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a p u b l i c a t i o n o f tA I A /A C S A C o u n c i l o n A r c h i t e c t u r a l R e s e a r


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Disclaimer

Crx;

Council on Architectural Research5 New York Avenue, NW

0-935502-08-4

1994. NHRP.ouncil on Architectural Researchrights reserved.s, AIA, Director - Editor

- Editorial Assistant- Adm inistrative Assistant

Christopher Arnold, FAIABuilding Systems Deve lopment, Inc.Palo Alto, CAontributing AuthorsRichard Eisner, AIA, AIC PGovernor's Office of Em ergency ServicesOakland, C A

This manual was prepared under Contract Number EM W-90 -C-3355 withEmergency Managem ent Agency.The photographs, draw ings, charts and other information containe d in this have been obtained from a variety of sources including go vernm ent age ncof architecture, professional architects, architectural firms, and others. The Council on Architectural Research has made every reasonable effort to proviinformation, but does not warrant, and assumes no liability for, the acompleteness of the text or its fitness for any particular p urpo se.Furthermore, the statements and descriptions contained in this manual do nily reflect the views of the U.S. Government in general or the Federal Management Agency in particular. The U.S. Government and FEMAwarranty, expressed or implied, and assume no responsibility for the acompleteness of the information herein.

AIA/ACSA Council on Architectural ResearchOn May 2,1 986, the Presidents of the American Institute of Architects (AIAssociation of Collegiate Schools of Architecture (ACSA) signed an agcreate a Council on Architectural Resea rch. The purpo se of the Counc il isa link between the schools of architecture, where significant resea rch a ctiand the profession, wh ich can utilize the results of this research in practic e.to foster research w hich benefits both the architectural curriculum and ac tivthe profession of architecture, and which positively imp acts the built env irthe public at large. With offices in the AIA hea dquarters bu ilding in Washthe AIA/A CSA Co uncil on Architectura l Research serv es as a national focarchitectural research in all its form s.The Council is organized into a number of constituentp rogram s w hich identipriorities, initiate research projects and disseminate findings. The CouncHazards Research Program coordinates research on the effects of naturalbuildings and was responsible for the develo pme nt of this publication .

AcknowledgementsMany people made valuable contributions to this publication, both durinpreparation and throughout the workshops w hich precede d it.We w ish to first acknowledge the guidance provided by M arilyn Mac Cabe,Emergency Ma nagemen t Agenc y' s technical representative for the project.-also due to Jane Bullock, Assistant to the Director, and Gary Johnso n, AFinancial Officer, both at FEMA .Likew ise, special thanks for their unique contribu tions to this effort are oHenry Lago rio, University of California, Berke leyDon Geis, AIA, Geis Design Research As so c, Potomac, M DFinally, the Council gratefully acknowledges support from the NatioFoundation. Under grant number 910 1564 , the Council was able to undertof research tasks focused on expanding the current architectural knowleseismic design. Results from this research were used to support and einformation contained in this publication.


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Buildings at Risk: Seismic Design Basics for Practicing Architects 1

e of Contents

IntroductionChapter 1: The Nature of Ground Motion and itsEffect on Buildings

Geologic BackgroundGround FailureGround MotionThe M easurement of Ground MotionThe Effects of Ground MotionResisting the Effects of Ground MotionChapter 2 : Site Issues

Siting of a Structure - Where does the Site Begin?Seismic Risk as a Siting CriteriaActive Earthquake Faults >Impact of Regional Geology on Site PerformanceRegional Damage and its Impact on a SiteThe Architect's Role in Site Selection and E\Chapter 3: Building Configuration:The Architecture of Seismic Design

Building Configuration EffectsConfiguration DefinedRegular ConfigurationsIrregular ConfigurationsConfiguration Irregularity and the CodeFour Common Configuration ProblemsThe Bottom LineChapter 4: Seismically ResistantStructural System s

Basic Seismic Engineering ConceptsResistant SystemsBuilding ResponseSelecting a Structural SystemFinal ConsiderationsChapter 5: The Basics of Seismic Codes

Performance ObjectivesPresidential Executive Order57 Chapter 6: Nonstructural D amage

IntroductionThe ProblemTypical Nonstructural DamageReducing Nonstructural DamageDesign and/or Selection ResponsibilityDesign for Nonstructural Damage ReductioInterior Nonstructural Hazard Assessment73 Chapter 7: Seismic Rehabilitation ofExisting Buildings

Existing StructuresHazardous Building TypesMethods of Rehabilitation79 Chapter 8: Seismic Design Process

Roles and ResponsibilitiesChecklists for Coordination87 Chapter 9: The Planning Process

The Planning ProcessThe General PlanSeismic Safety PlanningUrban DesignPlanning to Reduce Seismic RiskSeismic ZonationInventory Existing Hazardous Buildings andRetrofitThe Architect's Role in Disaster PreparedneResponse and Recovery

95 Glossary of Terminology102 Selected References


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2 Chapter 1: The Nature of Ground Motion and its Effect on Buildings

Introduction

Earthquakes are among the most frightening and devastating of natThe 1994 Northridge, California quake, for exam ple, resulted in ovmore than 5,000 injuries and over 25,000 people left homeless. 10,000 homes and businesses lost electricity, 20,000 lost gas and 5without water. Direct economic losses are estimated at over $ 20 bAnd this was not theBIG ONE! It was a large (mom ent magnitudea great earthquake, of relatively short duration (the main shoc k last15 seconds). The 1906 San Francisco quake, by contrast, was estRichter Magnitude 8.3 event, lasting 45 seconds, and the 1964 A(a Richter 8.4) lasted over three minutes. Larger events can texpected in the U.S., and they will not be confined solely to the region.In fact, two of the severest earthquakes in U.S. history occurredRockies: one in Charleston, South C arolina in 18 86; the other, a seshocks, in New Madrid, Missouri in 1811-12. The latter have theof being the greatest series of earthquakes in U.S. history. Meestimated 8.5 on the Richter scale, they sent shock waves as far Rocky Mountains and as far east as Washington, D.C. an d Boston. to occur today, astronomical loss of life and property d am age, estias much as $50 billion over a 200,000 square mile area, would liTraditionally, the structural engineer has been regarded as the pwith primary responsibility for the seismic performance of a buildno longer true, and architects are now seen as having a critical androle to play inmitigating earthquake damag e. Architectural dec isioing site planning, building form and con figuration, structural an d system layouts, construction details, and nonstructural com ponentdeterminants of the overall performance of a building during an To provide architects with the information they need to make thdecisions, the AIA/ACSA Research Council has, over the lastconducted a series of workshops, funded by the Federal Emergenment Agency, on the fundamentals of good seismic design. BuildiSeismic Design Basics for Practicing Architects hasevo lved direcinformation presented in these workshops, supplemented by researed under a grant from the National Science Foundation. The puintended to provide practitioners with a solid introduction to badesign principles.


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Buildings at Risk: Seismic Design Basics for Practicing Architects 3i

at Risk is organized into eight key chapters:The Nature of Ground Motion and its Effects on Buildings providesan overview of how earthquakes impact buildings and how structuresreact to seismic forces.Site Issues reviews the potential impact of site conditions on seismicvulnerability.Building Configuration: The Architecture of Seismic Design examines die best and worst building forms and layouts, concentrating onhow to "design in" seismic resistance.Seismically Resistant Structural Systems covers basic engineeringconcepts and what architects need to know to work effectively withstructural engineers.Basics of Seismic Codes discusses recent advances in seismic designcodes and specifically addresses FEM A's National Earthquake HazardsReduction Program Provisions.Nonstructural Damage deals with the "h idden" risk in buildings andhowarchitects, who often have primary responsibility for nonstructuralcomponents, can improve performance and avoid costly damage andrepair.Seismic Rehabilitation of Existing Buildings provides a brief introduction to incorporating good seismic design into the growing field ofremodeling/retrofitting existing structures.Seismic Design Process takes a look at howarchitects and engineers canbetter interact to produce seismically safer buildings, andThe Planning Process reviews key land use and urban planningconcepts to decrease seismic vulnerability.

is intended as a comprehensive primer on seismic design,mation that any architect practicing in earthquake countryds to know. The book is not intended as an exhaustive treatment of thet, and for this reason the publication ends with a glossary of key termsa list of selected references. The reader is encouraged to access any and allat Risk.


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Chapter 1: The Nature of GroundMotion and its Effect on Buildings

GEOLOGIC BACKGROUNDAccording to the now generally accepted theory of Plate Tectonics, the earth'scrust is divided into several major plates, some 50 miles (80km) thick, thatmove slowly and continuously over the interior of the earth.Earthquakes are initiated when, due to slowly accumulating pressure, theground slips abruptly along a geological fault plane on or near a plate boundary.The resulting w aves of vibration w ithin the earth create ground motion at thesurface w hich begins to vibrate in a very complex manner. This, in turn, inducesforces w ithin buildings that are determined by the precise nature of the groundmotion and the construction characteristics of the building.The point where the fault first slips is termed the "focus" or "hypocenter." Atheoretical point on the earth 's surface directly above the focus is termed the"epicenter." (Figure 1.1) The epicenter for the January 17,1994 Los Angelesearthquake w as located in the city of Northridge in the San Fernando Valley.The initial break in the fault moves rapidly along the line of the fault, and thedistance of this movement largely determines the intensity of ground shaking.Thus the 1906 San Francisco earthquake ruptured along some 250 miles(400km) of the San Andreas fault. The Loma Prieta, California earthquake of1989 w as unusual since no surface faulting occured, although a broad area ofground cracking indicated a w ide distribution of strain. The fault rupture movedupward to within about 6km of the ground surface area and then spreadapproximately 20km along the fault to each side of the initial rupture. (Figure 1.2)

GROUND FAILURESurface FaultingSlippage along a fault line deep in the earth 's surface may eventually result insurface faulting, the crack or split on the earth's surface that provides thelayperson 's vision of earthquakes. Surface faulting may result in large earthmovements: in the 1992 Landers earthquake east of Los Angeles, the earthoffset as much as 18 feet at the surface. A building located across a surface fault,no matter how w ell designed, is almost certain to suffer very severe damage.(Figure 1.3) However, the large disturbance of the ground near a fault isgenerally quite narrow in w idth on either side of the fault: in Landers themaximum width of severely disturbed ground was only about 40 meters.

Si V ana I

Epicenter su

Focus ' 'Figure 1.1: Earthquake location

TkM Jill

AO\


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School straddling a landslide-induced rupture,

1.4:House on Turnagain slide

1.5: Turnagain Heights, Alaska

compared to the likelihood of significant ground motion. So, in seismicwe design against the vibrations caused by fault slippage and try to ensbuildings are not built over fault zones.Landslides, Liquefaction an d SubsidenceThe energy released by an earthquake can also trigger ground failure in tof landslides, liquefaction and subsidence which can have devastating on a structure. Even well-built structures, designed to withstand earthforces, if built on an unstable site or in the path of a landslide, can fall vThe Alaskan earthquake of 1964 provides examples of structures winherent strength to withstand ground shaking that were devastated as of the instability of the sites they were built on. (Figure 1.4) While an aand contractor could take pride in the performance of their buildinTurnagain Heights or on 4th Street in Anchorage, the decision to bugeologically unstable sites produced catastrophic results. (Figure 1.5) Aance of sites with a potential for liquefaction, landslides or subsirepresents the best design approach.

\Ground shaking can also trigger subsidence and liquefaction in soils thunconsolidated and/or saturated with water. When sandy, water saturateare shaken, the bearing capacity of the soil is reduced as the soil liquefiflows laterally and vertically. Liquefied soils can produce volcano-like saat the ground surface or flow laterally if the soil is not contained. The gsurface and structures built on shallow foundations can subside severalbe torn apart as spreading occurs. Dramatic examples of liquefactionrecent earthquakes illustrate again, that even well built structures are vable if adequate attention is not paid to site conditions and foundation d(Figures 1.6 and 1.7)


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G R O U N D M O T I O NWhile ground failure can be an im portant consequence of any earthquake, theprimary effect buildings are designed to resist is ground motion. During anearthquake, wave vibrations emanate from the line of fault rupture and soapproach the building from a given direction. The w aves begin like ripples ina still pond when a pebble is thrown into it, but the seismic waves rapidlybecome more complex.There are four main w ave types, of which "bod y" waves, within the earth, arethe most imp ortant for design pu rpose s. (Figure 1.8) First to arrive at the surfaceis the P or primary wave . In this wave the ground is successively pushed andpulled alon g the w ave front. T he effect is of a sharp punch - it feels as if a truckhas hit the building. The P wav e is followed by the S, secondary or shear wave,which is a lateral motion, back and forth (but sideways to the wave front).The nature of the waves and their interactions are such that actual movem entat the ground w ill be random : predominantly horizontal, often with considerable directional emp hasis, but sometimes with a considerable vertical compo nent. The actual horizontal ground displacem ent is small, only inches even ina large earthquake, except in the immediate area of the fault rupture wheredisplacements of several feet may occur.

T H E M E A S U R E M E N T O F G R O U N D M O T I O NMeasurement of ground motion is important for design purposes because itprovides the basis for determining forces, and assessing the relative seismichazard at different locations.Earthquake motion is recorded by a seismo graph, an instrument that records themovement, over time, of a freely supported pendulum within a frame: theinstrument m ay be placed on the ground or within a structure.In modern seismographs, pendulum movement is converted into electronicsignals on tape. Strong-motion seismographs, called accelerometers, aredesigned to directly record nearby rather than distant ground movem ent, andthey produce a record called an accelerogram. Instruments are normally placedso as to measure movements along the two horizontal axes as well as onevertical. Three measures are of major interest: acceleration, velocity, anddisplacement.A cce l era t i o n , V e l o c it y , D i s p l a cem en tAcceleration is the rate of chan ge of velocity: when m ultiplied by m ass it resultsin the inertial force that the building must resist. This is a key measure, andforms the basis of the estimation of earthquake forces on buildings: Newton'sSecond Law of Motion states in essence, that an inertial force, F, equals mass(M) multiplied by the acceleration (A).

Figure 1.7: Sand boil in a lettuce ield,W

^ ~

j j r f#f f lK^^^M Si^^^R5rigure 1.8: "P" and "S" wavesJ^M


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time| l l l l | l l l l | l l l l | I IM|ll l l lUU|MII| l l l l | l l l l | l l l l | l l l l | l l l l | IMI|I IM|S eC0 10 20 30 40 50 60 70

2 0 - |mm

Figure 1.9: This accelerogram illustrates the size of theseismic waves and can be used to derive acceleration,velocity and displacement.

Figure 1.10: A l.Og design

y

Acceleration is comm only measured in "g's"- the acceleration of a body due to the earth's gravity (approx. 32ft/sec/sec, or 980 cm/l.Og.). Velocity, measured in inches or centimeters per second, referof ground motion at any time. Displacement, measured in inches oters, refers to the distance a particle is removed from its "at rest(Figure 1.9) The level of acceleration generally taken as sufficient to produce so mto weak construction is O.lOg. The lower limit of acceleration perpeople is set by observation and experiment at approximately 0.00sec2; at around 0.20g and above most people will have difficulty keefooting and sickness symptoms may be induced. An earthquakacceleration approaching 0.5g on the ground is very high. On upp ebuildings, maximum accelerations will often be higher, dependidegree to which the mass and form of the building act to damp theeffects. A figure of l.OOg, or 100% of gravity, may be reached, forof a second. To design for l.OOg is diagrammatically equivalent, sense, to designing a building that projects horizontally from a vertic(Figure 1.10) When the behavior of real buildings is observed, sevemodify this diagrammatic equivalence, and structures that could nelever from a vertical surface can briefly withstand l.Og earthquakeAcceleration is the measure comm only used to indicate the possible dpower of an earthquake in relation to a building. A more significanis that of acceleration combined with duration, which takes into aimpact of earthquake forces over time. In general, a number of moderate acceleration, sustained over tim e, can be much more diffbuilding to withstand than a single peak of much higher valueinstrumentation also measures the duration of strong ground motigenerally relates to the length of the fault break.Typically the extreme vibration will occupy only a few seconds; bothLoma Prieta and 1994 Northridge earthquakes lasted only a littleseconds, yet they caused much destruction. In 1906, in San Fransevere shaking lasted about 45 seconds; in Alaska in 1964 the sevquake motion lasted for over 3 minutes.Two earthquake measurement systems are in common use and nevarious reasons, is really satisfactory from the building design vieM agnitude: The S ize o f the W aveEarthquake magnitude is the first measu re: it is expressed as Richterbased on the scale devised by Professor Charles Richter of the Institute of Technology in 1935. Richter's scale is based on theamplitude of certain seismic waves recorded on a standard seismodistance of 100 kilometers from the earthquake epicenter. The scaletells nothing about duration, which may be of great significance i


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ermining dama ge. Because the instrument is unlikely to be exactlykm from the source, Richter developed a method to allow for the diminishof wave amplitude (or "attenuation") with increased distance, just as the

of a star appears dimmer with distance. (Figure 1.11)

of earthquakes varies enorm ously, the graphic range of waveon seismographs is compressed by using, as a scale, theto base ten of the recorded wave amplitude. Hence, each unit of

a 10 times increase in wave amplitude. But therepresented by each unit ofscale is estimated by seismologists

31 times. Since Richter magnitude is a measured quantity, theis open-ended, but seismologists believe that a Richter magnitude of9 represents the largest possible earthquake. A given earthquake will

one Richter magnitude, though differences in recording result inas to what this will be.*

y: The Amount of Damageirectly related to local shaking and building damage,

scales areused. These scales arebased on subjective observation ofof the earthquake on buildings, ground and people, hi the United

es the most comm only used scale is the ModifiedMercolli (MM) originallyin Europe in 1902, and modified in 1931 to fit construction

ons then prevalent in California and other parts of the United States.a result the MM scale is somewhat dated, with no references to common

is not much of a disadvantage becauseis most likely to be concentrated in older buildings, oftenthe very type that the scale describes. (Figure 1.12) The MM Scale is a twelve

/ - XII. The descriptions for MM I are, in abbreviated form, "Notof large earthquakes." For MM XII the

ptor reads, "Da mage nearly total. Large rock m asses displaced. Lines ofand level distorted. Objects thrown into the air." Because earthquake

cts vary depe nding on distance from the epicenter, nature of the ground, andmany M M values. The M M scale has been

For example, MM VII corresponds to aO.lg and 0.29g.TION

ile the effects of ground failure can be extremely severe, the most commonof earthquake damage is ground shaking. Seismically

and torsion. Shaking causes damage by internally generatedby vibration of the building's mass.

B 5 0 0 -4 0 0 -. . 3 0 0 -2 0 0 -10 0 -6 0 -4 0 -

2 0 -0 - 5 -

DISTANCEkm

- 5 0- 4 0- 3 0^ 2 0- 1 0- 8- 6- 4

- 2S-P

sec3

IT 30 AMPe _.

6 -

'4 -3 -2 -

V- o

1 -

0 -MAGN I T UDE0

A | 7 2 0 23|K)^ ^ * ^ ^ P P !

" '

AMP

Figure 1.11 : Richter magnitude

Figure 1.12: Damag e to an older masonry b


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Figure 1.13: The P-e or P-delta effect. Lateral forcescause structure to drift ( A ). This displaces the verticalloads rom PtoP}: extreme displacement causes collapse.

Figure 1.14: Fundamental periods

m.65 a E n g E D ao.\o

As noted above, inertial forces are the product of mass and ac(Newton's F = MxA). Acceleration is the change of velocity (ocertain direction) over time and is a function of the nature of the emass is an attribute of the building and, at ground level, is equivabuilding weight. (On the moon, building mass and weight wouldifferent.) Since the forces are inertial, an increase in mass will rincrease in the force for a given acceleration. Hence, there is an advantage-when lightweight construction is used as a seismic designfantage-wrAnother detrimental aspect of mass, besides its role in increasing loads, is that failure of vertical elements such as columns and w allby buckling, when the mass pushing down due to gravity exerts itsmember bent or moved out of plumb by the lateral forces. This phis known as the P-e, or P-delta effect. (Figure 1.13)Period and Am plificationAnother important characteristic of earthquake waves is their pinverse of frequency), for example: Are the waves quick and abrupand rolling? This information is particularly important for the deteof seismic forces.All objects have a natural, or fundamental, period. This is the rate awill vibrate if they are given a horizontal push. In fact, w ithout dragand forth, it is not possible to make an object vibrate at anything otnatural period. When a child on a swing is started with a push, to bthis shove must be as close as possible to the natural period of thecorrectly gauged, a very small push will set the swing going eSimilarly, when the earthquake ground motion starts a building vibrtend to sway back and forth at its natural period.When a vibrating structure is given further pushes that are also at period the structure tends to resonate. Its vibrations increase dram

g o o o g qQ q p q p q. O O D g D QI Nil *

o&o-im m i n m i i

1-2 C\J\COR.f> -


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Buildings at Risk: Seismic Design Basics for Practicing Architects 1 1

d, in fact, its accelerations may increase as much as four

0.05 seconds for a piece of0.10 seconds for a one story

0.50 second period, andr buildings betw een 10 and 20 stories will swing at periods of about 1 to

An app roximate rule of thumb is that the building period equals thePeriod is primarily a function of height. The

y story Citicorp b uilding in New York has a measured period of 7 secondsgive it a push and it will sway slowly back and forth, completing a cycle every

iffness of the buildiiig's con struction m aterials and

so the buildings sway back and forth in a very complex1.15) For seism ic purposes the natural period is generally the

0.4 seconds to 7.5 seconds,arily on the hardness of the ground. Very soft ground may have

seconds. The ground cannot sustain longer periods exceptertain unusual co nditions.

learly, this range is well within that of comm on b uilding periods, so it is quitethat the motion the ground transm its to the building will be at its naturaleriod. This may create resonance and the structure may have to deal with

ccelerations of perhap s 1.0g when the groun d is only vibrating with acceleraions of 0.2g.

on in building vibration is very undesirable. The possibility of thisoincide with the grou nd. Th e design of a short stiff (short period ) building, if

located on soft (long period) grou nd, wou ld be appropriate. This is acceptableor new buildings but, there are many inappropriate buildings which were

phen om ena was fully und erstood. The terrible destructionn the 1985 Mexico City earthquake was primarily the result of responsemplification caused by coincidence of the building and ground motion

Earthquake shaking tends to be greater on soft ground than on hard ground -such as rock. As a result, earthquak e dam age tends to be more severe in areasof soft ground. This characteristic became very clear when the 1906 SanFrancisco earthquake was studied, and maps were drawn which showedbuilding damag e in relation to known groun d geology . Studies after the 1989

i * d Mo

F0NDAM&NT>1rr

VBipmmrm'Figure 1.15: M odes of vibration

Figure 1.16: Total collapse caused by extreMexico City.


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12 Chapter 1: The Nature of Ground M otion and its Effect on Buildings

0.76

0.6

oil

fCT WH fW *7 PAMPi>J|

1 1 1 1 1o.% i.o i.s I.O j.5Peit iop-coNPi>

Figure 1.17: Sample response spectrum

P ^ V f*' r^yBB^~ Wj^ ^ ^ T ^ T i l lM g : ^ ^ TA |

^ K S S S M ^ ' ^ SFigure 1.18: Furniture upset by motion in a high-risebuilding.

If the period of a new building is close to that of the site, curves cafor the site, based on information about the nature of the ground, ththe periods at which maximum building response is likely. That is ,periods for which maximum shaking can be anticipated. Such a curthe site response spectrum. (Figure 1.17) Each site will have its ownresponse spectrum.The response spectrum generally shows the accelerations (on ordinate) that may be expected at varying periods (the horizontaThus the response spectrum illustrated here shows a maximum reperiod of about 0.5 seconds - a mid-rise building. Based on this knobuilding design might be adjusted to ensure that the building pericoincide with the site period of maximum respon se. For the figure sa maximum response at about 0.5 seconds, it would be appropriata building with a longer period, of 1 second or more . Of course, it ipossible to do this, but the response spectrum shows clearly what taccelerations at different periods are likely to be, and the builddesigned accordingly.Response spectra are commonly of the form shown that is, thresponse is at the short period end, and then the respo nse tails ofsignificantly as longer periods are reached. Cu rrently our codes recbeneficial aspect of flexibility (long period) by permitting lowcoefficients. However, the amount of motion experienced by thesemeans that they may suffer considerable damage to their nonstructunents such as ceilings and partitions, and contents such as filing cbookshelves, in even a modest earthq uake. (Figure 1.18)The design technique of base isolation, discussed in Chapter 4, ishifting the building period towards the long period of the spectrumresponse is reduced. Most base isolation system s shift th e building the 2 second range. Of course, if the building was located on a site wof 2 seconds, base isolation would not be effective and would, indeedlead to serious resonance.It is generally true that locations closer to the fault from where threleased will experience higher frequency (i.e. shorter period) grouand at large distances the motions w ill probably be of lower frequencwhich eye-witnesses describe as rolling, slowly rocking, swa ying, edistance of a building from a fault also affects th e kind of ground mmay be encountered. High-rise buildings located a long distancehundred miles) from the earthquake focus may be subjected to colong-period motion. In Mexico City, the earthquake focus was two hfifty miles from the area of highest damage. (Figure 1.19)Tors ionThe center of ma ss, or center of gravity, of an object is the point at wh


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Buildings atRisk: Seismic Design Basics for Practicing Architects 13

c center.the mass within a floor is uniformly distributed, then the resultant force ofhorizontal acceleration of all its particles of mass is exerted through the

If the resultant of the resistance (provided by walls or frames)back through this point, dynam ic balance is m aintained.

the mass is eccentrically disposed with respect to the center of resistance, theke force w ill be eccentric as w ell, since the earthquake only generates

ional to the am ount of m ass. In this instance the floor will tend to rotatethe cente r of resistanc e, thus creating "torsion" - a twisting action in plan,results in a very und esirable kin d of stress conce ntration. (Fig ure 1.20)

a building in which the mass is approximately evenly distributed in plan

in all direc tions , so no matte r which direction th e floors are pushed , the

practice, some degree of torsion is always present, and the building codeprovision for this.

I N G T H E E F F E C T S O F G R O U N D M O T I O Nbasic cha racteristic s of buildings he lp resist and dissipate the effects of

ng or a pendulum in response to ground m otion, its acceleration will greatly

is , they are rather inefficient in theirset in motio n tend to return to their starting position qu ickly.

in a building depends on its connections, nonstructuralion m aterials, and design assumptions are com monly

ledge of previous structures.is introduced, the general response remains the same, but the

Figure 1.19: Damage caused by long peMexico City.

,C*\*CR-

v ^ WklMWJ *MtOP tCAHHiCt UNW.UFigure 1.20: Torsion: balanced and resistance


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TTKAIfJDUCTILE 0&TTLE.G j \ ^

Figure 1.21: Ductile materials undergo considerablepermanent deformation before failure.

Figure 1.22: Brittle failure caused by lack of ductility inthe structural system.

DuctilityEven if resonance is avoided, and the building is well damped, a severe earthquake may still be subject to forces that are much highprovided for in the building codes. To design for maximum forcesin a very uneconomic design, and the size and placement of resistwould pose planning and architectural problems.The gap between design capacity (the computed ability of a withstand calculated forces) and possible actual forces is largely drelying on the material property of "ductility". This is the propertmaterials - steel in particular - to fail only after considerable inpermanent) deformation has taken place. This deformation, ordissipates the energy of the earthquake. (Figure 1.21) To achieve galso requires special, and sometimes expensive, detailing of joint

Brittle materials, such as unreinforced masonry, or inadequatelyconcrete, fail suddenly, with a minimum of prior distortion: theyequate ductility. (Figure 1.22)There are, however, many instances when buildings have encounmore than the forces for which they were designed and yet havsometimes with little damage. This can be explained by the fact thaof forces is not precise and deliberately errs on the conservative sidbuilding can really survive higher forces than is apparent. In adbuilding often gains additional strength from components, such asthat are not considered in the analysis. Finally, materials are often sthe engineer assumes in his calculations: all these factors add up terable safety factor, or uncalculated additional resistance.Strength and StiffnessStrength and stiffness are two of the most important characterisstructure. Although these two concepts are present in non-seismidesign and analysis, the distinction between strength and stiffnessmost critical, and its study most highly developed, in structural engapplied to the earthquake problem.A familiar measure of stiffness in the vertical sense is deflection;of floor joists, for example, deflection rather than strength oftbecause a "bouncy" floor is undesirable, though perfectly safe. Thlateral force condition is when limitations on drift, the horizontal sdeflection, impose more severe requirements on members than trequirements. (Figure 1.23)The strength problem is that of resisting a given load without exceestress in the material: the stiffness or horizontal deflection problempreventing the structure from moving out of vertical alignment m


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if the structure can tolerate more, because of its effect on nonstructuralcomponents - particularly partition, cladding and ceiling elements - and itseffect on the comfort of occupants.T h e relative rigidities of members a r e a major concern in seismic analysis. Assoon a s a rigid horizontal element, or diaphragm, such a s aconcrete slab, is tiedto vertical resisting elements, it will force those elements to deflect the sameamount. If tw o elements (two frames, walls, braces, or any combination) areforced t o deflect th e same amount, and if one is stiffer, that o n e will take moreof th e load. Why this is so can be visualized from the diagram which shows aheavy block supported away from a wall b y t w o short beams: clearly, th e thickstiff beam will carry much more load than th e slender o n e , a n d t h e same is trueifthey are turned 90 degrees t o simulate the lateral force situation. (Figure 1.24)Mathematically, the stiffness of a column varies approximately as the inverseof the cube of the length. In this diagram the columns have the same crosssection but the short column is half th e length of th e long o n e . Therefore, theshort column will be eight times stiffer (2 3 ) instead oftwice a s stiff, and will takeeight times the load of the long column. This has serious implications forbuildings with columns of different lengths. (Figure 1.25)Only if th e member stiffnesses are identical can it b e assumed that they sharethe load equally. Since concrete slab floors or roofs will generally fit into the"rigid diaphragm" classification, and since it is unusual for all walls, frames,or braced frames to be identical, the evaluation of relative rigidities is anecessary part of most seismic analysis problems to determine the relativedistribution of th e total horizontal force to the various resisting elements.

(CANTILEVER)

J j j p p n ^ - TTTTTU -cCfc J hart oofitmiia

P W F T

|^ Qf.\fT

= " I f

ft "ir

imrnrnrmmrrnm'I

\nHin

Figure 1.23: Story-to-story drift

J Liiiik f

v e p r i c M . l o w ? H o R iz otfT fc

Figure 1.24: Whether a beam or a colusupporting member will carry much more

Figure 1.25: Configuration, short columns


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: & v . : , ' - . ' ';;.

limitsSiMf| fc: ;


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2: Site Issues

ING OF A STRUCTURE - WHERE DOES THE SITE BEGIN?

ure will be b uilt; and w hile it is critical that a structure not be built across

ity, survival of life lines and potentially hazardou s adjacent land uses.seismic design is not limited to an analysis of the factors within the

nes of the site boun dary; it extends to a broad environm ental analysis of

SMIC RISK AS A SITING CRITERIA

andslides; adjacent structures and land uses that could pose a threat during orfter an earthqu akes; an d, the potential for inundation resulting from tsunamir dam failure.

From a site and urban planning standpoint, however, concern should not belimited to the identification on the site of a fault or potential fault rup ture, butto the broader im pact of ground shak ing and geologic failures that could occurin the region. The failure of the regional transportation n etwork, disruption of

ower or water supply or the isolation of building as a result of ground failure,can be as devastating to a business as actual structural dam age.Therefore, seismic risks from beyond the building site property line must beconsidered as design criteria for a structure . These criteria address the relativedesirability or risk of an individual site, that is , is one site safer for a particularuse than another site; and what factors beyond the site boundary, such asadjacent land uses, geologic stability of adjacent land, or the survivability oflifelines or access, could im pact site developm ent?

ACTIVE EARTHQUAKE FAULTSIf a structure is built over an active fault trace, it should be designed toaccommodate displacement or fault offset. (Figure 2.1)


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18 Chapter 2: Site Issues

several decades. It has been a critical element increasing our underegional seismicity: the frequency of seismic activity, the mprevious seismic events, and the potential for future seismic activmaps indicate where active surface faulting is identified and wherepotential exists. Where identified, designers should provide a seidentified faults for new construction.In many areas, development is limited or prohibited within deadjacent to active faults. Programs to map fault zones and limit netion within established zones have proven successful in reducingrisks to new construction. Unfortunately, earthquake fault tracesignored when land was subdivided and developed, presenting a costo owners of existing structures in a fault zone.Where existing structures are built across fault lines, their structmance, occupancy and continued use should be reviewed to evaluthey pose. Those sections of structures built across a trace can be occupancy types and loads can be reduced to reduce risk exposure

IMPACT OF REGIONAL GEOLOGY O N SITE PERFORThe geology of a region plays a significant role in determining theshaking and ground failure damage. In relatively old geological reas the eastern and midwestern United States where weathering ahave leveled the terrain and laid deep deposits of unconsolidated sground shaking resulting from fault rupture thousands of feet belowsurface can extend for thousands of square miles. Deep soils cground shaking intensity, extend duration of violent shaking and lation of shaking; resulting in greater damage over a larger area result in younger or bedrock regions.For example, in the central United States, the violent shaking oMadrid,Missouri earthquakes of 1811 and 1812 extended acrossand was felt as far away as the eastern seaboard. The earthquakes w2,000,000 square miles! In contrast, the 1906 San Francisco estimated to have released 30 times more energy, was felt over onsquare miles. It impacted a much smaller area because the regionCalifornia limited propagation and increased attenuation of shakiexamples, one without surface manifestations of faulting, and thevisible surface faulting, regional geology rather than presence of a determined the extent of potential damage. (Figure 2 .3)While not building across an earthquake fault is certainly a good radjacent to a fault may not pose as great a risk as one would expeof recent earthquakes have emphasized that regional and local geolack of attenuation of ground shaking are often more important tha


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was severe, resulting in several thousand deaths. Again in 1989, thePrieta earthquake, centered in the Santa Cruz M ountains resulted in theofmore than 40 persons on the CypressViaduct, 60 miles north of Santain Oakland. In both cases, the most violent ground shaking did not occur

the epicenter of the earthquakes, but a significant distance away as a resultthe propagation of the ground waves, the geology of the region and local soilons. Understanding the regional and local geology can tell the designeran individual site.

ONAL DAMAGE AND ITS IMPACT ON A SITEa building depends on more than merelyperformance of the structure. Damage to lifeline systems providing water,, pow er, transportation and communication services can isolate a struc

ning functions where pow er, water and/or communications is vital foronal lifelines serving the site. If access to the site or to regional transpor

is critical for ongoing operations or for reaching and maintainonal transportation system. (Figure 2.4) While these issues cannot be

the clients will provide as for their understanding of the strengths and limits of a specific site, andr determining the need for back-up facilities, water and power sources, andion systems that may prove critical to safety and post-earthquakee, recovery, and continued business operations.

c disruption, even without damage to the structure.

Figure 2.3: Comparison of isoseismals oearthquakes

if

Figure 2.4: Ground failure occurred at toverstressing column/slab connections.

9 ***' *Ur*i% *T

- *anrs " ^^aHT *"" " ' Sk V V I HHT


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20 Chapter 2: Site Issues

earthquakes. A large number ofsuffered poundingdamage during the 1985event, leading in many cases to partial or ull

T H E A R C H I T E C T ' S R O L E IN S I T E S E L E C T I O N A NE V A L U A T I O N

Only occasionally is the architect respon sible for site selection . Mosarchitect is provided with a site by a client unaware of its vulneseismic forces. The more traditional site analysis would includinformation on zoning and planning restrictions on the site. A "sana lysis" should include an evaluation of local site condition s, adjaceand regional geology, to assist the architect in briefing th e client on thperformance of the selected site, the survivability of transportation to the site, and the vulnerability of lifelines serving the site. Thiprovide valuable insights for the client and design team in establishparameters and in defining expected seismic performance of the stIf, however, the architect participates in site selection, desiredbuilding performance and post earthquake function can be me asurexpected site performan ce, life line survival and site access in determmost appropriate location.In either situation, a site analysis should inc lude an asse ssm ent of thment beyond the property line and include adjacent structuresconditions that could "spill over" onto your site. (Figure 2.5) Aanalysis should address the issues identified in the Site Analysis C


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Buildings a t Risk: Seismic Design Basics for Practicing Architects 21

Site Analysis Checklist Is there an active fault on or adjacent to the site?

\ , ^> W ill the site geo logy increase ground shaking?Does the site contain unconsolidated natural or man-made fills? Is the site geo logy stable? Is the site susceptible to liquefaction? Are adjacent up-slope and down-slope environments stable? Are post-earthquake acce ss and egress secure?

) Are transportation, comm unication and utility lifelines vulnerable to disruption and failure? Are there adjacent land uses that could be hazardous after an earthquake? Are hazardous materials stored or used in the vicinity? Are building setbacks adequate to prevent battering from adjacent structures?

Are adjacent structures collapse hazards? Would they collapse onto your site or would their failure o timpact the functions of your structure? Is the site subject to inundation from tsunami? Seiche? Dam failure flooding? Are there areas of the site that should be left undeveloped due to:

Landslide potential? Inundation potential? High potential for liquefaction? Expected surface faulting? More violent or longer duration ground shaking expected? Areas necessary to provide separation from adjacent uses or structures?

Is there adequate space on the site for a safe and "defensible" area of refuge fromhazards for building occupants?

Does the site plan increase potential for earthquake-induced landslides by: Cutting unstable slopes? Increasing surface runoff?


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3: Building Configuration:Architecture of Seismic Design

L D I N G C O N F I G U R A T I O N E F F E C T S

earthq uakes has show n that the architectural form of a buildinga major influence on its performance under ground motion. This influence

result of the three dimen sional in teraction of all the structural system s andFor. certain architectural

response of the building can become very complex, and the

building configuration is used in seismic design to define thectural form of a buildin g. Architectu ral features of concern are defined

kinds of unusu al cond itions an d buildings tha t are of concern resu lt fromconfiguration of the building.

these purpo ses, configuration can be defined as : building size and shape,size and location of structural e lements, and the nature, size and locationents tha t may affect structural performanc e. (Figure 3.1)

both the w ay forces are distributed throughou t the structure and alsorelative magnitude of those forces. Seismic codes distinguish between

and irregular configurations, and it is the latter that may have ainfluence on the effectiveness and cost of seismic engineering and

building seismic perform ance itself. Code forces, discussed in Chapter 5, are

Figure 3.2: The impact of building configuseismic performance has only begun to be rebuilding code s. A wide range of existing buibeen designed without taking such consideraaccount, such as this soft first story exampFrancisco.

Figure 3.1: Configuration components


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24 Chapter 3: Building Configuration: The Architecture of Seismic Design

. i. . . ."_ I T* ** * J . * ~

*


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^- Ske^r */*///'

tepdrtf/o/t/)

3.4: Building size will affect seismic response.large) in plan may have difficultyas one unit to seismic forces.

Buildings of circular planform are, in theory, evenbetter configurations because of their total symm etry, but they are structurally more complex and, ingeneral, not very useful inplanning and urban designterms.Not all regular buildingforms, however, are equally effective as seismic configurations. The size andgeometrical proportions ofa building also affect itsseismic response.

ings that are very large in plan (such as some industrial or warehouse typengs) may have difficulty in responding as one unit to seismic vibration .

y large forces may build up in the diaphragms that must be resisted by shearor frames. Th e solution is to add shear walls or frames (to reduce the spandiaphragm ), recognizing that this may present internal planning prob

, the building may be broken u p into smaller units, separatedjoints. This is an ideal solution seism ically, but introduces difficultitectural detailing problem s at the joints betwee n bu ilding units. (Figure 3.4)

may develo p large forces in shear walls or frames, and in the diaphrag m.solution is the sam e: add cross w alls or frames, o r subdivide the buildin g.long wings of Frank Lloyd W righ t's Imperial Hotel in Tokyo, for example,

tion problem s in very slen der buildings primarily relate to the possibilityW hile this may apply to the form of the entire building it more

proportio ns of resisting eleme nts such as shear walls.


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26 Chapter 3: Building Configuration: The Architecture of Seismic DesignProm: US. Coast and Geodetic Survey

Figure3.7: Variations inperimeter resistance can causetorsional failures such as this J.C. Pen ney store in the1964 Alaska quake.

I R R E G U L A R C O N F I G U R A T I O N S

Irregular configurations occur when the building deviates froregular, symmetrical form in plan and section. This deviation tetwo basic kinds of problems:

Torsion Stress concentration

Torsional problems are most typically associated with plan irgeometries, where the size and location of vertical eleme nts produity between the centers of mass and resistance. Torsional forcesuncertainty in analyzing the building's resistance.Stress concentrations occur when an undue proportion of the ovforce is concentrated at one or a few locations in the buildinparticular set of beams or columns.Many building failures occur because of the lack of balanced res isresults in undue stress being placed on a mem ber or memb ers, withoverstress or failure. Torsional forces and stress concentrationsconfiguration irregularities, such as abrupt change s of strength or the prime cause of such imbalances. (Figure 3.7)Configuration irregularities often arise for sound planning or ureasons and are not necessarily the result of the desig ner's whim (oFor example, the re-entrant corner forms are very useful in achdensity housing solutions on small lots. (Figure 3 .8) High first stonecessary for buildings such as hotels or offices in which large firstrequire much higher ceilings than smaller rooms on upper floors.ing the seismic effect of configuration irregularity will enabirregularity to be accommodated without significant detrimentperformance.

C O N F I G U R A T I O N I R R E G U L A R I T Y A N D T H E C O

To establish seismic forces for practical design purposes by use code, a num ber of assumptions must be made , typ ica l of these assthat the forces are analyzed in two directions, loads are analyzed inand aggregated by simple addition, and structures are assumed to pload paths and be regular inform (defined in Chapter 5).

) -The code procedures comm only in use for establishing earthqua keto structures based on these simplifying assum ptions. Until the 19the Uniform Building Code the configuration p roblem w as not despecific clause at all, and until the 1988 edition of the code the probl


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Buildings at Risk: Seismic Design Basics for Practicing Architects * 27

3.8: Re-entrant corner shapesthe lateral forces in structures which have highly irregular shapes, largedifferences in lateral resistance or stiffness between adjacent stories orfother unusual features shall be determined considering the dynamiccharacteristics of the structure" (italics added ).

1975 Com mentary to the Recommended Lateral Force Requirements,Uniform Building Code, explained

the problem w as dealt with only in a judgme ntal way:"These minimum standards have, in general, been written for uniformbuildings and conditions. The subsequent application of these m inimumstandards to unusual buildings or conditions has, in m any instances, led toan unrealistic evaluation." ^"... Du e to the infinite variations of irregularities that can exist, theimpracticality of establishing definite pa rame ters and rational rules for theapplication of this section are readily apparent."

Comm entary: Recomm ended Lateral Force Requirements (1975) SEAOC.essence, the seismic design problem has historically been too complex to be

How ever, starting with the 1988 Uniform Building Code and theNEH RP Prov isions, configuration irregularities are defined on a quanti-

modal analysis is required where certainegularities occur. T he requirem ents that trigger modal an alysis are complex

ation irre gularities are defined in 2 tables, reprinted in Figures 3.9 and

TABLE NO. 23 -MVERTI CAL STRUCTURAL I RREGULARI TI ESIRREGULARITY TYPE AND DEFINITION

A. Stiffness IrregularitySoft StoryA soft story is one in w hich the lateral stiffness is less lhan 70percent of that in the story above or less lhan 80 percent of theaverage stiffness of the three stories above.

B. Weight (mass) Irregula rityMass irregularity shall be considered to exist where theeffective mass of any story is more than 150 percent of theeffective mass of anadjacent story. A roof which is lighter thanthe floor below need not be considered.

C. Vertical Geometric IrregularityVertical geometric irregularity shall be considered to existwhere the horizontal dimension of the lateral force-resistingsystem in any story is more than 130 percent of that in anadjacent story. One-story penthouses need not be cons idered.

D. In-plane Discontinuity In Vertical Lateral Force-resistingElementAn in-plane offset of the lateral load-resisting elements greaterthan the length of those elements.

E. Discontinuity in CapacityWeak StoryA weak story is one in which the story strength is less than 80percent of that in the story above. The slory slrcnglh is the totalstrength of all seismic resisting elements sharing the story shearfor the direction under consideration.

TAB LE NO . 23-NPLAN STR UC TUR AL IRREGULARITIES

IRREGULARITY TYPE AND DEFINITIONA. Torsional Irregularity to be considered w hen diaphragmsare not flexible.

Torsional irregularity shall be considered to exist when themaximum story d rift, computed including accidental torsion, aione end of the structure transverse to an axis is more than 1.2times the average of thestory drifts of the two ends of thestructure.B. Reentrant Corners

Plan configurations of a structure and its lateral force-resistingsystem contain reentrant corners, where both projections of thestructure beyond a reentrant corner are greater than 15 percentof theplan dimension of the structure in the given direction.C. Diaphragm Discontinuity

Diaphragms with abrupt discontinuities or variations instiffness, includ ing those having cutoul or open areas greaterlhan 50 percent of the gross enclosed area of the diaphragm , orchanges in effective diaphragm stiffness of more lhan 50percent from one story to the next.D. Oul-of-plane Offsets

Discontinuities in a ateral force path, such asout-of-planeoffsets of thevertical elements.E. Nonparallel Systems

The vertica l lateral load-resisting elements are not parallel tonor symmetric about the major orthogonal axes of the lateralforce-resisting system.

Figures 3.9 and Figure 3.10: Irregular CoTables. Reproducedfrom the 1991 edition oBuilding Code 1991, with permissionpublishers, the International Conference oOfficials.


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"IRREGULAR STRUCTURES OR FRAMING SYSTEMS" (SEAOA. BUILDINGS WITH IRREGULAR CONFIGURATION

Other complex shap

Setbacks Multip le towers Split levelsOutwardly uniform ance but nonuniforUnusually high story Unusually low story distribution, or con

B. BUILDINGS WITH ABRUPT CHANGES IN LATERAL RESISTANCE


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hould also be noted that while the modal analysis will give a better definitionthe problem, the problem is not alleviated. The 1990 Commentary to theLateral Force Requirements now includes an extensive discussion oftypes of configuration irregularity, their effects and importance, and howey should be dealt with. (Appendix lD5b, Irregularity and Setbacks) In someBut, in general, the solutions are design and concept oriented and stillly on the engineer's understanding and experience. Figure 3.11 is reprinted

se irregularities vary in importance as to their effect, and their influenceso varies in degree, depending on which particular irregularity is present.while in an extreme form the re-entrant corner is a serious type of plany, in a lesser form it may have little significance. The determinationhe point at which a given irregularity becomes serious used to be a matterjudgment, but the new codes now attempt to define the issue in a quantitative

er, the code determinations are as yet largely untested and somewhat5 story exception is open to question, and theing that configuration irregularities are not significant below that height ist borne out by practical experience. Thus the recent codification of irregurity does not relieve the designer from the responsibility of understanding theys in which the irregularity impacts seismic performance.< >the irregularities shown in Figures 3.9 and 3.10 and 3.11 may serve aschecklist for ascertaining a problem configuration, four of the more serious

tion, some suggestions as to their solution are also provided, recognizingat it may not be possible to totally eliminate an undesirable configuration.

OUR COMM ON CO NFIGURATION PROBLEMS

ahe most prominent of the set of problems caused by discontinuous strengthd stiffness is the soft story. This term has commonly been applied to buildingsse ground level story, while adequate in strength, is less stiff than thoseThe building code distinguishes between soft stories, discontinuities inffness and weak stories (discontinuities in vertical load capacity or strength).uctures with weak stories are limited by code to two stories or 30 feet inght. A soft story at any floor creates a problem, but since the forces areally greatest towards the base of a building, a stiffness discontinuityween the first and second floors tends to result in the most serious condition.

he problem that all these variations of the soft story share is that most of the


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3.13: Soft story failure

instead of being more uniformly distributed among all the stories.Instead of a building's deflection under horizontal forces being equally among the upper floors, it is accommodated almost entirelyfloor. Tremendous distortion in the floor, and stress concentration afloor connections, can cause failure at this line, resulting in the cpartial collapse of the upper floors. (Figure 3.13)The best solution to the problem of the "soft" story is to avoid the dthrough architectural design. If, for some programmatic or compelreasons, this is not possible, the next step is to investigate ways ofdiscontinuity by other means, such as increasing the number of cadding bracing. (Figure 3.14)Discontinuous Shear WallsWhen shear walls form the main lateral resisting elements of the bumay be required to resist very high lateral forces. If these walls do in plan from one floor to the next, the forces cannot flow directly dothe walls from roof to foundation, and the consequent indirect loaresult in serious overstressing at the points of discontinuity. Often ttinuous shear wall condition represents a special, but common, c"soft" first story problem. The programmatic requirements for anfloor may result in the elimination of the shear wall at that levreplacement by a frame.

c

A discontinuity in vertical stiffness and strength leads to a concenstresses and ultimately to damage and collapse, and the story whicup the remaining stories in a building should be the last, rather thacomponent to be sacrificed.

omit

s >

add toftf/M? stiffen rac/tia


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Figure 3.15: Soft first stories are a com mon form of earthquake-induced building failure,as in the Olive ViewHospital.

Olive View Hospital, which was severely damaged in the 1971 San Fernando,California earthquake, represents an extreme form of the discontinuous shearwall problem. (Figure 3.15) The general vertical configuration of the mainbuilding was a "soft" two-story layer of rigid frames on which was supporteda four story (five, including the penthouse) stiff, shear wall-plus-framestructure. (Figure 3.16) The second floor extended out to form a large plaza.The severe damage occurred in the soft story portion, which is to be expected.The upper stories moved as a unit, and moved so much that the columns atground level could not accommodate such a huge displacement between theirbases and tops and hence failed. The largest amount a column was leftpermanently out-of-plumb was 2-1/2 feet!The solution to the problem of discontinuous shear walls is unequivocally toeliminate the condition. To do this may create architectural problems ofplanning, circulation or image. If this is so, then this clearly indicates that thedecision to use shear walls as resistant elements was wrong from the inceptionof the design. Conversely, if the decision is made to use shear walls, then theirpresence must be recognized from the beginning of schematic design, and theirsize and location must be the subject of careful architectural and engineeringcoordination.

-TX5 r- LDs

EFigure 3 .16: The soft first story at Oliveshowing the discontinuity between thelower levels and the stiff shear w all stru


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32 Chapter 3: Building Configuration: The A rchitecture of Seismic Design

storefront corner wedge

---y+ \ torsion4>perimeter strength and stiffness

moment fa/ve x #& OtapkratfMto resist torsion

Variations in Perimeter Strength and StiffnessThis problem may occur in buildings whose configuration is georegular and symm etrical, but nonetheless irregular for seismic d esignA building's seismic behavior is strongly influenced by the natperimeter design. If there is wide variation in strength a nd stiffness aperimeter, the center of mass will not coincide with the center of resistorsional forces will tend to cause the building to rotate around theresistance. This effect is illustrated in Figu re 3.17.Open front design is comm on in buildings suc h as stores, fire stationsmaintenance shops, where it is necessary to provide large doors for tof vehicles. The problem can be particularly acute when the opeasymmetrical, as in a corner, or wedge shape building. T he large imperimeter strength and stiffness around the building can resu lt in largforces.The purpose of any solution to this problem is to reduce the postorsion. Four possible alternative strategies are shown in Figure 3.1The first strategy is to design a frame structure of appro ximate ly equaand stiffness for the entire perimeter. The opa que portion of the peribe constructed of nonstructural clad ding, designed so that it doe s notseismic performance of the frame. This can be done either by using licladding, or by ensuring that heavy m aterials, such as co ncrete or m aisolated from the frame.A second approach is to increase the stiffness of the open facades shear walls at or near the open face.A third solution is to use a very strong mom ent-resistan t or braced fraopen front, which app roaches the solid wall in stiffness. T he abilitywill be dependent on the size of the facades: a long steel frame compare to a long concrete wall in stiffness. T his is, howeve r, a goofor wood frame structures, such as apartment houses with ground floareas, because even a comparatively long steel frame can be made plywood shear walls.Finally, the possibility of torsion may be accepted and the structureto accept it, with careful analysis of the diaphragm design and its transfer forces back to an inadequate resisting structura l system . Thiwill apply only to relatively small structures with stiff di aphr agm s, be designed to act as a unit.


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Buildings at Risk: SeismicDesign Basics for Practicing Architects 33

igure 3.20: Collapse ofre-entrant corner of he L-shaped San Marco Building, 1925 Santaarbara, California earthquake.

Re-entrant CornersThe re-entrant corner is the common characteristic of building forms that, inplan, assume the shape of an L ,T,H, etc., or a combination of these shap es.This is a most useful and tra ditional set of buildin g shap es, which enable larg eplan areas to be accommodated in relatively compact form, yet still provide ahigh percentage of perimeter roo ms w ith access to air and light.These config urations, p ictured in Figure 3 .19, are so common and familiar thatthe fact that they repre sent on e of the mo st difficult problem areas in seismicdesign may seem surprising. Examples of damage to re-entrant corner typebuildings are com mo n, and this problem was one of the first to be identified byobservers. (Figure 3.20)There are two problems created by these shapes. The first is that they tend toproduce variations of rigidity, and hence differential motio ns between differentportions of the buildin g, resulting in a local stress concentration at the re-entrantcorner. This effect is shown in Figure 3.21.The second prob lem of this form is torsion. This is caused beca use the centerof mass and the center of rigidity in this form can not geom etrically coinc ide forall possible earthqu ake direc tions. The result is rotation, that will tend to distortthe building in ways that will vary in nature and magnitude depending on thecharacteristics of the ground motio n. The resulting forces are very difficult toanalyze and predict.


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Figure 3.22: Structurally separate the building intosimple shapes.

< fttDHsf be>

tkrwifhein "hifffe'1Figure 3.23: Tie the building together more strongly.

stttf

Figure 3.24: Stiffen the free ends of the building.

The stress concentration at the "notc h" and the torsional effects areThe magnitude of the forces and the seriousness of the problem wil

the mass of the building, the structural system, the length of the wings and their aspec t ratios, and the height of the wings and their height/depth ratio s.

There are two basic alternative approaches to the problem of thcorner forms: structurally to separate the building in to simple r shathe building together m ore strongly. (Figure 3.22 and 3.23) When FWright divided the Tokyo Imperial Hotel into a number of rectanghe was adopting the former solution. Once the decision is mseparation joints, they must be designed and constructed co rrectlthe original intent. Structurally separated entities of a building mcapable of resisting vertical and lateral forces on their ow n, and theiconfigurations must be balanced horizontally and vertically.To design a separation joint, the maximum drift of the two uncalculated by the structural consultant. The worst case is whindividual structures would lean toward each other simultaneouslythe sum of the dimension of the separation spac e must allow for thbuilding drifts.Several considerations arise if it is decided to dispense with the sepand tie the building together. Collectors at the intersection can traacross the intersection area, but only if the design allows for thesmembers to extend straight across without interruption. Even collectors, are full-height continuous walls in this same location.Since the portion of the wing which typically disto rts the most is tit is desirable to place stiffening elem ents at that location to reduc e (Figure 3.24)The use of splayed rather than right angle re-entrant corn ers lessenconcentration at the notch. (Figure 3.25) This is analogous to the whole in a steel plate creates less stress conce ntration than a recta ngthe way a tapered beam is structurally mo re desirable than an abrupone.


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OTTOM LINE

If the configuration is good the seismic design will be simple andl and good performance is more likely to be assured. If the configu-ion is bad the seismic design will be expensive and good performance willless than certain. v.not to say that all buildings should be symmetrical cubes. The architect

ay towards assuring feasible solutions, and early consultation betweenher than adversarial stubbornness.


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IS


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y Resistant

C S E I S M I C E N G I N E E R I N G C O N C E P T S

App ropriate safety factors are used which ensu re that the materials neveructure will behave in the m anner predicted by the designer.

earthquake region s, we have a confused tradition which is an outgrowththe concept of treating earthquake loads as static, similar to winds. By doing

and by initially assum ing that earthquakes can be represented by lateralads similar in magnitude to w ind forces, we have evolved a simple concept,

behavior concepts is unde rdesign ed for the real earthquake forces it mayter deman ds that force it to perform

the inelastic or non-lin ear range. In this range, structural behavior is neither

obvious, and sim plistic, solution to overcome the discrepancy betweenstic behavior in all earthquakes. However, this approach cannot be econom i

we have a uniqu e situation in seismic de sign, where we do not design theucture for the anticipate d load s but rather try to rationalize the p erforman ce

per curves) and the building code elastic design basis (lower curves). The

i


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38 Chapter 4: Seismically Resistant Structural Systems

RESISTANT SYSTEMS

Figure 4.2: Diaphragm s are the prime horizontal resistingsystem.

4.3: Horizontal diaphragms typically ail at theironnections to vertical resisting elements.

In designing to resist seismic forces, the structural engineer uses small vocabulary of components which are combined to form resistance system.In the vertical plane three kinds of components resist lateral forces: braced frames, and moment resisting frames (sometimes called "rigiIn the horizontal plane diaphragms are used, generally formed by roof planes of the building, or by horizontal trusses. (Figures 4.2 andelements are also the basic architectural components of thebuildiDiaphragmsThe term "diaphragm" is used to identify horizontal resistance(generally floors and roofs) that act to transfer lateral forces betweresistance elements (shear walls or frames). The diaphragm acts asbeam: the diaphragm itself acts as the web of the beam, and its edflanges. (Figure 4.4)Floors and roofs often have to be penetrated - by staircases, elevashafts, skylights, or other architectural or mechanical features. Th


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4.4: Diaphragms act like horizontal beams.

loca t ion o f thesepenetrations is criti-cal to the effective-ness of the dia-phragm s. The reasonfor this is not hard t osee when the dia-phragm is visualizedas a beam: we can,for example, easilysee that openings cutin the tension flange

this beam will seriously weaken its load carrying capabilitity. (Figure 4.5)ctors, or "drag struts ," are diaphragm framing mem bers which "collect"

"drag" diaphragm shear forces from laterally unsupported areas to vertical

e location of a hole (core, skylight, etc.) at the intersection of the componenti

ar W alls

phragms and transmit them to the ground are common ly termed shear walls.e forces in these walls are predominantly shear forces, though a slender w all

re 4.8 show s a simple building with shear w alls at its ends. Ground motioners the building and creates inertial forces which m ove the floor diaphragm s.k down to the foundation.

thebuilding is visualize d as rotated so that it extend s horizontally, it is clear

ze and location ofar walls are extre me -

s which have vary-

and which are placedying distances from

Figures 4.5 and 4.6: Holes in beam s or ininterrupt structural continuity.

Inert ia l forces,.> d iaphragm

*M->/>&i^ ^

Shear forces

y> G ro u n d mo t io n "7?77

Figure 4.7: Shear walls


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Figure 4.9: Typical bracing configuration

mi I t t t f l M UM I Ml l i ! I B | rmi lllmill||||l lk1l | lMr5)*'.W-!,.-> b,-, - | B i nS< iBtKtiiiaiiM|.|g *vmml t t ii 'BlNI* IM-*M* IKf !!|sl !!*.'-.VIliMIIIMl lib ft>"* |p: ' RIMilt ISIPtttlillR*|$ ! ! Ifff 'i -'IIIBIII IIMBI'tff* ^Ml l l l f; me i *i i & n * ( I I S : ' !1Hl f t : . l . : ; ' -Tf J i ' ^P f j .: : tS ' n p p p p a i

Figure 4.11: Mom ent resistant ram es are typically steelstructures w ith stiff welded joints.

Braced FramesBraced ramesact in the same manner as shear walls though theylower resistance and stiffness depending on their detailed designforces may cause the bracing to elongate or compress, in which caeffectiveness and permits large deformations or collapse of tstructure. Bracing can be designed in a variety of configurations,on the forces to be resisted and on architectural limitations. It genthe form of steel rolled sections, circular bar sections, or tubes. (FDetailing to ensure complete load paths for the high forces is veryand detailing which causes eccentricity may greatly reduce the effebracing.Moment Resistant Frames

When seismic resistance is provided by mom ent resistant rames,are resisted by rotations of the beam/column joints. This inducesbending forces in the frame members. The joints become highly stheir design and construction becomes critical. (Figure 4.10) Ibehavior of the frame in the inelastic, or plastic, range becomes anfeature in resistance strategy, by using the energy absorption obtaiductility of the structure prior to ultimate failure. For this reasoresistant frames are generally conceived as steel structures with sjoints, in which the natural ductility of the material is of advantageproperly reinforced concrete frames will also act as ductile frames:will retain some resistance capacity in the inelastic range, prior toThe use of moment resistant frames is of architectural significance iOne is that their use obviates the need for shear walls or braced frthe possible restrictive planning implications of both. The other is tresisting frame structures tend to be much more flexible than sheastructures, with consequent implications for the design of accompatectural elements such as curtain walls, partitions, and ceilings. (F

SiMn.5

Figure 4.10: Joints are critical in m oment resistant frames.


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w and Emerging R esistant Systemsar walls can be constructed of masonry (and have been for ages), reinforcedete, plywood or wood boards, and even of steel plates. Diagonal bracesbeen made of timbers, steel tension rods or steel compression members,

d of reinforced concrete bracing struts. Reliable moment resisting framesve evolved with technological improvements in both welding of steel andnement of concrete by steel reinforcing. Recently, however, several newved. These new concepts acknowledge the need for ductility and energysipation. Some notable new systems are:

1. EccentricBracing, which combines the ductility of the moment framewith the rigidity, or drift control, of the conventional brace.

. Dual Moment-Frame/Shear Wall, combines ductility with rigidity.3. Progressive Resistance Systems, which combine two or three systemsthat progress in load-carrying capacity from rigidity to ductility atpredetermined load levels.. Base isolation, in which the superstructure of the building is partiallyisolated from ground motion by the use of bearings, generally ofspecially formulated rubber or rubber and steel laminates. The superstructure must, still, be designed using conventional seismic resistantmethods, but the force to be resisted w ill be substantially reduced.

eful to understand when these various structural systems evolved. TableTable 4.A

ime1800

19001910192019301940195019601970

Bearing WallsAdobeTimber FrameMasonry(Timber Frame)Masonry(Steel Frame)Concrete W allsand Framing

Concrete Walls(Light Framing)

RGBM or CMU(Light Framing)Ductile Shear Walls

HISTORY OF SEISMIC RESISTING STRUCTURAL SYSTEMSBuilding Frame

Steel Frame(Masonry Walls)Concrete Frameand WallsSteel Frame(Concrete Walls)

X-Braced SteelFrame (Light Walls)1

Moment Frame

Steel Frame(Ordinary)Concrete Frame(Ordinary)

Steel (Ductile)Concrete(Ductile)

Dual System

Steel Frame andConcrete Shear Walls

X-Braced andSteel Frame

Steel Frame andSteel WallsProgressiveResistance System

Controlled Behav

-


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42 Chapter 4: Seismically Resistant Structural SystemsChriaopher Arnold ^ ^ a

responsethe performance failures ofpreceding systems.

resistance; it is only since about 1960 that specific systems havemeet the unique needs generated by the earthquake problem. Spewere developed for ductile reinforced concrete momentframes neconomic steel braced frames were perfected in the late 1970isolation concepts and details are currently being developed in thAs new concepts evolve, some old ones cease to be used, either for technical reasons. It is also important to note that some of the osuch as unreinforced masonry infill around a simple structural steefrom the 1890s to 1920 and now abandoned, did perform well. Ohand, the concept of dual systems with brittle energy dissipaticontained by ductile steel, which once proved successful, are reconsidered, especially in retrofit applications. (Figure 4.12)

BUILDING RESPONSEIt is most important to be able to understand and predict a building'reponse. Will it perform well relative to other buildings? Will it yiebut be repairable? Will it be severely damaged, but not collapse?to be able to accurately predict earthquake behavior.Table 4.B provides a subjective view of seismic behavior of variosystems based on earthquake observations and laboratory tesinteresting and surprising observations should be noted. First, manand highly regarded momentframebuildings have not been testedearthquake. Second, although the old nominal steel frame inunreinforced masonry walls performed very well in San Franciscohave abandoned its use because we no longer build that way, andhave difficulty modeling and analyzing that kind of com posite co

SELECTING A STRUCTURAL SYSTEM jKey FactorsThe selection of an appropriate structural system for a building lregion of moderate to high seismicity is a complex task. It is a problbe shared by the owner, architect and stuctural engineer if a succesis to result. The issue is made complex because the response earthquake motions of both the structuralframework, he entire buand the contents is only just begnning to be properly understood.is confused by the numerous variables and thousands of combimportant factors, and by occasional inappropriate solutions.Selection of any structural solution must be made by an informed dand this is particularly important in a seismic region. The factors


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SUMMARY OF SEISMIC PERFORMANCE OF STRUCTURAL SYSTEMSStructuralSystem EQ Performance TestData Specific Bldg. Perf.& Energy Absorption GeneralCommentsWood Frame SF 1906, etc.

ALA 1964Variable to Good1950'sDFPAetc.

SF Bldgs. performedreasonably well eventhough not detailed.Energy Absorption isexcellent.

Connection detailare critical.Configuration issignificant.

UnreinforcedMasonry Wall SF 1906SB 1925LB 1933LA 1994Variable to PoorRecentSEADSC

Unreinf.masonry hasperformed poorly whennot tied together.Energy absorption isgood if system integrityis maintained.

Continuity and tiebetween w alls anddiaphragm is esse

Steel Framew/Mas Infill SF1906Variable to Good

SF Bldgs. performedvery well.Energy absorption isexcellent.

Bldg. form must buniform, relativelysmall bay sizes.

R/CWall SF 1957ALA 1964JAPAN 1966LA 1994Variable to Poor

Bldgs. in Alaska, SF andJapan performed poorlyw/spandrel and pier failure Brittle system.

Proportion of spanand piers is criticadetail for ductilityshear.

Steel Brace


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1. the anticipated level of earthquake ground motion;2. the site geology and its impact on the structure;3. the building occupancy and its impact on building form asystem;4. the building configuration, which may be arbitrary or diczoning, or program;5. the structural system relative to the configuration;6. the structural details;7. nonstructural components (cladding, ceilings, partitions,tion to the primary structure; and8. construction quality and its impact on structural continuit

Each of these issues is important, to varying degrees, for specifictall structure, for example, will require a dominance of the strucwork, and consequently will usually have a clarity of structural performance. In contrast, a low 2- or 3-story building may have tframework compromised to achieve a unique but irrational form.performance may not be satisfactory unless a significant amouncollaboration is involved to overcome the decision to use the irraGoalsBecause the range of options can be so varied when designing aresist an earthquake, it is first essential to establish goals for the must usually be done with the mutual agreement of the owner design team, and is a necessary step because simple complianminimum provisions of the building code does not assure suearthquake. To establish the project goal, we must ask what theto be: Life safety? Property protection? Or continued post-earthtion? (Figure 4.13)The seismic requirements stated in most building codes are intenassure life safety. To achieve this, only the primary structures musto prevent collapse. With this goal, nonstructural elements may and structural damage incurred even though substantial economresult. In contrast, to protect the building from damage usuallyupgrading from the minimum code loads and concepts, and toperformance must be understood. The lateral movement of the strand its impact on partitions, cladding and equipment must be adhigh accelerations experienced in a flexible, but strong, ductile stebe translated to strong appropriate mountings for cladding andminimize damage. This is clearly a different level of design andeffort, and will be more expensive than a minimum code solutioAn even higher level of performance is required by some buildingrequire continued post-earthquake function without damage oFrequently computer facilities, laboratories and, in California, hdesigned to this performance level. To achieve this goal, a s


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AL CONSIDERATIONS

ng a system w hich will perform well is , therefore, a demanding task. Thes must be establishe d; a join t effort of architects and enginee rs is required

a building form responsive to the program; site characteristics andmicity mus t be considered; a structure com patible with the above issuesds to be selected and analyzed; and, finally, the details m ust be de veloped.

are hundre ds of structural system com binations to choose from. Figurefor structural elements. Which one is

for a given project? D o you select the one in current favor? Or theyou use d on the previous project? Or do you invent a new idea based on

The answer is a combination of the above. Consideration Figure 4.14: Generic locations for seismstructural elements.

1

Perimeter

Interior

Core

Random

Walls

I

>

D

Frames

i


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46 Chapter 4: Seismically R esistant Structural Systems

must always be given to the unique requirements of the project, occupancy, the geometry, the anticipated life of the building. Sevissues are translated into structural systems in Table 4.C, where sand occupancy issues are listed. tThe structural resisting systems must be well understood to deveand reliable solutions. (Figures 4.15 and 4.16) The engineer can conventional or static systems which rely on uncontrolled yieldinto satisfy the earthquake demand, or use the new energy dissipatdiscussed below. These structural systems must be paired with eration (and building program) to complete the solution. Struct"tuned" to overcome some adverse situations, such as re-enbuildings, but tuning will not overcome major problems.Flexible steel moment frames are economical but rely on lateral to dissipate seismic energy. Recent post-earthquake observationdesigned 12-story steel frame in San Jose, California, confirmed thhowever, several negative aspects became apparent and reinforceabout flexible steel frames. The building continued to vibrate at ltudes at its fundamental period of 2.0 seconds for 60 seconds aftemotion stopped. The building was dissipating energy with very liThe result of this undamped vibration was severe internal damageand contents at the upper stories. So we have a clear example ostructural performance without damage to the steel frame, but uperformance of the building contents and great trauma for theoccimplications of this performance data are significant and may alterabout preferred systems.Alternatives to the steel moment frame are those systems which inrigidity to limit lateral drift and corresponding nonstructural damframes introduce more stiffness and rigidity. Shear walls are very sbe designed to higher forces. They will tend to directly transmit grforces with an abrupt motion, but they are also less likely to deveand to amplify the motion. If the building requires a number planning purposes - as most residential buildings do - then sheareconomical solution.

Dual

buildings at risk

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