Design for Lateral ForceResistance with PrecastConcrete Shear Walls

Prepared by

PCI Ad Hoc Committee on Precast Walls

NED M. CLELANDChairman

THOMAS J. D’ARCY DONALD R. LOGANDEGAN G. HAMBACHER RAPHAEL MAGA6JASIMON HARTON MICHAEL G. OLIVA

Principal Author:

Ned M. Cleland, Ph.D., P.E.Vice President - EngineeringShockey Industries, Inc.Winchester, Virginia

For many years, design engineers have successfully used shear wails in precast/prestressed concrete buildings to resist lateral forces both from wind effects andseismic motion. As building codes have developed in this decade, however, precastconcrete systems have been subject to more detailed scrutiny to ensure that theirapplication in regions of high seismic risk is confined to a prescription that emulatesmonolithic cast-in-p/ace concrete. On the positive side, during the last decade, manyresearch projects have addressed aspects that can improve the reliability andperformance of precast concrete systems with shear walls. With this report, the PCIAd Hoc Committee on Precast Walls has collected information from currentexperience and research and formulated a comprehensive design methodology forprecast concrete buildings using lateral force resisting systems of precast concreteshear walls. This design approach is conservative in the use of the unique, beneficialfeatures of precast construction. Designs developed following these principles,however, will perform well. The committee wishes to solicit comments from thedesign profession concerning the philosophy and methods proposed herein.

CONTENTS

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..4 6

2. Historical Performance of Shear Wall Buildings . . . . . . . . . . . . . . . . .46

3. New Building Code Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

4. Lateral Force Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49

5. Characteristics of Precast/Prestressed Concrete Building Systems . . . . 50

6. Design Process for Shear Wall Systems . . . . . . . . . . . . . . . . . . . . . . . . 51

7. Functicinal Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51

8. Development of Shear Wall Options . . .. . . . . . . . . . . . . . . . . . . . . .51

9. Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..5 3

10. Seismic Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54

11. Selection of Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56

12. Final Lateral Force Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

13, Evaluation of Loading Effects on Walls . . . . . . . . . . . . . . . . . . . . . . . .59

14. Wall Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60

15. Diaphragm Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60

16. Consideration of Framing and Connections Not Part of LFRS . . . . . . . 61

17. Needs for Additional Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62

18, Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62

References.. . . . . . . . . . . . . . . . . ..:............................6 2

Appendix I__ Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .64

1. INTRODUCTIONThe precast/prestressed concrete industry has for decades

used building systems with shear walls as the lateral forceresisting system (LFRS) to provide a safe, serviceable andeconomical solution for structures subject to wind and earth-quake loads. This selection and design has historically fol-lowed principles used for monolithic cast-in-place concretestructures, with appropriate modifications made in recogni-tion of the jointed assembly of precast concrete elements.Design methods to achieve successful performance havebeen largely left to the ingenuity and judgment of the designengineer.

With the development of more definitive prescriptive pro-visions in the codes, particularly for high risk seismic re-gions, more formal consideration of the design principlesand methodology of precast concrete construction is needed.A primary goal of the PC1 Ad Hoc Committee on PrecastWalls is to develop a design methodology that will providedesigners with direction in achieving satisfactory perfor-mance for walls in seismic regions. The excellent past per-formance of well constructed precast shear wall structurescan provide valuable guidelines for their successful designand construction in North America and throughout theworld.

2. HISTORICAL PERFORMANCE OFSHEAR WALL BUILDINGS

Historically, the primary focus in designing structures forearthquakes has been to ensure that there is sufficient ductil-ity in the individual elements and connections. That ap-proach developed after observations of structural failuresduring earthquakes that could be attributed to insufficientductility, either in the members or at their joints. Sufficientductility is currently a primary goal of most designers andprescriptive code requirements, whether the structure ismade of cast-in-place concrete, structural steel, or precastconcrete.

Systematic investigations over the past three decades haveshown that shear wall structures have performed remarkablywell in the most severe earthquakes around the world. Anexcellent review of the seismic performance of shear wallbuildings is given by Fintel in the PC1 JOURNAL.’

The primary intent of design and detailing has been toproduce a structure that will not collapse. Sufficient provi-sion for life safety and avoidance of collapse has beenachieved with ductile framing systems allowing the dissipa-tion of energy and internal forces. This ductility, however,has frequently not been satisfactorily achieved in the struc-tural details, thus allowing buildings to collapse or suffer se-vere damage. The recent poor performance of some framedstructures intended to be ductile, even in structural steel,shows there is much yet to be learned about developing ac-ceptable performance through controlled yielding and defor-mation in structures.?,’

The development of ductility has often been at the ex-pense of satisfactory post-earthquake performance. Build-ings that survived collapse were so badly damaged thatmany of them had to be demolished. Structural steel framed

structures have also experienced severe damage. Recently.some owners have found this life-safety approach to hc ofquestionable value when they find that their buildings arenot salvageable after a seismic event.“’

Earthquakes in Chile,” Mexico,’ and the recent Kohr.Japan,‘“.” earthquake illustrate the excellent performance dshear wall buildings. In Chile, a design philosophy based onproviding many shear walls without excessive or sophisti-cated reinforcement has shown that combined strengih andstiffness without large ductility may be more effective in rc-ducing non-structural damage and repair costs than r∈on energy dissipation alones By providing stiffness andstrength, displacement and damage in earthquakes has herolimited, allowing the continued use of structures with hllfunction.

Fig, 1 shows the 22-story Torres de1 Sol Building in Vibde1 Mar, Chile, after it survived the 1985 earthquake Khichregistered 7.8 on the Richter Scale. This reinforced concretebuilding, with shear walls, sustained only very modest dam-age during the severe seismic tremors. Fig. 2 is a structuralfloor plan of the building which shows the configuration ofthe shear walls. It is theorized that the increased stift’nesr.strength and configuration of the shear walls played a m;ljorrole in the excellent performance of the building.

It is interesting to note that seven other reinforced con-crete high rise buildings with shear walls in about the samearea survived this same earthquake with only slight damape,indicating their superior seismic performance as comparedto other structures.*

On-site inspection of precast concrete buildings at Koix,Japan, after the January 17, 1995, earthquake showed thatthe performance of shear wall buildings was also superior 10that of many framed structures.‘“~” In particular, the buildingstiffness appears to have forced inelastic behavior into thesoil below the foundations. Without particular application ofconfinement or special boundary elements, many well con-structed precast buildings with shear walls withstood theearthquake without structural damage and were returned 10habitation as soon as utility connections were restored. It isnecessary to study this experience, to correct deficienciesthat have created problems, and to provide a rationa\ bifor the design of shear walls and their connections to with-stand earthquakes.

Figs. 3 and 4 show one of the several precast concreteapartment buildings with shear walls which suffered nodamage to the structure, glass or interior finish in the Ian.uary 17, 1995, Kobe, Japan, earthquake. The precast build-ings were situated in Takarazuka, a town near Kobe vvhoxbuildings suffered very heavy damage. One theory for thislack of damage in the precast buildings is that the stit’t’ne~sinherent in the shear walls played a major part during theearthquake in forcing inelastic behavior back into theground below the foundations. Severe cracking in the pan.ment close to the buildings and disruption of utility lines

* For further d&ads on the performance of structures III the Chde eartbqu~kc. wih(specod report tttled “The 1985 Chde Earthquake: Obwrvahons on E.wthqu&Resistant Construction m ViAa de1 Mar,” by Sharon L. Wood, Jame\ K WIghI adJack P. Moehle. Civd Engineering Studies, Structural Research Sew\ Nn 512Univewty of llbnois at Urbana-Champaign, Urbana, IL, 1987.

IW the ground lend further credence that this behaviorId have taken place.hear walls have been widely utilized in buildings up to 27ies. but typical precast structures are usually ten stories or*Shear walls are used as the most common lateral forcesting system. There has been a general acceptance of thisiem in areas of low to moderate seismicity. Designersdare new to seismic considerations will find a familiar,rt, and effective approach for lateral resistance if thepue demands of earthquakes are considered in the details.bnventional wisdom, however, emphasizes that build-scannot be expected to withstand the effects of signifi-rearthquakes without damage. The ductile frame ap-rh accepts this damage, but imposes special detailing toure that the yield mechanisms do not lead to collapse andI the action of the yielding results in energy dissipationtreduces the effective forces.his approach requires a high level of skill and an empha-on the quality of construction, particularly in regions oficomponents congested with steel or welding in steelned structures. Such designs have resulted in the poorfonnance of many structures due to the failure of details.rdesign approach implicitly accepts high levels of ancil-ydamage that may save the structure from collapse, butimately results in complete loss of function after the211. These structures are inherently subject to highly un-

tain deformations. This can cause premature local crack-lin columns and result in the formation of soft stories,ding to collapse.

Fig. 1. Torres del Sol Building, Vitia del Mar, Chile. This 22.story, reinforced concrete structure with shear walls survived

the 1985 Chile earthquake with only modest damage. See Fig.

2 for structural floor plan of building. [Courtesy: ProfessorSharon Wood (see footnote on previous page)]

ai

-------------- -B-w---B-w--

-a------a----- --w-----w---

7

a45

i7.80

DIMENSIDNS IN m.

12. Structural floor plan (Floors 3 and 5) of Torres del Sol Building, ViWa del Mar, Chile, showing configuration of shear walls.

LFig. 1 for panoramic view of building. [Courtesy: Professor Sharon Wood (see footnote on previous page)]

Fig. 3. One of several precastconcrete apartment buildings with

shear walls in Takarazuka (near Kobe,Japan) that survived the Jamtan/ 17,

1995, Kobe earthquake with nodamage to the structure, glass or

interior finish. Kourtesy: Dr. S. K.

Ghosh (Portland CementAssociation)1

There have been cases’ in which shear wall buildings haveshown poor performance. The source of the weaknesses isfrequently not in the walls themselves, but in other parts ofthe buildings. Some failures are related to poor details thatresult in obviously brittle local failure or incomplete load

Fig. 4. Precast concrete shear wall apartment bullding in

Takarazuka (near Kobe, Japan). Despite considerable ground

movement that disrupted utilities serving the structure andseverely damaged many nearby flexible buildings, thisbuilding suffered no structural damage. LCourtesy: DonaldLogan (Stresscon Corporation)]

paths. There may also be local failure by instability in theweak direction of the walls. Many failures can be attributedto inadequate diaphragm behavior. Each of these problemareas will be discussed further in this report.

3. NEW BUILDING CODE CRITERIARecent developments in seismic provisions of the model

building codes are having an impact on the design of precastconcrete structures. Each of the model building codes hasbeen changed as a result of a series of recommended pro&sions developed through the National Earthquake Hazard Re-duction Program (NEHRP) under the Building SeismicSafety Council (BSSC). The Congressional Act that estab.lished NEHRP was intended to develop national standards

and lead to a uniform national code for improved earthquakeresistant design. The BSSC has already prepared a series of

recommendations, the latest of which is the 1994 revision.The model building codes have begun adopting portions of

the NEHRP suggested methods. For some areas of the COWtry, this change has only been one of format and detail. Forothers, the NEHRP provisions have resulted in new requite.ments for full seismic designs in areas where structures had

previously been exempt. The process is not a static one;BSSC has embarked on a regular three-year cycle of recom.mended provision updates to parallel the model code updates,

As engineers look for precast concrete structural systemsthat have previously performed well, the demonstrated per-formance of wel l constructed shear wal l bui lding systemsstands out. For many reasons that are not addressed by theold or new code provisions, these buildings have withstoodsignificant earthquakes. Designers of precasu’prestressedconcrete building systems will choose to use shear walls todevelop direct, economical approaches to lateral force resist-

ing systems (LFRS) that perform well during earthquakes.Although the new codes may provide new challenges and

suggest new system approaches, it certainly appears thatprecast concrete shear walls will be the solution of choice.They are particularly well suited for typical low to mid-risestructures. In this class of buildings, moment frames are ex.

nsivc and may be damaged as they develop ductility. Theti performance of concrete shear wall structures points tob ~ulution for meeting the needs of most precast structurest&ring lateral forces.h ic helpful to consider recent code progress as a way toarxluce the regional differences in code criteria that willfeel details of design. There are three model codes that areti in part or in whole in different regions of the Unitedties. The Uniform Building Code (UBC)” has generallyn adopted west of the Mississippi River. The BOCA Na-aal Building Code” is used in the northeast. The Standardhiding Code (SBC)‘” is used in the southeast. Fig. 5 is aop of the United States showing the areas of jurisdiction ofkvarious model building codes.llte UBC has been based on the Structural Engineers As-ciation of California (SEAOC) Blue Book. Althoughmte of the fundamental terms and concepts parallel theMRP provisions, the Blue Book is not based on NEHRP.It 1997 edition of UBC includes modifications of the ap-kh of Chapter 2 1 of AC1 3 18-95 for the design of shearplls. which provides a more rational approach to boundaryme detailing as compared to previous methods. Thebnges are currently being carried forward in the provi-mb updating process for NEHRP 97.The 1987 BOCA Code had seismic provisions, but pro-drd a general exemption for areas without historic damage&!d on United States Geological Survey mapping. TheW BOCA removed this exemption, but still exemptedi&t structures in Zone 1. The 1993 BOCA reflects theEHRP provisions, written into code format language. SBCrhorrowed from the efforts of the BOCA Ad Hoc Com-mitee on Loads to create code language from NEHRP, soLSBC provisions are very close to BOCA. The changebile for updating to NEHRP recommendations was notsnpleted in time for the 1996 BOCA change cycle, so191 NEHRP remains the basis of BOCA, with 1994EHRP as an option.Budding codes are predominantly prescriptive in order topride building officials with enforceable provisions. They

vary regionally in the detail of the prescription and level ofenforcement. Many perceive distinct differences betweeneast and west practice in this regard. Historically, east coastpractice has allowed more liberal interpretations and appli-cation of engineering judgment to areas not specifically ad-dressed in detail.

Although NEHRP provisions have primarily been used toform the basis of earthquake requirements of the easterncodes of the United States, the approach of the provisions re-flects a more detailed prescription common to the approachof UBC. In these provisions, precast concrete constructionhas received little direct attention. It is vital, then, that the de-velopment of the new code provisions to precast constructionbe considered.

4. LATERAL FORCE CRITERIADesign lateral forces on structures may be governed ei-

ther by wind load or seismic criteria. For precast struc-tures, where the mass is large, seismic loads are likely tocontrol except where the earthquake risk is so low as toallow an exemption or where there are extremely highwind loads such as in hurricane wind regions. There aredifferences between the codes for both wind and seismiccriteria. Each code permits a static force procedure forshear walls in all buildings of low and moderate risk re-gions, and in most buildings of high risk regions withoutdefined irregularities. That is the method that will be dis-cussed in this report.

Primary to the development of seismic resistance is thephilosophy that it is neither economical nor necessary to de-sign the lateral force resisting system to sustain earthquakeforces elastically. Fintel” summarized this philosophy,which is implicit in earthquake code provisions:

1. Resist minor earthquakes without damage.2. Resist moderate earthquakes with minor structural

damage and some non-structural damage.3. Resist major catastrophic earthquakes without

collapse.

Fig. 5. Map of the United Statesshowing areas of jurisdiction of

various model building codes.

[Courtesy: Dr. S. K. Ghosh (Portland

Cement Association)]

I0 T b

-.+---- --.--&.- -..1 ’ 0UNSTABLE WALL GROUP STABLE WALL GROUP

IFig. 6. Plan of building showing three non-colinear walls.

OVERTURNING RESISTANCE

exW/‘2>yxH

1WA- pi

L

I

e I-H

b

Pd +- w/2

Fig. 7. Overturning resistance in shear wall.

2. For most practical applications of shear walls, the gov-erning criteria that will define or limit the wall capacity isresistance to overturning (see Fig. 7).

3. It is important to look for configurations that providefor the balanced behavior between torsional stability andvolume change flexibility (see Fig. 8).

4. Successful designs balance the requirements and ca-pacities of the shear walls with requirements of the tloor di-aphragm, which must collect and transfer the lateral loads(see Fig. 9).

5. Walls must be sufficient in number and size to limitdrift to about one percent of height after inelastic magnifica-tion of displacement is considered to allow for further dis-placement in the diaphragm.

It is important to consider the mobilization of dead load toresist overturning because tension anchorage capacity at thefoundation-to-soil interface can be difficult to develop. Eachof the options discussed previously in considering the func-tional layout will mobilize the direct dead load of the floorsystem for resistance against overturning.

It is also important to find additional locations that canmobilize dead load overturning resistance. This might be

Fig. 8. Volume change and torsion stability of buildingdepends on configuration of shear walls.

Fig. 9. Shear wall building with long span diaphragm (top)andshort span diaphragms with continuity (bottom).

done by replacing a bay of beams with loadbearing walls(see Fig. IO). Cross-walls can be located at interior columnsto allow the mobilization of the column loads for overturning resistance (see Fig. 11). If it is architecturally accept.able, the exterior faces of the building make good shear walllocations because they do not affect the interior traffic flowor sight lines (see Fig. 12). These walls can also be tiedccolumns or replace columns to mobilize dead load overtunr-ing resistance.

When inelastic response is expected, the desired modeoiresponse must be considered. If the bending deformationdthe wall is the preferred mode, then the overturning reslktance must exceed the capacity provided in the wall. If&rotation is acceptable, by taking advantage of the provision+that consider soil/foundation interaction and rocking, Theothe foundation must be designed for a force level consisawith this mode.

The balance between shear wall capacity and diaphyforce transfer requirements must be considered. It mayappear to be desirable to establish the fewest number of SWwalls that will meet the resistance requirements for the lat.era1 load overturning moment; however, distant spacingtdvertical elements may create unrealistic demands on titconnections in the diaphragms, which must transfer theloads as flat, deep beams. This is particularly true in a paAing structure when the diaphragm is broken into parts by thtinterruption of the ramp bay.

I

,L- ---. /I

IO. To provide additional strength, a bay of beams can beaced with loadbearing walls.

111. Cross-walls can be mobilized to provide additionalngth.

overturning (M,,,,) should be evaluated in an approximatemanner to develop a sense of available wall options as theload requirements are developed. This resistance may beconsidered in two steps:

The first step is the consideration of precast wall resis-tance only, where the direct and indirect dead load that canbe mobilized for resistance is determined along with the mo-ment arm about the edge of the precast base.

The second step includes consideration of the weight ofthe foundation and the soil overburden, with the momentarm taken about the edge of the footing. It may not be pos-sible to determine all of the viable options for shear walls atthis stage without determining the load demands, Some it-eration between these steps in the procedure might beneeded.

9 . L O A D I N G

The next step in the design procedure is the determinationof loads. Vertical loads are needed in lateral load analysis todefine the dead load resistance to overturning and for massand seismic load calculations. The building weight must bedetermined level-by-level to permit the vertical distributionof seismic base shear.

To understand the relative magnitudes of loading for in-creasing seismic risk, compare the loads for a single struc-ture in different regions of the United States, Sample loadswill be evaluated for a typical precast parking structure thatwill have six levels. The plan of the example building willbe 184 x 460 ft (56.1 x 140.3 m) with an expansion jointseparating the ends 200 ft (61 m) from one end, as shown inFig. 13.

112. Occasionally, shear walls can be placed at exteriormoia building.

With the lighter loads typical of wind, long spans in thephragm may be tolerated because the chord forces andti are low. As the seismic category or region increases.tincrease in magnitude of forces will require more walls.tdiaphragm forces may become excessive unless verticaltments are more closely spaced. The closer distribution ofI walls gives a higher degree of rigidity and redundancythe precast structure, increasing the “egg crate” effect,

iich has proven so successful in resisting many past catas-phic earthquakes.his important for the engineer to get a sense of the struc-al value of a variety of options available at an early stagedesign. For several preliminary schemes, the resistance to

9.1 BOCA Wind Loading

For wind loading, the BOCA Code follows ASCE 7. It isnecessary to determine the appropriate wind exposure basedon the terrain of the site. For comparison, the exposure cate-gory will be assumed to be C. The roof height, h, for sixframed levels plus a railing height is 70 ft (2 I .3 m). The topof the footing is taken as 2 ft (0.61 m) below grade. Theschematic building section in Fig. 14 shows the positions ofthe floors and the wind loading.

The lateral analysis should be made on a level-by-levelbasis, so the loads are calculated by level. Although theloads above I5 ft (4.56 m) vary linearly with height, it issufficiently accurate to calculate the pressure at each leveland to use this load as the average pressure for that level.

Because the structure has an expansion joint, each sectionmust be evaluated as a separate structure. Some parametersare dependent on building geometry, which requires consid-eration of the dimensions of the building as a whole.

It is important to note in this analysis that for thenorth/south loading, the terms for internal pressure in thewindward and leeward pressure equations will cancel eachother because a rigid diaphragm at each level will connectthe windward and leeward walls. For the east/west loading,however, the expansion joint separates the windward andleeward walls, so the governing load will come from thewindward pressure equation with all its terms. The effective

”

W E S T S E C T I O N 4 EAST SECTION

N

I .I.

prc\\ures are cnlculatd level hy keel. The applied \vind

load\ are then calculated along \vith the h;~\t: overturning

moment for each section.

It should be re~ognid that these wind loads ;trt: \t‘r\ ice

loads. which must be djustt‘d by load fxtors for ultimntc

strength design of concrete. It is also important to recognize

the BOCA rquircment in Paragraph IhO9.I.2.“ Overturning

and Sliding: “The o\ crturning moment due to ~Yut/ lotrtl

IO'-6'

--l-Q

1

Fig . 14 . E levat ion oi Ixlilding showing wind distribution.

shrill not eu~retl two-thirds of the t/cd-kd stabilizing nt+

mt‘nt tml~ss the building or structure is anchored to resin

the C’YCCSS moment.” Thus. the cal~ulattxl Grid overturning

momc‘nt ;thout the base must he increased hy SO percent for

comparison with the resistance to ovrrturning.

It’ the dead Ioxl rc‘sistxncc is ru~txltxl. ;I positive tension

tie must he made to the tbtmdation and cxried to the sup

portins soils. While some anchorage may he feasible to sup

pkmcnt the dcud lad resistance. the \veight of the foolin

and soil ovtxhurdrn may not tx2 sufficitxt to offer signif.

cant anchorage capacity.

A&lit ionnl :unchorqt‘ may he devrloprd in the supportin

soils through pilin,0 or bclkd c;iissons. hut these measure

may not he economical unless the lixmdation system alread

requires the mobilization of these treks. It is important

this early stafe in the design Jr\~rlopmrnt to find overturnin

rcsistancc Gthout the need for positive anchorqe.

10. SEISMIC LOADING

The location of the site d rrgionnl expected soismicit

will determine some ot’ the basic paramctus used in the&

terminution of seismic loads. For BOCA. the location ofthr

site determines the values of the velocity-related accelerb

tion codicient. A,., and the uccdt‘ration cot’fficient. A,.FnUBC.“ the zone detrrminrs the sdsmic zone factor. Foricomparison ot’ Ids. the loution vilriilhks for three reginu

in the llnited Stiltes ;lrc‘ summxGxi in Table I.

The soil conditions ;tt the site will dstuminc the sitecurf.

fiknt. 3’. for exh code. For this comparison. the soil pmk

type will he taken ;IS Type S?. with ;I site ~oefficientofl.!

ere are no plan or vertical structural irregularities. Thettal force resisting system will include loadbearing andn.loadbearing precast reinforced concrete shear walls.Under BOCA. the base shear is proportional to A,. or A,, ifperiod exceeds the cut-off plateau of the response spec-The structural irregularity and detailing requirements

respond to the Seismic Performance Category. The re-irements are “nested.” Each level of increasing hazard in-des the requirements of all of the lower categories, plusditional requirements.By BOCA Section 16 12.3.5.1. “Regular or irregular build-

1s a&ned to Category A are not required to be analyzedseismic forces for the building as a whole.” For the lowcase, wind load criteria will govern the lateral force resist-

gsystem analysis. Seismic Performance Categories C and Dads are calculated from the static equivalent force procedure.

Ihe basic concepts for determining and applying theuivalent static lateral loads by the UBC are the same. Forilance. the calculation of the building period is the same.ay of the other details, however. are different. BOCA

1s two equations to establish base shear, with an A,, equa-nprovidinp a maximum load requirement. UBC has a@e equation, but sets an upper limit on the effects of theilding period and soil factor. In the 1994 UBC. the con-pof seismic zones is still used. The calculated loads arevice loads, so ultimate load factors are required in loadnbinations for strength design methods.lnboth codes, the base shear is inversely proportional to alponse modification factor. “The concept of a responsealification factor was proposed based on the premise thatdl-detailed seismic framing systems could sustain largeelastic deformations without collapse (ductile behavior)develop lateral strengths in excess of their design

ength (often termed II~.WI’\V strength). The R factor is as-tied to represent the ratio of forces that would developLr specified ground motion if the framing system were tohve entirely elastically (termed hereafter as c4astic de-/n) to the prescribed design forces as the strength levelsumed equal to the significant yield level).““InBOCA. R is defined in Table 1610.3.3. along with thetlection amplification factors and structural system limita-ns. In UBC, R,,. is defined in Table 16-N. The factors arefmed on the basis of defined structural systems.Engineering judgment is required in selecting the appro-iate system for defining R or R,,. because the systems de-ribed are generally traditional systems in cast-in-placencrete. steel or masonry construction. This fact must bensidered in precast detailing, even in regions of low orDderate seismicity where Chapter 21 provisions do not

pply. The basic structural system defined as “loadbearing1 system.” illustrated by Ghosh and Domel,?h implies astem where the walls provide both vertical and lateral loadjistance and there is no separate gravity loadbearingmme. In BOCA, reinforced concrete shear walls in such astem are given an R of 4’/:.In the example chosen for this report, the precast concretemostly supported by an independent vertical load struc-e. but some of the shear walls will directly carry verticalds to the foundation. Indeed, walls with axial dead load

Table 1. Location related seismic oarameters

I BOCA UBC

~ Effectivepeak

Level of 1 velocityseismicity 1 A,

Low ~ < 0.05

Moderate ’ 0.12 1 0.12 1 c 2A 0 . 1 5I

High 0.30 1 0.30 i D 3 1, 0.30

forces are preferred for resistance to wind load overturning,as previously noted. The engineer must determine if thiscreates a loadbearing wall system, or if the building shouldbe defined as a “building frame system,” which uses shearwalls for lateral load resistance that incidentally share inpart of the vertical support. Reinforced concrete shear wallsin this second system under BOCA have an R of 5’/2. Thislatter interpretation is implied in the examples given byGhosh and Domel.?h It is also possible that the responsemodification factor, R, may have to be decreased to reflectpost-yielding behavior more accurately, depending on theconnection system used.

The distinction is important, because the difference in theR values assigned results in 20 percent more lateral load forthe loadbearing wall system. The lower R value for a load-bearing wall reflects the added danger of collapse that existsif seismic damage to the lateral load resisting system re-duces the vertical load capacity. Such a system has less re-dundancy. The appropriate value should be a matter of judg-ment, which is somewhat dependent upon how much of thevertical load is carried by the lateral force resisting system.

If the shear walls are truly incidental to the vertical loadframe, then the higher R value is appropriate. If the shearwalls carry a large portion of vertical load, then the lower Rvalue should be used. It is important to recognize that theexample parking structure has two independent structures,separated by an expansion joint. Each section is treated sep-arately. It may be appropriate to use different values for R inorthogonal directions within each section. For this prelimi-nary step in the analysis, both values will be calculated.

In UBC, the R,,. values differ from the BOCA values to re-flect service load results. Because there are differences, thecalculation of the seismic lateral force and its vertical distri-bution is developed here for comparison. An important dif-ference in the application of R values affects shear walls.UBC Section 1628.3.3, Combination Along Different Axes,states “In Seismic Zones 3 and 4 where a structure has abearing wall system in only one direction, the value of R,,used for design in the orthogonal direction shall not begreater than that used for the bearing wall system.“‘J

The BOCA loads are intended for direct use in strengthdesign methods. The load combinations are given in Section16 13 by reference to ASCE 7-95. Load combinations thatapply are as follows:

(1 .l + O.SA,.) Dead -+ Floor Live + (0.7)Snow + Seismic

(0.9 - 0.5A,) Dead + Seismic

Table 2. Ultimate strength design base shear and moments.

East West-- - - iPredicted velocity 1 Calculated over- 1 Predicted velocity ) Calculatedow-i of earthquake~ motion, V(kips)

: turning moment I of earthquake ( turningman&! MO, (kip-ft) motion, V (kips) M,, Wp-f\\

& ~~~ ~~ _~ - t 1 IEast/West 246 1 0 , 4 9 2 246 10,4Y!

BOCA 4 --.- - 1. -.--North/South : 329 13,885 427 17,987

I - -R,,.=8155

1 35.789 885 ~

(JBC !-

44j17

- ’- .-.R,=6 -I --1009 [ --50,544 I 1 1 8 2 59.&7

- - --.-

.-.

i-5’/> +R 1 Go5 1411 69,14359,590

BOCA -

!

i -R=4’12 1 7 2 5 84,;‘) I

I.-

1473 --j 12,841 1

R,,.=8 ! 1 5 1 3 .75.732 .i 772 I 88,7!XUBC I

-‘. --- R,,. = 6- .---- ---2011

+

1 0 0 , 9 7 8 2363 I I8.250-

I--- -k 51/--= 3011-- 1 148,900 3526 i I72jOY

BOCA ‘m R = 4’12 ~ 3 6 8 I 1 8 2 , 0 3 3 4 3 1 1 2 II ,404-

-R,,=8 3024 -;-4

1 5 1 . 3 6 0 3 5 4 1 171,227

UBC -- - -I R, ~6 4049 202,686 4740 237,26-1 I

Low

Moderate

High

Note: I kip = 4.44X kN: I kip-ft = I.365 kN-m

In Section 1903.1.1, modifications are made to AC1 3 18,Section 9.2.3. The load combination for seismic design isreplaced with BOCA Section 1613.0 in its entirety.

UBC requires that the load combinations in AC1 3 18 bemodified to:

U= 1.4(D+L+E)

U=O.9D+ 1.4E

A comparison between lateral loads can only be made byapplying a 1.4 factor to the UBC loads and a 1.3 factor to thewind load from BOCA. Table 2 provides this comparison.

It should be noted that for BOCA in regions of high seis-mic risk (Seismic Performance Categories D and E), thesimplified method to ensure adequate capacity design forthe critical direction is to apply the full seismic load in oneorthogonal direction, while simultaneously applying 30 per-cent of the forces for the perpendicular direction.

11 l SELECTION OF SHEAR WALLSIn this design step, the loading demand is matched to

available options for possible wall configurations. “Analy-sis” is simplified by using only approximate load distribu-tion based on the position of the vertical resisting elementsand the requirements of the codes to assume minimum ec-centricities for torsion effects. A more thorough review ofdiaphragm rigidity may be required for selecting the methodof final load distribution.

11.1 Wind Load

The governing conditions for the design for lateral resis-tance for a low risk site will be the 50 percent safety factoragainst overturning for the wind loading. There is no ex-plicit provision in BOCA requiring the consideration of loadeccentricity from the center of resistance for wind loads.Unlike the requirements for seismic analysis, there is no ex-plicit requirement for the use of linear elastic modeling forload distribution. While it is usually convenient to perform

the lateral load distribution using linear elastic assumpth,the consideration of load redistribution when yielding cc&create a distribution in proportion to strength.

A preliminary analysis can then be performed by camping the calculated values of overturning resistance for SINIwall options with the total overturning requirement deter.mined from the analysis. If the walls are selected to be ge@erally concentric with the loads and with reasonable tar-sional stability, the final design should show agreement wih

little iteration.

11.2 Seismic loads

For moderate seismicity, a primary limitation on the&sign configuration and requirements will be overturning, ~IIIrecall that the load combinations are different. The seistiload computed under BOCA requirements is meant tokused with limit state technical codes, so the load calculatiis already an “ultimate” load.

In the final design, the resisting dead load is reduced byzfactor of (0.9 - OSA,.). BOCA Section 1610.4.3.1 requiresthe design to include “torsional moment (M,) resulting fmmthe location of building masses plus the accidental torsionalmoments (M,J caused by assumed displacement of the IMEeach way from its actual location by a distance equal to!percent of the dimension of the building perpendiculartotidirection of applied forces.”

At this design stage, choices for resistance should be ~1.ficiently conservative to allow accommodation of this mdated eccentricity in the final design. To simplify the initialselection of walls, it may be sufficient to simply cornpartthe required seismic overturning load to the dead load resis.tance and make sure that sufficient excess capacity or opportunity for added hold-down is provided to accommobha final design.

The seismic design requirements include limitationofdrift. The analysis is made using a linear elastic model,anlthen the lateral displacements are magnified by Cd valut

comparison to the code design limits. Using overturningthe basis of preliminary selection of walls. then, is cer-nly only a first step in an iterative process of optimiza-n. In this regard, it is also important that the effect of thealIs of precast construction on the final effective stiffnessIhz wall be considered. It should be recognized that thelues of R and C,/ have been established for monolithic\t-m-place concrete construction.To be considered for the concrete values, precast walls in[Idings of Seismic Performance Categories D and E mustlulate monolithic behavior. Emulation is not only confor-ince to the appropriate detailing requirements of AC1 3 18.eluding Chapter 21 for moderate and high seismic areas,also to have comparable deformation characteristics.

I < will be discussed in more detail below. For now, keepmind that the displacement calculation for all seismic per-mance categories may be more sensitive to accurate stiff-

I F than the lateral load distribution.l’he final solution will derive total resistance from bothad loads and supplementary positive hold-downs. Ati distribution of walls through the length of the struc-e will reduce diaphragm demand and assist in torsionalbdity.Similar to BOCA specifications, UBC Section5 1628.5d 1628.6 also require the design to include torsional mo-mt resulting from the location of building masses plus therldental torsional moments caused by assumed displace-tnt of the mass relative to the location of the lateral loadtments each way from its actual location by a distanceual to 5 percent of the building dimension perpendicularthe direction of the force under consideration.To simplify the initial selection of walls, it may be suffi-nt to {imply compare the required overturning load multi-led by 1.55. because the seismic load is a service leveld. to the dead load resistance and make sure that suffi-mt excess capacity or opportunity for added hold-down isollded to accommodate a final design.Kith the higher loads required by the Zone 3 criteria. it ispcially important to remember that successful designslance the requirements and capacities of the shear walls

i t h requirements of the floor diaphragm, which must col-and transfer the lateral loads. Rather than concentrate

kemely large resisting elements at a large distance apart,sill be more effective lo distribute shear walls throughoutstructural plan so that there is significant redundancy invertical elements and moderate requirements for the di-

hragm in rigidly connecting the vertical elements.For a parking structure layout. the normal functions sup-fl this strategy because of the need for long continuouslies and the regular spacing of parking stalls. The loca-PS of shear walls can be a balance of interior and exterior1,Itions that allow overall openness.

12. FINAL LATERAL FORCE ANALYSISIhe final lateral force analysis follows the requirements

om the building codes. The BOCA Code specificallyguires that “the corresponding internal forces in theNctural components of the building shall be determinedalinear elastic model.” Shear walls act as vertical can-

tilevered beams that transfer lateral forces from the super-structtire to the foundation.“2x It is common practice toassume that the floor acts as a rigid diaphragm, so that theloads are distributed to the walls based on their relativestiffness. The approach used in the Second Edition of thePC1 Design Handbook (Section 4.7.2)“’ considers stiffnessdirectly:

1 1 I

CK=E+ZK,

One problem with the original equations in the PC1 De-sign Handbook was that flexural stiffness was defined as12E,I,lh J (where E and I are properties of the wall), whichis based on conditions which rarely. if ever, occur in precastconstruction. The “12” factor assumes fixed/fixed wall spanbetween floors (third case. Table 3.7.1, PC1 Design Hand-book, Fourth Edition). The h, uses the span of the wall asonly floor to floor.

The use of this stiffness relationship is rarely appropriatefor almost any precast alternative. The case of a wall can-tilevered from the base and free to bend at the level of load(first case, Table 3.7.1, PC1 Design Handbook, Fourth Edi-tion:“) with flexural stiffness of 3E,I,lh: is a much bettermodel of behavior. Multiple levels can be handled by apply-ing this case independently for each level and summing theresults.

Taking the shear modulus G = 0.4E, then the stiffness ofeach Wall i at a given Level j can be expressed as:

KL- 1

E - 3h h7’ +A‘4 3 1 ,

When the aspect ratio of height to base is less than 0.3,shear stiffness will predominate. Between 0.3 and 3.0, thebehavior is mixed. Above 3.0. the behavior is primarily intlexure. Although shear failure is not generally ductile, nodistinction is made for this factor in assigned R values forshear walls in the model codes, as discussed above.

Using the stiffness relationships directly, the lateral dis-placements due to both bending and shear can be derived,and so the evaluation of lateral drift that is needed to satisfyseismic code provisions can be made, Using the simplemodel of a cantilever beam will allow a relatively simplecalculation for load distribution and drift to be performedwithout a sophisticated finite element analysis. Appropriatevalues for the effective moment of inertia for the walls may,however, require more consideration.

12.1 Unsymmetrical Shear Walls

The layout of shear walls may produce a center of stiff-ness in a framing plan that does not coincide with the centerof mass, which is the center of seismic load for that floor.Even if with a symmetrical layout, the center of stiffness isat the center of mass, the codes require the addition of an al-lowance for “accidental” torsion “caused by assumed dis-placement of the mass each way from its actual location bya distance equal to 5 percent of the dimension of the build-ing perpendicular to the direction of applied forces.“zl’ In

57

Category D and E structures with torsional irregularities,BOCA increases the accidental torsion at each level by atorsional amplification factor.

It is necessary to consider this torsional loading in theevaluation of the forces on walls. An approximate methodbased on a “polar moment of stiffness” is simple and direct.Symbolically, the distribution can be written:

Force in the y direction distributed to Wall i at Level j dueto the force in the y direction at Level j:

W,.K~.qjy =: -.p +

TWy xKiy

ZKiy ZKiyX2 +gK,Y*i=l i=l i=l

Force in the x direction distributed to Wall i at Level j dueto the force in the y direction at Level j:

whereWY = lateral load

x = distance of Wall i from center of stiffness in x directiony = distance of Wall i from center of stiffness in y directionK = stiffness of wall defined above

This approach has the benefit of using the stiffness rela-tionships with both shear and bending contributions to dis-tribute the torsional effect of lateral loads.

To apply these equations, it is important to determine ap-propriate values for the wall section properties. The shearresisting area of the walls is not necessarily the total area ofthe walls. Because the shear stress in flanges of beams islow, only the web really resists shear. Similarly, in wallgroups with walls perpendicular to the direction of load, theperpendicular walls or flange walls will not be effective inshear stiffness, and their area should be discounted.

The configuration and assembly of walls or wall groupsmay have an effect on the bending stiffness, and the effec-tive moment of inertia. It has been common practice to as-sume that a wall or connected group acts as a combined unitas long as the connections in vertical joints between wall el-ements develop the required capacity to resist overturning orthe VQ/Z shear force. Recent codes, however, now prescribethe evaluation of lateral drift and its magnification (inBOCA using the Cd deflection amplification factor) for thedetermination of P-A effects and for comparison to maxi-mum limits. Therefore, there is a need to make a more accu-rate evaluation of the stiffness properties to allow this evalu-ation to be valid.

For precast shear walls that are stacked and connectedacross horizontal joints, it must be recognized that thesejoints create a zone of reduced stiffness. Rather than crack-ing uniformly along the edge of the wall near the base, pre-cast walls tend to deform by joint opening. The curvaturedistribution is discrete. The degree to which this occurs isnot simply a function of the number and spacing of jointsbut also depends on the direct vertical loads that act to closethe joints. At the phase of initial analysis, the reduction ofstiffness may be approximated as 75 or 80 percent of thegross untracked cross section. Subsequent verification

should be made of the section along the height of the HJIIbased on the axial load/bending interaction. The final efktive stiffness may be determined using a conjugate himanalysis of the wall with varying cross section to deteruwa moment of inertia that produces equivalent displacematThe shear wall configuration has been subject to se\crnltests to evaluate the details that can be used.“-15

Walls that are assembled with vertical joints, ho\\c\rr,may behave differently. The use of ductile co~lt~~~l~clnsacross vertical joints may have very beneficial efl&% ,I\ ohlocation of clearly defined sites for inelastic beh,l\lnr godenergy dissipation without collateral damage to th< ma/alateral force resisting elements. Research into the Jl,ltacrrr-istics of these connections has been carried out h! Schuluand Magafia.‘6

It should be recognized, however, that the deformationoithese connections may change the effective deformn!loncharacteristics from those of a comparable monolithic \\1A commentary discussion from the 1994 NEHRP pro%r,\ashould be considered relative to this type of system, thoyhthe intention of the commentary is to address limitatiomenconnections for cast-in-place emulation. The NEHRP ill\cussion is paraphrased as follows:

Precast concrete frame and wall systems designed usingcast-in-place emulation with ductile joints shall satis@the following condition: The deformed shape of thestructure under specified lateral loads shall emulate htfor the same structure constructed in monolithic rein.forced concrete.

This requirement is intended to make the designer eon.sider the likely deformations of any proposed preca,tstructure vis-a-vis those of the same structure compose&’monolithic reinforced concrete before claiming that thrprecast form emulates monolithic construction. For exam.ple, a designer might propose a shear wall composed01multiple precast panels over its length and height that aconnected vertically but not horizontally. With ductile V!II.tical connections, that precast wall could be made to mtiall the requirements of emulation with ductile joints exc?pthat the deformed shape would differ from a monoliKsystem. That wall could, therefore, not be designed usin;this provision.97

It has been proposed by Dr. Alex Aswad (UniversityotPennsylvania) that a precast wall with vertical joints CFI.netted by ductile connections should actually be viewed RI.ative to a monolithic wall with coupling beams.” The problem, then, becomes the determination of the effecti\rreduction of stiffness of the wall system with the conruttions as a shear medium. This may be done using methodithat are already in the literature, or it may require the useliEa finite element analysis to classify the load-defonnatilincharacteristics for specific configurations. With reduceistiffness established, these walls can be analyzed for lotidistribution by stiffness using the equations above.

Without this detailed study, reasonable assumptions tibe made based on connection detailing. Connections inw~.tical joints may be considered either “soft” or “strong.” %Hconnections are ones that are detailed for ductility andin.tended to yield under the design event. These connection,

td to retain their load-carrying capacity at deformations intess of the elastic demand. They will provide the functioncontinuing to mobilize dead load resistance for overturn-even after yielding has occurred. When these connec-are applied in vertical joints, the wall stiffness can con-

natively be modeled as if each vertical pane1 actsependently. After yielding, each vertical pane1 has onlyown section for resistance as a limit. In considering theactive section, however, the component of “hold-down”ad load developed across the joint should be considered inveloping the stiffness properties.lo come cases, it may be necessary or desirable to developong connections. These connections are proportioned tontmue to carry loads elastically beyond the yield primarytchanism in the walls. The design forces for this type ofnnection would be based on VQ/Z with essentially elasticdmg. ASCE 7 considers a load combination with thesmic term 2R/S applied, removing the response modifica-factor reduction. This kind of joint has been achievedoverlapping hairpins and longitudinal steel in a cast-in-

ce joint that develops a capacity in excess of a compara-monolithic wall. as shown by Park.‘” For this condition,stiffness of the resulting wall is equivalent to the mono-ic wall.Using the model of the walls as cantilevered beams withffness appropriate to the connections to be used, the dis-bution of force< is derived. By applying the loads on ael-by-level basis and accumulating the effects of dis-cement from both shear and flexure by superposition,ry drift and total lateral displacements can be derived. Bycumulating the forces, the level-by-level shear and over-mine moments can also be calculated.his possible to use the selection of jointing and connec-details to define post-yield stiffness of groups of walls.

ith this technique. walls can be detailed to accept thefor which they can develop overturning resistance orloads for which such capacity cannot be mobilized. In

way, the designer of precast wall systems can “tune” thestiffness through detailing and connection definition ofpa ths .

13. EVALUATION OFLOADING EFFECTS ON WALLS

3ffirst concern in the evaluation of the effects of the dis-buted loads is the overturning moment. In an earlier step,capacity for the resistance to overturning, M,,,,, for the

n&date walls was determined. Now the overturning re-irement at the wall base derived from the analysis can bempared to that which is available.lfthe resistance to overturning is exceeded by the analysisgutrement. the design may not be at fault. There is a needlook further to determine if a positive hold-down and anpie foundation size can be developed to overcome anyual wall base overturning capacity deficiency.While the connections in vertical joints may reduce stiff-

they still may be fully effective in the mobilization ofoverturning resistance. Inability to determine a feasible

ution after considering the anchorage and foundation ca-city may indicate the need to add additional walls, or to

modify or extend walls in the locations already selected.For seismic loads, the prescription for the safety factors

for overturning is not directly specified. The BOCA seismicforces are already strength design levels. Section 1610.4.4simply states that “The building shall be designed to resistoverturning effects caused by the seismic forces determinedin Section 1610.4.2.” The implication here is that it isenough for overturning moment resistance, M,,,,, with theload combination reduced dead load, to exceed the calcu-lated overturning moment, M,,.

The BOCA Code further states that “The foundations ofbuildings, except inverted pendulum structures, shall be de-signed for foundation overturning design moment (LM$ at thefoundation-soil interface... with an overturning moment re-duction factor (5) of 0.75 for all building heights.” This im-plies that either the vertical load distribution overestimatesthe base moment or that some base “rocking” is acceptable,even if not directly considered or calculated as soil/founds-tion interaction.

From the analysis, the base shear is also determined. Ef-fective methods of anchorage must include the transfer ofthis shear. The principles of shear-friction may be applied.Although one side or the other of the wall may have anopened joint due to flexure and the anchorage acting in ten-sion, there will be a region of compression that is the combi-nation of the compression couple and the effective verticalforce on the wall. Tests have shown more ductile behaviorin concrete reinforced with spirals to protect the compres-sion zone.

This compression force contributes to the shear transfer.Additional connections or mechanically spliced reinforce-ment may be required at the wall-to-foundation joint to pro-vide additional shear capacity. This reinforcement shouldnot be concentrated at the ends with the flexural reinforce-ment, but should be uniformly distributed along the lengthof the wall.

Finally, the drift effect must be evaluated. In AC1 318-95,Section 21.7.1 requires that frame members that are as-sumed not to contribute to lateral resistance be detailed de-pending on moments induced by lateral displacements twicethe calculated displacement from factored lateral forces.

The BOCA Code, from NEHRP, has superseded this re-quirement and requires application not only for those struc-tures in areas of high seismic risk, but also for low and mod-erate risk structures that require lateral force analysis. Thedrift used to evaluate the P-A effects is the linear-elasticanalysis drift, based on strength design loads, multiplied byC,,, as discussed above. For shear walls with R of 4’12, this is4, and for R of 5’/2, it is 5. If the stability coefficient (O), ex-ceeds 0.10, P-A effects must be considered.

BOCA also provides for an upper limit to 0:

&Lo25-PC,, . -

When C, is 5, the upper limit is 0.10 if the shear demandto shear capacity (m is 1.0. With shear walls, the objective

is to derive the benefit of reduced lateral displacement andavoid the need to consider P-A effects, but the drift muststill be calculated and evaluated.

14. WALL DESIGNS

In general, walls in low and moderate seismic areas canbe designed using the requirements of Chapter 10, Chapter14 and Chapter 16 in AC1 3 18-95. In these areas, special de-tailing requirements are not imposed by the Code. For Cate-gories D and E structures, the special provisions for mini-mum transverse and longitudinal reinforcement and forboundary elements are included in Section 2 1.5.

Provisions based on NEHRP currently require that thesewalls meet the conditions imposed by emulation. The 1994UBC has changed the AC1 requirements for boundary ele-ments to reflect work by Wood” and Wallace and Moehle,”which has demonstrated the confinement requirements to beexcessive. These changes are currently under considerationfor the 1997 NEHRP provisions.

S. K. Ghosh has summarized the UBC provisions asfollows:“’

1. A shear wall is designed for flexural and axial loadconsidering the entire cross section, including the web(s), tobe effective, as in a short column. Shear resistance is stillprovided by the web, without contribution from the over-hanging flanges. This concept requires the connection to bealong the length of the wall instead of being concentrated atthe ends.

2. The wall is screened to eliminate cases where specialboundary zone detailing is not required. Walls having P,, 5O.lOA,? and either M,,IV,,I,,. S 1.0 or V,, I 31,,.hfi are ex-empt. Walls with P,, > 0.35P, are not permitted to resistearthquake-induced forces.

3. Two options are provided for cases where boundary el-ements with special details are needed: (a) Conservative ap-proach - provide boundary elements over 0.251,,. at eachend; (b) Alternatively, determine the compressive strains atwall edges when the wall is subject to design earthquakedisplacements, using cracked section properties. Provideconfinement wherever compressive strain exceeds 0.003.

The maximum spacing of transverse reinforcement is 6 in.(152 mm) or six times the longitudinal bar diameter in UBC,whichever is smaller, rather than 4 in. (102 mm) as in AC1318-89. Connection designs for walls should follow the de-tails that have been the subject of published research. Whenwall groups use connections that cross vertical joints, it isimportant to ensure that the total required capacity is met.

If the adjacent walls were modeled as independent can-tilevers for stiffness, it is sufficient to check the connectiondesign for the mobilization of dead load which is required tosupplement the direct load overturning capacity. If the wallswere considered as a unit, with an effective shear medium inthe joints, then the capacity will need to develop the sheartransfer derived from the analysis. Where it is desired thatthe wall group develop the elastic capacity as if the wallwere monolithic, it is appropriate to evaluate the “horizontalshear” demand on the connections through the calculation ofVQll for each joint. The final design should ensure that the

capacity of connections crossing the joint is capable ofsus.taining this shear flow.

15. DIAPHRAGM DESIGN

According to the BOCA Code, the floor and roofdi-aphragms must be designed to resist a minimum forceeqnato 50 percent the effective peak velocity-related acceleration(A,) times the weight of the diaphragm and other elementsof the building attached to it.” Although not specificallymentioned, it is also clear that the diaphragm must becapble of collecting and transferring the lateral force assipedto its level. The diaphragm must provide for both shearandbending in its plane resulting from these forces.

For precast concrete, it is common practice to differenti.ate between chord reinforcement and connections and sheaconnectors in connecting the flanges of double tees to formthe diaphragm. For wind load conditions, it is also commonpractice to use welded plate connections between doubletees to develop chord forces. Except in short spans andinlow load conditions, this method alone becomes questicm.able for higher seismic forces. For Category C structures,iiwill almost always be appropriate to at least place reinfotcr.ment in the topping pour strips at the ends of untoppeddtm.ble tees to embed and develop a continuous load path\GtImild steel reinforcement for the chord. At this level ofloadit is still possible to use welded connections at close spacin![4 to 5 ft (I .22 to 1.52 m)] to provide the capacity andt.dundancy required by shear transfer.

For structures in areas of high risk or high lateral fltnforces, it will be appropriate to develop the diaphragmrebforcement through the use of a full concrete topping withaminimum depth of 2’/~ in. (63 mm). With camber coosid.ered, the thickness of topping at the chords will be 3 to%in. (76 to 89 mm) minimum.

Inherent in the analysis described above is the assumpticthat the diaphragm acts as a rigid body. When precast cmCrete diaphragms are connected with welded connections.hwill be important to limit the aspect ratio of the plate tentsure that the accumulation of local strains does not resultitflexibility that negates the rigidity assumption.

In the past, the diaphragm has been treated as just anotkrelement of the lateral force resisting system. Its elementicomponents and connections have been designed usine~ksame forces and the same load factors as the rest ofth:structure. This approach may be flawed. In many cases.tt:diaphragm is designed as a statically determinate elemen.or one with simple continuities.

If there is yielding in the diaphragm, the load path forkera1 load transfer is compromised. If there is continuity,dwyielding may simply turn a “rigid” diaphragm into a “tldble” one. This could result in a serious redistributioniloads for which the walls or frames are not designed. Iftlt:diaphragm is a simple span, then the yielding of thedi.aphragm may result in failure. This may occur with oih:systems or materials that also assume a rigid diaphragm.

The California Northridge earthquake of 1994 creaitldamage in numerous structures that were designed to ILZ~modern codes and were presumed to be earthqiiat

%nk3 Some of the damage to precast structures, includ-parking garages, appeared to be induced by diaphragmblems. Problems that suddenly appeared to be critical

the Northridge earthquake were most often related todetailing in connections of diaphragms. Failures haveblamed on lack of reinforcement at critical locations in

Jhragm topping, lack of connection between precast ele-ts, poor connections between the diaphragm and the

wall system, and shear failure in precast girders whereprestress was less than expected.decent in-depth studies at Lehigh University, sponsored!he National Science Foundation, have shown that the di-rngm flexibility might also have been a contributing3 to distress in parking structures.J2 The diaphragm flex-ity was partially attributed to long spans between shear1s and to flexibility of joints between the double teesiing the diaphragm where mesh in the topping was thegconnection between the precast elements. In some:s the diaphragms, and particularly the reinforced top-;j, were found by analysis to yield in flexure before theporting lateral force was developed in the resisting shear!Is. Yielding of the diaphragms dramatically increasedstory drift that had to be sustained by the vertical load

iiling frame system.illgoing studies at the University of Wisconsin at Madi-Ihave identified the diaphragm regions at the ends of

:ps as critical areas for distress. In parking garages thattramps bounded on both sides by level parking bays, the:I bays act as two separate relatively stiff diaphragms.:level floors at the ends of the ramp are relatively flexible\veak but still attempt to connect the two separatehragms on either side of the ramp. If a relatively small

illlIt of eccentricity exists between the floor’s center ofs&e., the center of seismic load) and the center of loadiltance from the lateral force resisting system, then the

p end regions develop tremendous forces in trying toIiect the separate level floor diaphragms.he Lehigh studies”’ also noted that the sloped ramps

:d as a weak and flexible tie (brace) system betweenKS. The “bracing” forces, transferred by the diaphragmreen floors, induced an added internal force to the level!r segments at the ends of the ramps and caused some,of-plane twisting and flexure.Is a result of the diaphragm distress during recent earth-I;es, both NEHRP and AC1 have made changes or estab-:d subcommittees to re-examine the role of diaphragmsuccessfully resisting seismic loading. Current research is:cted to improve our understanding of diaphragm actionprovide improved design methods in the near future. At

ient, it is clear that more attention must be placed on di-rngm design, particularly where large openings causes of force concentration or split a floor into separate rel-:ly stiff diaphragm segments.ilthough the codes do not differentiate between designtile remainder of the structure and the diaphragm, and,ait strength design concepts to be applied, the designeruld consider the implication of ultimate strength designpossible effects of yield and ductility in the diaphragm.ielding is permitted with the aim of achieving inelastic

ductility, what is the effect on the assumption of rigid bodybehavior?

In regions of low and moderate seismic risk, this behaviormay be of less concern. In areas of high risk, however, it maybe appropriate to take a more comprehensive approach indesigning the diaphragm. There are two possible approaches.

First, the diaphragm can be designed to perform in an es-sentially elastic manner based on the code-derived loading.Under the load combinations in ASCE 7-95,“’ cited byBOCA 96,” the appropriate combination for this approach is:

E = +(2R/5)QE - 0.5AvSD where (2R/5) 2 1 .O

This would be the expected lateral force on the diaphragmif the walls or lateral load system remained essentially elas-tic. Then the connections could be designed to carry the re-sulting shear and moments.

The second approach is to make a detailed evaluation ofthe probable yield capacity of the vertical elements (thewalls) to determine how much lateral force the lateral forceresisting system could possibly transfer when it is at its ulti-mate strength. From this capacity, a reasonable vertical dis-tribution of forces to floors can be made using code meth-ods. The diaphragm would be designed for a yield capacityhigher than the loading applied for the lateral force resistingsystem. Diaphragm yielding such as is found in the Lehighstudy42 should be avoided by this approach.

The lateral force resisting system capacity must be evalu-ated by removing capacity reduction factors and consider-ing the actual ultimate tension capacity of steel in the com-ponents and connections. In precast systems where firstyielding in wall connections may be more reliably deter-mined, the jointed nature of the material may prove to be anadvantage.

From this load demand, it is possible to back out the forcerequirements for the floors, based on the code prescribedvertical distribution. The design would then produce a di-aphragm whose ultimate capacity exceeds the yield capacityof the walls and would ensure that the loads are distributedto the walls consistent with the assumptions. The wallswould be part of the system where first yield and ductilitydevelopment occurs.

If the load limit is set by a form of first yielding in the di-aphragm and does not create a mechanism, the redistributionof loads from loss of rigidity should be considered.Research into diaphragm connections and their load-deformation characteristics is currently being carried outunder the direction of Dr. Michael Oliva at the University ofWisconsin at Madison.

16. CONSIDERATION OF FRAMING ANDCONNECTIONS NOT PART OF THE LFRS

Even in structures in areas of very low seismic risk (Cate-gory A), it is recognized that there are minimum require-ments for ties and continuity, BOCA Section 1610.3.6.1prescribes these requirements, as well as minimum require-ments for the anchorage of concrete or masonry walls.Chapter 16 of AC1 3 18-95 has increased the detailed consid-erations of structural integrity connections in precast con-

struction, which then follows as a minimum requirement.Additionally, it is important that the structure be capable ofsustaining the drift calculated above. Connections, if notductile, must at least be flexible and accept inherent move-ment without creating collateral damage.

It is important for the design engineer to consider poten-tial conditions of unintended stiffness. If conditions dictatethat elements will receive forces or moments as a result ofmovement, even if they are not intended to be part of the lat-eral force resisting system, they must be detailed to acceptthese forces without failure. This is particularly true of verti-cal elements of the gravity load carrying framing, such ascolumns.

17. NEEDS FOR ADDITIONAL RESEARCHThis evaluation of the design and detailing of successful

shear wall structures constructed of precast concrete haspointed to the need for additional research to better quantifyour experience and to address unknowns. A summary ofsome of these topics should include the following:

1. True demand for ductility in stiff wall buildings.2. The effect of ductile connections to vertical wall as-

semblies to develop energy dissipation with reduced effects

on the body of the wall components. The cftccts (111 therda~tive stiffness of structures with walls so constructed.

3. The actual period of jointed systcrns with tiotlllnra!connections.

4. The energy demand for longer pcriotl prerrn\;tructtirec .

5. Deformation demand and control for precast coxrdshear wnll buildings.

6. Diapl~rqyn behavior after connection yielding.

18. CONCLUDING REMARKS

Historically, properly designed and constructcrl prca\lipbstressed concrete structure\ with shear wall< have pcrli~rmrdwell. In reviewing the design of these struoturcs, thr ut@features of precast/prestressed concrete constructlot h,i\cbeen considered. Although prescriptive design rules Inr pre.cast system design are still developing. thcro is sufficbt In.format ion to build such structures safely and ccono~n~c;~II! inlow and moderate seismic regions. As the results ol’ OII-~~III;

and future rcsettrch become tlanslated into practical ~~~I~III~IIISprccast/prestressed structures with shear walls will be builtmore frequently in the highest seismic arcas.

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APPENDIX - NOTATION

A, = effective peak acceleration coefficient (derived fromseismic maps)

A, = gross area of wall sectionAi = shear area of Wall iA, = effective peak velocity coefficient (derived from seis-

mic maps)C = exposure coefficient

C, = deflection amplification factorD = dead load (unfactored)E = earthquake load (unfactored) or modulus of elasticity

Ei = modulus of elasticity of Wall iFtiX = defined in equation in Section 12. IEij~ = defined in equation in Section 12. I

f,t= specified compressive strength of concreteG = shear modulush = roof height

hj = height of Wall j above its baseIzV = story height

h,T.r = story height below Level xI = moment of inertiaji = moment of inertia at Wall iK = stiffness factorKf= flexural stiffness

Ki,r = stiffness of Wall i in x directionKi, = stiffness of Wall i in y directionK,v = shear stiffness

KY = stiffness in y directionL = live load (unfactored)

I, = length of wallIV, = overturning design moment at foundation-soil

interface

4, = calculated overturning moment (see Table 2)

Mm = moment resistance to overturning

W = torsional momentMN, = accidental torsional momentM,, = ultimate moment load

P,, = total vertical design load at Story Level x

P(! = dead loadI I .,P,, = ulttmdte dxtdl load

Q = moment of area in shear stress calculationQE = earthquake load effect (ASCE 7)

R = response modification factorR,,. = response modification factor (UBC)(see Table 2)

S = site coefficientT = fundamental period of a buildingU = ultimate load (general)V = see Table 2

V,, = ultimate shear loadV’, = seismic shear force between Levels x and x - IWI = lateral load in y direction

wVj = lateral load in y direction at Level jx = distance of Wall i from center of stiffness in )‘dirff.

tiony = distance of Wall i from center of stiffness inadiree.

tion2 = seismic zone factor6= story drift by elastic analysisA = design story drift/3 = ratio of shear demand to shear capacity for story br.

tween Levels x and x - l (BOCA)0 = stability coefficientr = overturning moment reduction factor (BQCA)