Seismic Behavior ofLarge Panel Precast

Concrete Walls:Analysis and Experiment

Ray W. CloughNishkian Professor Emeritus

Department of Civil EngineeringUniversity of California

Berkeley, California

Faris MalhasFormer Graduate Student

Department of Civil EngineeringUniversity of Wisconsin

Madison, Wisconsin

Michael G. OlivaAssociate ProfessorDepartment of Civil EngineeringUniversity of WisconsinMadison, Wisconsin

S tructures have traditionally been de-signed for earthquake resistance by

providing a sufficient strength level toresist a prescribed static lateral load.Though previous codes only defined therequired lateral loading, there was animplicit assumption that a certainamount of post-yield deforinabilitywould be available within the structurewhile a yield level resisting capacitywas maintained. Recent codes have spe-cifically addressed the need for ductilityto be provided during the design inconjunction with the specified designIoading. It is unclear, however, whatlevel of ductility might actually beavailable in precast concrete construc-tion. These recent changes have leftproducers of precast components pon-

dering the problem of how to provide ausable product and caused design en-gineers to become reluctant to designprecast structures in seismic regions.

The competitive advantages of precastsystems in building construction havecreated a demand for their use in seis-mic as well as nonseismic locations.Moreover, sonic areas of previous non-seismic use have recently been rezonedas seismic regions and seismic resis-tance design requirements will nowoften control over wind design criteria.The problem in design is that precastsystems typically have weak connectionregions. They cannot currently be easilyjoined in a manner which would re-semble monolithic concrete and still beassembled efficiently. Their behavior

42

under strong seismic excitation willgenerally be substantially different frommonolithic systems, but their actualductility level and failure sequence hasnot been well documented yet. The re-search program described here investi-gated the performance of one type ofprecast system, the panelized buildingsystem, under replicated seismic motionto determine the demand for ductilitywhich would he produced and the fail-ure condition which might develop in awall of a prototype 13-story building.

Use of Precast ConstructionPrecast concrete has been used in

markets around the world to satisfy thetremendous demand for housing by pro-viding a rapidly built system using fac-tory fabricated quality controlled com-ponents. The competitive edge gainedby precast manufacturers in the UnitedStates and other countries has comefrom the development of refined modu-lar building systems with standardizedcomponents and simple connections.

The panelized system is an exampleof a method which uses standardizedwall and floor/roof precast panels con-nected together to form a complete boxtype structure without a separateframework., While individual panels inthe system may be connected togetherby a variety of means, all the connectionschemes tend to form points of weak-ness in the structure.

Seismic Resistance and Designof Precast Structures

Many of the precast systems in usethroughout the world are not well suitedto resist the force and deformation de-mands caused by earthquake loading.The systems were often originally de-signed for nonseismic regions, but theiradvantages led to later use in low risebuildings in seismically active areas.Now, high rise buildings are being builtwith precast component systems in thesame active regions. The use of precast

Synopsis

The demand for economicallycompetitive precast building sys-tems has been increasing aroundthe world at the same time that con-cerns have grown regarding theirseismic resistance capacities. Re-cent codes have rezoned portionsof the United States to reflect higherseismic hazard and to require seis-mic resistant design where windloading prevailed in the past. De-signers have been left in the dilem-ma of having to provide seismic re-sistant design without having codeprovisions specifically addressingthe unique characteristics of precastconstruction.

The seismic resistant capacitiesof one form of precast construction,the large panel wall system, aredescribed in this report. An inves-tigation consisting of shaking tabletests with earthquake motion andsubsequent analytical investigationsof the seismic response are in-cluded.

A set of conclusions and sugges-tions for improved performance ofprecast large panel construction aregiven, based on the tests, analyticalsimulations, and results from otherresearchers, along with a detaileddiscussion of problems in precastconcrete design and behavior of theprecast concrete system.

systems for lateral resistance in strongseismic areas has been precluded inmost areas of the United States by codeprovisions which would require con-nections similar to monolithic cast con-crete. The simplicity of the connections,which makes precast concrete econom-ically viable, causes a lack of continuityin stiffness and concentrated deforma-

PCI JOURNAt-1September-October 1989 43

FElAs1ICI 0 .i^ I

Q 1 FYIELpOJ ^

ALAS11C AIaD OMAXMUM

DISPLACEMENT DISPLACEMENT

Fig. 1. Assuming that the earthquake transfers equal energy (area under curve),elastic system (left) has small displacement, inelastic system (right) has largerdisplacement.

tion demand in some locations may de-velop during an earthquake.

Precast panelized wall systems have alack of continuity in the horizontal con-nections between vertical wall ele-ments. The wall elements are reliedupon to provide both vertical loadcarrying capacity and lateral load resis-tance as shear walls. Yet, the shear wallsare characterized by cantilever beamtype behavior with a lack of redundancy.Under lateral load the panel wall sys-tem's ability to carry vertical loads maybe jeopardized by the wall's naturaltendency to yield and deform inelasti-cally within a few weak horizontaljoints.

Monolithic construction of joints maybe appropriate in strong seismic zones;however, in zones of lower seismicity,structures with weak joints may stillperfprm satisfactorily. The present diffi-culty for design is in determining whatrelation between strength and ductilitymust be provided in such joints. Unfor-tunately, there is not a single answer tothis design dilemma. For any givenyield strength which a designer mightprovide within a joint, the earthquakeenergy level will dictate how much de-form nation capacity at the yield level (i.e.,

ductility) the joint must have availablein order to perform without failure. Ifthe earthquake transfers a specificamount of energy to the joint, thatenergy must be accommodated withinthe joint by either remaining elastic,with a large force and small deforma-tion, or by yielding, with a lower inter-nal force but larger deformation. Thisrelation may be understood by viewingFig. 1.

The relation described above is theapproach used in PCI Technical ReportNo. 5,' where a simplified approach toseismic resistant design of precaststructures is outlined. The PCI methodis not necessarily readily usable at thistime, however, because it may be dif-ficult to envisage a single-degree-of-freedom inelastic response mechanism.Thus, it could be hard to determine howmuch energy will be transferred into astructure, and more importantly, theactual yield capacity and available duc-tility in most common precast connec-tions is not known yet. The yield leveland available ductility must both beknown to use the PCI approach or to as-sign a proper force reduction factor (R)in design by current code equivalentstatic load methods,

44

Objectives of Research ProgramShaking table tests would provide the

best means to study the seismic behav-ior of precast systems, particularly therelation between provided strength andthe demand for deformation induced bythe earthquake. Static tests of individualcomponents or joints can provide in-formation on stiffness and strength, butnot the amount of deformation whichwill be needed. The actual forces trans-ferred into the structure and deforma-tion developed during an earthquake,however, depend on the strength andductility of the structure. The actualforces and deformation can only bedetermined by dynamic shaking testsor analyses. The first objective of thisinvestigation was to quantitatively mea-sure the response of a large scale panelwall system during an earthquake.There have been no previous shakingtests of large scale precast wall assem-blages. The shaking table tests con-ducted in this program were intended tofurnish a complete quantitative de-scription of inelastic mechanisms andthe effects of such mechanisms on thesystem's dynamic response.

The second objective of the researchprogram was to test analytic methodswhich have been proposed for use inpredicting the inelastic seismic re-sponse of large panel precast wall as-semblages. Acceptable analysismethods could then be used to predictthe response of a complete prototypewall system which would be too expen-sive and complex to test. Schrieker, Bec-ker, and kianotish 2 °3.4 developed com-puterized techniques based on statictests of wall connections and on as-sumed forms of system deformation forestimating inelastic response in precastwall systems. Shaking table experi-mental results combined with correla-tion studies would check the abilities ofthe existing programs and aid in im-proving analytic techniques. Analyticmethods could then be used to developimproved designs by evaluating per-

formance of structures whose joints havevarious yield strength-deformationlimits.

Scope of Research ProgramThree different 3-story assemblages of

wall panels were tested under earth-quake motion on the shaking table.Since the cost of shaking table testinglimited the number of specimens whichcould be investigated, a few basic con-figurations had to be selected whichwould represent the most common typesfound in panel wall structures. Eachspecimen was a single wall section builtat one-third scale to represent a portionof a 10 to 20-story precast shear wall.The measured test deformations in thewalls were compared with computeraided predictions using various analyticmodels. These correlation studies indi-cated which specific inelastic mech-anisms had to be accurately representedin the analytic model. The analyticmodel was then used to predict the be-havior of a 13-story prototype wall sys-tem.

This report describes the basic re-search and development work whichhas been completed on large panel pre-cast wall systems. The shaking table testprogram is outlined with an explanationof the model design, the testing system,the test procedures, damage observa-tions and measured test results. The an-alytic correlation work is then summar-ized with a discussion of modelingtechniques. Finally, knowledge gainedfrom the tests and analytic work is usedto predict likely seismic limit states for a13-story prototype wall system,

LARGE PANEL PRECASTWALL SYSTEMS

Large panel building systems arecomposed of vertical wall panels whichsupport horizontal roof and floor panelsto form a box like structure as dia-grammed in Fig. '? The vertical panels

PCI JOURNAL/September-October 1989 45

are stacked and joined to create axialloadhearing shear walls while the hori-zontal panels act a diaphragms andgravity load collecting roof and floors. stems, Martin, Patnian and Zcck57have assembled reviews of various basiclarge panel precast systems and thetypical joint configurations.

The primary difference between thesystems currently used is in the mannerin which the vertical and horizontalpanels are joined. Panel wall systems inthe United States typically use a "plat-form" construction in which hollow-core slabs form a horizontal layer simplysupported on the wall panels below.The tipper wall panels then rest on theplatform formed by the hollow-corefloor slab as shown in Fig. 3. Panel sys-tems in Europe and elsewhere, how-ever, have connections which fre-quently use cast-in-place concrete toform the joint between the wall andfloor panels. Nearly all systems suffer asimilar weakness when used for resist-ing seismic loads, namely, economicallyefficient systems have only a limitedamount of vertical reinforcement con-tinuous across the joints and seismicloading creates shear and flexural de-inands which can easily exceed thecapacity of that steel.

Earthquake Performance andExisting Data

The existing use of large panel precastsystems was obviously accomplishedwith a considerable amount of testingand analysis. Fortunately, very few largepanel buildings have ever been sub-jected to strong seismic motions. Thelack of records of performance, however,has created uncertainty regarding theirability to withstand the large forces orenergy levels which could result. Thebest account of' large panel buildingperformance may be found in the after-math of the 1988 Armenian earthquakenear Yeravan in the Soviet Union. Thelarge panel building system used in Ar-

Fig. 2. Box like structure formed by alarge panel cross wall system.

menia was very similar to the systemtested in this research project.

Information from disaster inspectionshas indicated that three of the hardesthit cities (Spitak, Leninakan andkirovakan) experienced strong shakingover period ranges which would likelyencompass the natural period of largepanel buildings. Not a single large panelbuilding, however, was categorized ashaving collapsed or been damaged to adegree requiring demolition though anumber of 4 to 9-story large panelstructures existed.* In the same areanumerous precast frame structures col-lapsed and damage was even found inthe steel frame system of an industrialbuilding. Unfortunately, none of thelarge panel buildings was instrumentedand little is known regarding their de-sign strengths and likely level of seismicinduced shear force.

There were a number of Bulgarianlarge panel buildings which were notdamaged by the 1977 Vrancea earth-quake in Romania,' but the ground mo-tion was reported to have been pre-dominantly long period which wouldnot excite short period shear wall struc-tures. Shapiro 9 noted that significant

"Harris, J. R., "Precast Building PerformanceDuring the Armenian Earthquake," presentedduring PCI Committee Days, April 13, 1989.

46

4p n' WALL PANEL•a

DRY PACK n o'•F' PRECAST SLAB

GROUTUOUSAL

Fig. 3. Platform type horizontalconnection used in the United States

damagedid develop in 2 and 4-storylarge panel structures during the Gazli1976 earthquake in the Soviet Union.Some of the vertical panels had residuallateral shifts of 4 to 6 in, (10-15 cm) andfloor slabs had slipped off of walls.Damage to panels themselves was neg-ligible. It was postulated that the panelshifting was due to the opening of hori-zontal joints during the initial motion.Then aftershocks caused gradual pro-gressive displacements. Shapiro alsonoted that a portion of an actual 9-storybuilding had been tested with shakers atthe top to a displacement of 1.8 in. (4.6cm), causing joint cracking and neardoubling of the first natural period.

Other test work has determined thenatural periods of in-situ full sizebuildings through low amplitude shak-ing with vibration generators. Measuredperiods of buildings with 4 to 12 storiesranged between 0,17 and 0.52 seconds.Low amplitude tests, however, reflectthe natural periods in an "uncracked"state since axial loads generally keepexisting cracks closed. Polyakov 1O notedthat large changes in period can developwith large motion and damage- Manytests ofjoints between panels have beenreported, such as those by Hanson, Vel-knv, Verbic" ' x ' a and others. Subas-sernblage tests, with statically appliedloads, have been completed by Borges,

Gavrilovic, and Suenaga. " •15• `6 Only tworeports described dynamic testing ofsubassemblages; Harris' 7 has tested 1/16

scale models on a shaking table andPolyakov 1 ' noted that vibro-platformtests had been completed in the SovietUnion but he did not provide any mea-sured data. Though there is widespreaduse of precast large panel construction,there apparently have been virtually nofull scale investigations to determinethe capacity demands which might bemade upon the systems during earth-quakes.

Methods for Predicting ResponseThe precast walls act in a manner

similar to monolithic shear walls in re-sisting axial loads and shear forces in-duced by wind or low amplitude earth-quakes. As long as the internal forces donot cause nonlinear response, a precastwall could he analyzed like a monolithicshear wall. When precast large panelwall systems are used in low seismic re-gions, it may be practical to design themfor elastic linear behavior. Elastic be-havior may be achieved by ensuring thatthe forces determined from an elasticanalysis, using a response spectrumwhich has not been reduced, are lessthan the panel and joint strengths.

Nonlinear response may be createdwhen either the shear force or over-turning moment surpasses a Iimitingvalue. Horizontal slip starts in the walljoint when the shear becomes too large.The vertical reinforcing steel yields androcking of the panels starts when themoment becomes too high. Based on thelimited existing test data, it appears thatslip and flexural distortion are the twomechanisms of damage associated withnonlinear response in a simple wall unitwhen called upon to resist strong seis-mic motion. These two types of concen-trated deformation are not common inmonolithic shear walls because theuniform continuous vertical and hori-zontal steel tends to distribute the

PCI JOURNAL/September-October 1989 47

flexural and shear distortions over alarger region.

Several investigators have used theavailable test data and analytic tech-niques to attempt to predict and investi-gate the behavior of wall systems duringstrong ground shaking. Llorente, andPowell and Schricker 38.19 proposedanalytic means to investigate the sensi-tivity of panel wall response to variousdesign parameters during strong earth-quakes.

Llorente and Becker" examined theeffects of the postulated rocking motionin a series of analytic studies. They de-scribed rocking as an undesirable typeof motion since acute shear concentra-tions at the neutral axis and the severecompressive strains at the closed end ofthe rocking joint may induce failure andlateral instability. Despite this possibledrawback, Llorente's analyses indicatedthat rocking may be helpful to the wallsystem. It exhibits an isolation behaviorwhich limits the force which can betransferred into the wall, and softens thewall system, moving the period to alower point in a response spectra.

Shear slip has been determined ex-perimentally to be a function of the totalvertical axial load being transferredthroiigli the joint." Since a major portionof the shear may be resisted by purefriction before slip starts, it is likely thatslip might occur in joints near midheightof the wall where the total axial force isless than at the base but shear is stillhigh. Llorente investigated the effects ofshear slip in analytic studies and con-eluded that while it represents a sourceof energy dissipation and force isolation,it should not be counted on as a reliableresistance mechanism because ac-cumulated unrestrained slip could re-sult in enough eccentricity to threatenthe stability and integrity ofa building.

SHAKING TABLE TESTSA series of tests was conducted on

three large panel wall models as part of

this research program to determine whatdeformation demands and base forceswould be developed in such jointedstructures under earthquake motion andwhether the structure could withstand astrong motion without developing in-stability. The specific design of connec-tions in the test models was intended tobe an improvement on previous forms ofjointing for better seismic behavior. Thejoints were not built in a "platform"fashion as most connections in Americansystems, but many facets of the responseof the tested joints could occur in plat-form or Other types as well. The jointswere purposely designed to avoid ashear slip mechanism, for the reasonsnoted above, while the platform type isexpected to fail first in slip. 21 Only asmall portion of the description of thetests and deformation mechanisms inthe large panel systems can he pre-sented here; a detailed description ofthe test program may be examinedelsewhcre.22

Test Models

The three large panel test specimenswere one-third scale, 3-story high wallsegments axially loaded to represent aportion of a wall near midheight of a 15-story building. Each of the specimenswas composed of three individual I-story high wall panels. Every one of thesubassemblies was 10i.6 in (2bl cm)high overall. 'i rue scale modeling wasemployed at a size sufficient to allowuse of normal concrete, though withsmall aggregate. The overall mass of themodel had to be artificially enlarged topreserve the correct force ratios. Thefirst model specimen was a simple as-semblage of three wall panels. The sec-ond model was similar to the first butalso included short perpendicular"flange" walls at the ends of the mainwall. The third wall included dooropenings and strengthened lintel beamsbut it will not be included in the testsdescribed here.

48

OF

O° WALL PANEL

0

GROUTED JOINT o °

o ° a a 'o; p 9

Q

v PRECAST SLAB

° p CONTMUOUSVERTICAL

Ij WALL PANEL i STEEL

I JOINT

KEY CONTINUOUSVERTICALSTEEL

PANEL

Fig. 4. The wall connection used in the test specimens.

The simple wall had a total verticalreinforcing content across horizontaljoints of0.4 percent and the flanged wallsystem had 0.7 percent of the wall crosssection area. In each case the verticalreinforcement which continued acrossthe joint was concentrated at the ex- #treme ends of the walls as detailed inFig. 4. The precast panels contained anadditional amount of well distributed Jj ^^ ►horizontal and vertical reinforcement --==throughout their interiors. Each test --specimen had a steel platform and a set - _

of mass blocks attached at its top to pro- . J Rr."

vide the desired level of internal axial s - -^^force and to induce lateral inertial forces `. IL' r`'in the correct scale ratio. The walls were t

provided with an accessory lateral sup-port frame which allowed vertical andlateral motion parallel to the walls butprevented out-of-plane motion. A test Fig. 5. Three-story model on the shakingmodel and support frame are shown in table with a lateral support frame andFig. 5. mass blocks above.

PCI JOURNAL(September-October 1989 49

Testing SystemThe tests were performed in the

Earthquake Simulator Facility at theUniversity of California EarthquakeEngineering Research Center in Rich-mond, California. The 20 ft (6.1 m)square shaking table was controlled toreproduce the horizontal motion of a re-corded earthquake. The wall modelswere attached to a special foundationwhich was bolted directly to the shakingtable.

Instrumentation was selected tomonitor three types of dynamic re-sponse; (1) shaking table motion, (2) ac-celerations and displacements of themodels, and (3) local deformations andstrains within the models. Horizontaldisplacements and accelerations weremeasured at the base, at each floor level,and at the top of each assemblage. Localdeformations measured within the testspecimens included the shear slip atvertical and horizontal joints, uplift athorizontal joints, and panel shear dis-tortions. Strains were measured in se-lected reinforcing bars. The specialfoundation included a set of force trans-ducers to determine the magnitude ofbase shear transferred into the structure.

Test ProgramEach of the specimens was subjected

to a series of simulated earthquake mo-tions. The applied motion was propor-tional to the N-S component of theearthquake recorded at El Centro,California, in May 1940, the intensitybeing modified by adjustment of thetable control system. The El Centroearthquake had considerable energy inthe period range near 0.5 seconds char-acteristic of prototype structures. Therecorded earthquake record had to betime scaled to the ratio determined bythe true scale modeling of the testspecimens. In each case the test signalwas first applied at a low intensity todetermine the system's elastic responsebehavior. Then each specimen was

subjected to a base motion of sufficientintensity to cause appreciable damage.The test sequence for the two speci-mens is summarized in Table 1.

The free vibration frequencies of eachtest structure were measured before andafter each simulated earthquake to as-sess the degree of damage inducedduring the test. A low intensity "whitenoise" motion was applied through thetable and the acceleration response ofthe structure was analyzed by Fouriertransform procedures.

Damage ObservationsObservation of the specimens during

and after the tests provides an indicationof what damage and inelastic action oc-curred, supplementing the instrumentaldata. There was no visible damage dur-ing the low intensity shakes. A summaryof damage for the intense shakes fol-lows.

Simple wall: The obvious visible re-sponse mechanism was rocking motionassociated with uplift at the lowest hori-zontal joint. No shear slip was noted. Up-lift was accompanied by apparent com-pression damage at one end of the wall.Two of the through joint reinforcing barsat the damaged end of the wall buckledand the third had ruptured. Fig. 6 is a pho-tograph of the damaged area. Only minorcracks existed at the opposite end.

Flanged wall: The visible responsewas again dominated by rocking asso-ciated with alternate uplifting of thewall ends and flange walls. The princi-pal damage consisted of crushing orspalling at one end of the wall and in theadjacent flange; lesser but similar dam-age occurred at the opposite end. Care-ful examination of the damaged end re-vealed that all but one of the five verti-cal continuous bars in the horizontalconnection below the flange and wallhad ruptured; the remaining bar wasbuckled. The! opposite end had oneflange tar ruptured and two other barsbuckled.

50

Table 1. Test sequence

TableTest acceleration

Model No. Earthquake (g's)

Simple 1 EICentro 0.18wall 2 El Centro 0,67

Flanged I El Centro 0.22wall 2 El Centro 0.69

Test Results

The reduction in frequency (Table 2)shows that the stiffness decreased bynearly 50 percent as a result of the dam-age during the strong tests and verifiesconcerns noted by Polyakov andHawkins 1°' regarding the large periodchanges which can occur in panel wallsystems. Hawkins suggested that theground motion may not be significantlyamplified in panel structures if theirperiod remains at 0.3 seconds or less (a

Table 2. Measured natural frequencies.

ModelTime of

measurementFrequency

(Hz)

Simple Prior to test 5.20wall After Test 1 5.10

After Test. •.9{)

Flanged Prior to test 6.30wall After Test 2 4.32

period of 0.16 seconds for a one-thirdscale model, slightly lower than the ac-tual measured period). Softening canlengthen the period if joint slip or rock-ing starts and could result in the struc-ture showing major amplification of theground motion.

The instrumentation confinned thepredominance of the rocking responsedescribed above during the strongshakes. The structures remained elasticduring the low intensity tests. Peakdeformation quantities in Table 3 indi-

Fig. 6. Close-up view of damage at the end of the simple wall; the bar which is notbuckled had ruptured.

PCI JOURNAJSeptember-October 1989 51

Table 3. Peak deformation quantities.

imple Flanged wall

' Test 1 Test _>Loading conditions

'fable acceleration (g's) 0.18 0.67 0.22 0.69

Acceleration at top of wall (g's) 0.28 0,80 0.4.2 1.08

Iiisplacementattop of wall (in.) 0.13 1.48 0.09 0.80

0.02 0.70 Not (1.36Uplift at end of wall (in.)available

Base shear (kips) 7.4 15.7 9.1 21.9

Base moment (in.-kips) 1525 3516 1688 4056

cate that an amplification of the groundmotion occurred in the structure. Therocking response during the strong mo-tion tests was indicated by the upliftgages at the base of the wall across thelowest joint. The nature of the upliftmay he deduced from Fig. 7 whichshows the measurement at three pointson one side of the simple wall. Gage U6is located at the extreme end; it clearlyshows that the wall uplifts a maximum of0.34 in. (0.86 ern) as the top displacessideways in one direction. It is also clearthat the uplift never completely returnsto zero. The displacement at Gage U8shows similar but less uplift in synch-ronization with Gage U6 indicating thatthe point of rotation is near the oppositeend of the wall. The smaller peaks,which are out of synchronization withGage U6, occur when the opposite endof the wall uplifts to a maximum of0.70in. (1.8 cm).

The second type of primary motionmeasured at the lower joint was slip, i.e.,sliding of the two vertical walls relativeto each other. The average sliding mo-tion, though only reaching an amplitudeof 0.04 in. (0.1 cm), tends to be toward thesouth — the end which experienced con-crete damage and spalling.

A final indication of the performanceof the simple waIl may be obtained froman examination of its base shear vs. top

displacement history as shown in Fig. 8.Overturning moment controlled therocking response, but the shear waslinearly related to the moment. Themoment-shear ratio was 204 in. or 2.7times the length of the wall. First yieldin the joint steel was measured at a mo-ment of 1790 in.-kips (202 kN-m). Thecalculated yield strength of the wall wasestimated as 2070 in.-kips (234 kN-m) orwith a shear of 10.1 kips (45 kN) usingnormal beam theory. The difference inpredicted and actual first yield was a re-sult of the plane section assumptionused in nonnal beam theory, The behav-ior of the wall was essentiall y linear untilthe shear exceeded –12.8 kips (-57kN). On reversal, the stiffness decreasedand further damage occurred at +15 kips(67 kN), The sudden drop in shear cor-responded with rupture of one of thejoint bars. Oil subsequent cycles the po-sitive shear never reached the samemaximum value.

Similar rocking and slip response wasmeasured in the tests of the flanged wallthough its stiffness and strength wereobviously higher than in the simplewall. A very small amount of defonna-tion occurred in the vertical joint be-tween the main wall and the flanges butdid not dissipate significant energy. Afull description of the test results anddata measurements is given in Ref. 24.

52

C)

C-0GasZ

cnCDaCD3am

0C)0a-CD

CD

CDCD

0.

8.e 6.0 12.0

time - seconds

UPLIFT AT U6

0.r

U6 U1 U13 o t41 Uf2 0.

LOCATION KEY

0. 0.

0. r e.

J

J

0. Qa

0.0 6.0 12.0

UPLIFT AT U8

18.0 24.0

lime - seconds0.0

6.0 12.8 18.0 24.0

time - seconds

UPLIFT AT U7thW Fig. 7. Uplift histories of the panel at the lower connection.

16,

4a^

a.

QQwI e.

w

a-s.

-16.e--1.5 -e.8 @8 a8 1.6

TOP DISPLACEMENT (inches)

Fig. 8. History of the base shear plotted with the top displacements.

ANALYTIC CORRELATIONPROCEDURES

Extensive computer aided studieshave been completed at the Universityof Wisconsin using data from the shak-ing table tests to verify the ability of anexisting analytic method. The experi-mental vs. analytic correlation studiesidentified the essential response mech-anisms which had to be duplicated forsuccessful prediction of the seismic re-sponse of the precast panel wall systemat various levels of excitation.

Analytic Modeling

The primary portion of the analyticmodel consisted of the three stories ofprecast walls and joints. A lumpedtranslational and rotational mass wasplaced above the wall elements and at-tached to the wall with rigid links tosimulate the mass blocks above the testspecimens. The wall elements were con-nected to a spring foundation whichmodeled the flexibility of the actualfoundation member and force trans-ducers. Finally, the shaking table itselfexhibited a detectable amount of pitchduring each test (due to structural inter-

action) even though the command mo-tion specified only a horizontal compo-nent. The analytic model included arigid table with rotational mass sup-ported on springs to simulate thedynamic table pitching. This entiremodel is diagrammed in Fig. 9.

Certain assumptions were initiallymade during the modeling: (1) the pre-cast panel elements were assumed tomaintain a linear stiffness and remainelastic; (2) all inelasticity was assumedto be concentrated within the connec-tion regions, and (3) the connection re-gions were assumed to be precrackeddue to likely shrinkage between thejoint grout and the precast panels.

Techniques for Joint Modeling

Horizontal joints between verticalprecast panels were assumed to be theIocations where all inelasticity wouldtake place and the analytic joint ele-ments had to represent the inelasticmechanisms likely to develop. Insuffi-cient information exists at present tomodel the entire range of possible jointbehavior with a single analytic 1. con-nection element." Two approaches havebeen used to model this connection re-

54

C)

0

Z

CDDCD

2O0CrCD

CD

CD

TOP MASS

4 _czZI j4::::: RIGIDLINKS

RIGIDLINKS TOP CAST

WALL

UBSTRUCIURE 0o------- 6 PANEL WALL

PRECRACKEDJOINT

(ELASTIC)

---------------- 1

.----- ------ -^------ - A -------INELASTICNONLINEAR JOINTSPRVNG-SYSTEM

FOUNDATONRIGID SHAKINGTABLE

• ACTIVE NODE° PASSIVE NODE --• TABLE PITCH ^a

SPRINGS _R

SPRING C SPRING 5d SPRING ST(CONCRETE) (SHEAR KEY) (STEEL)

Detail A: bottom horizontal jointflexural spring-system

Precast concrete panel wallfinite element model.

DETAIL 'A'

CAST WALL

n Fig. 9. Diagram of the 3-story analytic model.

gion: (1) as a continuous media, withmiiltinode inelastic rectangular contactor interface finite elements, or (2) withdiscrete inelastic nondimensionalspring elements. Becker, Mueller andLlorente 3 discuss the advantages ofusing interface or contact finite ele-ments in obtaining gradual opening ofthe joint and avoiding impact problemsand the required discretization if springelements are used.

Regardless of the modeling approach,the simulation of the joint concrete mayneed inclusion of initial compressivestiffness, crushing with stiffness andstrength degradation, and opening of agap when under tension. The concreteis represented with finite elements or aseries of discrete springs. The verticaltension reinforcement is usuall y ex-plicitly modeled with truss elements.Shear slip resistance may be included inthe continuous concrete finite elementsor by special nondimensional slipsprings.

The use of discrete springs in the formsuggested by Powell and Schricker 19 wasapplied in this study with a modifiedversion of the DRAIN2D-MkII $ com-puter program. Use of discrete nondi-mensional springs makes it easier to de-velop new elements to model specificcharacteristics.

Our test results clearly indicated thatsuccessful modeling would requireduplication of the rocking mechanismwith attendant gap opening and closing.Moreover, it appeared as if the modelshould be able to reproduce rupture ofthe steel reinforcement across the hori-zontal joint since rupture occurred veryquickly once the bars started yielding.As a consequence of these two require-ments, particular attention had to begiven to the form of the timewiseanalysis algorithm since each of the pre-ceding events would abruptly alter thestiffness of the system. If a constant timestep method were used, small time in-crements would be necessary, resultingin significant computation time. In other

joint configurations, without large shearkeys, shear slip may he more significantand require special simulation.

ANALYTIC CORRELATIONRESULTS

Elastic BehaviorThe earthquake motion reached a

peak acceleration of 0.17g during theIow amplitude test of the simple wall;this level of motion was not sufficient tocause visible cracking or yielding. Ex-cellent correlation was achieved be-tween the measured experimental andpredicted top displacements throughoutthe time history when the correct sec-tion properties were used in the analyticmodel. To obtain "correct" sectionproperties, the lower panel's effectivecross section had to be able to changefrom gross to cracked when momentsexceeded the cracking level. It was thennecessary for the model to regain itsgross section stiffness as a condition formatching the low amplitude responsecycles after cracking had occurred. Theaxial gravity load causes the wall to re-develop its gross section when theoverturning moment is low.

Inelastic BehaviorSimulation of the simple wall's in-

elastic response during strong groundmotion required an exceptionally com-plex modeling approach. The system'sresponse involved cracking of panels,yielding of steel in the joint, gap open-ing, buckling of steel rods, rupturing ofsteel, and spalling and destruction ofunconfined concrete near the buckledvertical bars. The effect of these variousmechanisms in modifying the predictedresponse of the wall system is discussedin this section.

The simplest inelastic model used bi-linear yielding truss elements to repre-sent the joint's steel reinforcement, Thepredicted response to the 0.67g earth-

56

1.5H

L 1.0UC

I-zLU

w 0.0U4

U)

0 -1.0

5 1CI

1

IIf 5'

Cl

Ii

4RUPTURE 5„'

SOLID = EXPERIMENTALDASH = ANALYTICAL

-1.5

0.0 1.0 2.0 3.0

TIME (seconds)

Fig. 10. Response history of the analytic mode( without rupturecapability (rupture occurs at 1.42 seconds in the test specimen).

1.5SOLID = EXPERIMENTALDASH = ANALYTICAL

1.0U

I-2

W

LU2 0.0

1.[/UI

U

-1.0

0I--1.5

0 2 4 6 a

TIME (seconds)

Fig. 11. Response history of the analytic model whichincluded rupture (rupture occurs at 1.42 seconds).

quake using a time step of 0.001 secondsis compared to measured response inFig. 10. The simulation gave good cor-relation until the joint reinforcementruptured in the test specimen at 1.42seconds.

The inclusion of steel rupture was es-sential for accurate modeling of the truebehavior since the predominant rockingmechanism became active after rupture.Fig. 11 shows the change in simulatedresponse which occurred when the truss

PCI JOURNAL(September- October 1989 57

24 +4

12

IIF ^ ^• ^^

ROCKING

-24

-1.28 -0.32

0.32 1.28TOP DISPLACEMENT (in.)

Fig. 12. History of the base shear and top displacement aspredicted by the analytic model.

element had a Iimiting value at whichrupture occurred. The predicted baseshear vs. top displacement plot, which isshown in Fig. 12, produces the samenonlinear elastic rocking response afterrupture as was seen in the experimentaldata of Fig. 8.

Many additional effects had to he con-sidered to achieve a satisfactory simula-tion of the precast system's inelastic re-sponse. For instance, the wall panels donot bend in a manner consistent withthe simplified theory of "plane sectionsremain plane." The gap at the tensionend, during rocking, becomes largerthan expected and the compression zoneat the opposite end becomes smallerthan expected. Reliable modeling of thejoint resistance requires use of numer-ous closely spaced discrete concretecompression gap elements at the ends ofthe wall due to the tremendous strainvariation in a short distance. MalhasG5has listed items which had to be specifi-cally simulated or considered in devel-oping the analytic model with the re-sponse shown in Fig. 10 including:

I. Nonlinear strain variation over a

cross section.2. Yielding across the thickness of the

joint or over the unbonded length forreinforcement which is made continu-ous across the joint.

3. Rupture of steel crossing a joint.4. Buckling of unruptured steel after

it has undergone tension stretching andis then reconipressed.

5. Concrete cracking and gap open-ing.

fi. Degradation of concrete includingstrength deterioration and loss of stiff-ness.

7. Decrease in concrete compressionstiffness after a gap opening has oc-curred,

8. Simulation of shear slip mech-anisms if necessary.

9. Inclusion of closely spaced nodesat the interface between panels and jointelements to avoid force concentration atparticular points in the panel and tosimulate the rapidly varying joint forcescaused by the nonlinear strain variation.

10. Use of small time steps or variabletime stepping to avoid integration errorswhen stiffnesses abruptly change.

58

RESPONSE PREDICTIONFOR 13-STORY PROTOTYPE

]laving achieved successful correla-tion between the measured responseand predictions for the test model, theanalytic techniques were employed inestimating strong motion response anddefining certain limit states for the full13-story prototype wall system uponwhich the model sections in the experi-mental work had been based. Panel andjoint stiffnesses were assigned on thebasis of experience with the one-thirdscale model, Because of the height ofthe prototype system, the initial elasticfirst mode natural period was nearly 0.€3seconds.

The limiting earthquake level for thewall system to avoid yielding was foundto be at a maximum acceleration of 0.36gif the ground motion was proportional tothe El Centro motion used in the walltest program. The peak accelerationwould vary considerably for other typesof motion since the yielding level wasfound to be very sensitive to the matchbetween structural natural frequencyand the earthquake spectra. A secondlimiting level of ground motion, againfor a motion proportional to the El Cen-tro record, would be the amplitudewhich would cause rupture of the verti-cal reinforcement. The predicted pro-totype rupture would occur when theground motion reached an accelerationamplitude of 0.9g though concretecrushing initiates in the joint at anamplihide of 0.5g and may cause deter-ioration of the system's stability beforerupture could occur.

SUMMARY AND DISCUSSION

Project Description

Three precast panel wall subassem-blages were tested under simulatedearthquake motion. Each of the speci-mens was a 3-story one-third scalemodel of walls from near midheight of a

high rise building. The individual pre-cast panels had very limited verticalreinforcement (0.4 to 0.7 percent) madecontinuous across joints at each floorlevel. The reinforcement was concen-trated at the ends of the wall with a pro-totype spacing of 206 in. (5.23 in). Themodels experienced shaking fromground motion proportional to the ElCentro earthquake with their responsemeasured and compared to analyticallypredicted behavior.

Base Shear DuringEarthquake Testing

The 3-story simple precast wall suhas-semblage examined here was designedto elastically resist a base shear equal to45 percent of the system's weight. If de-signed by the Uniform Building Code(UBC), 28 the wall would be required toresist a base shear of approximately 20percent of the system's weight for con-struction in a high seismic area (Zone 4).Thus, the test subassemblage was de-signed to resist a force of just over twicethe UBC's required minimum, Amoderate seismic motion with an accel-eration amplitude of 0.2g was success-fully resisted by the system without per-ceptible damage or measured yieldingwhile peak base shears as high as 32percent of the system's weight were de-veloped. When the ground motion wasincreased by a factor of 3 to 4, to a peakacceleration of 0.7g, significant observ-able damage developed with yielding,uplifting, and rocking at the lower hori-zontal joint. At the higher level of mo-tion a base shear equal to 70 percent ofthe weight was developed, indicatingthat the force demand created by theearthquake was approximately equal tothe product of ground acceleration andsystem mass. The base shear reached150 percent of the design base shear.

The base shear which develops dur-ing seismic shaking of a structure de-pends on the dynamic nature of thestricture, specifically its natural periods

PCI JOURNAUSeptember-October 1989 59

and damping. Thus, the characteristics adesigner gives to a structure can influ-ence the base shear or strength demandand displacements which an earthquakewill create. In the test structure the de-sign shear capacity at yield was less thanthe earthquake induced force andyielding ensued, The yielding resultedin a 50 percent reduction of the system'sstiffness causing a significant change inthe natural period. The yielding andchange of period has a direct influenceon what level of force demand theearthquake will tend to create withinthe structure.

Structural Capacity Demand andDesign Strength

Minimum design forces such asspecified in UBC are lower than the ac-tual forces which would be induced in astructure by a design earthquake if thestructure were to remain elastic. Thecodes allow design for reduced forcesunder the assumption that during a de-sign earthquake the structure will yieldwhen the reduced design force level isreached and plastic deformation willtake place until the reversing nature ofthe earthquake causes the forces to de-crease. It is implicitly assumed that thestructure will be able to withstand thisyielding and plastic deformation with-out failing. Unfortunately, our under-standing of the dynamic mechanismsand available plastic deformation iiiprecast concrete structures is very in-complete, making it difficult to take ad-vantage of reduced force design.

The connection below the lowest pre-cast panel was forced to sustain largedeformations in the wall system tested.The connection yielded because thebase shear which the earthquake motionwould have created, if the structure hadremained elastic, was more than threetimes the yield strength of the connec-tion. Since the connection was not ableto remain elastic, it was forced toundergo plastic deformation with dam-

age. The amount of top displacementwhich accompanied the extensiveyielding was dependent on the designyield force level and reached more thanfive times the yield displacement beforethe strength capacity became reduced(displacement ductility = 5). In fact, themaximum top displacements, withrocking after joint reinforcement hadbroken, became as high as ten times theyield displacement. Seventy-five per-cent of the top displacement was due tothe rocking of the bottom joint.

Connection CapacityDemand — Rocking Motion

A rocking mechanism effectively iso-lated the test walls from the ground mo-tion and limited the amplitude of baseshear which could be transferred intothe structures. During rocking the sys-tem's weight, acting along a path nearthe center of the wall, provided the onlyrestoring moment to counteract theoverturning moment caused by inertialeffects. This constant resistance capacityis clearly evident in the negative dis-placement cycles of Fig. 8. This mech-anism isolated the wall above the jointfrom receiving any greater moment orshear. The load limiting effect, initiatedby opening of the lower joint, however,prevented the spread of inelasticity toany other locations in the wall system.The reinforcement which was continu-ous across the lower horizontal joint ex-perienced tremendous elongation andrupture since nearly all the deformationdemand created by the ground motionwas concentrated within the single joint.Strains in the limited cross joint rein-forcing bars surpassed 4 percent, thelevel at which strain gages became de-fective. The average bar strain calcu-lated from the joint uplift if the bar hadnot ruptured would have been as high as32 percent, far beyond the bar's straincapacity.

A maximum uplift of 0.7 in. (1.78 cm)occurred in the model test wall. The

60

equivalent uplift in the 13-story proto-type building would have been 2.1 in.(5.3 cm). Under these conditions theclosed end of the joint is under tre-mendous compressive stress since theaxial load and flexural compression isresisted within a very small concen-trated compression zone. This zonemust be able to resist high compressionforces without brittle crushing. Thecompression zones in the test specimensexhibited limited crushing of the jointconcrete. The crushing naturally startedat the outer fiber and proceeded inwardas material was lost. Crushing only oc-curred over a limited distance near theends of the walls in the test specimens.The amount of crushing which occurredappeared to be limited by the shortlength of time within which the wallwas at a high uplift. Reversal of the up-lift, caused by the reversing ground mo-tion, reduced the compression force andlimited the degree of concrete crushing,

Connections play the most importantrole in controlling the behavior of a pre-cast large panel wall system duringseismic loading regardless of the designapproach used, Though current prac-tice in aseismic design is aimed at de-veloping strong connections and forcinginelastic behavior away from connec-tions, just the opposite behavior occursin precast systems. Weak connectionscan, however, operate successfully iftheir design explicitly provides for theinelastic demands of earthquake motion.The level of yield strength has to be bal-anced with sufficient deformability andfailure strength to allow repeated cyclesof shaking without collapse.

The connections between panels ofthe wall system described in this paperwere designed to yield and fail in flex-ure before shear, Large shear keys pre-vented a premature shear failure. Panelwall systems using the platform con-nection of the United States are likely tofail first in shear. Llorente andBecker's`" ° investigations of the bene-fits and disadvantages inherent in

flexural or rocking type failures andshear failures showed that both types offailure could lead to instability,

The shear failure mechanism (shearslip) is capable of dissipating energyvery efficiently so that the energytransferred into the structure by theground motion does not create largeforces or large displacements. Llorentefound nevertheless that shear slip maybe undesirable because resistance de-pends largely on friction and whensliding starts it is liable to lead to ac-cumulated unrestrained displacementsunder certain earthquakes when the slipoccurs predominantly in one direction.There is certain danger in having unre-strained displacement because largesecondary (P-Delta) moments developand eccentricity will occur in perpen-dicular walls.

A rocking mechanism dissipates littleenergy as could be seen in Fig. 8, andcreates severe force concentration in thecompression region. As the wall rocksopen, all the axial load and the compres-sion force of the flexural couple has tobe resisted in a small compression stresszone at one end of the wall. Compres-sion crushing of the concrete may occur,leading to instability. Rocking's mainadvantage is that it should not result inaccumulated displacements.

The importance of connection designin precast large panel wall structureswith the two mechanism alternatives,decisions regarding design strength andassociated deformation demand, andIimited knowledge of available ductilityleave a designer in a quandary whenattempting to provide an efficient andsafe system. Both of the mechanismsnoted above create softening of thestructural system with increasing dis-placement and may act to isolate the re-mainder of the wall from increased forcetransfer, however, they also prevent thespread of inelasticity in the wall. Overallit appears that the flexural or rockingtype of mechanism is preferable in itsresistance to developing accumulated

PCI JOURNALISeptember-October 1989 61

displacements. If the flexuralmechanism is selected, then it remainsfor the designer to insure that the jointhas a sufficient balance of strength andductility or deformability to survive theseismic demands, The best balance ofthose quantities has not been deter-mined but the model test walls havesufficient capacities to withstand theeffects of a major seismic motion.

Toughness of Large Panel SystemThe ability to survive the effects of an

earthquake, through strength, energydissipation, ductility and defbrmability,has often been referred to as toughness.Though the walls examined in this studyexhibited little energy dissipation andonly moderate ductility in their rockingmechanism, they did endure a strongearthquake test. The combination offorce isolation, varying stiffness andperiod, and ability of the system toundergo large displacements associatedwith rocking while maintaining its sta-bility allowed it to maintain vertical loadcarrying without collapse. The tough-ness exhibited in the walls appears to beprimarily a result of the deformabilityand the force isolation effect.

The large panel building system canalso he designed to withstand a majorearthquake by providing toughness inthe form of a large elastic energy ab-sorption capacity. The precast panelwall buildings which survived the De-cember 1988 Armenian earthquakewere very similar in construction to thewalls described here and appeared tohave been provided with such a capac-ity. The very nature of the application ofmany large panel systems, to provide

housing, tends to make it easier toachieve high elastic strength capacities.In apartment buildings many of theinterior walls can be load resisting panelwalls. With numerous walls available,the lateral load capacity may becomequite high. The horizontal joints be-tween panels need reinforcementwhich is continuous hetwen panels, buta small amount of reinforcement whichis connected at the end of each wall canprovide a considerable moment resist-ingeapacity due to its Iarge moment armcombined with the axial compressivestresses in the wall from gravity loading.

Evaluation of Analytic TechniquesA set of criteria which needs to be

included in an inelastic analysis to cor-rectly simulate response has been de-term rued for wall rocking mechanisms.Particular attention must be given to thespecial conditions which exist duringthe rocking type of response: nonlinearcross section, opening and closing of thejoint gap, rapid changes in stiffness, anddegrading material characteristics, Newelements had to be added to an existingpanel wall analysis program to simulateall the required joint stiffness char-acteristics. The integration time step ina step-by-step analysis had to be care-fully chosen to avoid errors caused byabruptly changing stiffness. Beam mod-eling of the cantilever wall system wasnot successful once the joint crackopening reached a stage where non-linear strains exist across the section.Even in the elastic range (before yield-ing) the wall's response is particularlysensitive to changes between gross andcracked section stiff iesses.

62

CONCLUSIONS AND RECOMMENDATIONS

Based on the three specimens testedin this program and the correspondinganalytic simulations, certain conclusionscan be drawn.

1. Shear walls of large panel precastconstruction can be designed to resistloads induced by moderate earthquakeswhile remaining elastic and being easilyconstructed. Even though the systemshave weak connection regions, withonly a small amount of continuous rein-forcement through the joints, significantcapacity can be achieved. When the de-sign is based on full elastic loads, it isnot necessary to provide connectionswhich resemble monolithic concrete.

2. When shear wall systems, and par-ticularly the joints, are designed to resistforces which are lower than the likelyelastic force which would be induced bythe design earthquake, such as designforces which are often given in codes,inelastic action will likely result duringa design earthquake. If the constructionis similar to the test specimens, then thejoints may undergo extreme deforma-tions, such as the rocking noted in thetests, and joint reinforcement ma y nip-ture, but there is a good likelihood thatthe system will he able to survive theground motion without collapse. Arocking mechanism will result andwould need special provisions to main-tain stability.

3. The particular large panel systemtested in this program appears to havethe potential to he able to survivestrong seismic motions without collapsethough experiencing serious deforma-tion and damage.

4. Energy dissipation, which is verylow with a rocking mechanism, may notneed to be a prime objective in the de-sign of the large panel precast seismicresistant systems if sufficient deform-ability is provided while maintainingstability. Minimum ties as suggested byPCA27 are essential to maintaining sta-

bility in a three-dimensional structureand must be provided when inelasticbehavior is anticipated.

5. Analytic techniques exist whichare capable of predicting the behavior oflarge panel wall systems but are u.un-practical or unreliable for normal usebecause modeling is complex and theresponse of the system is very sensitiveto the changing stiffness of the connec-tion region.

The design of precast structures tohave acceptable seismic resistance is aperplexing problem at this time becauseof their weak jointed nature. Recentchanges in design codes will requirebuildings to be designed for seismicload in portions of the United Stateswhere wind Ioading once controlled.Additionally, the demand for economicalbuildings, particularly for housing, hascreated a need for means of seismic re-sistant design in regions of strong mo-tion. Certain steps may he taken to treatthe current design problems in regionsof low seismicity and to develop designsin the future for regions of high seismic-ity.

A. Large panel buildings should hedesigned to elastically resist the seismicforces in regions of low seismicity. Cur-rent technology allows such designwhen seismic forces are not high. Whencodified equivalent static loadingmethods are used, the design base shearshould not he a "reduced" force. TheATC 21 approach for defining the baseshear could be employed without theload reduction factor "R". Since it isoften possible to use many walls in thebuilding as Iateral load resisting struc-tural walls, the actual force developed ineach will often be relatively small as inthe panel wall buildings surviving theArmenian earthquake.

B. The rocking mechanism, which oc-curs when the elastic capacity is passed,appears to be the more desirable of the

PCI JOURNAL^September-October 1989 63

two likely inelastic mechanisms possi-ble in a horizontal joint between wallpanels (i.e., shear slip or rocking). Therocking mechanism is unlikely to de-velop unrestrained motion or accumu-lated deformation which could Iead toinstability.

It appears that it would be desirable tomodify the platform system of construc-tion used in the United States in a man-ner which would limit slip and createflexural motion. This might be achievedby providing grouted keys or shear re-sisting links between the stacked wallelements. If the platform system can bemodified to force the inelastic mech-anism to become a flexural one, thensteps must also be taken to provide anincrease in the compressive strength inthe joint region. Once rocking starts,high compressive forces become con-centrated near the ends of the walls. Theplatform system would probably nothave sufficient compression capacity toresist the necessary loads in a ductilemanner and development of a modifiedjoint would be necessary.

C. Particular attention must begiven to tying the entire building sys-tem together as a means of preventingaccumulated deformations from de-veloping and for maintaining stability. Asa minimum, the PCA recommendationsfor ties around the periphery andthrough the diaphragms of the systemmust be provided.

D. Attention should be aimed at theuse of vertical connections betweenstacks of panels as the first location ofinelasticity rather than rocking or slip inthe horizontal joints between panels.Vertical joints could be used to form acoupled shear wall system analogous tothe system recently developed formonolithic construction_ A limitedamount of research has been dedicatedtoward this end 2A but apparently has notbeen successful yet.

A mechanism within the vertical joint

would solve three of the basic problemsin inelastic panel wall response. First,since the vertical connection would notbe an essential link in the gravity loadbearing system, its loss would not en-danger the stability of the building sys-tem. Secondly, the joint could be asource of energy dissipation since itwould serve primarily as a shear transfermechanism. This would complementthe lack of energy dissipation and duc-tility of the existing rocking mechanism.Third, having a vertical coupling jointbetween walls would create redundancyin the wall system. It is well recognizedthat redundancy is a very desirable fea-ture in any structural system which mayhe loaded beyond its elastic limit.

E. Large scale tests and analyticalsimulations, except for the Armenianearthquake results, have involved onlytwo-dimensional assemblages. It is un-clear how the complete three-dimen-sional building system will act, Eitherthe slip or rocking mechanism, occur-ring in walls in one direction, will affectthe strength and stability of other wallsrunning in a perpendicular direction,since those walls will presumably bebent about their weak axis. Analyticalapproaches will have to be developed tomodel the three-dimensional behaviorand tests should be used to verify pre-dictions since stability effects are dif-ficult to simulate in an inelastic system.

F. Large deformation in the hori-zontal joints between vertical panels,either slip or particularly rocking, willexert serious deformation demandsupon the floor or roof diaphragms.Rocking in a three-dimensional struc-ture may literally tear the floor dia-phragms apart. Special reinforcementand ties may be necessary to ensure theintegrity of such diaphragms but insuffi-cient information is available at presentto provide any design guidelines. Thebending deformability of precast floordiaphragms should be substantiated.

64

ACKNOWLEDGMENTSThe test program described here was

funded by the National Science Foun-dation. Drs. M. Velkov and P. Gav-rilovic of the Institute of EarthquakeEngineering and Engineering Seismol-ogy (IZIIS) of Skopje, Yugoslavia, pro-vided aid in obtaining and assembling

the wall models which were supplied byRAD Construction Company of Bel-grade, Yugoslavia. Bahrain Shahrooz, AliBelhadj, and Cherif Baleh, graduatestudents at the University of Wisconsin,provided help in data reduction and an-alytic studies.

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PCI JOIJRNALSeptember-October 1989 65

Portugal, 1966.15. Gavrilovic, P., and Velkov, M., "Ex-

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NOTE: Discussion of this paper is invited. Please submityour comments to PCI Headquarters by June 1, 1990.

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