Concepts for the Developmentof Earthquake Resistant

Ductile Frames ofPrecast Concrete

Robert E. Englekirk*PresidentRobert Englekirk Consulting Engineers Inc.Chief Executive OfficerEnglekirk & Hari Consulting Engineers, Inc.and Adjunct ProfessorCivil Engineering DepartmentUniversity of CaliforniaLos Angeles, California

P recast concrete ductile moment re-sisting frames have traditionally

been permitted by building codest pro-vided that they comply with the provi-sions contained in a building code de-

*Dr. Englekirk serves on PCI's Board of Directorsand Technical Activities Committee and is amember of the PCI Seismic Committee.

t"Precast concrete frame members may he used ifthe resulting construction complies with all of theprovisions of this section." The section referencedis the section describing ductile moment resistingspace frames of cast-in-place reinforced concrete.Uniform Building Code, 1983 Edition.'

CA reinforced concrete structural system notsatisfying the requirements of this Appendix' maybe used if it is demonstrated by experimental evi-dence and analysis that the proposed system willhave strength an t i toughness equal to or exceedingthose provided by a comparable monolithic rein-forced concrete structure satisfying this seedon.'Uniform BuiI lir g Code,1985 Edition.-'

veloped for the design of cast-in-placeconcrete ductile frames.'

Prescriptive provisions for concreteductile frames are contained in Appen-dix At of the ACI Building Code (ACI318-83). 2 Unfortunately, these provi-sions severely restrict the developmentof precast concrete ductile frame con-struction and, indeed, if strict compli-ance were required, a ductile frame ofprecast concrete would be impossible toattain,

More recent codes permit the de-signer considerably more latitude in thedevelopment of a precast ductile mo-ment resisting frame. Therefore, tocomply with the intent of recent codeprovisions, there must be a return tobasic concepts. This paper will discussthese fundamental principles and showhow they might reasonably be applied

30

to the development of a precast ductileframe.

The PCI Seismic Committee is in theprocess of developing a recommendedpractice for the design and constructionof precast and prestressed concretestructures to resist earthquake forces. Amajor part of this report is to focus onconceptual aspects.

To provide such input, a workshopwas held in conjunction with the PCIConvention in Los Angeles in October1986 on the Effective Use of PrecastConcrete for Seismic Resistance. Theworkshop was structured from theviewpoint of the constructor, design en-gineer, researcher and building official.Papers dealing with basic issues suchas building systems, ductile frames andsegmental shear walls were presented.Highlights of the workshop were pub-lished in the November-December PCIJOURNAL.4

Precast concrete offers a wide varietyof fabrication and assembly processes. Itis only through the innovative use ofthese fabricated components thateconomical and functional buildings ofprecast concrete can be developed.Ductile frames are the best way to pro-vide the flexible open spaces requiredof many building functions. Precast con-crete is most effectively applied as abuilding system only when all its com-ponents are precast including the brac-ing system.

This required design flexibility makesprescriptive codification difficult. Un-fortunately, without some form of pre-scriptive guidelines the engineeringprofession will not readily accept pre-cast concrete ductile frames as a viableconstruction alternative.

The key to prescriptive codificationlies in the relationship between the lo-cation of the joint between precast com-ponents and the point at which the duc-tile hinge will form. The criterion forconnecting precast elements at pointswhere the ductile behavior is typifiedby strain reversals in the post yield

SynopsisPresents the basic concepts in de-

veloping a precast concrete ductilemoment resisting frame for a buildingsituated in a region of high seismicity.

Describes the design considera-tions concerning hinged connectors,ductile energy dissipating connectorsand strong nonyielding connectors.

Discusses the various connectortype assemblages including a precastshell with cast-in-place core, coldjoints reinforced with mild steel,grouted post-tensioned assemblageof precast components and unbondedpost-tensioned assembly of precastcomponents.

range must be different from the criter-ion for connectors located in regionswhere the structure is expected or de-signed to remain elastic.

Connector types are crucial not onlyto the behavior of the completed struc-ture but also to the economical de-velopment of the building system.Cast-in-place or grouted connections arereadily accepted as design solutionswhen they closely emulate cast-in-placeconstruction. Welded and post-ten-sioned connectors are usually avoidedbecause of the brittle behavior whichhas historically been associated withthis type of connector. A generic codifi-cation such as this is not reasonablesince connecting precast components bypost-tensioning or welding does notnecessarily reduce system ductility.System economics require that accept-able connectors be identified in such amanner that maximum design freedomis preserved.

Clearly, understanding the basic be-havior of a ductile moment resistingframe is essential. Therefore, a brief re-view of the key elements involved willbe given.

PCI JOURNAUJanuary-February 1987 31

E f P E ^PE

JLh

(a) FORCES

Mu MuE

E/2 E/2^Mu nih/2

(b) DEFORMED SHAPE

Fig. 1. Ductile frame showing forces (left) and deformed shape (right).

BASIC CONCEPTS OFDUCTILE FRAMES

Post-elastic behavior is anticipated inall ductile frames. The system is de-signed so as to cause this post-elasticbehavior to occur in the part of the sys-tem which can best provide for post-elastic rotations. Post-elastic behavior isusually promoted in flexural members(beams or girders) as opposed to col-umns. Axial compressive loads tend toreduce available ductility. When hingesare allowed to form in columns, the po-tential for mechanism failures or "softstories" is increased.

Codes and spectral procedures de-velop a first yield criterion which pro-poses an elastic limit for ductile sys-tems. The development of ductileframes requires that the yield strength ofthe ductile hinge be then used as anelastic criterion for the design of allcomponents of the system. Thus, a hingelocation is created, post-elastic behaviorlocalized and elastic strength require-ments for all elastic components of thesystem identified.

Consider, for example, the ductileframe shown in Fig. 1. Ductile frameconcepts require that the design of thesystem conform to the following steps:

1. The lateral seismic force E is usedto establish the yield criterion for thebearn : *

M„ = (E/2) (h/2) (1)

in which,, is the ultimate moment de-

mand on the column which in this caseis the same as for the beam.

2. The shear strength of the beammust exceed the shear generated in thebeam when a plastic hinge (yielding)forms at each end of the beam:

V. _ 2Mp /1,1ear +Vp +Vy (2)

wherelctear = clear span of the beam, face of

column to face of columnM, = actual yield moment which

can he developed in a beamdesigned to an ultimate mo-ment of M. It is usually as-surned that M, = 125M5.

M„ = nominal moment capacity ofthe beam

3. The strength of the column mustexceed that which is required to resistloads imposed on it when yielding oc-curs in the beam:

M,,, > M,, when P, = P ±P&(3)

whereP = unfactored dead and live

loadsPE = 2 for all levels above

column in questionPe = ultimate axial design load on

column= ultimate design moment for

column

'Note that the lateral seismic force E in this caseis the ultimate base shear as prescribed by the Uni-form Building Code: E = 1.4 V = 1.4 ZICSKIV. See,for example, Ref. ! or3.

32

(I) ±

h/2(II}

^^s^—^tS Cr+—r^^h/2

(III)

(IV) 1/2

PRECAST UNIT CAST SOAS TO LOCATEPRECAST JOINT AT POINTS OF LEASTMOMENT DEMAND. THIS SYSTEM ISOFTEN REFERRED TO AS A TREECOLUMN FOR T-$E BEAM AND COLUMNARE USUALLY CAST AS ONE PIECE.

PRECAST JOINTS (MOMENT RESISTANT)TO BE LOCATED AT POINTS OF MAXIMUMMOMENT BUT HINGING WILL NOT OCCURAT THE PRECAST JOINT.

PRECAST JOINTS LOCATED AT POINT OFMAXIMUM MOMENT AND POINT AT WHICHA HINGE IS EXPECTED TO FORM.

PRECAST HINGE OCCURS IN MEMBER ATPOINT OF MAXIMUM MOMENT BUT HINGELOCATION IS NOTANTICIPATED AT THEPRECAST JOINT.

Fig. 2. Classification of precast frames according to component connector location.

The design of a ductile frame requiresthat the designer know where thestructure will yield and what forces thisyielding will impose on the componentsof the system.

CLASSIFICATION OFPRECAST SYSTEMSAND CONNECTORS

For the purpose of identifying genericproblems, precast systems can be iden-tified in terms of (a) component con-nector location and (b) connector type.

(a) Precast frames may be classifiedaccording to the precast connector loca-tion and the anticipated location ofyielding or ductile hinge location. Fourbasic types are identified in Fig. 2.

The following symbols are used toidentify behavior and construction char-acteristics:

A Strong, nonyielding connection

joining precast members.o Ductile, energy dissipating con-

nection joining precast members.q Hinged, free but guided connec-

tion joining precast members.• Plastic hinge location (first yield).(b) Connector type1. Precast shell with cast-in-place

core.2. Cold joint reinforced with mild

reinforcing steel.3. Cold joint partially prestressed

(grouted).4. Cold joint partially prestressed

(ungrouted).5. Mechanical connection.

DESIGN CONSIDERATIONSThe precast systems previously de-

scribed require that the treatment ofprecast ductile frame design deal withseveral basic issues, for example, hinged

PCI JOURNAL/January- February 1987 33

JOINT INPRECASTMEMBER

ELASTICCOLUMN

—PLASTIC HINGEiN THE BEAM

DUCTILE BEAM

Fig. 3. Deformation of a subassembly.

FACE OF BEAM

MID HEIGHT OF COLUMN

COLUMN VARIATION INCOLUMN FACE MOMENTS BEAM MOMENTS

(a)

(b)

Fig. 4. Variations in moment demand on precast components.

connectors, ductile energy dissipatingconnectors and strong nonyielding con-nectors.

Hinged ConnectorsConsider first the case where the con-

nector location is at a point of minimalmoment. The design must either allowfor the accidental moments that mayoccur or it must permit shear transfers tooccur in conjunction with the cyclic ro-tations the precast joint might be ex-pected to undergo during an earth-quake.

To illustrate the difference, considerthe members of the subassembly shownin Fig. 3. [see also Fig. 2(i)].

If the precast members are connectedby connections capable of resisting mo-ments, the design must consider the

member moment gradients to which thejoints will be subjected. Beam momentdemands will usually be larger than thatwhich will be imposed on the joint inthe column (see Fig. 4).

Observe that the moment in the col-umn at the joining point is not subjectedto the variations imposed on the joint inthe beam for the dead load, live loadmoments on successive floor beams areusually the same. The influence ofhigher modes of vibration and the asso-ciated variations in moment demandmust, of course, be considered. If, on theother hand, the joint between the pre-cast elements is free to rotate, the rota-tional demands imposed on the joint inthe beam and the joint in the column area function of geometry, elastic joint ro-tation, and post yield hinge rotation.

34

Recall that elastic rotation is a func-tion of MiIEI. Since M and E are thesame (or vary by a factor of two), jointrotation depends on the ratio of length toinertia. Observe, however, that theamount of rotation which occurs at theplastic hinge will have no effect on therotational demand on the joint whichoccurs between precast elements. As aconsequence, rotational demands onfree connectors will probably be quitesmall.

Ductile Energy DissipatingConnectors

Where connector locations occur at thepoint in which a plastic hinge will form,Fig. 2(iii), the connector must be capa-ble of undergoing cycles of post yieldrotation without reducing the momentor shear capacity of the joint. This is acomplex analytical problem and little ifany effort has been expended on con-firming analytical procedures with testprograms. Designers tend to avoid theissue by making sure that hinge loca-tions and connector locations are notcoincident, Fig. 2(u),

Unfortunately, locating the hinge andprecast connector at the same point, Fig.2(iii), probably produces the most eco-nomical construction type provided theconnector cost is not prohibitive. Theeconomics are derived principally fromthe fact that members are rectangular(beam extensions are not cast with col-umns) and hence are easy to transportand handle. Further, single columnpieces may be erected for several levelsthereby reducing the number of pieces[see Fig. 2(iii) ]. Clearly, an economicalconnector and an analytical procedurefor insuring adequate performanceshould be a major research objective.

The behavior of cast-in-place flexuralmembers subjected to post-elastic rota-tions has been studied in the testinglaboratory and their performance hasbeen emulated analytically. The proce-dures used in predicting post yield

+P

FIXED

1 p

END

^^ lyE

Fig. 5. Displacement-curvature relations ina hinge region.

hinge behavior start by estimating ahinge length. Curvatures and strains inthe hinge region are then estimated andcompared with test data so as to evaluatethe design procedure. The analyticalprocess follows from mechanics. Eq. (4)relates curvature, 0, to tip deflection fora component of the seismic resistingsubassembly.A = 4„ (1/3) + (0u – 0, )1 [1 -(1/2)] (4)where

A = tip deflectionI, = length of plastic hinge

= curvature at yield= ultimate curvature

Fig. 5 shows the displacement curva-ture relations graphically in the hingeregion of a flexural member.

If a connector is introduced in this re-gion, the validity of the mathematicalmodel must be either established or ap-propriately modified. Suppose that thebeam and column were to be simplyconnected with a mechanical connectorproducing a joint similar to that shownin Fig. 6.

Before the beam reaches its yieldmoment, a crack will undoubtedly format the face of the column. Post-elasticrotation will to a large extent be con-fined to the joint and any post yield de-flection of the member must produce anassociated opening of the joint. Cycleworking of a joint in this manner will

PCI JOURNAL`January-February 1987 35

REINFORCINGMECHANICALLYCONNECTED ^^^

J

GROUTEDJOINT ep=pp/,e

.L^

Fig. 6. Cold joint located at face of column.

/

CONVENTIONALREINFORCING

SHEARAT DUCTS CASTELLATIONS

Fig. 7. Post-tensioning is used to clamp beam column connection.

undoubtedly reduce its capacity totransfer shear forces. Repeated highstrains and subsequent strain reversalsin the reinforcement have been shownto lead to severe moment degradation ifnot controlled. Reinforcing bars strainedin tension when recompressed producehigh lateral pressure on surroundingconcrete which usually reduces thecapacity of the concrete to take sub-sequent compressive cycles of load.

In cast-in-place concrete this degra-dation is mitigated by confining ties.The effectiveness of confining tiesplaced near a cold joint has not, how-ever, been proven. Both the steel andconcrete industry groups have dealtwith the development of joints in re-gions subjected to post yield behavior.Based on extensive test programs, ac-ceptable joints were developed.

Precast concrete affords the designerconsiderable flexibility and imaginativealternative solutions exist to the post

yield connector problem. Post-tensioning provides one such alterna-tive.

Fig. 7 shows how post-tensioningtendons can be used to clamp the beamcolumn connection and yet permit ahinge to form close to the column.Grouted tendons have been successfullytested in New Zealand without supple-mental mild steel reinforcement. Teststo (late have been limited to straighttendon configurations. This type of con-nector is not economical principally be-cause shoring is required until tendonshave been placed, stressed and grouted.Alternatives such as that shown in Fig. 8need to be considered.

Strong Nonyielding ConnectorsA logical alternative to the problems

created when the joint between precastmembers is coincident with the plastichinge location is to force the hinge away

36

STRESS POINT

--SHEAR TRANSFER(UP AND DOWN)

A

B TEMPORARYSUPPORT

CONVENTIONALREINFORCING

A A

STRESSEDo 6 RODS

o 0BB

Fig. 8. Alternative beam column connection using mild steel reinforcementand stressed bars.

TENSIONEDTENDONS

CONVENTIONALM p REINFORCING

COLUMN

HP1REGION

0u

Fig. 9. Beam column connection with plastic hinge located awayfrom joint.

from the joint as shown in Fig. 2(11).Now the elastic capacity at the face ofthe column must exceed that generatedby hinging at the plastic hinge locationwhich has been created in the beam.The details of this scheme are shown inFig. 9.

The elastic moment capacity at theface of the column Mf must be largerthan the plastic moment capacity at thehinge location:

M„ (5)

PCI JOURNALIJanuary-February 1987 37

PLASTIC HINGE SHEAR TRANSFERLOCATION

PRECASTBEAM

REINFORCING

PRECASTCOLUMN

Fig. 10. Beam column connection with plastic hinge located atpoint of maximum moment.

Higher strains and shears will he im-posed on this member if the yield anddeformation criterion is the same as thatused for the same member were theplastic hinge to coincide with the jointbetween the precast members. Rota-tions which occur in the plastic hinge forthe beam shown in Fig. 9 will clearly begreater than those generated on thesystem described in Fig. 5. Variations onthis design approach are numerous andall that the designer need do is satisfyhimself that adequate elastic capacityexists at the joint between precast mem-bers and that rotations and strains at theplastic hinge can be sustained.

Placing the precast joint at the point ofmaximum moment on the column butyet outside the plastic hinge location,Fig. 2(iv), presents an interesting alter-native (for connection details, see Fig.10). Problems posed by the two previ-ous alternatives are avoided, since theprecast beam elements are continuousacross the column support locations andplastic hinges occur in monolithic con-crete sections, The only penalty is a lossof erection speed (single lift columns).In order to effect this alternative themoment capacity of the column at thejoint should be larger than that whichwould he generated when plastic hingesform in the beam.

Connectors can now be designed to anelastic criterion providing the designerwith a wide variety of alternatives in-cluding welding, post-tensioning orgrouted splices.

CONNECTOR CONCEPTSThe connector types previously iden-

tified must be designed to satisfy the re-quirements of the systems in which theywill be used. Three basic use categoriesidentified were:

—Strong (nonyielding).—Ductile (energy dissipating).—Hinged (free to rotate but guided).Connector types often adapt more

readily to a particular use category. Thedesigner must accordingly select theconnector type which best matchesfunction and intended construction pro-cedure.

CONNECTOR TYPES ANDSUGGESTED APPLICATIONS

This section will consider the fol-lowing connector type assemblages:

—Precast shell with cast-in-placecore.

—Cold joints reinforced with mildsteel.

38

—Grouted post-tensioned assemblageof precast components,

—Unbonded post-tensioned assemblyof precast components.

Precast Shell WithCast-in-Place Core

Cast-in-place cores of precast shellelements have been a popular means ofemulating cast-in-place concrete frameconstruction. In this system the "shell"unit is used to support other precastelements. Shell units essentially takethe place of beam forms, usually re-quiring shoring to support wet concreteplaced on top of the precast system.Supplemental reinforcement, usually allof the seismic reinforcement, is placedwithin the shell to create the desiredstrength.

Column shells are often used in con-junction with beam shells, in effect pro-ducing cast-in-place construction. Thedesign of this type of system is well cov-ered elsewhere' and the process willonly be discussed from the perspectiveof shell behavior. Where either thebeam or the column is a shell and theother component is solid, the joint be-tween the shell elements and the coldjoint produced requires special atten-tion.

Continuity is inherent in this system.Connector uses which require hinges,which are free to rotate, are not wellsuited to this construction process.Strong nonyielding joints designed to anelastic criterion require the effectivetransfer of shear across the natural coldjoint. The shear transfer at the joint mustexceed the shear demand which is cre-ated when plastic hinges form in thesystem. It is not necessary to developthe strength of the member being con-nected. For example, the shear transferin the column [Fig. 2(iv) ] is a function ofthe plastic moment capacity of thebeam. The plastic moment capacity ofthe column need not be developed.

Shear transfers across cold joints can

be accomplished in a variety of ways.When strong nonyielding behavior canhe ensured, shear transfer should presentno special design problem. Shear ismost economically transferred throughshear keys (or castellations) cast in theprecast element. Shear friction is notpopular because of the number ofevenly spaced bars which must cross thecold joint making precast element erec-tion considerably more complex. A typi-cal joint is shown in Fig. 11.

If, on the other hand, the beam of Fig.11 is to be subjected to post yield de-formations, special provisions must bemade to insure that the joint does notopen up and that shear transfer is in-sured through cycles of reverse curva-ture. An early crack at the weakenedplane near the column face would placelarge strain demands on tension steel inthis area and recycling to compressionstrains would produce large dilatationalforces on the shell. Shell constructionsimilar to that described herein hasbeen tested in New Zealand." The be-havior of the unit from the standpoint ofenergy dissipation and shape of thehysteresis loop is quite comparable tocast-in-place concrete construction.

Since bonding of the shell to the con-crete core can only achieve a nominalshear transfer, large compressive strainsshould not be introduced into shellcomponents. The New Zealand tests re-vealed that when the beam shell and theconcrete column were in contact and didtransfer compressive stresses, the col-umn shell spalled after several cycles ofload. The best behavior was achievedwhen a gap was maintained between thebeam and column. The New Zealandtests were performed on full size proto-type assemblies. The column was castbefore the beam core, thereby produc-ing a cold joint at the top of the column.Where a cold joint exists in the beam atthe column interface, cracking at theinterface will probably occur unlessspecial reinforcement is provided.

In the absence of test data to support

PCI JOURNALJJanuary-February 1987 39

GROUTEDSLEEVEIN PRECASTCOLUMN

POUREDCONCRETE

8EAM SHELL L: .PRECAST COLUMN PRECASTSOLID

SHELL

PLAIN GROOVED CASTELLATED

Fig, 11. Beam column connection with elastic behavior in beam.

CONFININGREINFORCING

SHELL GAPUNIT

SLEEVES

CASTELLATIONS

FLEXURALREINFORCING

SHEAR REINFORCING(SHEAR FRICTION)

Fig. 12. Modified reinforcement details for beam column connectionshown in Fig. 11.

the behavior of joints of this type, thetendency would be to provide uniformlydistributed horizontal reinforcementacross the joint. This reinforcementwould he intended to keep the jointclosed, improve the sheartransfer acrossthe castelIations and provide a shearfriction type shear transfer. Shear fric-tion has been shown to be effective in

transferring shear across joints subjectedto cyclic strain reversals especiallywhere cold joints or cracks occur. Shearkeys and aggregate interlock, which canbe counted on to transfer significantshears when strains are low, deterioraterapidly under cyclic loads when strainsare high. The connection in Fig. 11should he modified as shown in Fig. 12.

40

CONFINING HOOPSCAST-IN-PLACECONCRETE

Hwm

PRECAST BEAM

Fig. 13. Wet splice (beam elevation) with confinement reinforcement.

Providing evenly distributed rein-forcement across a joint affects con-structihility and, for that matter, columnstrength. The number of column pene-trations should, therefore, beminimized. A uniform spacing of barsdoes not seem to be as important as try-ing to control the crack width. Barsshould then be placed close to theflexural face (top and bottom). Quantifi-cation by shear friction needs study butat least at this writing the method seemsreasonable.

Horizontal reinforcement should ex-tend through the length of anticipatedhinging to control cracking. Confine-ment in the hinge region ensures betterbehavior of the concrete because it in-creases the ultimate stress and strainvalues in the concrete and controls dila-tional stresses.

Cold Joints Reinforced WithMild Steel

This type of joint is typically placed inregions of low moment demand. Themost common use type is hinged, free torotate because they are usually installedin regions where analytical hinges occur(see Fig. 3 for example.)

Beams are most often connected bywet splices (Fig. 13). The design criter-ion for this connector requires thequantification of moment demand as de-scribed in Fig. 4 and the shear imposedon the beam when plastic hinges form.From the perspective of designing theprecast interface, the objective is to

create a strong, nonyielding connection.Minimizing joint length is advantage-

ous from a constructibility perspectiveand also to reduce the moment demand.Bond transfer of stress from tensionreinforcement is usually preferred be-cause it eliminates tolerance problemsassociated with the installation of me-chanical splices. Stress transfer is con-siderably improved by providing con-finement reinforcement as shown inFig. 13.

Wet splices are not practical for col-umn splices. Dowel splices and me-chanical splices as shown in Fig. 14 arethe most common connectors whenmoment demands are low. Moment de-mand at the midheight of a column istypically low as described in Fig. 4.Tension transfer through dowel splicescan be quite high especially if epoxygrouts are used. As a consequence, ten-sion stresses induced by overturningmoments and accidental column Tno-ments can be transferred.

Shear transfer must also occur acrossthe joint. Analytically it would be dif-ficult to establish much shear transferthrough the joints shown in Fig. 14.Even castellation or shear keys are notcapable of transferring shears over theIimited area available for shear transfer.This often forces the designer to usecastellated shear plates (Fig. 15). Me-chanical splice sleeves such as the oneshown in Fig. 16 are becoming morepopular as their price becomes morecompetitive.

Cold joints which are truly free to ro-

PCI JOURNALIJanuary-February 1987 41

A 11 ^r

RI

II II

II II

IIII IIo II

f I ^'II

II 4 IILJ I I iJ

(a) DOWEL SPLICE { b) MECHANICAL SPLICE

Fig. 14. Column connections showing dowel splice (left) and mechanicalsplice (right).

tate are seldom used in ductile frameconstruction. Seat type connections forbeams must be significantly modified totransfer large shears acting in either di-rection while shear, compression andtension transmission through a columnconnector which is free to rotate wouldundoubtedly cost more than the con-nectors previously described and wouldnot offer a performance advantage.

Current construction economics seemto dictate that where splicing of precastsubassemblies in columns is en-visioned, joints such as those shown inFigs. 14 and 15 are most practical. Forbeam splices, the wet splice shown inFig. 13 is most popular.

Grouted Post-TensionedAssemblage of PrecastComponents

Grouted post-tensioning as a means ofconnecting precast elements has beenstudied in New Zealand and Japan. In-

vestigators have concluded that, if prop-erly used, it is an effective and oftensuperior way of assembling precast ele-ments. A variety of tests have been con-ducted in New Zealand on precast duc-tile frames incorporating post-tensioning as a ductile energy absorbingconnector.' Lateral load resisting sys-tems joined by post-tensioning are per-mitted by the New Zealand Code andseveral buildings of this type have beenbuilt including one three-story struc-ture.

Two basic alternative systems exist,the continuous column system (Fig. 17)and the continuous beam system (Fig.18). The New Zealand tests7 involvedonly the continuous column systemshown in Fig. 17. Concerns that thelength of the plastic hinge might be sig-nificantly reduced did not materialize.Large cracks did not form at the beamcolumn interface.

Both systems must be considered forconstruction benefits and constraints

42

Fig. 15. Column splice with grouted joint.

NONSHRINK

__1'

GROUT

SPLICECj SLEEVE

Fig. 16. NMB column splice sleeve system.

exist with each. The erection of multi-story columns is clearly an advantage butthe horizontal post-tensioning of this as-sembly is probably considerably more

costly than for the continuous beamsystem. From a design perspective, thecontinuous column system of Fig. 17 re-quires that the plastic hinge form at a

PCI JOURNAL/January-February 1987 43

PRETENSIONED TENDONSAND/OR NONPRESTRESSEDREINFORCEMENT

MORTAR DUCTILE ENERGYMORTAR JOINT ABSORBING JOINT.JOINT

POST TENSIONED MIDSPANTENDONS

NOTE; ONLY LONGITUDINAL STEEL IN MEMBERS SHOWN

Fig. 17. Precast concrete units connected by post-tensioning(continuous column).

joint between precast members. As aconsequence, the yield moment must besupplied by grouted post-tensioning.Shear transfer across a cold joint must beaccomplished. Post-tensioning must herelied upon to make a castellated jointperform well in a region where yieldlevel strains will occur.

Alternatives do exist which forceplastic hinges away from the connector(Fig. 19). However, more reinforcementand a higher moment and shear demandat the precast interface result if the sameyield criterion is used to design bothsystems. The continuous beam system(Fig. 18), though more expensive toerect, has significant advantages in thatthe beam element may be reinforcedwith mild steel, post-tensioning or acombination of the two. The column ismore easily post-tensioned and subjectonly to an elastic design criterion.

The continuous beam system adheresto the basic philosophy of ductile framedesign when conventionally reinforced.A post-tensioned column section can bedeveloped which would remain elasticwhen subjected to a mechanism in-duced by moments and shears. Castel-

lations or shear keys can be incorporatedinto the beam column joint which com-ply with accepted theories for sheartransfer. In effect, the only current rea-son for not permitting this type of con-struction is the rather arbitrary exclusionof post-tensioning from ductile frames.

The continuous column system clearlyrequires an in-depth investigation ofseveral issues. Not only must it beshown to he equivalent to concrete duc-tile frames as described by currentpractice but major construction con-straints must also be dealt with. Col-umns will be several stories high. Whenbeams are erected they must be quicklyreceived by the system. A simple shearconnection is required and ideally boththe beam and the column should rapidlybecome a part of the erection bracingsystem. Quick vertical load transfer isusually accomplished by corbels.

Corbels are not effective in frame as-semblies where load reversal is ex-pected. Shear transfer for reverse load-ing is most easily incorporated in theinterface, thereby making the corbel re-dundant. In post and beam precast sys-tems temporary bracing is usually ac-

—COUPLED POST-TENSIONED TENDONSAND/OR

COUPLED NONPRESTRESSED REINFORCEMENT

MORTAR II I I ^P-O T TENSIONED MIDSPAN

JOINT

STRONG MORTAR/

NON -YIELDING JOINT PRETENSIONED TENDONS

JOINTAND/OR

T NONPRESTRESSED REINFORCEMENT

NOTE: ONLY LONGITUDINAL STEEL IN MEMBERS SHOWN.

NOTE : OTHER PRESTRESSING TENDONS AND/OR NONPRESTRESSEDREINFORCEMENT IS AS SHOWN IN FIGURE 17.

Fig. 18. Precast units connected by post-tensioning (continuous beam).

complished by tieing the precast systemto the previously constructed shearwalls, Temporary support systems suchas that shown in Fig. 20 are probably thebest solution to both problems. Thesetemporary supports should he installedin such a way that erection stability canbe attained.

Complex ducts such as those shown inFig. 18 are not easily connected nor isthe post-tensioning easily installed.

Stressing losses will prevent long con-tinuous stressing operations. Ideallystraight ducts should be used. If thecommitment to a precast interface at thepoint of plastic hinging is accepted andthe construction constraints are under-stood, reliable joining details must bedeveloped.

Beam column joints with mortarplaced in the gap which are subse-quently post-tensioned have been

PCI JOURNALiJanuary-February 1987 45

COLUMN BEAM

IjIL

MILDPOST REINFORCINGTENSIONING

Fig. 19. Strong nonyielding precast connection.

EGHRENGTHLTS

SLEEVECAST IN BEAMAND COLUMN

Fig. 20. Temporary beam column connection erectionassembly.

shown to behave favorably when sub-jected to cyclic loads." Plastic curvatureextends throughout the hinge region asopposed to being concentrated in themortar joint. Subassembly tests did re-veal a somewhat lower hystereticenergy dissipation when compared toconventionally reinforced concrete sub-assemblies of the same size. Park s con-cludes that higher deflections will occurwhen the system is subjected to earth-quake loading and as a result a baseshear coefficient which is 25 percentlarger than that used for conventionallyreinforced concrete ductile frames ap-pears warranted.

Energy absorption would appear to bea better way of evaluating structural

systems than deflections. Clearly, moretesting must be done before suitablecriteria can be developed. The most en-couraging aspect of this test program isthat a mortar joint (presumably withoutspecial castellations) was able to transfershear through repeated cyclic excur-sions well into the plastic range. Fur-ther, finite hinge lengths developed andlarge strains or rotations did not occur inthe mortar joint.

This type of construction appears tohe a viable means of constructing duc-tile moment resisting frames from pre-cast concrete components. Input fromprecasters as to the constructibility ofgrouted post-tensioned precast framesystems is required.

46

Unbonded Post-TensionedAssembly of Precast Components

The use of unbonded post-tensioningin a concrete ductile moment resistingframe has not been accepted- The basicconcern focuses on the ability of theanchorage to perform adequately whileundergoing seismic deformations. Par-tial post-tensioning should improve theperformance of a ductile frame by pro-viding a compression over the section. Itis hard to imagine that a continuous un-grouted tendon which passes throughmany strain reversals along the length ofthe member could be elongated to thepoint that the anchorage would fail. Un-grouted strands cannot provide anyyield moment strength in a plastic bingeregion and must, therefore, be used incombination with the same amount ofmild reinforcing steel which would berequired of an unpost-tensioned section.

Reservations as to the ability of post-tensioning to withstand seismic stressreversal should not prevent its use inthose portions of ductile frames inwhich yielding will not occur. The per-formance of the system shown in Fig. 18should not be lessened if unbondedpost-tensioning is used in the columns.If the beam were reinforced with non-stressed reinforcement in the hinge re-

gion (top and bottom) and ungroutedpost-tensioning placed as shown in Fig,18, the performance of the systemshould be comparable to a system whichis not stressed.

Test programs for grouted post-ten-sioning of precast ductile frames shouldalso include ungrouted assemblies. Theassembly of precast components by un-bonded post-tensioning would seem tooffer significant economic advantages.

CONCLUDING REMARKSThe purpose of this paper has been to

present basic concepts for developing aprecast concrete ductile moment re-sisting frame suitable for buildings lo-cated in severe earthquake regions.These ideas have been extracted frompublished articles, s- e accepted practiceand the author's experience. Inevitably,some of the frame systems, connectionsand post-tensioning schemes need fur-ther development work and testing be-fore they can be applied in actual pre-cast buildings. Nonetheless, it is theauthor's hope that this paper will sparksome new ideas thereby making precastand prestressed concrete a more viableand competitive building material in re-gions of high seismicity.

NOTE: Discussion of this paper is invited. Please submityour comments to PCI Headquarters by September 1, 1987.

PCI JOURNAUJanuary-February 19B7 47

REFERENCES1. Uniform Building Code (1983 Edition),

International Conference of Building Of-ficials (ICBO), Whittier, California.

2. ACI Committee 318, "Building Code Re-quirements for Reinforced Concrete (ACI318-83)," American Concrete Institute,Detroit, Michigan.

3. Uniform Building Code (1985 Edition),International Conference of Building Of-ficials (ICBO), Whittier, California,

4. Englekirk, Robert E., "Overview of PCIWorkshop on Effective Use of PrecastConcrete for Seismic Resistance," PCIJOURNAL, V. 31, No. 6, November-De-cember 1986, pp. 48-58.

5. Park, Robert, and Bull, D. K., "SeismicResistance of Frames Incorporating Pre-cast Prestressed Concrete Beam Shells,"PCI JOURNAL, V. 31, No. 4, July-August1986, pp. 54-93.

6. Park, Robert, "New Trends in the Designof Buildings and Bridge Structures for

Earthquake Resistance," Fourth Interna-tional Seminar on Seismic Engineering,Universidad de Los Andes, Bogota, Col-ombia, September 1985.

7. Park, Robert, "Design Concepts for Pre-stressed and Partially PrestressedlPrecastConcrete Frames," Proceedings of theApplied Technology Conference, ATC-8,Los Angeles, California, April 27-29, 1981,pp. 145-182.

8. Sorensen, Svend A. K., "Precast ConcreteFrame Construction in Zones 3 and 4,"Proceedings of the Applied TechnologyConference, ATC-8, Los Angeles, Califor-nia, April 27-29, 1981, pp. 183-201.

9. Andrews A. L., "Design of Partially Pre-stressed Precast Concrete Ductile Mo-ment Resisting Frames," Proceedings ofthe Applied Technology Conference,ATC-8, Los Angeles, California, April27-29, 1981, pp. 225-249.

APPENDIX-NOTATIONE = lateral seismic forceM = moment

= ultimate design moment for col-umn

^i^f^ = moment at joint between precastmembers

= nominal moment capacity ofbeam

M„ = actual yield moment (plasticmoment capacity)

= actual yield moment for beamMw = ultimate moment demand on

memberP unfactored dead and ] ive loadsP' = ultimate axial design load on

columnP^ = maximum possible earthquake

induced axial load on columnVr, = shear created by imposing dead

loadsVL = shear created by imposing live

loadsV„ = ultimate shearh = story heightl = lengthhua,. = clear span of beam, face of col-

umn to face of columnl„ = length of plastic hinge0 = curvatureC,u = ultimate curvature4,y = curvature at yieldA = strong, nonyielding connection

joining precast memberso = ductile, energy dissipating con-

nection joining precast membersq = hinged, free but guided connec-

tion joining precast members• = plastic hinge location (first yield)

48