Seismic Design and Constructionof Precast Concrete Buildingsin New Zealand

Robert ParkProfessor EmeritusDepartment of Civil EngineeringUniversity of CanterburyChristchurch, New Zealand

Trends and developments in the use of precast reinforced concrete inNew Zealand for floors, moment resisting frames and structural wallsof buildings are described. Currently, almost all floors, most momentresisting frames and many one- to three-story structural walls inbuildings are constructed incorporating precast concrete elements.Aspects of design and construction, particularly the means of formingconnections between precast concrete elements, are discussed. Thepaper emphasizes seismic design since that is where the majordifficulties exist in using precast concrete in New Zealand. Confidencein the use of precast concrete in an active seismic zone has requiredthe use of an appropriate design philosophy and the development ofsatisfactory methods for connecting the precast elements together.

Sincethe early 1960s in New

Zealand, there has been asteady increase in the use of

precast concrete for structural components in buildings. The use of precastconcrete in flooring systems has become commonplace since the 1960s,leaving cast-in-place floor construction generally uncommon. Also, precast concrete non-structural claddingfor 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 precast concrete in moment resistingframes and structural walls. This cameabout because the incorporation ofprecast concrete elements has the advantages of high quality control, a reduction in site formwork and sitelabor, and increased speed of construction. In particular, with high interestrates and demand for new buildingspace in New Zealand in the mid1980s, the advantage of speed gaveprecast concrete a distinct edge in cost.

Contractors readily adapted to pre

60 PCI JOURNAL

cast concrete and the new constructiontechniques resulting from on and off-site fabrication of building components.Also, the availability of tall, high capacity cranes and other equipment havemade precast erection more efficient.

New Zealand is in a zone of high tomoderate earthquake activity and theuse of precast concrete in seismic regions requires special provisions fordesign and construction. It is of interest that moment resisting frames andstructural walls incorporating precastconcrete elements have been observedin some countries to perform poorly inearthquakes.

The observed failures have beenmainly due to brittle (non-ductile) behavior of poor connection details between the precast concrete elements,poor detailing of components and poordesign concepts (see Fig. I). As a result, the use of precast concrete inframes and walls was shunned in NewZealand for many years.

Confidence in the use of precastconcrete in moment resisting framesand structural walls in the 1980s inNew Zealand required the development of satisfactory methods for connecting the precast elements together.The then current New Zealand concrete design standard, NZS3 101:1982,’ like concrete design standards of many countries, containedcomprehensive provisions for the seismic design of cast-in-place concretestructures but did not have seismicprovisions covering all aspects of precast concrete structures. Hence, the in-

crease in the use of precast concrete inthe 1 970s required a good deal of innovation.

The design methods introduced forthe connections between precast elements of moment resisting frames generally aimed to achieve behavior as fora monolithic concrete structure (cast-in-place emulation). The design methods for structural walls aimed at behavior as for either a monolithicstructure or a jointed structure with relatively weak joints between elements.

A Study Group of the New ZealandConcrete Society, the New ZealandNational Society for Earthquake Engineering and the Centre for AdvancedEngineering of the University of Canterbury was formed in 1988 to summarize and present data on precastconcrete design and construction, toidentify special concerns, to indicaterecommended practices, and to recommend topics requiring further research.The outcome of the deliberations ofthe Study Group was the publicationof a manual entitled “Guidelines forthe Use of Structural Precast Concretein Buildings,” which was first printedin August 1991.

A second edition incorporating research undertaken in the first half ofthe 1 990s was published in December1999.2 A revision of the New Zealandconcrete design standard was published in 1995. This revision containsadditional provisions for the seismicdesign of structures containing precastconcrete based on that research.

This paper describes aspects of the

design and construction of buildings inNew Zealand incorporating precastconcrete structural elements in floors,moment resisting frames and structural walls. It emphasizes design andconstruction for seismic resistance,since that is where the greatest difficulties exist in the connection of precast elements.

SEISMIC DESIGN CONCEPTSFOR PRECAST CONCRETE

IN BUILDINGS

General Requirements

For moment resisting frames andstructural walls incorporating precastconcrete elements, the challenge is tofind economical and practical meansof connecting the precast elements together to ensure adequate stiffness,strength, ductility and stability. Thedesigner should consider the loadingsduring the various stages of construction and at the serviceability and ultimate limit states during the life of thestructure.

In common with other countries, theseismic design forces recommendedfor structures in the current NewZealand standard for general structuraldesign and design loadings for buildings, NZS 4203:1992, are significantly less than the inertia forces induced if the structure responded in theelastic range to a major earthquake.

The design seismic force is related tothe achievable structure ductility factor

(a) Tangshan, China, 1976 (b) Leninakan, Armenia, 1 988

Fig. 1. Examples of damage to precast concrete buildings caused by major earthquakes.

September-October 2002 61

frames in the post-elastic range.

= 4I4 where 4 is defined asthe maximum horizontal displacementthat can be imposed on the structureduring several cycles of seismic loading without significant loss in strength,and is defined as the horizontal displacement at first yield assuming elastic behavior of the cracked structure.

According to the New Zealand loadings standard,4 structures may be designed as ductile or structures of limited ductility or elastically responding.

For ductile structures, i = 5 or 6 isused to determine the appropriatespectra of seismic coefficients andthe design horizontal seismic forcesat the ultimate limit state typicallyvary between 0.03g and 0.20g, depending on the seismic zone, thesoil category, the importance of thestructure and the fundamental period of vibration of the structure.For structures of limited ductility,= 2 or 3 is used and the design hori

zontal seismic force at the ultimatelimit state typically varies between0.03g and 0.39g.For elastically responding structures, i = 1.25 is used.Note that the design seismic forces

for cast-in-place concrete structuresand for structures incorporating precast concrete elements of the sameavailable ductility recommended inthe New Zealand loadings code4 areidentical.

Capacity Design

Before about the mid-1970s, it wascustomary in the seismic design ofstructures to use linear elastic structural analysis to determine the bendingmoments, axial forces and shear forcesdue to the design gravity loading andseismic forces, and to design themembers to be at least strong enoughto resist those actions.

As a result, when the structure asdesigned and constructed was subjected to a severe earthquake, the manner of post-elastic behavior was a matter of chance.

Flexural yielding of structural members could occur at any of the regionsof maximum bending moment, andshear failures could also occur, depending on where the flexural andshear strengths of members and jointswere first reached. Hence, the behavior of such structures in the post-elastic range was somewhat unpredictable.

For example, for monolithic moment resisting frames, overstrength ofthe beams in flexure or understrengthof the columns in flexure could resultin column sidesway mechanisms (seeFig. 2b) (soft story behavior). Also,the flexural overstrength of membersleads to increased shear forces whenplastic hinges form, which could resultin shear failures (see Fig. 2c). Theseundesirable failure modes could causecatastrophic collapse of the frame.

In the case of monolithic cantileverstructural walls, there are a range ofpossible undesirable modes of behavior. Overstrength of the wall in flexurecould cause failure modes in either diagonal tension shear, sliding or hingesliding (see Figs. 3b, 3c and 3d) whichhave limited ductility.

There is no doubt that the confidence of New Zealand designers thatadequate ductility may be achieved instructures, either totally cast-in-placeor incorporating precast concrete elements, has come about mainly as a result of the introduction of the capacitydesign approach. The capacity designapproach is the result of research anddevelopment in New Zealand.

The method was developed by discussion groups of the New ZealandNational Society for Earthquake Engineering in the 1970s and by Park andPaulay.5 The capacity design approachwas first recommended by the NewZealand loadings standard in 1976 andby the New Zealand concrete designstandard in 1982.’ The capacity designapproach is described in more detailby Park,6 Paulay and Priestley,7 andPark, Paulay and Bull.8

To ensure that the most suitablemechanism of post-elastic deformationdoes occur in a structure during a se

-

-

Seismicloading

(a) Frame withgravity andseismic loading

• Plastic hinge

(b) Columnsideswaymechanism

X Shear failure

(c) Mixed sideswaymechanism withplastic hinges andshear failures

Fig. 2. Undesirable modes of behavior for tall seismically loaded moment resisting

(a) Wall actions

r1 ii

vi 1 :1:

1‘

1*11L&(b) Diagonal (c) Sliding (d) Hinge

tension shear slidingshear

Fig. 3. Undesirable modes of behavior for seismically loaded cantilever structuralwalls in the post-elastic range.

62 PCI JOURNAL

vere earthquake, New Zealand designstandards3’4require that ductile structures be the subject of capacity design.In the capacity design of ductile structures, the steps are:

I. First, appropriate regions of theprimary lateral earthquake force resisting structural system are chosen andsuitably designed and detailed for adequate design strength and ductilityduring a severe earthquake.

2. Next, all other regions of thestructural system, and other possiblefailure modes, are then provided withsufficient nominal strength to ensurethat the chosen means for achievingductility can be maintained throughoutthe post-elastic deformations that mayoccur when the flexural overstrengthdevelops at the plastic hinges.

The use of capacity design hasgiven designers confidence that structures can be designed for predictableductile behavior during major earthquakes. In particular, brittle elementscan be protected. Yielding can be restricted to ductile elements as intendedby the designer.

The steps of the capacity design approach according to the New Zealandconcrete design standard,3 as describedabove, require consideration of threelevels of member strength, namely, design strength nominal strength S,and overstrength S0 as defined below.

Design strength, cS0, is the nominalstrength S multiplied by the appropriate strength reduction factor 1.0,where • is to allow for smaller material strengths than assumed in designand variations in workmanship, dimensions of members and reinforcement positions. Note that in Europeanstandards, material factors y and y areused to reduce the characteristic steeland concrete strengths, respectively,instead of 0 factors.

Nominal strength S, is the theoretical strength calculated using the lowercharacteristic strengths (5 percentilevalues) of the steel reinforcement andconcrete and the member cross sections as designed.

Overstrength S0 is the maximumlikely theoretical strength calculatedusing the maximum likely overstrengthof the steel reinforcement and of theconcrete including the effect of confinement, and reinforcement area in-

cluding any additional reinforcementplaced for construction and otherwiseunaccounted for in calculations.

Recommended Mechanisms ofPost-Elastic Behavior forMonolithic MomentResisting Frames

For moment resisting frames ofbuildings, the best means of achievingductile post-elastic deformations is byflexural yielding at selected plastichinge positions, since with proper design and detailing the plastic hingescan be made adequately ductile.5’6

Significant post-elastic deformations due to shear or bond mechanisms are to be avoided since withcyclic loading they lead to severedegradation of strength and stiffnessand to reduced energy dissipation dueto pinched load-displacement hystere

sis loops. Post-elastic deformationsdue to flexural yielding at well designed plastic hinge regions result instable load-displacement hysteresisloops without significant degradationof strength, stiffness and energy dissipation.

The preferred mechanism for precast concrete equivalent monolithicmoment resisting frames is a beamsidesway mechanism (see Fig. 4a). Abeam sidesway mechanism occurs as aresult of strong column-weak beamdesign.5 The ductility demand at theplastic hinges in the beams and at thecolumn bases is moderate for thismechanism and can easily be providedin design. A column sidesway mechanism is not permitted (except for theexceptions given below), since it canmake very large demands on the ductility at the plastic hinges in thecolumns of the critical story.5 Column

-

Seismicloading

(a) Beam sideswaymechanism

• Plastic hinge

(b) Column sidesway (c) Mixed sideswaymechanism mechanism of a gravity

load dominated frame

Fig. 4. Desirable mechanisms of post-elastic deformation of monolithic momentresisting frames during severe seismic loading, according to the New ZealandStandard.3

I

I

-

—

-

(a) Weak Couplingof Walls

(b) Stronger Couplingof Walls

Fig. 5. Desirable mechanisms of post-elastic deformation of monolithic coupledstructural walls.8

September-October 2002 63

sidesway mechanisms (soft stories)have often led to the collapse of buildings during earthquakes.

To ensure that failure in flexure cannot occur in parts of the structure notdesigned for ductility, or that failure inshear cannot occur anywhere in thestructure, the maximum actions likelyto be imposed on the structure shouldbe calculated from the probable flexural overstrengths at the plastic hingestaking into account the possible factorsthat may cause an increase in the flexural strength of the plastic hinge regions.

These factors include an actual yieldstrength of the longitudinal reinforcing steel, which is higher than thelower characteristic yield strength

used in design and additional longitudinal steel strength due to strain hardening at large ductility factors (thesum of these two factors is referred toas the steel overstrength). The flexuraloverstrength in New Zealand is takenas l.25M, where M is the nominalflexural strength.

To avoid column sidesway mechanisms, the design column bending moments need to be amplified (strongcolumn-weak beam design) to takeinto account beam flexural over-strength, higher mode effects and concurrent earthquake loading.3-8

The New Zealand concrete designstandard3recommends that the columnbending moments at the center of

beam-column joints derived by elasticstructural analysis for the equivalentstatic design seismic forces acting in aprincipal direction of the frame bemultiplied by an amplification factorof at least 1.9 for one-way frames or atleast 2.2 for two-way frames. The amplified column bending moments sodetermined are to be resisted by thenominal flexural strength of thecolumns in uniaxial bending. Theseamplification factors take into accountthe possible flexural overstrength ofthe beams, the effects of higher modesof vibration of the frame and the effectof seismic loading acting along bothprincipal axes of the frame simultaneously in the case of two-way frames.

The New Zealand concrete designstandard3 has only two exceptions tothe requirement of the strong column-weak beam design approach:

1. For ductile frames of one- or twostory buildings (see Fig. 4b), or in thetop story of multistory buildings, column sidesway mechanisms are permitted (that is, plastic hinges occurring simultaneously at the top and bottom ofall the columns of a story). In suchcases, the design seismic forces arethose associated with a structure ductility factor i = 6 since the curvatureductility demand at the plastic hingesin the columns in such cases of lowframes is not high as can be providedby proper detailing.

2. 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 cases, ductile frames threestories or higher may be designed todevelop plastic hinges in any story simultaneously at the top and bottomends of some columns, while plastichinges develop in beams at or near theother columns in that story which remain in the elastic range. The columnsthat remain in the elastic range willprevent a soft story failure (see themixed sidesway mechanisms in Fig.4c). Such frames are required to be designed for the design seismic force associated with equal to 12 times theratio of the total shear capacity of thecolumns remaining in the elastic rangeto the total story shear to be developed, but not more than = 6.

-

-

Plastic hinges

(b) Strong beam-weak columnbehaviour of frame

(a) Weak beam-strong columnbehaviour of frame

Fig. 6. Desirable mechanisms of post-elastic deformation of monolithic dual systems.6

Type 1 Type 2

Cast-in-placeconcrete

zjPrecast

concretehollow-corefloor unit

Type 3

Fig. 7. Type of support of precast concrete hollow-core floor units by precastconcrete beams used in New Zealand.2

64 PCI JOURNAL

It should be appreciated that themechanisms of Fig. 4 are idealized inthat they involve possible post-elasticbehavior obtained from static “pushover” analysis with the frame subjected to code type equivalent staticseismic forces. The actual dynamicsituation may be different, due mainlyto the effects of higher modes of vibration of the structure.

For example, the curvature ductilitydemand at the plastic hinges in thebeams in the lower region of the framemay be greater than in the upper region. However, considerations such asthose shown in Fig. 4 can be regardedas providing the designer with a reasonable feel for the situation. Nonlinear dynamic analyses indicate thatmechanisms such as those shown inFig. 4 do form.

Recommended Mechanisms ofPost-Elastic Behavior forStructural Walls

Ductile capacity of structural wallsis required to be obtained by plastichinge rotation as a result of flexuralyielding, with a displacement (structural) ductility factor i of 6 or less, depending on the height-to-length ratioof the wall, and in the case of coupledwalls also depending on the ratio ofthe overturning moment resisted bythe coupling walls to that resisted bythe wall bases.3

The preferred mechanism for amonolithic cantilever structural wall involves a plastic hinge at the base. Fig. 5shows desirable mechanisms of post-elastic deformation of monolithic structural walls during severe seismic loading with coupling beams between them.

If the coupling is weak (for example, only from floor slabs), the wallswill act as individual cantilever wallsconnected by pin-ended links (see Fig.5a). If the coupling beams are stifferand have significant flexural strength,but not sufficient to cause shear failureof the walls, plastic hinging will alsodevelop in the coupling beams (seeFig. 5b).

Jointed precast concrete wall construction (with relatively weak jointsbetween the precast elements) coulddevelop other mechanisms of post-elastic deformation and need to be de

Fig. 9. Examples of types of support and special support reinforcement at ends ofprecast concrete hollow-core floor units.2’3

012 orDl6 Serviceability deflectionand crack control

Barrier or “dam”in the cell

“Paperdilp” seismic tiereinforcement, two per

1.2m wide slab.

(a) For Type I Support2

Support beam formingpart of two way ductile

moment resistingframe structure

RIO bars asrequired for

vertical shear

cast-in-placeconcrete

(b) For Type 2 Support

September-October 2002 65

Fig. 10.Arrangements of

precast reinforcedconcrete members

and cast-in-placeconcrete forconstructing

reinforced concretemoment resisting

frames.2’1416

f,Ii•l

j.:. 1.1

signed as structures of limited ductilityor to respond in the elastic range.

Recommended Mechanisms ofPost-Elastic Behavior forMonolithic Dual Systems

For dual systems (combined momentresisting frames and structural walls),the deformations of the frames will becontrolled and limited by the muchstiffer walls. Fig. 6a shows the mechanism preferred for frames, that is, weakbeam-strong column behavior withplastic hinging occurring in the beamsand at the bases of the columns andwalls. However, strong beam-weakcolumn behavior may be admitted inevery story of the frame (see Fig. 6b)because a wall proportioned using capacity design principles will remainelastic above the plastic hinge at thebase and its stiffness will prevent a“soft story” failure from developing inthe frame. Without a wall, such a framesystem designed for ductile responsewill normally have to be restricted tobuildings of one or two stories.3

Detailing for Ductility

Structures need to be detailed so asto possess sufficient ductility to match

the ductility required by the seismicforces used in design.

The most important design consideration for detailing plastic hinge regions of reinforced concrete membersfor ductility is the provision of appropriate quantities of transverse reinforcement in the form of rectangularstirrups, or hoops with or withoutcross ties, or spirals. The transversereinforcement needs to be adequate toact as shear reinforcement, to confineand hence to enhance the ductility ofthe compressed concrete, and to prevent premature buckling of the compressed longitudinal reinforcement.3

Joint core regions of beam-to-column connections also need special attention because of the critical shearand bond stresses that can developthere during seismic loading.3

PRECAST CONCRETEIN FLOORS

Types of FloorCurrently, the majority of floors of

buildings in New Zealand are constructed of precast concrete units,spanning one way between beams orwalls. The precast concrete units are

either of pretensioned prestressed orreinforced concrete (solid slabs,voided slabs, rib slabs, single tees ordouble tees), and generally act compositely with a cast-in-place concretetopping slab of at least 50 mm (2 in.)thickness and containing at least theminimum reinforcement required forslabs.

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. Probably, themost common floors are constructedfrom precast concrete hollow-corefloor units typically 200 mm (8 in.)deep, or deeper.

As well as carrying gravity loading,floors need to transfer the in-plane imposed wind and seismic forces to thesupporting structures through diaphragm action. The best way toachieve diaphragm action when precast concrete floor elements are usedis to provide a cast-in-place reinforcedconcrete topping slab over the precastunits.

Where precast concrete floor unitsare used without an effective cast-in-place concrete topping slab, in-plane

Cast-in-placeconcrete and lT:: Cast-in-place concrete

steel in and top steel in beamcoiuJ

Midspan

Precast orcast-in-placecolumn unit

Mortar orgrout joint\

I ‘“!

Precastbeam unit

•, Midspan

Cast-in-place concrete Cast-in-placeand top steel in beam 1,..,joint

Precastbeam unit

Precast orcast-in-placecolumn unit

Precast beam unit

(a) System I Precast Beam Units Between Columns (b) System 2- Precast Beam Units Through Columns

Vertical leg ofprecast T-unit

Mortar orgrout joint

MidspanCast-in-place

— joint

Precast T- unit

(c) System 3- Precast T-Units

Notes: E Precast Concrete Cast-in-place concreteReinforcement in precast concrete not shown

66 PCI JOURNAL

force transfer due to diaphragm actionmust rely on appropriately reinforcedjoints between the precast units. Thismay be difficult to achieve in somefloors unless the connections betweenthe precast units are specifically designed and constructed.

Support Details

The supports for precast concretefloor units may be simple or continuous.

Three types of support for precastconcrete hollow-core or solid slabflooring units seated on precast beams,identified by the New Zealand Guidelines,2 are shown in Fig. 7. The differences between these types are the depthof the supporting beam prior to thecast-in-place concrete being placed.

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 imposedmovements caused by earthquakes orother effects which reduce the seatinglength (see Fig. 8).

One source of movements duringsevere earthquakes, which could causeprecast concrete floor units to becomedislodged, is that beams of ductile reinforced concrete moment resistingframes tend to elongate when formingplastic hinges, which could cause thedistances spanned by precast concretefloor members to increase.2’9’10

The elongation is due to the tensileyielding of the reinforcement associated with plastic hinge formation.Longitudinal extensions of beams inthe order of 2 to 4 percent of the beamdepth per plastic hinge have been observed in tests in which expansion wasfree to occur.2’9The compression induced in beams by restraint againstthis expansion can enhance the flexural strength of beams and cause cracking of the topping slab.

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 and

Fig. 11. Somedetails of mid-span connectionsbetween precastreinforcedconcrete beamelements.2’14-16

!Ii

ColumnCast-in-place joint

Column

beam

(a) Conventional Straight Bar Lap

beam

ColumnCast-in-place joint

Column

I

Precastbeam

tJJ4d.J L Precastbeam

(b) Hooked Lap

ColumnCast-in-place joint

÷ ColumnJ

Precastbeam

j -fr Precast- beam

(c) Double Hooked Lap

111111

- -

Fig. 12. Construction of a building frame using System 1 in New Zealand.

September-October 2002 67

Fig.1 3. Hookedlap of bottom bars

within joint corefor System 1 2

temperatures effects, as well as for theeffects of earthquakes.

Some concern has been expressed inNew Zealand that there were cases inconstruction where the support pro-

vided for precast floors was inadequate. In the 1980s, the New ZealandCode for design1 had no specific requirements for the support of precastconcrete floors.

As a result, the current New Zealandconcrete design standard, NZS310l:1995, provides that for precastconcrete floor or roof members, withor without the presence of a cast-in-place concrete topping slab and/or continuity reinforcement, normally 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 /180 of the clear span but notless than 50 mm (2 in.) for solid or hollow-core slabs or 75 mm (3 in.) forbeams or ribbed members. However, ifshown by analysis or by test that theperformance of alternative support details is adequate, the above specifiedend distances need not be provided.

The above recommendation, whichrequires proven alternative support details unless the specified end distancesare provided, is similar to the recommendation in the building code of the

4%

I‘ ..\‘

Y

__________

Fig. 14. Construction of the 22-story perimeter frame of the Price Waterhouse-Coopers building using System 2 in Christchurch, New Zealand.

Beam longitudinalreinforcement onlyshown

Cast-in-placecolumn

Top bars slidInto place

..

I.,:.:

Precast beam 2L or £dh+8db gvihichever is less

Cast in place- g (hooks to terminate

column at the far side ofthe joint core)

Note: db = bar diameter

= development length ofhooked anchorages

-r-- ___•“___•__i.—.- 4

// ii-’,çg)2’’

68 PCI JOURNAL

American Concrete Institute, ACI31899.h1

One method of providing the alternative details which permit smallerseating lengths is to use special reinforcement between the ends of the precast concrete floor units and the supporting beam, which can carry thevertical load in the event of the precastconcrete floor units losing their seating. The special reinforcement shouldbe able to transfer the end reactions byshear friction across the vertical cracksat the ends of the units if the crackwidths are relatively narrow or bykinking of the reinforcement crossingthe cracks if the crack 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,2and recommended earlierby the Precast/Prestressed ConcreteInstitute and the Fédération Internationale de la Précontrainte. For example, for precast concrete hollow-core

units, the special reinforcement maybe either placed in some of the coreswhich have been broken out at the topand filled with cast-in-place concreteor grouted into the gaps between theprecast units.

Tests conducted at the University ofCanterbury12’13 on special reinforcement, placed in filled cores at the endsof hollow-core units and passing overprecast supporting beams, have investigated a number of types of specialsupport reinforcement. They werefound to be able to support at least theservice gravity loads of the floor, inthe event of loss of end seating.

The plain round straight or drapedreinforcement with hooked endsshown in Fig. 9 are favored.2 Plainround end hooked reinforcement wasfound to perform better than deformedreinforcement since bond failure propagating along the plain round bars allowed extensive yielding along thebar, therefore allowing substantialplastic elongation before fracture. 12

Moment resisting frames incorporating precast reinforced concrete elements have become widely used inNew Zealand. The design aim has beento achieve behavior of the frame as formonolithic cast-in-place construction(cast-in-place emulation).14’15’16

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 less of the horizontalforces, the exact amount depending onthe relative stiffnesses of the perimeterand interior frames. If the perimeterframes are relatively stiff, the columnsof the interior frames will carrymainly gravity loading. Also, the inte

MOMENT RESISTINGFRAMES INCORPORATING

PRECAST REINFORCEDCONCRETE ELEMENTS

September-October 2002 69

— —. — _..._

Fig. 16. Construction of the 13-story perimeter frame of Unisys House using two-story high cruciform-shaped precast units(System 3) in Wellington, New Zealand.

nor columns can be placed withgreater spacing between columns.

For the perimeter frames, the depthof the beams may be large without affecting the clear height between floorsinside the building and the columnscan be at close centers. The use ofone-way perimeter frames avoids thecomplexity of the design of beam-to-column joints of two-way moment resisting frames. References 17 to 22give details of several buildings constructed in New Zealand which incorporate significant quantities of precastconcrete in their frames and floors.

Arrangements of PrecastConcrete Members andCast-in-Place Concrete

The precast reinforced concreteframe elements are normally connectedby reinforcement protruding into regions of cast-in-place reinforced con-

crete. Three arrangements of precastconcrete members and cast-in-placeconcrete, forming ductile moment resisting multistory reinforced concreteframes, commonly used for strong column-weak beam designs in NewZealand, are shown in Fig. 10. Fig. 11shows some midspan connection detailsused with Systems 2 and 3 of Fig. 10.

The precast concrete beam elementsof System 1 of Fig. 10 are placed between the columns and the bottomlongitudinal bars of the beams are anchored by 90-degree hooks at the farface of the cast-in-place joint core (seeFigs. 12 and 13). Figs. 14, 15 and 16show structures under constructionusing Systems 2 and 3 of Fig. 10.

For System 2, the vertical columnbars of the column below the jointprotrude up through vertical ducts inthe precast beam unit (see Fig. 14),where they are grouted, and pass intothe column above. The white plastic

tubes over the bars in Fig. 14 are thereto help pass the bars through the vertical ducts. The plastic tubes are thenremoved. Those vertical bars are connected to the bars in the column aboveby splices if the column is of cast-in-place concrete or by steel sleeves orducts which are grouted if the columnis of precast concrete (see Fig. 17).

The columns of the precast elementsof System 3 are connected by longitudinal column bars which protrude intosteel sleeves or ducts in the adjacentelement and are grouted (see Figs. 16and 17). The beams are connectedusing a cast-in-place joint at midspan.

It should be noted that the capacitydesign procedure for these three systems will ensure that yielding of thecolumn bars at the connections is keptto a minimum. Fig. 18 shows a furthersystem using pretensioned prestressedconcrete U-beams and cast-in-placereinforced concrete.23

70 PCI JOURNAL

Many of the currently used connection details shown in Figs. 10 to 18have had experimental verification.1o2324 The verification involvedsimulated seismic loading tests conducted on typical full-scale beam-to-column joint specirn,ns, designed forstrong column-weak- beam behavior,to determine the performance of thehooked bar anchorage of the bottombars of the beam in the cast-in-placeconcrete joint core in ‘System 1 of Fig.10, the performance of the groutedvertical column bars which passthrough vertical ducts in the precastbeam in System 2 of Fig. 10, and theperformance of the composite beamshown in Fig. 18.

Simulated seismic loading testshave also been conducted to determinethe performance of the cast-in-placemidspan connections between precastbeam elements shown in Fig. 11. Thetest results indicated performance asfor totally cast-in-place construction.

Fig. 1 7. Steel sleeve splices and corrugated metal ducts used for column-to-columnand slab-to-slab connections.

Precast Reinforced ConcreteMoment Resisting Frames andCast-in-Place Reinforced ConcreteStructural Walls

Structures comprising both reinforced concrete structural walls andflexible moment resisting frames canalso be used to advantage. The structural walls, normally of cast-in-placeconcrete, can be designed to resist almost all of the horizontal forces actingon the building. The frames, beingmuch more flexible than the walls,will be called on to resist only a smallportion of the horizontal forces, theamount depending on the relative stiffnesses of the walls and frames. Thecolumns of such frames in the building mainly carry the gravity loading.

When such systems are used in seismic regions, the frames can be designed for limited ductility, providingit can be shown that when the ductilewalls have deformed in the post-elastic range to the required displacementductility factor or drift during severeseismic loading, the ductility demandon the frames is not large. A buildingso designed in New Zealand is shownin Fig. 19. The central cast-in-placereinforced concrete walls, forming theservice core of the building, were de

signed to resist the seismic loading.The perimeter frame of precast reinforced concrete beams and the reinforced concrete columns were designed mainly for gravity loading.

PRECAST CONCRETESTRUCTURAL WALLS

Construction Details

Most structural walls for multistorybuildings in New Zealand are of cast-

in-place concrete, but significant use ismade of precast concrete walls forsmaller buildings (see Fig. 20). Precastreinforced concrete structural wall construction usually falls into two broadcategories: “monolithic” or ‘jointed.”

In monolithic wall construction, theprecast concrete elements are joinedby “strong” reinforced concrete connections which possess the stiffness,strength and ductility approaching thatof cast-in-place concrete monolithicconstruction.

Rebar

Precastconcretemember

Rebar (upper)—‘ (lapped beside)

Grout and air

Precastconcretemember

High strength‘grout

Splice sleeve

(a) Steel sleeve

(lower)

(b) CorrugatedMetal Duct

Rebar 25 mm minimum(upper) and code spacing

requirements

Rebar (lower)

Duct

Section A-A

Proprieta,y floor systemand cast-in-placereinforced concretetopping

Fig. 18. A structural system involving precast pretensioned prestressed concreteU-beams and cast-in-place reinforced concrete.23

September-October 2002 71

(a) Typical floor plan

Fig. 19. The Westpac Trust building constructed with a precast reinforced concrete perimeter frame with seismic forces resistedby cast-in-place reinforced concrete interior structural walls in Christchurch, New Zealand.

Fig. 20. TheWest Fitzroy

buildingconstructed

using precastreinforced

concretestructural walls

in Christchurch,New Zealand.

In jointed wall construction, the connections are “weak” relative to the adjacent wall panels and, therefore, governthe strength and ductility of the building.

Monolithic Wall Construction

Monolithic precast reinforced concrete structural wall systems are designed according to the requirementsfor cast-in-place reinforced concreteconstruction.3

At the horizontal joints between precast concrete wall panels or foundation beams, the ends of the panels areusually roughened to avoid slidingshear failure, and the joint is madeusing mortar or grout. The vertical reinforcement protruding from one endof the panel and crossing the joint isconnected to the adjacent panel orfoundation beam by means of groutedsteel splice sleeves or grouted corrugated metal ducts (see Fig. 17).

122.33m.. 1 73m

Precastbeam

122.33m

I 73m

—— .“—J — —‘ .— —

(b) The perimeter structure

72 PCI JOURNAL

Cast-in-placeconcrete tdh All horizontal

‘bandage” I I reinforcement splicedjoint I [./ with 90° standard

—1%.. 1F hooks i.e.

Fig. 21. Somevertical joints usedfor monolithicprecast concretewall construction.2

Vertical joints between precast concrete wall panels are typically strips ofcast-in-place concrete into which horizontal reinforcement from the ends ofthe adjacent panels protrude and arelapped. Fig. 21 shows some possiblevertical joint details between precastwall panels that make use of cast-in-place concrete. The widths of thestrips of cast-in-place concrete are determined by code requirements for laplengths of horizontal reinforcement.

Fig. 21 a shows a joint with sufficient

width to accommodate the lap splicelength of the straight horizontal barsthat protrude from the precast wallpanels. Fig. 21b shows hooked lapsplices that enable the width of joint tobe reduced. Figs. 21c and 21d showhairpin spliced bars, which may not beconvenient to construct since once thelapping bars have been overlapped, theability to lower the precast panels overstarter bars is very restricted.

At exterior walls, support for precast floor units can be achieved in a

number of ways — for example, on asteel angle anchored to the wall panel,on a concrete corbel, or on a recess inthe wall panel. These connections aredesigned to transfer the floor inertiaforces to the walls and to avoid loss ofseating. Typical floor-to-wall connection details are described elsewhere.2

jointed Wall Construction

In jointed wall construction, theconnection of precast reinforced con-

Fig. 22. Typical low rise buildings constructed in New Zealand incorporating tilt-up precast concrete walls.

Cast-in-placeAll horizontal

concretefor reinforcement

“bandag’4°bap)Pliced

JPrecasti Side of wallsconcrete panels keyed

wall and roughenedpanels

Vertical reinforcementtypically lap splicedabove floor level

(a) Straight Lap Splices

Precast-1concrete

wallpanels

Sides of wallspanels keyed

and roughened

Cast-in-placeconcrete

“bandageS’joint

Vertical reinforcementtypically lap spliced

above floor level

(b) Hooked Lap Splices

All horizontalreinforcement

,,___— hairpin spliced

‘‘‘I

Cast-in-place Sealant All horizontal

grout filled joint hairpin splicedconcrete or / reinforcement

Precast—”” ) — Vertical reinforcementconcrete Sides of joint typically lap spliced

wall keyed and above or belowpanels roughened floor level

(C) Hairpin Lap Splices

Precastconcrete

wallpanels

Vertical reinforcementSides typically lap spliced

of joint above or belowroughened floor level

(d) Hairpin Lap Splices

(a) Lecture theatres (b) Commercial buildings

September-October 2002 73

Crete components is such that planesof significantly reduced stiffness andstrength exist at the interface betweenadjacent precast Concrete wall panels.Jointed construction has been extensively used in New Zealand in tilt-upconstruction, generally of one- tothree-story apartment, office and industrial buildings.

The buildings are normally designedas structures of limited ductility or aselastically responding structures whichrequire only nominal ductility. Generally, tilt-up concrete walls are securedto the adjacent structural elementsusing jointed connections comprisingvarious combinations of concrete inserts, bolted or welded steel plates orangle brackets, and lapped reinforcement splices within cast-in-place joining strips.

The concrete inserts and bolted orwelded steel plates need to be fixed tothe concrete in a manner which ensures ductile yielding of the reinforcing bars, bolts or plates before a brittle pullout failure from the concreteoccurs. This requires a capacity design approach for the fixing to ensurethat the desired yielding of the steeloccurs under the most adversestrength conditions.

Tilt-up construction has been usedto build structures with complex geometric and appealing architectural features. Some photographs of typicallow-rise buildings constructed usingprecast concrete walls are shown inFig. 22.

Unfortunately, the current NewZealand concrete design code3 doesnot have design recommendationscovering all aspects of tilt-up construction. However, a research projecthas been in progress at the Universityof Canterbury,25’26’27which has the aimof cataloguing currently used connection details, assessing and testing themwhere necessary, and recommendingappropriate details for tilt-up andjointed construction.

GENERALPrecast concrete is also commonly

used in New Zealand for a variety ofindustrial buildings, tanks and sportsstadiums, including components suchas stairways.

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 construction specification.28The New Zealand Guidelines2suggests considering three differenttypes of tolerances, namely, product,erection and interface tolerances.

THE FUTUREThe building industry in New

Zealand has embraced the use of precast concrete. All indications are thatprecast concrete construction will continue to be used extensively in the future. The use of the capacity designapproach and the development of appropriate methods for the detailing ofconnections and members have givendesigners the confidence that precastconcrete can be used in an active seismic region such as New Zealand. Theadvantages of using precast concretehave given it a cost advantage.

CONCLUSIONSBased on accumulated design and

construction experience during the lastthree decades, the following conclusions can be made:

1. Confidence in use of precast concrete in structures in an active seismiczone such as New Zealand has required the application of appropriatedesign approaches and the development of satisfactory methods for connecting the precast elements together.

2. The advantages of using precastconcrete have given it a cost advantage. Currently, almost all floors, mostmoment resisting frames and manyone- to three-story walls in buildingsin New Zealand are constructed incorporating precast concrete.

3. A capacity design approach, developed in New Zealand, is used toensure that in the event of a majorearthquake, yielding of the structureoccurs only at chosen ductile regions.In particular, this means that for structures incorporating precast concreteelements, ductility can be provided inregions away from potentially brittleconnections.

4. Experimental and analytical research conducted during the lastdecade in New Zealand has led to seismic design provisions in the concretedesign standard NZS 3101:1995 forthe seating of precast concrete floorunits and the design of the connectionsbetween precast elements in momentresisting frames. Guidelines have alsobeen written for aspects of the seismicdesign of tilt-up wall constructionbased on experimental and analyticalresearch.

5. The future of precast concreteconstruction in New Zealand appearsto be bright.

ACKNOWLEDGMENTThe author gratefully acknowledges

the helpful discussions he has had withmembers of the design and construction profession in New Zealand, particularly Mr. R. G. Wilkinson of theHolmes Group, Christchurch, Dr. A. J.O’Leary of Sinclair, Knight Mertz,Wellington, and Mr. G. Banks of AlanReay Consultants, Christchurch. Theauthor also gratefully acknowledgesthe financial support provided by theEarthquake Commission of NewZealand. Miss Catherine Price isthanked for her assistance in preparingthis manuscript.

74 PCI JOURNAL

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