Turners patent. One layer ofslab reinforcement was placed parallel to thespan of the bridge, and the other layer wasplaced perpendicular to the abutment walls.Some transverse reinforcement was present,which correlated with the patent, but it was sowidely spaced over five feet on center thatits intended purpose was likely just to sup-port the draped geometry of the two primarylayers of slab reinforcement. e profile of the

Figure 3: Original construction drawing of plan and elevationof Bridge No. 92297.

8/11/2019 2014-01 Jan


slab and abutment reinforcement correlatedclosely with the design illustrated in Turnerspatent. Because of the geometry of the bridgespan, the flat slab of Bridge No. 92297 moreclosely resembled a one-way structural system,rather than the four-way systems found inTurners published designs.Considering its age, Bridge No. 92297 was

in remarkably good structural condition and

continued to perform as intended by car-rying heavy vehicular traffic even into thestart of demolition. Despite the somewhatdeteriorated condition of the bridge, includ-ing concrete spalling and substantial graffiti,its continued use had demonstrated that theearly design was not only adequate for the

streetcar loads at the time of construction,but also remained suited for the loadingdemands imposed by modern traffic.

Conclusion

Researching the history of engineering hasunique and persistent challenges: structuraldetails are concealed, drawings are often not

available, and the field is relatively new com-pared to the more established scholarshipof architectural history. However, programssuch as the MHPR and HAER provide aframework for expanding this field of study.

When demolition of a resource is unavoid-able, documentation can partially mitigate its

loss by recording and allowing for the futurestudy of its features. Understanding the his-tory of a profession can provide a valuableperspective on how its common practices andphilosophy have evolved.Likewise, engineers seeking to preserve or

rehabilitate existing structures can benefitfrom studying previously documented anddemolished examples for the insights that they

provide into design and construction. BridgeNo. 92297 offered a unique opportunity todocument the details of the steel reinforcementin a historic reinforced concrete structure, atask that is for obvious reasons gener-ally infeasible for such structures that are toremain intact.

Figure 4: Careful demolition of the bridge revealed the reinforcement, facilitating its documentation in selected areas.

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8/11/2019 2014-01 Jan

20/60January 201420

renovation and restorationof existing structures

STRUCTURAL

REHABILITATION

By D. Matthew Stuart, P.E., S.E.,F.ASCE, F.SEI, SECB, MgtEng

and Ross E. Stuart, P.E., S.E.

D. Matthew Stuart, P.E., S.E.,F.ASCE, F.SEI, SECB, MgtEng([email protected]), isthe Structural Division Managerat Pennoni Associates Inc. inPhiladelphia, Pennsylvania.

Ross E. Stuart, P.E., S.E.

([email protected]), isa project engineer at Pennoni

Associates in Philadelphia,Pennsylvania.

The Triage, Life Support

and Subsequent Euthanasia

of an Existing Precast

Parking Garage Part 3

Prescription for Repair

As a part of Pennonis on-call contractwith an existing client, the Philadelphiastructural division investigated anddeveloped repair bid documents for an

existing, three-level, 1,200-space precast concreteparking garage during the last quarter of 2012.Part 1 of this series (September 2013) describedthe existing structure and summarized Pennonisobservations and material testing results. Part 2(November 2013) presented an analysis of those

findings. Tis article conveys Pennonis conclusionsregarding the feasibility of repairing the garage inorder to extend its service life.Te third level precast, prestressed inverted

concrete girders that supported the 16-inch deepdouble tees were in extremely poor condition.Te corresponding second level girders were alsoin poor condition due to high chloride content,and several locations exhibited large subsurfacedelaminations. Te cast-in-place, post-tensionedconcrete inverted girders that supported theconventional 24-inch-deep precast double tees

were generally in fair

condition due to limiteddeterioration at isolatedareas. In addition, theextremely high chloride

content of all of the concrete, in conjunction withongoing carbonization, was expected to causedeterioration to accelerate in the near future.Material testing indicated that the existing concrete

in the garage girders had a chloride content thatwas significantly greater (25 times) than the limitrecommended by ACI for prestressed structures ina moist environment that are exposed to chloridesin the form of either admixtures or deicing salts

(0.06%). None of the previous testing performedin 2002 or 2005 included chloride testing from thebeams, therefore the previous reports failed to revealthe true nature of the current rapid demise of thegarage. Extrapolating the results of carbonizationanalysis indicated that the depth of carbonization

would reach the embedded reinforcing in approxi-mately two to three years. Significant repairs wouldbe required within that time to prevent permanentdamage to the embedded reinforcing.

Service Life Analysis

Pennoni determined that the practical remainingoperational service life of the existing parkingstructure was approximately two years. Tis wascommensurate with the large spalls and severelycorroded reinforcing, observed during the sitevisit, at the girders associated with the barricadedportion of the third level. In addition, the servicelife calculations were considered representativeof the remaining portions of the garage, whichindicated that some if not all of the other thirdfloor girders would also begin to corrode in thesame fashion within the next two years, and thesecond level would follow shortly thereafter.

From an engineering perspective, the service life ofa structure is considered to be over when the extentof deterioration renders the facility inoperable dueto impending hazards to public safety, and reme-diation is required in the form of complete repairor replacement. Terefore, the end of a structuresservice life does not mean that it is in a state ofimminent collapse, but instead implies that thestructure can no longer safely function or supportminimum loads as required by the building code.

In the case of this particular garage, vehicles andpedestrians would no longer be able to use theentire garage for parking, similar to the currentpartial loss of service at the third level. Pennoniestimated that, within the next two years, thegarage would have to be progressively closed asadditional areas became unsafe, until eventuallythe entire facility would be completely out ofoperation. Te eventual and unavoidable lossof use of the entire garage by the current occu-pants would therefore have a direct impact onthe practical everyday operations of the facilityin the very near future.

ypically, a garage constructed with precast con-crete components should have a useful lifespanof 40 to 50 years before significant repairs wouldbe required. In this case, the actual service lifeof the garage in the absence of any remediation

will be approximately one-half of this duration.Te shortened lifespan of the garage is directlyattributable to the use of chloride-containingadmixtures in the main girders.

Feasibility of Repairs

It is clear from the results of the condition assess-

ment, material testing and investigation that theprimary source of the internal reinforcing and con-crete deterioration in the garage was the presence ofexcessive chlorides in the concrete in conjunction

with continued exposure to deicing salts. In addi-tion, it was anticipated that further carbonizationof the concrete would cause additional deterio-ration of the structure. Terefore, any solutionsinvolving the repair and restoration of the garageto extend its service life would have to address thepresence of the high chloride content.ypically, a chloride extraction process, such

as Norcure by Vector Corrosion echnologies,

or an active galvanic protection system, such asEbonex or Vectrode iape by Vector Corrosionechnologies, could be used to reduce or removethe chloride ion content or arrest the current rateof deterioration in conjunction with conventionalconcrete repairs. However, the presence of thehigh-strength prestressing steel precluded the useof chloride extraction processes or active galvanicprotection systems due to the potential for hydro-gen embrittlement of the strands, as describedby State-of-the-Art Report: Criteria for CathodicProtection of Prestressed Concrete Structures, pub-lished by NACE International Te Corrosion

8/11/2019 2014-01 Jan

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23/60STRUCTURE magazine 23

discussion of constructionissues and techniques

CONSTRUCTIOISSUES

John Hlinka, P.E., is Senior ProjectManager/Structural Engineer atQualEx Engineering in Paducah,Kentucky. He can be contacted [email protected].

By John Hlinka, P.E.

AWS D1.4/D1.4M:2011

Welding Reinforcing Steel

On a recent chemical plant project for

which the author was the Engineerof Record, an electrical contractor,contrary to contract specifications,

manually arc welded electrical grounding conduc-tors to reinforcing steel for a pipe rack foundation.Te electrician explained that the NationalElectric Code (NEC) allows welding to concreteencased reinforcing steel, and he frequently does

so in lieu of independent electrical ground rodswhich were specified on this project. Paragraph250.52 (A) (3) Concrete-Encased Electrodeof theNEC does permit welding to reinforcing steel.However, NEC does not reference AWS D1.4/D1.4M Structural Welding Code-Reinforcing Steelor provide any guidance to the special rules, regu-lations, and procedures prescribed by AWS D1.4/D1.4M. If AWS D1.4 is not followed for manualarc welding reinforcing steel, the structural integ-rity of reinforced concrete may be jeopardized.Unfortunately, this particular contractor did notconform to D1.4 and the reinforcing steel was

encased in concrete before a visual inspectioncould be conducted.Tis article covers AWS requirements for

welding reinforcing steel in reinforced concreteapplications. It summarizes the main themes ofthe various sections as they pertain to weldingreinforcing steel and contains guidelines for work-ing with the body of rules and procedures forstructural welding of reinforcing steel to reinforc-ing steel, welding reinforcing steel to structuralsteel, and welding reinforcing steel to electricalgrounding electrodes. Implications to improvefuture projects are also addressed.

Fusion welds in shop fabrication of reinforcingsteel and CADWELDs are outside the scope ofthis article. Electric resistance welds found in thefabrication process of welded-wire reinforcementare conducted by computer controlled weldingmachines within a controlled environment. Acombination of pressure and heat generated byelectric impulses fuse intersecting wires together.Shop personnel are never engaged in the actualwelding process and no filler material or otherforeign matter is introduced. CADWELDs donot apply because the steel-filled coupling sleeveof a CADWELD is a mechanical splice in whichmolten metal interlocks the grooves inside thesleeve with the deformations on the reinforcing bar.Weldability of reinforcing steel and compatibil-ity of welding procedures need to be consideredand closely supervised when manual arc weld-ing of reinforcing steel is required. Weldabilityis determined by the chemical composition ofsteel and described by the Carbon Equivalent(CE) number. Carbon is the primary hardeningelement in steel. Hardness and tensile strengthare inversely related to ductility and weldability.As carbon content increases up to 0.85%, sodoes hardness and tensile strength. As carbon

content decreases, ductility and weldabilityincreases. CE is an empirical value in weightpercentages, related to the combined effects ofdifferent alloying elements used in making carbonsteel, of an equivalent amount of carbon. Tisvalue can be calculated using a mathematicalequation. Te lower the CE value the higher theweldability of the material. Te welding Codeprovides two expressions for calculating CE.

Te first expression (Equation 1) only consid-ers the elements carbon and manganese, and isto be used for all bars other than ASM A706material. A second more comprehensive equation(Equation 2) is given for ASTM A706 and consid-ers carbon, manganese, copper, nickel, chromium,molybdenum, and vanadium content. Chemicalcomposition is obtained through certified milltest reports or independent chemical analysis.Chemical composition varies for each produc-tion run, so it is important to obtain the analysisthat matches the specific material to be welded.Once the CE number is

calculated, the minimumpreheat and interpasstemperature is deter-mined from able 5.2 ofthe Code. If material testreports are unavailable and chemical compositionis not known, which is particularly common inalterations and building additions of existingstructures, the Code prescribes the highest preheatand interpass temperature for desired reinforcingbar size: 300 F (150 C) for number 6 bars andsmaller, and 500 F (260 C) for number 7 barsand larger. If the chemical composition for ASTM

A706 is not known or obtained, then preheat andinterpass requirements are somewhat relaxed; nopreheat is required for number 6 bars and smaller,50 F (10 C) for number 7 to number 11 bars,and 200 F (90 C) for number 14 and larger.

As with all welding, when the material is below32 F (0 C), the Code prescribes the materialto be preheated to at least 70 F (20 C), andmaintained during the welding process.

CE = %C + %Mn/6 (Equation 1)

CE = %C + %Mn/6 + %Cu/40 + % Ni/20 +%Cr/10%MO/50%V/10 (Equation 2)

Standard specifications for low-alloy steelASM A706 limit chemical compositionand CE to enhance weldability. However, itis permissible to weld other base metals, suchas ASM A615, which is commonly used inreinforced concrete, as long as the appropriate

weld procedure specification (WPS) is followedand correct filler weld metal is used. Manyother permissible base metals are listed underparagraph 1.3.1 of the Code. High strengthreinforcing steel such as ASTM A615 materialis susceptible to cracking when not adequately

Te online version of thisarticle contains detailedreferences. Please visit

www.STRUCTUREmag.org.

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preheated. Welding of ASTM A615 mate-rial should be approached with caution, sinceno specific provisions have been included toenhance its weldability. e Tablecompareschemical composition, CE, and preheattemperatures for sample ASTM A615 and

ASTM A706 materials. As shown, the pre-heat requirements are lower for A706 than

A615 material. A lower carbon percentageand the addition of molybdenum and vana-dium contribute to a lower CE number for

A706. Bar size also is considered in determi-nation of preheat temperature. e smallerthe bar size, generally, the smaller the pre-heat temperature. With all rebar welding,allow bars to cool naturally. Never acceleratecooling; accelerated cooling will change themetallurgy of the reinforcing steel.Sections 2 and 3 of the Code provide

allowable stresses and structural details,respectively. A wide range of details are pro-vided, including Direct Butt Joints, IndirectButt Joints, Lap Joints, and Interconnectionof Precast Members. e effects of eccentric-

ity should be considered when designing LapJoints, if external restraint is not provided.AWS D1.4 does not provide details for weld-ing reinforcing steel to electrical groundingconductors. If unavoidable, the authorsuggests using the CADWELD method toattach the grounding conductor to an ASTMA36 plate and then using the AWS Lap Jointdetail to attach the plate to the reinforcing

steel, or CADWELD the conductor directlyto the reinforcing steel. Other mechanicaltype attachments provided by NEC are pref-erable to manual arc welding.Section 4 of the Code addresses work-

manship in regards to preparation of basemetal, joint assembly, distortion, and quality.Welding of bars which cross and weldingwithin two bar diameters from the pointsof tangency for the radius of bent bars arenot permitted. Cross bar welding can lead

to local embrittlement of reinforcing steel.When welding on bars that are alreadyembedded in concrete, allowances must bemade for thermal expansion of the steel toprevent spalling or cracking of concrete ordestruction of the bond between the concreteand steel. Acceptable and unacceptable filletand grove weld profiles are illustrated inSection 4 of the Code.Section 5 of the Code discusses welding

technique. Technique includes selection offiller metal, minimum preheat and inter-pass temperatures, welding environment,

arc strikes, cleaning, progression of welding,coated base metal, and welding electrodes.Allowed welding processes include shieldedmetal arc welding (SMAW), gas metal arc

welding (GMAW), or flux cored arc welding(FCAW). Other processes maybe used whenapproved by the Engineer of Record. Specialstorage conditions are required for low-hydro-gen electrodes. Low-hydrogen electrodes must

be purchased in hermetically sealed con-tainers or must be baked prior to use.Selection of correct welding electrodeswhich are compatible with base metalmaterial is critical. An incorrect choicemay lead to micro cracking in the heataffected zone, which may lead to jointfailure. Generally speaking, tack weldsare prohibited unless they conform to alldesign and control requirements of D1.4.Tack welding can create a metallurgicalnotch effect and weaken a bar at the weld.Sections 6 and 7 of the Code pertain

to welder qualifications and inspections,respectively. All structural welding mustbe performed by qualified welders. WPSqualification by testing must include

specific joint type and size to be welded.Inspectors must also be qualified. Acceptablequalifications include AWS certification,Canadian Welding Bureau certification, oran Engineer/Technician trained or experi-enced in metal fabrication, inspection, testing,and who is competent to perform inspection

work. It is not unusual for the Engineer ofRecord to request evidence of welder quali-fications prior to starting a project. Annex Aof the Code includes the following sample

forms for informational purposes: ProcedureQualification Record (PQR), WeldingProcedure Specification (WPS), and WelderQualification Test Record.

Conclusion

Welding of reinforcing steel should beapproached with caution to prevent crackingof base metal and potentially jeopardizingthe integrity of a reinforced concrete founda-tion or structure. AWS D1.4/D1.4M coversthe design, workmanship, technique, quali-

fication, and inspection requirements forwelding reinforcing steel in most reinforcedconcrete applications.NEC paragraph 250.52 (A) (3) allows weld-

ing of electrical conductors to reinforcingsteel without reference to AWS D1.4/D1.4M.Electrical contractors can potentially damagethe structural integrity of reinforced concretefoundations if the requirements of AWS D1.4are not followed. Proposed Tentative Interim

Amendments (TIAs) were submitted to theNFPA Standards Council on August 6, 2013.Hopefully, the NFPA Standards Council willadopt these amendments.In the case presented at the beginning of

this article, the minimum amount of rein-forcing steel required by ACI 318 providedgreater than two times the strength neededfor design loads. erefore, if the integrity ofone reinforcing bar was reduced, the foun-dation would still be structurally adequate.e author suggests that a note be addedto future concrete drawings that specificallyprohibit welding of electrical conductors toreinforcing steel without the approval of theEngineer of Record.

Sample Material Comparison Table

MaterialGrade

RebarSize

Chemical Analysis (Percent) PreheatTemp. F (C)C Mn P S Si Cu Cr Ni Mo Cb V CE

ASTMA615

#7 0.39 1.00 0.018 0.037 0.21 0.39 0.20 0.13 0.038 0.00 0.00 0.56 200 (90)

ASTMA706

#7 0.28 1.18 0.028 0.028 0.17 0.29 0.19 0.09 0.02 0.00 0.24 0.48 50 (10)

Sample mill test report data with calculated CE numbers and minimum preheat and interpass temperatures from Table 5.2 of AWS D1.4.

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Each year, the American Society of Civil Engineers (ASCE)publishes a Report Card for Americas Infrastructure. Teirrating system uses familiar academic grading (A through F) toreport on the condition of Americas aging infrastructure. Last

year, Americas infrastructure received a D+, indicating deterioratinginfrastructure. Included in this report are waterfront structures, whichare vitally important. According to ASCE, America has over 3,700maratime terminals serving as commerce and transportation hubs.Often, steel sheet pile walls are incorporated as part of cost-effective

waterfront earth-retaining structures for these harbors. Many of thesewalls are subjected to salt-water exposure, tidal fluctuations, and ahost of other environmental factors that accelerate corrosion. Evenif the coatings are maintained and a cathodic protection system is

employed, the corrosion near the waterline eventually necessitatesexpensive repairs and, often, replacement of the structure. Cellularsheet pile cofferdams are particularly diffi cult to repair because of thelack of redundancy in the tensioned cell walls. Tis article highlightsa unique method to repair a cellular cofferdam using a reinforcedconcrete facing installed over the aging sheet piling.

Existing Structure Configuration

Te submarine pier is located in the Hood Canal waterway inWashington States Puget Sound region. Considered a vital naturalresource, Hood Canal provides vessel passage and is home to manyaquatic species, some of which are fished for human consumption.

Te triangular shaped pier, accessible from land by two pile-supportedtrestles serves as a vessel docking-surface on two sides and a dry dockis integrated into the third side of the triangle (Figure 1). Te concretedeck supports gantry cranes, mooring hardware, and a number of build-ings. Most of the deck is supported by concrete piles, but portions aresupported by a cofferdam that surrounds the dry dock structure. Tecofferdam is formed by a series of circular interlocking steel sheet pilecells that form the outer perimeter of the dry dock structure. Tesesheet piles were vibrated or driven into the underlying seabed, and

each cell was backfilled to form a cylindrical earth retaining structure.Additional sheet pile arcs connect each cell. In total, the dry dock uses20, 75-foot diameter cells connected by 19 arcs in a U-shaped patternthat forms three sides of the dry dock structure. Te fourth side is opento accept vessels and configured to accept a caisson (gate) that closesthe dry dock off from the surrounding water. Te internal cast-in-placeconcrete dry dock walls envelope portions of the cofferdam cells andarcs. However, at the outer perimeter, the sheet pile walls are fullyexposed to the marine environment above and below the water line.

Field Observations

Te first step in developing a comprehensive design strategy was to per-form field investigations of the existing pile condition. From boat- anddeck-based observations, the design team documented the conditionof each exposed sheet pile face and noted special conditions that wouldneed to be considered through the design process. Te existing coatings

were in a state of failure at most locations (Figure 2). Te facility wasequipped with an active cathodic protection system that consistedof anodes suspended from cantilevered beams extending over thecofferdam face. It is important to note that this type of cathodicprotection system is fully effective only below the low water line,

with reduced effectiveness in the tidal zone. Te field observationsalso illuminated a number of construction constraints that wouldhave otherwise been diffi cult to discern from a review of the designdocuments alone. Te most surprising and important constraint wasthe restricted conditions under the deck. As shown in Figure 3, someexisting concrete piles were less than 1 foot from the sheet pile face.Te number and proximity of existing concrete piles would presenta substantial access and construction challenge for under-pier work.

Additionally, the design team observed a number of deck-based andwater-based operations associated with the day-to-day function ofthe pier that would present a construction staging and sequencingchallenge for the contractor.

Figure 1: Submarine pier aerial view. Courtesy of the U.S. Geological Survey.

Figure 2: Existing failing cofferdam coating.

By Brian Robinson, P.E.

Concrete Paint Arrests CofferdamCorrosion at Submarine Pier

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Corrosion Mitigation Selection

and Design Considerations

Te owner considered a number of coating systems to mitigate further

sheet pile corrosion: a high performance marine coating, thermal sprayaluminum, copper-nickel cladding, and a concrete facing. Due tothe considerable durability and low life-cycle cost, the owner chose aconcrete facing extending from the top of the cofferdams to two-feetbelow Mean Lower Low Water as the preferred repair alternative. Teowner called for a concrete service life of at least 50 years. Consideringthe cofferdam pile structure appeared to be performing well anddid not exhibit excessive corrosion, the concrete facing would notbe structural, but rather would provide an overlay to passivate andprotect the steel cofferdam.Te concrete facing would present a number of different technical

design challenges, but most of them would be related to one centralissue: crack control. In this case, the design team had two related tools

for controlling concrete cracking: rebar configuration and concretespecification.

Rebar Configuration

Te design team considered that, during dryi

2014-01 jan

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