A Perspective on the SeismicDesign of Precast ConcreteStructures in New Zealand

The author describes trends and developments in the use ofprecast concrete in New Zealand for floors, moment resistingframes, and structural walls of buildings. Currently, almost allfloors, most moment resisting frames, and many one- tothree-story walls in buildings are constructed incorporatingprecast concrete elements. Aspects of design andconstruction, particularly the means of forming connectionsbetween precast concrete elements, are discussed. Thepaper emphasizes seismic design; the aim of the designmethods for frames is to emulate monolithic construction.Examples of recent precast concrete buildings using theabove discussed methods are presented.

In addition to his duties as a facultymember in the Department of CivilEngineering, Professor Park is DeputyVice-Chancellor of the University ofCanterbury and Chair of the ConcreteDesign Committee of Standardsof New Zealand. During his longprofessional career, he has receivedmany awards and honors, includingPCI’s Martin P. Korn Award (twice)and ASCE’s T. Y. Lin Award. Underhis leadership, the University ofCanterbury is well known throughoutthe world for its pioneering researchin the seismic design of structures.

Since the early 1 960s, there hasbeen a steady increase in theuse of precast concrete for

structural components in buildings inNew Zealand. The use of precast concrete in flooring systems has beencommonplace since the 1 960s, makingcast-in-place floor construction generally uncommon. Also, precast concrete non-structural cladding for buildings has been widely used.

During the boom years of buildingconstruction in New Zealand in themid to late 1980s, there was also a significant increase in the use of precastconcrete in moment resisting framesand structural walls. Precast concreteelements have the advantages of highquality control, a reduction in site

formwork and site labor, and increased speed of construction. In particular, with high interest rates andpressure for new building space in themid 1980s, the speed advantage gaveprecast concrete a distinct cost advantage. Contractors adapted to precastconcrete construction with increasedcrane capacity, new construction techniques, and off-site fabrication.

The increase in the use of precastconcrete in the 1980s required considerable innovation because of NewZealand’s location in an active seismiczone; the seismicity of most of NewZealand is similar to that of California.At the time, the New Zealand concretedesign standard, NZS 3101:1982,’ likethe concrete codes of many countries,

IRobert Park, Ph.D.Professor of Civil EngineeringUniversity of CanterburyChristchurch, New Zealand

40 PCI JOURNAL

contained comprehensive provisionsfor the seismic design of cast-in-placeconcrete structures but did not haveseismic provisions covering all aspectsof precast concrete structures. The design ultimate seismic forces used forthe design of ductile moment resistingframes in the most seismically activeparts of New Zealand are very similarto those recommended by the UniformBuilding Code2 in Zones III and IV ofthe United States.

In the past, some framed structuresincorporating precast concrete elements have performed poorly in earthquakes in many countries because ofpoor connection details. As a result,precast concrete in moment resistingframes was excluded in New Zealandfor many years. Confidence in the useof precast concrete in moment resisting frames required the developmentof satisfactory methods for connectingthe precast elements together. The design methods that were introduced inNew Zealand in the 1980s for framesof buildings incorporating precast concrete elements generally aimed toachieve behavior equivalent to that ofa completely cast-in-place concretestructure. That is, the objective of thedesign method is to emulate monolithic construction.

With the increase in the innovativeuse of precast concrete elements inbuildings in New Zealand came an increasing concern that some of the design solutions should be more fully researched. Even if there were no reasonto doubt the validity of extrapolatingthe results of design and constructionprocedures that were originally developed for cast-in-place concrete, thelarge number of important buildingsemploying precast concrete for seismic resistance demanded that more research and testing be done to justifyconfidence in the structural systems.

In February 1988, a seminar at theUniversity of Canterbury, attended bydesigners, researchers, fabricators andconstructors, highlighted a growingneed to investigate and verify aspectsof the performance of precast concretein building structures designed forseismic resistance. Following the seminar, a Study Group, jointly funded bythe New Zealand Concrete Society,the New Zealand National Society for

Earthquake Engineering, and the Centre for Advanced Engineering at theUniversity of Canterbury, was formedwith the following objectives:

1. Summarize data on precast concrete design and construction

2. Identify special concerns3. Indicate recommended practices4. Recommend topics requiring fur

ther researchThe outcome of the deliberations of

the Study Group during 1988-91 wasthe publication of a manual authoredby the members of the Study Group titled “Guidelines for the Use of Structural Precast Concrete in Buildings,”which was first printed in August1991.

A new revision of the New Zealandconcrete design standard, NZS 3101:1995, is being published in 1995. Thisrevision contains more provisions forthe seismic design of structures incorporating precast concrete than itspredecessor.

This paper describes aspects of thedesign and construction of buildings inNew Zealand incorporating precastconcrete structural elements in floors,moment resisting frames, and structural walls. Design and constructionfor seismic resistance are also emphasized because that is where the greatest difficulties lie in the connection ofprecast concrete elements.

SEISMIC DESIGNCONCEPTS FOR PRECASTCONCRETE IN BUILDINGS

General Requirements

For moment resisting frames andstructural walls constructed incorporating precast concrete elements, thechallenge lies in finding an economical and practical means of connectingthe precast concrete elements togetherto ensure adequate stiffness, strength,ductility, and stability. The designshould consider the loadings duringthe stages of construction and at theserviceability and ultimate limit statesduring the life of the structure. The design should ensure that the structureperforms satisfactorily in the serviceload range, has a reasonable margin ofsafety before the ultimate load isreached, and will not fail catastrophi

cally at the ultimate load.In common with other countries, the

seismic design forces recommendedfor structures in the current NewZealand loadings standard for generalstructural design and design loadingsfor buildings, NZS 4203:1992, aresignificantly less than the inertiaforces induced if the structure responded in the elastic range to a majorearthquake. The design seismic forceis related to the achievable structureductility factor = maxMy, where

max is defined as the maximum horizontal displacement that can be imposed on the structure during severalcycles of seismic loading without significant loss in strength, and is defined as the horizontal displacement atfirst yield assuming elastic behavior ofthe cracked structure up to the designseismic force.

In the New Zealand loadings standard, NZS 4203:1992, for ductilestructures, ji = 5 or 6 is used to determine the appropriate spectra of seismic coefficients from the elastic response spectra. The design ultimatehorizontal seismic forces typicallyvary between 0.03g and 0.20g. depending on the seismic zone, the soilcategory, the importance of the structure and the fundamental period of vibration of the structure.

For structures of limited ductility,= 3 is used and the design ultimate

horizontal seismic forces typicallyvary between 0.03g and 0.39g. Thedesign ultimate seismic forces recommended in the New Zealand loadingsstandard5 for cast-in-place concretestructures and for structures incorporating precast concrete elements of thesame available ductility, are identical.Note that as an alternative to ductilestructures, designers can design structures of limited ductility with higherdesign ultimate seismic forces butwith less stringent requirements fordetailing for ductility.

The exact characteristics of theearthquake ground motions that canoccur at a given site cannot be predicted with certainty and it is difficultto evaluate all aspects of the completebehavior of a complex structure whensubjected to a major earthquake. Nevertheless, it is possible to design thestructure to ensure the most desirable

May-June 1995 41

behavior. The rational approach forachieving this objective in the designfor earthquake resistance is to choosethe most suitable mechanism of post-elastic deformation for the structureand to ensure, by appropriate designprocedures, that yielding of structural members will occur only in thechosen manner during a major earthquake and that the available ductilityis adequate.4’5

If all the elements of the structureresisting seismic forces are detailedfor ductility in accordance with theseismic provisions of the concrete design standard,4 adequate ductility isconsidered to be provided.

Capacity Design

To ensure that the most suitablemechanism of post-elastic deformationoccurs in a structure during a majorearthquake, New Zealand standardsNZS 4203:l992 and NZS 3101:l995 require that ductile structures bethe subject of capacity design. In thecapacity design of structures, elementsof the primary lateral earthquake loadresisting systems are suitably designedand detailed for adequate strength andductility for a major earthquake. Allother structural elements and possiblefailure modes are then provided withsufficient strength so that the chosenmeans for achieving ductility can bemaintained throughout the deformations that may occur.4’5

For moment resisting frames andstructural walls of buildings, the bestmeans of achieving ductile post-elasticdeformations is by flexural yielding atselected plastic hinge positions. Withproper design, the plastic hinges can bemade adequately ductile. To ensure thatfailure in flexure cannot occur in partsof the structure not designed for ductility, or that failure in shear cannot occuranywhere in the structure, the maximum forces likely to be imposed on thestructure should be calculated from theprobable maximum flexural strengths atthe plastic hinges. This is done by taking into account all the possible factorsthat may cause an increase in the flexu

The lower characteristic yield strength is defined as

the value of the yield strength below which not more

than 5 percent of production tests in each size fall.

loading.

ral strength of the plastic hinge regions.These factors include an actual yield

strength of the longitudinal reinforcingsteel, which is higher than the lowercharacteristic yield strength,* and additional longitudinal steel strength dueto strain hardening at large ductilityfactors. Due to these two factors, thesteel overstrength in New Zealand istaken to be 1.25 times the lower characteristic yield strength when calculating the probable maximum flexuralstrength in the plastic hinge regions.

As a result, the shear reinforcementin the plastic hinge regions, and allflexural reinforcement in parts of thestructure away from plastic hinge regions, will need to be designed forshear forces and bending momentsthat are at least 1 .25I times the shearforces and bending moments associated with the design bending momentsof the plastic hinge regions. This ensures that non-ductile failures do notoccur elsewhere, where 1.25 is thesteel overstrength factor and is thestrength reduction factor used for designing the flexural reinforcement atthe plastic hinge, taken as 085 in NewZealand.4 If plastic hinges in columnsof moment resisting frames are to beavoided (that is, strong column-weakbeam behavior is sought), the designcolumn bending moments may need tobe amplified by much more than1 .25/p in order to guard against highermode effects and concurrent earthquake forces as well as the over-

Fig. 2. Types of support using precastconcrete beams for hollow-core floorunits (Ref. 3).

strength of steel in beams.4The use of capacity design has

given designers confidence that structures can be designed for predictablebehavior during major earthquakes. Inparticular, brittle elements can be protected and yielding can be restrictedto ductile components as intended bythe designer. The capacity design procedure has enabled structures incorporating precast concrete elements to bedesigned for ductile behavior, because

irn

irniLL

Frame

Coupled structural wallsand mechanism

Fig. 1. Possible mechanisms of post-elastic deformation for equivalent monolithicmoment resisting frames and structural walls of buildings during severe seismic

Type 1earn

1-Precast concrete

hollow-core unitconcrete

Type 2

Cores broken out Cast-in-place

Na_.___ /‘“concrete topping

Precast concrete

ik• 4.1 unit

of cores to befilled with concrete

Precast

concretebeam

Type 3

42 PCI JOURNAL

any brittle connections between elements can be designed to remainin the elastic range during a majorearthquake.

Preferred Modes ofPost-Elastic Deformation

Fig. 1 shows mechanisms of post-elastic deformation that could occur inequivalent monolithic moment resisting frames and structural walls due tothe formation of plastic hinges duringa severe earthquake. These mechanisms are idealizations in that they involve behavior under the typicalequivalent static seismic forces recommended by codes, which are basedmainly on the first mode of vibration.The actual dynamic situation for moment resisting frames and coupledstructural walls can be different due tothe effects of higher modes of vibration that can result in the plastichinges in the beams forming in a fewstories at a time and moving in wavesup the structure during the earthquake.Nevertheless, the static mechanisms ofFig. 1 give designers a reasonablesense for the actual situation.

As shown in Fig. 1, if yielding begins in the columns of a moment resisting frame before it begins in thebeams, a column sidesway mechanismcan form. In the worst case, the plastichinges may form in the columns ofonly one story because the columns ofthe other stories are stronger. Such amechanism can make very large curvature ductility demands on the plastichinges of the critical story,6 particularly for tall buildings.

On the other hand, if yielding beginsin the beams before it begins in thecolumns, a beam sidesway mechanismwill develop, which makes much moremoderate demands on the curvatureductility required at the plastic hingesin the beams and at the column bases.Therefore, a beam sidesway mechanism is the preferred mode of post-elastic deformation, particularly because ductility can be more easilyprovided by reinforcing details inbeams than in columns.

As a result of the above considerations, the New Zealand concrete design standard4requires that columns ofmultistory ductile moment resisting

frames should have sufficient flexuralstrength to avoid the formation of column sidesway mechanisms as far aspossible. Thus, a strong column-weakbeam approach to design is advocated.

The New Zealand concrete designstandard4 has two exceptions to thisrule:

1. In some buildings in areas of lowseismicity and/or where beams havelong spans, the gravity load considerations may govern and make a strongcolumn-weak beam design impracticable. In such a case, the interiorcolumns of gravity load dominatedductile frames three stories or highermay be designed to develop plastic

hinges in any story simultaneously atthe top and bottom ends, while plastichinges develop in some beams only,typically only in the beams at or nearthe exterior columns (see the mixedsidesway mechanism in Fig. 1). Suchframes are required to be designed fordesign seismic forces which may behigher than for frames with beamsidesway mechanisms.

2. For ductile frames of one- or two-story buildings, or in the top story of amultistory building, the New Zealandstandard permits column sideswaymechanisms (that is, a strong beam-weak column approach) because thecurvature ductility demand at the plastic

Topping750

665 Mesh(5.3mm dIameter at 150mmcentres both ways)

420 750

7

Hollow-core unit j \—.ri Grade 300

—.. __—_Seating length50mm Test B0mm Test A

Precast Beam Type I

Topping750

TIE CONNECTIONTYPE I OFSPECIMEN I

TIE CONNECTIONTYPE 2 OFSPECIMEN 2

TIE CONNECTIONTYPE 8 OFSPECIMEN 3

TYPICALCROSSSECTION

10 10 665 MeshJ400[ /Zlv

__

LI\450\_2-D16Grade 300

L Precast Beam Type 2

Topping 10 10750 140011

665 Mesh750 /

Tie 016/

1Mesh

Topping

Hollow-coreunit

1197

Fig. 3. Types of special support reinforcement at the ends of hollow-core floor unitstested at the University of Canterbury (Ref. 13).

May-June 1995 43

hinges of the columns in such cases oflow frames is not high and can be provided by proper reinforcing detailing.

The preferred mechanisms of post-elastic deformation for structural wallsare also shown in Fig. 1. For cantileverwalls, a plastic hinge forms at the baseof the wall. For coupled structuralwalls, yielding also occurs in the coupling beams and, ideally, the beamsshould yield before the wall bases.

In summary, a capacity design approach is used in New Zealand to ensure that, in the event of a severe earthquake, flexural yielding of members at

the chosen plastic hinge position controls both the strength and post-elasticdeformation capacity of the structure.When the connections between theprecast concrete elements are placed incritical (potential plastic hinge) regions, the design approach in NewZealand ensures that the behavior ofthe connection region approaches thatof a monolithic cast-in-place structure(equivalent monolithic); thus, monolithic construction is emulated.

Possible brittle connections betweenmembers are made over-strong inorder to not be in critical regions. Re-

inforcing details and structural configurations can be arranged to ensure thatthe plastic hinging occurs away fromthe jointing faces of precast concretemembers and cast-in-place concretejoints, but plastic hinging in regionsincluding the jointing faces is permitted if appropriately designed.

Detailing for Ductility

The most important design consideration for ductility in the plastic hingeregions of reinforced concrete members is the provision of adequate longitudinal compression reinforcement aswell as tension reinforcement, and theprovision of adequate transverse reinforcement in the form of rectangularstirrups, or hoops overlapping or withcross ties, or spirals.

This ensures they act as shear reinforcement, to confine and, hence, toenhance the ductility of the compressed concrete, and to prevent premature buckling of the compressedlongitudinal reinforcement. A center-to-center spacing of transverse bars notexceeding six longitudinal bar diameters in plastic hinge regions is considered necessary to control bar budding.4

Failure modes to be prevented arethose due to diagonal tension or diagonal compression caused by shear, excessive plastic hinge rotation of heavily loaded columns, sliding shearalong construction joints or otherjointing faces or in plastic hinge regions, buckling of compressed longitudinal reinforcement, and bond failure along lapped splices or atanchorages. All of these undesirablefailure modes lead to prematurestrength degradation and reduced ductility. They can be avoided by use ofthe capacity design procedures.4

Joint core regions of beam-to-columnconnections need special attention because of the critical shear and bondstresses that can develop there duringseismic loading.46

PRECAST CONCRETEIN FLOORS

As is common in many countries,floors in New Zealand buildings inearly years were mainly of cast-in-place reinforced concrete construction.Significant use of post-tensioning was

Cast4n-placeconcrete andsteel In column I

Cast-In-place concreteand top steel In beam

Mldspan

Precastbeam unit

Precas1’beam unit

(a) System 1-Precast Beam Units Between Columns

MldspanCast-in-place concrete cast-in-placeand top steel In beam _,folnt

.1’

Precast beam unit

Precast orcast-In-placecolumn unit

Mortar orgroutJoint\

Precast orCast-In-placecolumn unIt

(b) System 2- Precast Beam Units Through Columns

precast •l

Mortar orgrout joint

MIdpanCast-In-place

joint

Precast T- unit

(C) System 3-Precast T-Unlts

Notes: Precast Concrete cast-in-place concrete

Reinforcement in precast concrete not shown

Fig.4. Arrangements of precast concrete members and cast-in-place concrete for

constructing reinforced concrete moment resisting frames (Refs. 14 and 15).

44 PCI JOURNAL

also made in cast-in-place concretefloors in the 1950s and 1960s. However, since the 1960s, precast concreteelements have become widely used infloors in New Zealand.

Currently, the majority of floors inNew Zealand buildings are constructed of precast concrete units,spanning one-way between beams orwalls. The precast concrete units aremade of either pretensioned, prestressed concrete or reinforced concrete (solid slabs, voided slabs, ribslabs, single tees or double tees) andgenerally act compositely with a cast-in-place concrete topping slab of atleast 50 mm (2 in.) thickness and containing at least the minimum reinforcement required for slabs.

Alternatively, precast concrete ribsspaced apart with permanent form-work of timber or thin precast concrete slabs spanning between are usedacting compositely with a cast-in-place concrete slab. The most common floors are constructed of precastconcrete hollow-core floor units. Themost frequently used depth of hollow-core unit is 200 mm (7.9 in.) plus a65 mm (2.6 in.) thick cast-in-placeconcrete topping. Typical spans are 8or 9 m (26 or 30 ft) long.

This trend of precast concrete floorsin New Zealand has come about because of the reduction in site costs resulting from reduced site labor andfast erection, and also because mostprecast concrete floors are lighter thancast-in-place floors, resulting insmaller dead loads and seismic forces.The current New Zealand practice ofusing mainly precast concrete floorscontrasts markedly with the practice ofmost overseas countries that usemainly cast-in-place concrete floors.

Support Details forPrecast Concrete Floors

The supports for precast concretefloor units may be either simple orcontinuous. Both supports have theiradvantages in different applications.Simple support suits long span orheavily loaded structures where itwould be difficult and costly to provide the required degree of negativemoment restraint at the supports. Support of precast concrete flooring with

moment fixity at the ends suits themore general commercial and residential type of construction, but requiresattention to detail in order to ensurethat the required degree of continuitycan be achieved.

The types of support for precastconcrete hollow-core or solid slabflooring units seated on beams, identi

fied by the New Zealand Guidelines,3can be divided into the three groupsshown in Fig. 2. The difference amongthese types of support is the depth ofthe supporting beam prior to placement of the cast-in-place concrete.

Some aspects of these three types ofsupport are:

Type 1 — The presence of well

Fig. 5. Rnforced concrete building frame incorporating precast concrete elementsin the beams between columns as used in System 1.

Cast-in-placecolumn

Cast-in-placejoint concrete

I,

Beam longitudinalreinforcement onlyshown

Top bars slidinto place

Precast beam

Cast-In-placecolumn

r

I—. 2 or h 8db - gwhichever is less

‘— g (hooks to terminateat the far side ofthe joint core)

Note: db = bar diameter

f= development length ofhooked anchorages

Fig. 6. Hooked lap of bottom bars within joint core for System 1.

May-June 1995 45

(c) Column barsafter being groutedin the joint core of aprecast concretebeam unit

Fig. 7. Constructionof a 22-story buildingusing System 2 inNew Zealand.

compacted cast-in-place concreteagainst the ends of the precast concrete floor unit enables reliable negative moment continuity to be developed. It is recommended that the coresof hollow-core flooring units be broken out at the ends and that the endsbe filled with cast-in-place concrete toenhance the shear strength. However,due to the reduced depth of the supporting beam at the stage when theprecast floor units are erected, moreshoring is generally required than withthe other support types.

Type 2 — If the vertical gaps between the supporting beam and thefloor units are too small, there may bedifficulty in compacting cast-in-placeconcrete both in the gaps and in the recesses of the hollow-core units for thistype of support. This can reduce the

shear strength and prevent the development of negative bending momentactions in the floor units. However,the depth of the supporting beam isgreater and, hence, less shoring is required when precast units are erected.

Type 3 — This support system maybe used for perimeter beams or walls.No formwork for the cast-in-placeconcrete topping slab is required.

Adequate support of precast concrete floor units is one of the mostbasic requirements for a safe structure.It is essential that floor systems do notcollapse as the result of imposedmovement caused by earthquakes orother effects that reduce the seatinglength. One source of movements during severe earthquakes that couldcause precast concrete floor units tobecome dislodged is the tendency of

the beams of ductile moment resistingframes to elongate when forming plastic hinges. This can cause an increasein the distances spanned by precastconcrete floor members.3’7

In the design of the length of theseating in the direction of the span, allowances must be made for tolerancesarising from the manufacturing process, the erection method, and the accuracy of other construction. Also, allowances must be made for thelong-term effects of volume changesdue to concrete shrinkage, creep andtemperatures effects, as well as for theeffects of earthquakes.

Some concern has been expressed inNew Zealand that there were cases inconstruction where the support provided for precast concrete floors wasinadequate. The New Zealand standards for design and construction inthe 1980s”8 had no specific requirements for the support of precast concrete floors.

As a result, the revised New Zealand concrete design standard NZS3l0l:l995 recommends that for precast concrete floor or roof members,with or without the presence of a cast-in-place concrete topping slab and/orcontinuity reinforcement, unlessshown by analysis or test that the performance of alternative details at thesupports will be acceptable, eachmember and its supporting systemshall have design dimensions selectedso that, under a reasonable combination of unfavorable construction tolerances, the distance from the edge ofthe support to the end of the precastmember in the direction of its span isat least 1/180 of the clear span but notless than: 50 mm (2 in.) for solid orhollow-core slabs or 75 mm (3 in.) forbeams or ribbed members.

The above recommendation requiring proven alternative support details,unless the specified end distances areprovided, is similar to that being considered by ACI Committee 318 for therevision of the current ACT BuildingCode.9 The above end distances aresimilar to those recommended byACI-ASCE Committee 550.10

One method of providing the alternative details that permit smaller seating lengths is to use special reinforcement between the ends of the precast

tconcrete beam corner unitbeing lowered into place usingtemporary plastic tubes as guides

A

46 PCI JOURNAL

Fig. 8. Construction of 152 m (499 ft)Coopers and Lybrand Tower using

System 2 in New Zealand.

concrete floor units and the supportingbeam that can carry vertical load in theevent of the precast concrete floorunits losing their seating. The specialreinforcement should be able to transfer the end reactions by shear frictionacross the vertical cracks at the endsof the units if the crack widths are relatively narrow or by kinking of the reinforcement crossing the cracks if thecrack widths are large.

This reinforcement can be in theform of hanger or saddle bars, or horizontal or draped reinforcement, as recommended by the New ZealandGuidelines,3the Precast/PrestressedConcrete Institute,’ and the FédérationInternationale de la Précontrainte.’2This special reinforcement passes overor is anchored to the supporting beam.For example, for precast concretehollow-core units, the reinforcementmay be either placed in some of thecores that have been broken out at thetop and filled with cast-in-place concrete or grouted into the gaps betweenthe units. Note that reinforcement in acast-in-place topping slab alone cannot be expected to provide an adequate load path to support the units,

should the seating be lost, because thetopping slab may split away from theprecast concrete units.

Recent tests conducted at the University of Canterbury3on special reinforcement. placed in filled cores at theends of hollow-core units and passingover precast supporting beams, haveinvestigated the three types of specialsupport reinforcement shown in Fig. 3.All three types were able to support atleast the service gravity loads of thefloor, in the event of loss of end seating, when no significant horizontaldisplacement of the floor occurred.

However, the plain round straight or

draped reinforcement with hookedends shown as tie connection Types 1and 3 in Fig. 3 are favored, because itwas found that they could undergosubstantial plastic elongation when theprecast concrete units were pulled horizontally off their 50 mm (2 in.) wideseating and subjected to significantvertical displacement. Plain round endhooked reinforcement was found toperform better than deformed reinforcement because bond failure propagating along the plain round bars allowed extensive yielding along thebar, thus allowing substantial plasticelongation before fracture.’3

/ su3;

(a) Construction overview of building

May-June 1995 47

Diaphragm Action

The floor system also has to act as adiaphragm during horizontal loadingdue to earthquakes in order to transferthe in-plane diaphragm forces fromthe floor into the lateral load resistingcomponents, such as frames andstructural walls. When floors are constructed of precast concrete units, it isessential to ensure that diaphragmforces can be transferred between theunits and to the supporting structureso that shear transfer over the floor isachieved. In New Zealand, a cast-in-place reinforced concrete topping slabat least 50 mm (2 in.) thick containingat least the minimum reinforcementrequired for slabs is considered an excellent means for transferring the in-plane diaphragm forces. Some limiteduse of precast concrete elements infloors without a cast-in-place toppingslab has also been made, but withadequate shear connection betweenthe elements.

MOMENT RESISTINGFRAMES WITH PRECASTREINFORCED ELEMENTS

Moment resisting frames incorporating precast reinforced concrete elements are widely used in NewZealand. The main challenge in thedesign of such structures is finding aneconomical and practical method forconnecting the precast concrete elements together. In New Zealand, if theconnections between the precast concrete elements in frames are placed incritical regions, such as potential plastic hinge regions, the approach is todesign and construct connections thatpossess stiffness, strength, and ductility similar to that of cast-in-place concrete monolithic construction.’4”5Inother words, monolithic constructionis emulated.

The general trend in New Zealandfor multistory buildings with momentresisting frames is to design theperimeter frames with sufficient stiffness and strength to resist most of thehorizontal seismic loading. The moreflexible interior frames will be calledon to resist only a small proportion ofthe horizontal forces, the exact amountdepending on the relative stiffnesses

of the perimeter and interior frames.If the perimeter frames are relatively

stiff, the columns of the interiorframes will carry mainly gravity loading. Also, the interior columns can beplaced with greater spacing betweencolumns. For the perimeter frames, thedepth of the beams may be large without affecting the clear height betweenfloors inside the building and thecolumns can be at relatively close centers. The use of one-way perimeterframes avoids the complexity of thedesign of beam-to-column joints oftwo-way moment resisting frames.

Note that if the perimeter framebeams are fairly deep, and thecolumns are close and small, it may bedifficult to ensure strong column-weakbeam behavior. Hence, the relative dimensions of the beams and columns intall ductile frames should be such thatstrong column-weak beam behaviorcan be achieved. Details of severalbuildings in New Zealand constructedin the late 1980s and early 1990s thatincorporate significant quantities ofprecast concrete in their frames andfloors are described elsewhere.’2

Several possible arrangements of

Fig. 9. Construction of a 1 3-story building using System 3 in New Zealand.

48 PCI JOURNAL

precast reinforced concrete membersand cast-in-place concrete formingductile moment resisting multistory reinforced concrete frames have beenidentified.’415 Arrangements commonlyused in New Zealand for strong colunm-weak beam designs are shown inFig. 4. These three arrangements canalso be used in a modified form whenone- or two-story frames with strongbeam-weak column design are permitted. The objective in design of the systems is to achieve behavior emulatinga monolithic structure. The three arrangements are described below.

System 1

An arrangement involving the use ofprecast reinforced concrete elementsto form the lower part of the beams isshown in Fig. 4(a). The precast beamelements are placed between columns,seated on the cover concrete of thepreviously cast-in-place reinforcedconcrete column below, and supportedunder the precast elements (see Fig.5). In some cases, there may be twoprecast beam elements per span with acast-in-place joint at midspan wherethe longitudinal beam bars are spliced.A precast concrete floor system isplaced seated on the top of the precastbeam elements and spanning betweenthem. The reinforcement is thenplaced in the top of the beam, the topping slab over the floor system, thebeam-to-column joint core, and thenext story height of column. Lastly,the cast-in-place concrete is placed.The frame can be designed using theprovisions for totally cast-in-placeconcrete structures.

This system leads to a large reduction in the quantity of site formworknecessary. A difficulty with the connection detail is that the bottom longitudinal bars of the beams, protrudingfrom the precast beam elements, needto be anchored in the joint cores (seeFig. 6). Hence, the column dimensionsneed to be reasonably large to accommodate the required developmentlength and to reduce the congestion ofthe hooked reinforcement.

Another possible problem is that thecritical section of the potential plastichinge region in the beam occurs at thecolumn face where there is a vertical

Column

Precast beam

joint between the cast-in-place concrete of the joint core and the end ofthe precast beam. This is permitted bythe New Zealand standard, NZS3lOl:l995.

To make the transfer of verticalshear possible, it is recommended thateither the end of the precast beamshould be clean, free of laitance andintentionally roughened to a full am-

Cast-In-place joint

coIumL][JYoIumn

,1 L:.J J54.-4 I \_•Precast

L4e 2d÷e.

•“—

Precast

(a) Conventional Straight Bar Lap

(çln.laceJointColu.j

Precastbeam

(b) Hooked Lap

Cast-In-place jColumn

Precast beam

(c) Double Hooked Lap

/oint Column

Overstrength stub

Column

[ 1DiagonPrecast J welded to Steel platesbeamsteelplate bolted together

(d) Diagonal Beam Reinforcement

Precastbeam

Notes: Transverse reInforcement Is not shown

i= development length of hooked anchorage

= development length of straight bar

Fig. 10. Some details for midspan connections between precast reinforcedconcrete beam elements that have been used in New Zealand (Ref. 3).

May-June 1995 49

plitude of not less than 5 mm (0.2 in.),or alternatively, a mechanical keyshould exist at the end of the beam. Awell prepared joint at the end of theprecast beam at the critical section atthe column face is no worse than asimilarly placed construction joint between pours of cast-in-place concretein totally cast-in-place construction.

Also, it is recommended that the topsurface of the precast concrete beamunit be clean, free of laitance and intentionally roughened to a full amplitude of not less than 5 mm (0.2 in.).

System 2

An arrangement that makes moreextensive use of precast concrete andavoids placement of cast-in-place concrete in the congested beam-to-columnjoint core regions is shown in Fig.4(b). The reinforced concrete columnscan be either precast or cast-in-placeto occupy the clear height betweenbeams. The precast portions of the reinforced concrete beams extend fromnear midspan to midspan, and, hence,include within the precast elementover the columns the complex arrangement of joint core hoop reinforcement that is fabricated at the precasting site. The precast portions ofthe beams are placed seated on theconcrete column beneath, with suitable material between, and supportedfor construction stability.

The protruding longitudinal columnbars from the reinforced concrete column below pass through preformedvertical holes in the precast beam element and protrude above the top surface of the element. The holes in theprecast beam elements are preformedusing corrugated steel ducting and aregrouted after the column bars havepassed through. The protruding bars ofthe precast beam elements are connected in a joint to be cast in place atmidspan.

A precast concrete floor system isplaced seated on the precast beam elements and spanning between them.The reinforcement is then placed inthe top of the beam and the toppingslab, and the cast-in-place concrete isplaced. Alternatively, the total depthof the beam can be precast, includingthe top and bottom longitudinal rein-

forcement in the precast beam, and theprecast floor system supported onledges on the sides of the precastbeams. The columns of the next storyare then positioned above the beamsusing grouted steel sleeves or ducts toconnect the vertical bars if thecolumns are precast or using normalreinforced concrete details if thecolumns are cast in place.

An advantage of this system is thatthe potential plastic hinge regions inthe beams occur within the precast elements away from the jointing facesbetween the precast elements. Also,this system makes extensive use ofprecast concrete and eliminates thefabrication of complex reinforcing details during construction. A possibledifficulty is the tighter tolerances necessary when assembling the precastconcrete elements.

Figs. 7 and 8 show two buildingsduring construction in New Zealandusing this system. The structure ofboth buildings consists of stiff moment resisting perimeter frames withinterior frames carrying mainly onlygravity loading. In both buildings, thecolumns between the precast beamunits were cast in place. The floorsconsisted of precast concrete with acast-in-place concrete topping. For theCoopers and Lybrand Tower, a construction time of 4 to 5 days per officefloor was achieved regularly throughout the construction period.2’

System 3

A third possible arrangement incorporating T-shaped precast reinforcedconcrete elements is shown in Fig.4(c). The vertical column bars in theprecast T units are connected usinggrouted steel sleeves or ducts. At themidspan of the beams, the bottom barscan be connected in a cast-in-placeconcrete joint. An alternative to the Tshaped precast concrete units is theuse of cruciform-shaped units with thejoints between columns occurring atthe midheight of the stories. Precastconcrete floor systems can be used aswith the other systems.

An advantage of System 3 is the extensive use of precast concrete and theelimination of the fabrication of complex reinforcing details during con-

struction. A possible constraint is thatthe precast elements are heavy andcrane capacity may be an importantconsideration.

Fig. 9 shows a perimeter frame of a13-story office building constructedusing precast concrete cruciform-shaped units with columns two storiesin height and two levels of beamstubs. Reinforcement projects from thebeam stubs to be incorporated in cast-in-place hooked splices at the midspanof each beam. The column joint between the precast units consists of anepoxy grouted bedded joint andgrouted steel sleeves. Note the longspans of the beams of the more flexible interior frames.

Midspan ConnectionsBetween Precast ConcreteBeam Elements

Some details for cast-in-placemidspan connections in beams thathave been used are illustrated in Fig.10. The New Zealand concrete designstandard, NZS 3101:1995, requiresthat no portion of any lap splice of thelongitudinal reinforcement in thebeam be located within a length of oneeffective depth of beam from the critical section of the potential plastichinge region. This normally meansthat lap splices in beams cannot commence closer than one effective beamdepth from the column face.

For short span beams of perimeterframes, a straight lap splice in themidspan region may be too long tomeet this requirement. In this case, theconventional straight bar lap of Fig.10(a) can be shortened using hookedlaps, as shown in Figs. 10(b) and (c).

The double hooked lap of Fig. 10(c)is the most convenient hooked lap toconstruct because the protruding endsof the reinforcement from the precastconcrete beam elements do not overlap, and therefore, the beam elementscan be positioned during constructionwithout difficulty. The lap is madeusing “drop in” bars, which consist ofshort lengths of bar with a hook ateach end.

Diagonal reinforcement has beenused where the shear forces in thebeams are large [see Fig. 10(d)]. Thedesign and detailing of this connection

50 PCI JOURNAL

detail require particular care. The endsof the diagonal bars are welded to steelplates that are bolted together atmidspan during construction to makethe connection. Significant vertical tiesare required between the bends in thediagonal reinforcement to resist thevertical component of the force in thediagonal bars. Also, bearing failure ofthe concrete should not occur under thebends of the diagonal reinforcement.

Grouting of aBeam-to-Column Joint

Fig. 11 illustrates a typical arrangement of a precast concrete beam element placed on a cast-in-place or precast concrete column, as used inSystem 2 of Fig. 4(b). The beam elements can be seated on leveling shims.The column bars pass through corrugated metal ducts, cast in the precastconcrete beam. The diameter of theduct should accommodate the tolerances, plus a recommended additional10 mm (0.39 in.) clearance betweenthe duct wall and bar surface to allowgrout to flow between the duct walland bar. Typically, the duct diametersrange from two to three times thenominal diameter of the bar. The twoprincipal methods of grouting a precast concrete beam-to-column jointare as follows:3

Method 1 — The horizontal joint atthe beam-to-column interface is firstsealed around the outside and thengrout (typically non-shrink cement-based) is pumped in at an inlet port (ortube) at one corner of the horizontaljoint to displace air progressivelyacross the interface (see Fig. 11). Ifthe grout has a high viscosity, it maystart to flow up the open ducts, starting at the duct closest to the groutinlet. It is recommended that outletports be provided at the other threecorners of the interface. These are progressively plugged once grout withoutair bubbles flows out.

When all the outlets are plugged,further pumping of grout will result inthe ducts being filled upwards fromthe bottom. The duct nearest the inletshould fill first while the one furthestaway (opposite corner) may requiretopping up by use of a tremie tube, orby pouring-in from a dispenser in such

a way that grout runs down in contactwith the reinforcing bar to avoid airlocks. The inlet tube or port is pluggedonce injection is completed.

Method 2 — The horizontal joint atthe beam-to-column interface is firstsealed around the outside and thengrout is poured in from a dispenser

down one corner duct. Progressively,the grout will flow across the interfaceand up the remaining ducts. It is recommended that, as with Method 1,outlet ports be used to confirm theprogress of the grouting of the interface. Topping off of ducts remotefrom the filling position may be neces

Grout ‘top off’as necessary

Precastconcretebeam

rouGmut

ColumnSeal

1,,/corruated ducts

Precast concretebeam

LW—Grout level

Air and groutGrout//

Sealed

Fig. 11. Grouting of abeam-to-column joint (Ref. 3).

Column

tconcrete

::::

(b) Cruciform UnitMid Height Column JointUpward Sleeving

beam

(a) Full Height Column

(c) ‘T’ Precast UnitBeam and ColumnCast Together

Fig.

(d) Cruciform UnitMid Height Column JointDownward Sleeving

12. Typical connections between precast concrete column elements (Ref. 3).

May-June 1995 51

sary. Again, care must be exercised sothat no air is trapped in the ducts.

The grouting operation must be conducted using proper quality assuranceprocedures to ensure that all voids areproperly and solidly grouted.

Connections Between PrecastConcrete Column Elements

Fig. 12 shows typical connectionsbetween precast concrete column elements using grouted splices.3 Someconnections are made by bars protruding downwards from the precast column element above, with the bars located in pregrouted corrugated metalducts or proprietary steel sleeves [seeFig. 12(d)]. The other configurationshave bars protruding upwards intocorrugated metal ducts or proprietarysteel sleeves that are post-grouted.The precast column elements may beseated on leveling shims. The horizontal joint can be grouted or can consist of a bed of cement or epoxy-basedmortar.

The two types of duct used in column splices, namely, proprietary steelsleeves and corrugated metal ducts,are shown in Fig. 13. When using theproprietary steel sleeves, the bars arebutted in the sleeve. When using corrugated metal ducts, the bar in the ductis lapped with the bar or bars in theadjacent precast concrete. For postgrouting with either type of duct configuration, it is necessary to have aninlet tube at the base of each duct andan outlet tube at the top for bleedingair and grout.

Pinned Joints

Pinned joints can be used to connectsecondary beams to primary beams.They are sometimes also used atbeam-to-column joints to reduce themoment input from the beam to thecolumn. An example of a pinned jointat a secondary beam to main beamconnection is shown in Fig. 14. Thetypical 20 mm (0.79 in.) tolerance gap,between the end of the secondarybeam and the side of the main beam,implies that the precaster and contractor are required to work to very stringent tolerances. Also, the weldingof reinforcing bars to rectangular hollow steel sections, or to other steel de

connections (Ref. 3).

tails, is a critical operation. Weldthroat thicknesses should be carefullymonitored.

Tests on ConnectionsBetween Precast ReinforcedConcrete Elements

A research project at the Universityof Canterbury has investigated theseismic performance of connectionsbetween precast reinforced concrete elements of moment resisting frame systems commonly used in buildings inNew Zealand.7 This research was necessary because the solutions proposedby designers and contractors for theseframes have normally been extrapolations from code provisions for cast-in-place concrete. This stems from thefact that New Zealand codes, even inthe had only limited provisions for precast concrete construction.

The design objective for theseframes incorporating precast concreteelements is to achieve behavior as fortotally cast-in-place concrete structures. That is, monolithic constructionis emulated. The research project investigated whether the frames couldachieve stiffness, strength and ductilebehavior similar to completely cast-in-place concrete frames when subjected

to cyclic loading in the post-elasticrange, which simulated the effects ofsevere earthquakes.

Six full-scale subassemblies offrames were subjected to simulatedseismic loading.7 Two of the sub-assemblies were of cruciform shapeand had precast concrete beam elements connected in two different waysat the beam-to-column joint, representing Systems 1 and 2 of Fig. 4.

At the vertical construction joints ofthe System 1 subassembly, which interfaced precast and cast-in-place concrete, the ends of the precast beamswere clean and free of laitance, butwith only a small amount of roughness, approximately 1 mm (0.04 in.)full amplitude, to represent the worstconditions normally encountered inconstruction practice. The precast concrete beam of the System 2 subassembly had corrugated metal ducts in thejoint region through which the verticalcolumn bars passed and were grouted.The columns of both subassemblieswere cast-in-place concrete. During thecyclic horizontal loading, horizontal interstory drifts of up to at least ±3 percent occurred. (Horizontal interstorydrift is horizontal displacement givenas a percentage of the story height.)Both subassemblies performed with no

1LRebar

Precast11 concretefj9ember

1 High strengthgrout

Rebar (upper)(lapped beside)

Gro Ut and air

Precastconcretemember

-‘Duct

Beddingmortar

(a) Steel sleeve

Grout

Shims

Rebar (lower)

Rebar 25 mm minimum(upper) and code spacing

requirements

(b) CorrugatedMetal Duct Rebar (lower)

Duct

Section A-A

Fig. 13. Steel sleeve splices and corrugated metal ducts used for column

52 PCI JOURNAL

significant difference in behavior frommonolithic construction.7

Another four full-scale subassemblies were of H shape and were testedsubjected to simulated seismicloading7 to investigate the performance of the four details for midspanconnections between precast concretebeam elements shown in Fig. 10. Thedetails shown in Fig. 10(a), (b), and(c) were found to perform very satisfactorily in that the beams showed nosignificant difference in behavior frommonolithic construction. The testsconfirmed that the splice could beginat a distance of one effective beamdepth from the critical section of thebeam at the column face. This findingmeans that beams with relatively smallspan-to-depth ratios can be used,which is often a desirable feature inthe configuration of moment resistingperimeter frames.

The test on the detail shown in Fig.10(d) indicated that a problem couldexist in the regions of the bends of thediagonal beam bars, if the design ofthat reinforcement does not considerthe possible bearing stresses on theconcrete at the inside of the bend.Also, the three-dimensional effectscaused by the arrangement of the longitudinal reinforcement in that regionrequires the presence of significant tiereinforcement in the form of closedstirrups. However, it was found thatthe detail can be designed to performsatisfactorily.7

An interesting aspect of the test results from the six subassemblies7wasthe large permanent elongation of thebeams that was measured after yielding of the longitudinal reinforcementoccurred. This was due to residualplastic tensile strains in that steel during cyclic loading.

The elongation of the beams gradually increased during the loading cycles. At horizontal displacements corresponding to interstory drifts of about±2 percent, the total length of thebeams of the subassemblies had increased by 20 to 30 mm (0.8 to 1.2 in.)as a result of the deformations of thelongitudinal bars in the two plastichinge regions of each subassembly.

A practical consideration is that thisbeam growth during a severe earthquake could result in loss of support of

precast concrete floor systems due toan increase in the span between supporting beams. Hence, there must bespecial support reinforcement at theends of precast concrete floors, asshown in Fig. 3, unless seating lengthsare adequate.

MOMENT RESISTINGFRAMES WITH PRECAST,

PRESTRESSED ELEMENTSAnother building system that has

become popular in New Zealand involves the use of precast concretebeam shells as permanent formworkfor beams (see Fig. 15). The precastbeam shells are typically pretensioned,prestressed concrete U-beams and areleft permanently in position after the

cast-in-place reinforced concrete corehas been cast. The precast U-beamssupport the self weight and construction loads and act compositely withthe reinforced concrete core when subjected to other loading in the completed structure. A building under construction is shown in Fig. 16.

The precast concrete U-beams aregenerally not connected by reinforcement to the cast-in-place concrete ofthe beam or column. Reliance is normally placed on the bond between theroughened inner surface of the precastU-beam and the cast-in-place concretecore to achieve composite action. Occasionally, protruding stirrups or tiesfrom the U-beams have been used toimprove the interface shear strength.During construction, it is very impor

Pair of RHS hanger bracketswith U-bars welded on

Cast-in-place concretetopping \

.Top continuityreinforcement

Precast concretefloor

dbsr)

—

t mont( 7aliy used)

Bearing angle

Precast concrete _......,7secondary beam 20mm gap ..4.. Precast concrete

main beam

Fig. 14. Example of secondary beam to main beam connection using a rectangularhollow steel (RHS) section seated on a steel angle (Ref. 3).

IProprietary floor systemand cast-In-placereinforced concrete

topping

Cast-in-placeconcrete beam

Reinforcedccncretecolumn

Pretenslonedprecast concrete U-beam

Fig. 15. Construction details of a structural system using precast, prestressedconcrete U-beams and cast-in-place reinforced concrete (Ref. 22).

May-June 1995 53

tant to ensure that the inside surfacesof the shell beams are clean when thecast-in-place concrete is cast, otherwise sufficient bond between the shelland core cannot develop.

Tests have been conducted in NewZealand in which full-scale subassemblies, typical of moment resistingframes constructed using this buildingsystem, have been subjected to simulated seismic loading.22 No stirrups orties protruded from the precast concreteU-beams. The resistance to seismicforces was designed to come from thecast-in-place reinforced concrete coreof the U-beams. The tests were conducted because doubts had been expressed by some designers and buildingofficials concerning the ability of thisform of composite construction to perform as ductile moment resistingframes. It was felt that cracking mayconcentrate in the beam at the columnface at the discontinuity caused by theend of the precast concrete U-beam.

However, the tests22 demonstratedthat during severe seismic loading,there is a tendency for the plastic hinging to spread along the cast-in-placereinforced concrete core within theprecast U-beam due to some breakdown of bond. Hence, the plastichinge rotation does not concentrate inthe beam at the column face and, as aresult, no undesirable concentration of

curvature occurs at that section. Seismic design recommendations for thistype of construction are available.22Reinforced concrete beams incorporating precast, prestressed concrete beamshells and cast-in-place reinforcedconcrete cores are suitable for use inductile moment resisting frames.

STRUCTURAL WALL-FLEXIBLE FRAME

SYSTEMS

Structures comprising both reinforced concrete structural walls andframes offer advantages. The structural walls, normally of cast-in-placeconcrete, can be designed to resist almost all of the horizontal forces actingon the building. The frames, which aremuch more flexible than the walls,will resist only a small proportion ofthe horizontal forces, the amount depending on the relative stiffnesses ofthe walls and frames.

The columns of such frames are present in the building mainly to carry thegravity loading. When such systemsare used in seismic regions, the framescan be designed for limited ductility ifit can be shown that when the ductilewalls have deformed in the post-elastic range to the required displacement ductility factor or drift during severe seismic loading, the ductility demand on the frames is not large.

A New Zealand building employingthis design is shown in Fig. 17. Thecentral cast-in-place reinforced concrete walls, forming the service coreof the building, were designed to resistthe seismic loading. The perimeterframe of precast concrete beams(formed in the shape of trusses forlightness) and the columns (formedusing precast concrete tubes infilledwith cast-in-place concrete) were designed mainly for gravity loading.

STRUCTURAL WALLSWITH PRECAST

CONCRETE ELEMENTSStructural reinforced concrete walls

in buildings have long been recognized in New Zealand as efficientstructural systems for resisting horizontal forces due to earthquakes.

Properly designed walls have a largeinherent strength and their ample stiffness means that displacements duringsevere earthquakes are reduced, thusproviding a high degree of protectionagainst damage to structural and non-structural elements.

Comprehensive design provisionsexist for cast-in-place reinforcedconcrete structural walls.4 In NewZealand, it is considered that well proportioned ductile cast-in-place reinforced concrete coupled walls formthe best earthquake resisting structuralsystem. The recent trend towards moment resisting frames, rather thanstructural walls, in New Zealand hasbeen mainly due to the preference ofarchitects for the more open spaces offloors when walls are not present.

Most structural walls for multistorybuildings in New Zealand have beenmade of cast-in-place reinforced concrete, but there has been significantuse of precast concrete walls forsmaller buildings.

Precast reinforced concrete structural wall construction usually fallsinto two broad categories:3monolithicor jointed. In monolithic wall construction, the precast concrete elements are joined by “strong” reinforced concrete connections thatpossess stiffness, strength, and ductility approaching that of cast-in-placeconcrete monolithic construction. Injointed wall construction, the connections are “weak” relative to the adjacent wall panels and, therefore, governthe performance of the building.

Monolithic Precast ConcreteStructural Wall Systems

Monolithic precast reinforced concrete structural wall systems aredesigned according to the code requirements of cast-in-place concreteconstruction.3

Horizontal joints between precastconcrete wall panels are usuallygrouted connections. The vertical reinforcement is usually connectedthere using either grouted steel sleevesplices or a lap formed by grouting abar extending from the end of oneprecast panel into a corrugated metalduct in the matching panel. Some typical details of monolithic horizontal

Fig. 16. Construction of a momentresisting frame using precast concreteU-beams.

54 PCI JOURNAL

joints are shown in Fig. 18.When corrugated metal ducts are

used, the starter bars that project intothe ducts are usually designed for afull lap length. In general, centralstarter bars are lapped with pairs ofsmaller bars, one on each face of theprecast concrete wall section. Alternatively, all of the main flexural reinforcement is lapped on the precastconcrete wall centerline and some additional basketing cover reinforcementis provided.

The horizontal joint between precastconcrete panels is usually roughenedto avoid a sliding shear failure.

Vertical joints between precast concrete wall panels are typically verticalstrips of cast-in-place concrete. Horizontal reinforcement from the ends ofthe adjacent panels protrude into thejoint zone and are lapped. The widthof the cast-in-place concrete jointzone is determined by the code requirements for lap lengths of horizontal reinforcement. Typical details ofmonolithic vertical joints are shownin Fig. 19.

Vertical joints shown as Types Dand E need to be detailed with extremecare. During construction, once thelapping bars have been overlapped,the ability for lowering the wall panelsover the starter bars is very restricted.These details will typically work onlywhen grouted steel splice sleeves areused to splice the vertical flexural reinforcement and when the laps of thevertical bars in the joints are madenear floor level.

As an example, Fig. 20 shows partof the construction of a two-storybuilding that uses full height precastconcrete structural wall panels toprovide support for the precast concrete floor system and the roof. Theprecast concrete walls, which provide the lateral load resistance of thebuilding, were designed as cantilevered structural walls of limitedductility to the seismic requirementsof what was the current New Zealandconcrete design code.’

The connection detail between thewalls and foundations was designed towithstand the larger seismic forcescorresponding to the elastic responseof the structure to a severe earthquake.The vertical joints between panels,

(a) Perimeter structure

(b) Typical floor plan

Fig. 17. Building with a precast reinforced concrete perimeter frame with seismicforces resisted mainly by cast-in-place reinforced concrete interior structural walls.

May-June 1995 55

Grout outlet vent

Monolithic Precast ConcreteWall ConstructionHorizontal Joint - Type A

Lap bars in preformed metal ductsor grouted steel splice sleeve.Area of lap bar 2 x Area ofvertical bars

Horizontal S

____________________

reinforcement :.:.:.:.:.:.:::.:.:::::.:::::.:.:.:::::::.:::::.:.:.:::::..::::......:.:l\Lap bars lap with pairsof vertical bars

Section A-AAlternative 1

Reinforcement same as Alternative 1‘- except horizontal shear reinforcement

I Y is tied directly to the preformed metal

I ‘ ducts or grouted splice sleeve

Section A-AAlternative 2

Full length bars lap within full height preformedmetal ducts or full length lap bars spliced withgrouted steel splice sleeves.

Vertical basketingSection A-A reinforcement

Alternative 3

Monolithic Precast Concrete Wall ConstructionAlternative Sections A-A

Fig. 18. Details of horizontal joints in monolithic precast reinforced concrete wall construction (Ref. 3).

Steel splicesleeve

)ut outlet

Preformed metal ductfilled with expansive grout

Lap bars to lap with‘pairs of vertical bars

inlet tuberung strips

Precast wall panels

Lap bar

topping overhollow corefloor planks

Precast wall panels

Monolithic Precast ConcreteWall ConstructionHorizontal Joint - Type B

56 PCI JOURNAL

shown during construction in Fig.20(a), consist of horizontal overlapping hairpin-shaped reinforcement thatprojects from each of the wall panelsand cast-in-place concrete creating amonolithic joint of Type E, as shownin Fig. 19.

A vertical steel bar was placed inthe space between the ends of the hairpins prior to casting the concrete. Details of the connection between thewall and the foundation are shown inFig. 20(b). Holes were formed in thebases of the panels so that horizontalreinforcing bars could be placedthrough to resist the design horizontalshear forces and tension forces resulting from overturning moments.

Jointed Precast ConcreteStructural Wall Systems

In jointed construction, the connection of precast reinforced concretecomponents is such that sections ofsignificantly reduced stiffness andstrength exist at the interface betweenadjacent precast concrete wall panels.This type of precast concrete wall construction is not common for high riseconstruction in New Zealand; however, it has been extensively used inthe tilt-up construction of typicallyone- to three-story apartment, officeand industrial buildings.3

For tilt-up construction, relativelylarge reinforced concrete wall panelsare cast horizontally on top of concrete floor slabs or casting beds adjacent to final wall panel positions.When the concrete has gained sufficient strength for the wall panels to remain uncracked during lifting operations, the walls are tilted up and liftedinto their permanent positions. Generally, tilt-up walls are secured to theadjacent structural elements withjointed connections consisting of various combinations of concrete inserts,bolted or welded steel plates or anglebrackets, and lapped reinforcementsplices within cast-in-place joiningstrips. Such walls are designed asstructural walls of limited ductility.That is, the design seismic forces areon the order of twice those used in thedesign of ductile walls.

Unfortunately, the revised NewZealand concrete design standard,

Cast-in-place d for horlz. bars All horizontal shearreinforcement lapconcrete bandaesplicedjoint

IF-,

Precast concretewall panels

Sides of wail”—. Vertical reinforcementpanels keyed typically lap splicedand roughened above floor level

Type A

All horizontal shearCast-in-placereinforcement splicedconcrete bandagej,,

I.e.with 900 standardJoint

IPrecast concretewall panels

Sides of we’II”Vertical reinforcementpanels keyed typically lap splicedand roughened above floor level

Type B

All horizontal shearCast-in-place tdh reinforcement splicedconcrete band_ with 900 standardjoint ,__—“hook lap bars.

4. _i.e.

1•Vertical reinforcementtypically lap splicedabove floor level

,‘ \j5 of wallPrecast concrete panels keyedwall panels and roughened

Type C

Cast-in-place concretejoint Sealantor grout filled

/All horizontal shearreinforcementhairpin spliced

Precastconcretewall panels

/Sides of jointroughened

Type D

‘Vertical reinforcementtypically lap splicedabove or belowfloor level

All horizontal shearreinforcementhairpin spliced

Cast-in-placeconcrete bandageJoint

3 Vertical reinforcementPrecast concrete Sides of Joint typically lap splicedwall panels keyed and above orbelow

roughened floor levelType E

Fig. 19. Some details of vertical joints in monolithic precast reinforced concretewall construction (Ref. 3).

May-June 1995 57

NZS 3lOl:l995, does not have design recommendations covering allaspects of tilt-up construction. However, a research project is currentlyin progress at the University of Canterbury23 with the objective of cataloging currently used connection details, assessing and testing themwhere necessary, and recommendingappropriate details for tilt-up andjointed construction.

(b) Detail of wall/foundation junction

Fig. 20. Construction of a two-story building incorporating full height precastconcrete wall panels (Ref. 3).

TOLERANCES

Successful precast concrete construction relies on a full understandingof the need for tolerances and the fullimplications of variations in dimensions. This understanding must be developed by designers, fabricators, andconstructors.

The New Zealand requirements fortolerances for precast concrete construction are given in the constructionspecification.8More complete recommendations for tolerances for precastconcrete used can be found in PCIpublications.24

The New Zealand Guidelines3 suggest three different types of tolerances,namely, product, erection, and interface tolerances, as defined below:

1. Product tolerances relate to thedimensions of an individual precastconcrete component. They are set bythe designer to control production inorder to achieve the structural and architectural requirements.

2. Erection tolerances are the allowances needed between the actuallocation of precast concrete components and the primary control surfaces,such as grids and datum levels.

3. Interface tolerances refer to theallowances needed for the jointing orattaching of material in contact withthe precast concrete components.

Experience in New Zealand showsthat designers should work together asclosely as possible with contractorswhen specifying tolerances so that appropriate allowances can be made,thereby reducing construction difficulties. When designing connectionsthat are sensitive to tolerances, thesum of the maximum reasonable tolerances can be used to define theworst design case.

I Precast concretewall/J /012 at 200

016 bar per holein precast panels

012 at4-H32

4-H20

F4H20

58 PCI JOURNAL

CONCLUSIONSBased on the preceding discussion,

the following conclusions can bedrawn:

1. Precast concrete floor systems,normally spanning one-way, havebeen commonplace in New Zealandsince the 1960s. Also, non-structuralprecast concrete has been widely usedfor the cladding of buildings.

2. The building boom in NewZealand in the mid-1980s produced asignificant increase in the use of structural precast concrete, particularly formoment resisting frames, because ofthe advantages of high quality factorymade units, speed of construction, andthe reduction of site formwork andlabor. This required developments inall aspects of the use of structural precast concrete as designers and contractors sought increasingly effectivemethods of design and construction.

3. The design approach for momentresisting frames incorporating precastreinforced concrete elements in NewZealand is similar to that for totallycast-in-place reinforced concretestructures. That is, the methods usedfor connecting the precast concrete elements together are generally aimed at

achieving behavior equivalent to thatof monolithic construction.

4. For moment resisting frames, thestructural arrangements include precastreinforced concrete beam elementsspanning between columns, precast reinforced concrete beam elements passing through columns, and T-shaped andcruciform-shaped precast reinforcedconcrete elements. Structural continuitybetween precast concrete elements isgenerally achieved using cast-in-placereinforced concrete. A structural system utilizing precast, prestressed concrete U-beams with a cast-in-place reinforced concrete core is also in use.

5. Significant use of precast reinforced concrete wall elements is alsobeing made for low rise structural wallconstruction, particularly for tilt-upwalls.

6. The successful use of precastconcrete requires close cooperationbetween designers, precasters andcontractors.

7. New Zealand design codes havetraditionally contained extensive provisions for cast-in-place reinforcedconcrete but are now being extendedto include design provisions for precast concrete.

8. Laboratory tests, involving the

simulated seismic loading of full-scalesubassemblies incorporating precastconcrete elements, have been conducted in New Zealand.7 These testsgive confidence in the design and construction of a range of connections between precast concrete members that,when first developed, went beyond thecodes of the time.

ACKNOWLEDGMENTSThe author gratefully acknowledges

the helpful discussions of many members of the design and construction profession in New Zealand. Particularlyappreciated are the members of theStudy Group of the New Zealand Concrete Society and of the New ZealandNational Society for Earthquake Engineering, which produced the “Guidelines for the Use of Structural PrecastConcrete in Buildings,” published bythe Centre for Advanced Engineeringof the University of Canterbury in August 1991. Thanks are also due to R. G.Wilkinson of the Holmes ConsultingGroup, and to A. I. O’Leary ofKingston Morrison Ltd., for providingmaterial. The efforts of graduate students J. I. Restrepo and J. C. Mejia arealso gratefully acknowledged.

May-June 1995 59

REFERENCES

1. Code of Practice for the Design ofConcrete Structures, NZS 3101:1982,Standards Association of New Zealand,Wellington, New Zealand, 1982.

2. Uniform Building Code, UBC:1991,International Conference of BuildingOfficials, ‘Whittier, CA, 1991.

3. “Guidelines for the Use of StructuralPrecast Concrete in Buildings,” Report of a Study Group of the NewZealand Concrete Society and theNew Zealand National Society forEarthquake Engineering, Centre forAdvanced Engineering, University ofCanterbury, Christchurch, New Zealand, 1991, T74pp.

4. Design of Concrete Structures, NZS3101:1995, Standards Association ofNew Zealand, Wellington, NewZealand, 1995.

5. Code of Practice for General Structural Design and Design Loadings forBuildings, NZS 4203: 1992, StandardsAssociation of New Zealand,Wellington, New Zealand, 1992.

6. Park, R., and Paulay, T., ReinforcedConcrete Structures, John Wiley &

Sons, New York, NY, 1975, 769 pp.

7. Restrepo, J. I., Park, R., and Buchanan,A. H., “The Seismic Behaviour ofConnections Between Precast ConcreteElements,” Research Report No. 93-3,Department of Civil Engineering, University of Canterbury, Christchurch,New Zealand, April 1993.

8. Specification for Concrete Construction, NZS 3109:1987, Standards Asso

ciation of New Zealand, Wellington,New Zealand, 1987.

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12. Fédération Internationale de Ia Précontrainte, Precast Hollow- CoreFloors, Thomas Telford, London,1988,31 pp.

13. Mejia, J. C., and Park, R., “Tests onSpecial Reinforcement for the EndSupport of Hollow-Core Precast Concrete Floor Units,” PCI JOURNAL,V. 39, No. 5, September-October1994, pp. 90-105.

14. Park, R., “Seismic Design Considerations for Precast Concrete Construction in Seismic Zones,” Seminar onPrecast Concrete Construction in Seismic Zones, V. 1, Japan Society for thePromotion of Science-United StatesNational Science Foundation, Tokyo,Japan, 1986, pp. 1-38.

15. Park, R., “Precast Concrete in SeismicResisting Building Frames in NewZealand,” Concrete International, V.12, No. 11, November 1990, pp. 43-51.

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22. Park, R., and Bull, D. K., “SeismicResistance of Frames IncorporatingPrecast Prestressed Concrete BeamShells,” PCI JOURNAL, V. 31, No. 4,July-August 1986, pp. 54-93.

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25. Recommended Practice for Erectionof Precast Concrete, Precast/Prestressed Concrete Institute, Chicago,IL, 1985, 87 pp.

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