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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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(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
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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
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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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September-October 2002 75