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Transcript of Canoa Bull
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Seismic Resistance of
Frames Incorporating Precast
Prestressed Concrete
BeamShells
D. K.Bull
Design
Engineer
Smith Leuchars
Ltd.
Consulting Engineers
Wellington,
NewZealand
Robert
Park
Professor
and
Mead
of
Civil
Engineering
Universityof
Canterbury
Christchurch,
NewZealand
T
he use of precast concrete in
build-
ing frames has a num ber of
attractive
features suchasbetterquality
control
of
the product and savings in formwork
and construction time. The basic prob-
lem in the
design
of
earthquake
resistant
building structures incorporating
pre-
cast concrete elements is in finding an
economical and practical
rnethod
for
connec t ing th e precast e lements to -
gether. The connect ion be tween the
e lements should
e n s u r e
sa t i s fac tory
strength and stiffness against seismic
l o a d s
and enable the s t r u c t u r e to
achieve th e
necessary ductility
during
cyclic loading
in the
inelastic range.
C o m p o s i t e s y s t e m s o f c o n c r e t e
buildings, combining precast
and
cast-
in-place
(cast in situ) reinforced
con-
crete,
have a number of
advantages
in
construction. The incorporation ofpre-
cast
concrete elements
has the
advan-
tageofhigh qua lity c ontroland speedof
co n s t r u c t i o n , a n d t h e c a s t - i n -p l a c e
reinforced concrete provides the struc-
tural continuity
and the
ductility neces-
sary
fo r
adequate seismic performance.
A
building
system
wh ich
has
become
popular in New Zealand involves the
use of
precast concrete beam shells
as
p e r m an e n t
formwork
for beams . The
precast shells are typically pretensioned
prestressed concrete
U-beams and are
left permanently
in
position after
th e
cast-in-place reinforced concrete core
has been cast.
The precast
U-beams
support
the
self
weightandconstruction
loads and act co m p o s i t e ly w i t h the
r e in forced concre te core whensub-
jected
to
other loading
in the
finished
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INS T IT UT O CHILENO Q FL
CEMENTO
structure.
The
precast U-beams
are not
connected
by steel to the cast-in-place
concrete
of thebeamor column.
The typicalstructuralorganization of
abuildingfloor and
frame system
incor-
porating
the
precast pretensioned
U-
beam
units
is
shown
in
Fig.
1.
Current
construction practice is to support the
U-beam units
on the
cover concrete
of
the
previously cast reinforced concrete
columnbelow,with
a
seating
of 40 to 50
mm
(1.6 to 2.0 in.) and toplace apro-
prietary precast concrete
floor
system
between the U-beams of adjacent
frames.
Some
propping may b-; provided
under
the
ends
of the
U-beam units
as a
back-up
measure in case the U-beam
seatingon the column should prove in-
adequate tocarrythe construction load.
Once the precast
f loor
system is in
place,
the
reinforcement
may be
placed,
and the
in-place
concrete cast, inside
thebeam units,the topping
slab
and the
colum ns of thenextstory.The sectiono f
the
composite beam
in the
finished
structure
is
shown
in
Fig.
2.
Precast
concrete
columns have sometimesbeen
used rather than cast-in-place concrete
columns.
The precast prestressed concrete
U-beam
illustrated in Fig. 2 has webs
tapered
from the bottom to the
top,
to
ensureease
of
removing internal form-
work
when
precast.
The inside surface
is
intentionally roughened,by the use of
a
chemical retarder
and the
removal
of
the
surface cement paste,
to faciltate
the
development
of
interface bond
be-
tween
the
precastU-beamconcrete
and
the cast-in-place concretecore.
The
U-beams
are
pretensioned with
seven-wire strandsand aredesigned to
carry
at least allof the self
weight
and
imposed loads during construction.
Note
that
the
strands termnate
in the
end of the
U-beam
and
henee
are not
anchored in the beam-columnjointre-
gions of the frames.
Initiallyin NewZealand, precast con-
crete U-beams were principally used
in
The performance of
frames
ncorporaing presast pre
concret
U beam
shells
to
leading,
is
invest-
gaied.
The ac as
formwork
and areno
oon
nected
by stee te
the
concreteof the
o.r
eolumii,
Three
fui eoi
performance
characteristicswhen
plstic
hinge
re-
gions
occur
in
he
bearns
adjacent
to
Provlsonsfor
the
of
such eomposite structures are
dte-
cussed andadditonal design
reeorn-
mendatiorts on-the test
are wiiere necessary.
A
numrica design
exampleis
tollustrae
the
design
the constructionoflowrise buildingsin
which the
horizontal seismic loads
are
resisted
primarily by other elements
such as totally cast-in-place reinforced
concrete structural walls
or
frames.
A n
early example ofthis type ofconstruc-
tion
is the Karioi
PulpM ili
1
(see Fig.
3).
Recent trends have seen this
form
of
composite beam construction used in
multistory momentresisting reinforced
concrete framed structures. In this ap-
plication,
the composite beams are re-
quired to be adequately ductile to act as
the
primary
energy dissipating members
during seismic loading. Doubts have
beenexpressed bysomedesignersand
building
officials
concerning
the ability
of
thisform
of
composite construction
to
be
able
to
fulfill that demand.
This
paper reviews seismic design
considerations
for f r a mes
with such
composite beams. The results of tests
PCI JOURNAL/July August 1986
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Proprietory
Floor
Sys tem
and
Cast-in-place
Concrete
Topping
Precast Concrete
U-Beam
Cast-in-place
Concrete
Beam Core
Reinforcing
Concrete
Column
Fig.
1.
Construction details
of a
composite structural
system
not
al l
reinforcement
is
shown).
conducted
on
three full scale composite
beam-exterior
co lumn
subassemblies,
subjected to simulated seismic loading,
are
summarized. Design provisions
basedon the
test results
are
proposed
and
a numerical
design example
is in-
cluded
to
Ilstrate
the
design approach.
The
results
of the
tests may
be
seen
re-
ported
in
more detail
in
Ref.
2.
SEISMIC
DESIGN
ONSIDER TIONS
In the
design
of structures forearth-
quake
resistance,
a
prime
consideration
is
to
ensure that
the structure is
capable
of deforming in aductile
manner when
subjected to
several cycles
of
horizontal
loading
well
into the inelastic range.
This
is
because
it is
generally uneco-
nomical
todesign astructuretorespond
in the elastic range to the large hori-
zontal
inertia
loads induced by the
greatest likelyearthquake.
The
recommended level
of
seismic
design loads
in
codes
is
generally sig-
nificantly
lower
than
the
elastic
re-
sponse inertia loads during severe
earthquakes
and the structuremay be
required to undergo horizontal dis-
placements which are
four
to sixtimes
the
horizontal displacement
at the
commencement
of
yielding
of the frame.
The
ratio
of the
mximum
displacement
to the
displacement
at first yield is
commonly
referred to as the displace-
ment
ductility
factor.
Ideally,
th e
design concept
for
mo-
ment resisting
frames
should
aim at
dis-
sipating seismic energy by
ductile
flexuralyielding
at
chosen plstic hinge
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C a s t - i n - p l a c e
Co n cre te
Beam Core
C a s t - i n - p l a c e
C onc re te Topp ing
Precast
Concrete
Floor Slab
Precast
Prestressed
Concrete U - B e a m
Unit
Fig.2. Sectionofcompositebeamin
finished
structure
reinforcement
is not
shown).
Fig.
3.
Precast concrete U-beams used
as
permanent formwork
for
cast-in-place
reinforced
concrete
frames (KarioiPulpMiliBuilding,
New
Zealand).
regions
when
th e
structure
is
subjected
to
the seismic design loads. The restof
the frame should be made sufficiently
strong
to
ensure that
it
remains
in the
elastic
range when flexuralyieldingoc-
curs at the chosen
plstic
hinge loca-
tions. This
means
ensuring that shear
failures
and
bond failures
do not
occur
and that the
preferred
energy dissipat-
ing
mechanism
forms.
M e c h a n i s m s
involving flexural
yielding atplstic
hinge
s are shownin
Fig. 4.
If
yielding commences in the
column
beforeitoccursin the
beams,
a
column sidesway mechanism can form
as illustrated
in
Fig.
4b.
Such
a
soft
story mechanism can make
very large
curva ture ductility demands on the
plstic
hinges
of the
critical story, par-
ticularlyin thecase
of
tall buildings.
On
the
otherhand,
if
yielding occurs
in
the
beams before
it
begins
in the
col-
umns a beam sidesway mechanism,i l-
lustrated
in
Fig.
4c, can
develop which
makes
more modrate demands
on the
curvatureductility requiredat the pls-
tic
hinges
in the
beams
and at the
col-
umn
bases, evenfortall frames.
There-
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1986
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jy/ -rl
i- -w- -l
U^N^-%Jt-w-'M-vl
l vi y ^br v yj
Plstic
hinge
Bending
moment
(a) Moment
resisting
trame
(b)Columnsidesway
mechanism
(c)Beamsidesway
mechanism
Fig.4.Momentresistingtramewith horizontal seismic
loading
andpossible mechanisms.
fore,
fo r
tall
frames,
a
beam sidesway
mechanism
is
the
preferredmodeof in-
e l a s t i c d e f o r m a t i o n
an d a
s t r o n g
column-weak beam concept is advo-
catedtoensure beam hinging.
For frames with
lessthan
about
three
stories, and for the top story of tall
frames,
the curvature ductility required
at the plstic hinges if a colum n side-
sway mechanism develops
is not
par-
ticularly high.
3
Henee, for one- and
two-story frames, and in the top storyof
taller frames, a weak co l u m n-s t r o ng
beam concept
can be permitted.
4
'
5
'
6
This
approach
would
protect the composite
beams from damage during seismicmo-
tions.
H o w e v e r , for tall
f rames
w h e r e a
strong column-weak beam concept is
necessary, the
composite beams
will
need to be designed for adequate duc-
tility. Seismic design considerations fo r
moment resisting frames when plstic
hinges form
in the
composite beams
are
discussed
in the
bllowingsections.
Flexura Strength
of
Beams
The critical section for
f lexure
in
beams inmoment resisting frames sub-
jected
to
gravity
and seismic loading is
at or near th e beam ends . In frames
w here gravity loadingeffects
domnate,
the
criticalsections
fo r
positive m om ent
due to gravity plus seismic loading
may
occur in the beams away
from
the col-
umn
faces.
The critical negative m o-
ment sections will always occur at the
beam ends.
A
distinctive feature
of the
behaviorof the composite beam-column
connection shown in Fig. 1 is that the
prestressing strands
of the
precast con-
crete U-beam termnateat the end of the
U-beam and henee are not
anchored
in
th e
beam-column joint core.
T h e
n e g a t i v e m o m e n t f l e x u r a l
strength
at the end of the
composi te
beam will be aided by the presence of
the U-beam since the bottom flange of
the
U-beam will bear
in
compression
against th e
cast-in-place
column con-
crete (see Fig. 5b). Henee,
the
upper
limit of the
negative moment flexural
strength
at the
ends
of the
beam will
be
thatof the composite section. How ever,
should
the
beam
end
bearing
on the
column
concrete
and/or
the interface
bondbetween
the
cast-in-place
and
pre-
cast
beam concrete break down during
seismic loading, the available negative
moment
flexural
strength will reduce
to
less
than the composite section valu.
The
lower
l imit of negative moment
flexural strength
at the
beam ends
is
that
provided
by the
cast-in-place reinforced
concrete core alone. The negative mo-
ment
flexural
strength away
from
the
ends will
be
that
due to the
composite
section.
Thepositive mom entflexuralstrength
at the end of the
beam w ill
be
provided
only by the longitudinal reinforcement
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and
the cast-in-place
concrete
in the
beam core
and slab
topping (see Fig.
5a). A wayfrom
the beam ends there
will
be
somecontribution from the precast
prestressed U-beam
to the
positive
mo-
mentflexuralstrength, but a
full
contri-
bution
from
the prestressing strands
(and henee
full
composite action of the
section) can
only occur
at a
distance
greater than approximately 150 strand
diameters from the beam end, whichis
the order oflength required to develop
thetensilestrengthof the strand.
Henee, the dependable negative and
positive
flexural
strengths
of the
com-
posite beamat the beam ends shouldbe
taken
as
that
p rov ided on ly
by the
cast-in-place reinforced concrete beam
core.
Plstic Hinge Behavior
of
Beams
The length of the plstic hinge regin
inthe beamsis ofinterest in seismicd e-
sign
sincethe plstic hinge lengthhas a
significant
effect
on the
level
of
dis-
placement ductility factor which
can be
achieved by f r a m e s . Longer p ls t ic
hinge
lengths
lead
to
greater available
d i sp lacemen t duc t i l i ty factors for a
given
ultmate section curvature.
3
In a
conventional reinforced concrete
frame
the length of the beam regin over
which
the
tensile reinforcement
yields
is typically about equal to the beam
depth
and
several
flexural
cracks
will
form
in
that regin.
In th e
composite system considered
here, in w hich there is no connection by
steel be tween the end of the precast
prestressed U-beamand thecolumn,the
length of the regin of reinforcement
yielding at the end of the composite
beam
when the bending
moment
ispos-
itive will be less than for a beam in a
conventional reinforced concrete
frame.
This
is
because when positive moment
is
applied,
the first
crack
to
form w i ll
be
atthe contact surface between the end
of
the precast U -beam and the face of the
column.It is possible that positive mo-
mentplstic rotatioiis
will concntrate
at
this
one
cracked section, since signifi-
cant crackingmay notoccurin the flex-
urally stronger adjacent composite sec-
tions during subsequent loading.
If
the flexural
cracking
in the
beamduring positive bending moment does
concntrate at the column face, the con-
s e q u e n c e w o u l d be h i g h e r b e a m
curvatures in the plstic hinge regin
than for conventional reinforced con-
crete
members .
Henee,
the
concrete
there
w o u l d be s u b j e c t e d to h i g h
localized
compre
ssive strains
and the
longitudinal reinforc em ent in the beam
there
w ould
suffer
high localized
plstic
tens i le s t ra ins which would perhaps
lead to bar fracture when signif icant
plstic hinge rotation occurs.
Further,
the extensive widening of that crack at
large plstic hinge rotations may mean
that
the
shear resistance mechanism
due
to
aggregate interlock alongthe (verti-
cal) crack will break down, leading to
sliding
shear
displacements
along that
weakened vertical plae.
Theseopinionsconcerning
the
plstic
hinge behavior d uring positive m om ent
have resulted in
reservations
being ex-
pressed by some designers about the
performance of this type of composite
beam when required to act as
primary
e n e r g y d i s s i p a t i n g m e m b e r s d u r i n g
seismic loading.
The possible shorteningof the length
of
the
regin
of
reinforcement
yielding
onlyapplies when the beam m oment is
positive. When
the
beam momen t
is
negative,thebehavior shouldbesimilar
to a
conventional reinforced concrete
beam, sincethe top of the cast-in-place
concrete core doesnot havethe precast
U-beam surrounding it and the plstic
hinge regin should be able to spread
alongthe beam.
One
possible approach,
aimed
at
im -
provingthe plstic hinge
behavior
dur-
ing positive moment, wouldbe to con-
struct a
compo site beam
in such a
man-
ner
that
in the
potential plstic hinge
re-
PCI
JOURNAUJuly-August1986
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4>
Diagonal
compression
T r strut
x . ,
lp||
irHr
Flange of
U-beam
(a)
Positive Bending
Moment Applied to Beam
Diagonal
compression
strut
Flange o f
U- beam
compression
strut
fb) Negative
Bending Moment
Applied to Beam
Note:
Not
all reinforcement
is
shown.
Fig.
5. Internal torces
acting
on a composite b eam-exterior
column jointcore during positive and
negative beam moments.
gionsat theendsofthebeamthebondat
the interface between the precast pre-
stressed U-beam and the cast-in-place
concrete core is intentionally elimi-
nated. The effect of such adetailwould
be to
allow
the plstic hinge regin to
spread along
the
cast-in-place
concrete
beam core without hindrance from the
U-beam,and soavoidthe
possible
con-
centration
of thebeam plstic hingero-
tation in the regin ciseto the end of
thebeam.
In the
plstic
hinge regions of the
beams
the reinforced concrete cast-in-
placecore should have longitudinal and
transverso
re in forc ing
steel which is
detailedaccordingto theseismic design
provisions fo rreinforced concrete duc-
tile
frames.
This
typically
means
a
limi-
tation on the mximum
rea
oftensin
steel,thepresence ofcompression steel
with
an rea
of at least
one-half
of the
rea oftensin steel, and stirrup
ties
with acise spacingso as toconfine the
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compressed concrete
and to
prevent
buckling
of
longitudinal bars
and
shear
failure.
In the New Zealand concrete design
code
5
the
spacing
of
stirrup ties
in the
potential plstic hinge regions
of beams
is required
not to
exceed
the
smaller
of
one-quarter
of the
effective
depth
o f the
beam or sixlongitudinal bar diameters
or 150mm (6
in.).
The potential plstic
hinge regin is taken to extend over a
lengthequal to
twice
the
overall beam
depth.
ShearStrength
of
Beams
In the
plstic hinge regions
at the
ends of composite beams the cast-in-
place rein force d concrete core
w ill
need
to resist all the
applied shear forc
alone, if the bond at the interface be-
t w e e n the precas t U - b e a m and the
cast-in-place concrete breaksdow n
or if
the
bond
is
intentionally el iminated.
Therefore,the beam core shouldbe de-
signed to have
suff ic ient
t ransverse
reinforcement
to
resist
the
design shear
forc,
using the seismic design provi-
sions for reinforced concrete ductile
frames.
Aw ay
from theendso f the beam,
th e
whole
composite section
m ay
be
considered to provide shear resistance.
In order toavoidashear failure,and
henee to ensure that ducti le plstic
hingingof the
beams
can
occur during
severe
seismic loading,thedesign shear
forc
for the
beams should
be
that
as-
sociated
with the likely
beam over-
strength in flexure. To calclate th e
l ike ly u p p e r
l imi t
of f lexura l over -
strength
of the beam, composite action
should
be assumed in plstic hinge re-
g i o n s
w h e r e n e g a t iv e m o m e n t i s
applied, since
the
flange
of the
U-beam
can
act as the
compression zone
of the
composite member, as previously dis-
cussed.
However , for positive bending mo-
mentinplstic hinge region sat the ends
of the member, only the cast-in-place
reinforced
concrete
beam core
need
be
considered.
If
positive moment plstic
hinges form away from the beam ends,
the composite section flexural
strength
should
be
used
if the
interface shear
and
strand development length require-
mentsare
satisfied.
The
stirrup ties provided
in the po-
tent ia l pls t ic h inge reg ions of the
beams
should be capable of resisting the
entire
design shear forc by
truss
action
alone, since
th e
shear carried
by the
concrete,V
c
, diminishes during severe
cyclic
loading. That is,V
c
tends to zero
due to abreakdow n in the shear trans-
ferred
by
dowelaction
of thelongitudi-
nal
bars, by aggregate interlock, and
across
the com pression zone.
Interface
Shear Transfer
BetweenPrecastU-Beam and
Cast-in-Place
Concrete
Core
Composite action of the beam can
only occur if shear can be
transferred
across
th e
inter face between
the ad-
joining
precast and cast-in-place con-
crete surfaces with practically
no
slip.
Shear stress
is
transferred across
the
interface
of concrete surfaces by con-
c r e t e adh es in , i n t e r l o ck o f mated
roughened contactsurfaces,and friction.
Friction
is
reliant
on a
clamping
forc
orthogonalto the contact plae. In the
composite
beam detail , reinforcement
does not cross the contact surface and
therefore does not provide a clamping
forc.
Some small
clamping
forc
may
be
generated
on the side
faces
of the
cast-in-place concrete core by the li-
be
amwebs resisting dilation caused by
re la t ive shear movement a long
the
roughened contact surfaces. Neverthe-
less,it
wouldseem
appropriate to ignore
friction
an d to
relyonly
on
shear
transfer
by
adhesin
and
interlock
of the
mated
roughened contact
surfaces.
The
imposed shear s tresses
at the
interface
of the
contact surfaces
of the
U-beam
and the cast-in-place concrete
coreare the summationofstresses
from
anumberofsources.The imposed hori-
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1986
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zontal shear stresses
at the
interface
of
contact surfaces between
th e
U-beam
and
cast-in-place
concrete core during
positive bending m om entarise from th e
transfer
of the prestressingsteel tensin
forc
from the U-beam to the core, and
during negative bending moment arise
from the transferof there inforcin g steel
forc from
the
core
to the
U - b e a m
flange.
The
horizontal interface shear stress
couldbefound from
V
u
/b
v
d,
where
V
H
= vertical shear
forc
at factored
(ultmate)
load
b
v
= total w idth of interface(tw osides
plus bottom
surface)
and
d
=
ef fect ive
depth
of compos i te
section
This
is a
simplistic approach
to the
more
complex real behavior
of the U-
shaped
interface.
The
imposed vertical shear stresses
at
the interface during service loading
arise from the superimposed live loads
being supported by the floor sys tem.
The
service live loads need
to be
trans-
ferred
from
the U-beam
unit,
on
which
the floor system is seated, to the cast-
in-place concrete core by vertical shear
stresses acrossthe interface (see Fig.2).
The self
weight
of the U-beamunit,the
precast concrete floor system, and the
cast-in-place concrete core and floor
topping during service loading are car-
ried
by the U-beamaloneand therefore
will not cause vertical shear stresses at
th e
interface.
However, the
transfer
ofvertical shea r
stresses across
the
interface will
be
more critical at the factored
(ultmate)
load if the end supp ort of the U -beam in
thecolum n cover concrete is
lost
during
seismic
loading.
Inthatcase the vertical
shear stresses will arise from
the
self
weight of the U-beam, the precastfloor
system
and the
cast-in-place concrete
core and floortopping, aswell as
from
the
live loads. This more critical case
at
ultmate should
be
used
to
determine
the design vertical shear stress at the
interface.
In the New
Zealand
concrete design
code
5
an interface shea r of 0.55 MPa (80
psi)
is
permitted
at the factored (ult-
mate) load fo r interfaces that have no
cross
ties,
but
have
th e
contact surfaces
cleaned and intentionally roughened to
a
full
amplitude
of 5 mm
(0.2
in.).
A de-
sign approach to check interface shear
transferw hen the inside
face
of the pre-
cast U-beams have been so roughened
would be to
find
the
vector
sum of the
imposed design horizontal
and
vertical
shear stresses at the interfa ce at the fac-
tored (ultmate) load
and to
ensure that
it is lessthan0.55 MPa (80psi).
Columns
Seismic design provisions for rein-
forced
concrete ductile frames should
be used to determine for the columns
th e
longitudinal reinforcem ent required
for
flexure and axial load, and the trans-
verse reinforcement required for shear,
co nc r e t e co n f inem en t a n d res t r a in t
against buck lingoflong itudinal bars.
For
tall
frames
a strongcolumn-weak
beam concept is adopted, in order to
prevent as far as poss ible a column
sidesway
mechanism
(soft story) from
occurring during a major earthquake.
Henee,
th e
column bending moments
found
from
elastic frame
analysis for the
code factored (ultmate) load combina-
tions
need to be amplified to give a
higher column design moment, to take
into account
the
l ike ly beam over -
strength
in flexure, the higher mode ef-
fects of dynamic loading which can
cause much h igher co lumn moments
than calculated fromcode static loading,
and the
possible effect
of
seism ic load-
ing acting along both principal axesof
th ebuilding simultaneously.
3
'
4
'
5
'
6
Similarly,
the
column shear
forces
found
from
elastic
frame
analysis
for the
code
factored (ultmate) load combina-
tions need to be amplif ied to give a
higher design shear forc so as toavoid
the
possibility
ofbrittle shear failure of
the
columns.
Transverse
reinforcement
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in the column ends is
also
necessary to
provide flexural ductility there, since
th e amplified column
design
moments
may not be
sufficiently
high to
elimnate
the possibility of
some
column hinging.
In
pa rticular,
a transverso bar
spacing
of
not
more
than
six times the longitudinal
bar
d iamete r
to
p re v e n t p r e m a t u re
buckling of
compressionsteel
is an im -
portantrequirement.
Beam-ColumnJoints
The d e s i g n s h e a r fo r c e s for the
beam-column joint corescan be based
on
th e
overstrengthinternal forces
from
beams.
During negative bending moment ,
the greatest beam flexural s t rength
arises from composite action when the
precast U-beam flange transfers most of
the
com pressiveforc
in the
beam
to the
joint core by direct bearing against the
column.Then both
the
upper
and
lower
layers of
longitudinal reinforcement
in
the beam m ay be in tensin .Ajoint core
diagonal compression mechanism in-
volvingtw o
struts
w hich
transfer
partof
thejointcore
forces
isshowninFig.5b.
One strutformsbetween the bends in
the upper tensin steel and the lower
concrete compression zone. The other
strutformsat a shallowangleto the h or-
izontal betw een the bendsin the lower
tensin steel
and the
lower compression
zone. Should the flange of the precast
U-beamceasetotransfer com pression to
the
column during seismic loading,
the
negative beam moment
will be due to
the cast-in-place concrete
core alone
and
the joint core behavior
wil l
be that
of a conventional reinforced concrete
frame.
During positive bending moment,the
cast-in-place reinforced concrete alone
transfers
the beam
forces
to the beam-
column joint core. Henee, for positive
moment in the beam the joint core be-
havior
is that of a conventional rein-
forced concrete
frame.
Adiagonal com-
pression strut mechanism which trans-
ferspartof thejoint core forces isshow n
inFig.
5a.
It is apparent thatthe code approach
for the designofcast-in-place re inforced
concrete beam-column joints could be
used ignoringforces from possible com-
posite
beam action.
That
is, the design
horizontal joint core shear forces could
be
found
for the
cast-in-place concrete
beam acting alone. This assumpton is
obvious for positive beam moments but
is an
approximation
fo r
negative
mo-
ments . However, for negative beam
moments the upper layers of bars intro-
ducethehorizontaljointcore shear forc
over
the
greatest part
of the
coredepth.
The horizontal shear
forc
introduced
by the
lower layers
of
bars
may be as-
sumed to be equilibrated by the very
sha l low d iagona l compress ion s t ru t
shown
in Fig. 5b if those bars are in ten-
sin.
The joint core mechanism resisting
th e applied forces is
made
up partly by
the diagonal compression strut mecha-
nismsdescribed above
and
partly
by a
truss
mechanism involving transverse
hoop reinforcement and intermediate
column bars. During
cyclic
loading
in
the inelastic range the joint core shear
transferredby the diagonal com pression
strut mechanism decreases, mainlydue
to
the presence of
fulldepth
cracking in
the beam at the column face, and the
shear transferred by the truss mecha-
nism
increases.
5
'
6
TEST
PROGRAM
Three
full-scale
compos i te beam-
exterior column units havebeentested
2
to
assessthe seismic performance char-
acteristics of the composite frame sys-
tem
described. The overall dimensions
of
the
units
are
shown
in
Figs.
6, 9 and
10. For ease ofconstructionof the un its,
the
T-beam
flanges typically resulting
from
the presence of the cast-in-place
concrete loor topping were not
mod-
elled.
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11/40
450
8
F B
\B
2645
250
,Cast-n-place
Beam
P recas t
U Beam
400
250
400
SECTION
A-A
SECTION B-B
SECTION
C-C
Fig.
6.
Elevation
and
sections
of
test units (dimensions
in
mm).
Note:1 mm =0.0394in.
Fig.
7.
View
of
beam
and
column reinforcement
in
place
during
construction
of
Unit
1.
64
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12/40
Dabonding
material
3.5
mm
thick
foam
rubber
Length of
debonded regin=
depth
of cast-in-ploce
core + U-beam sea ting
U-bam
seating
4O-50mm
Lowor column
Fig. 8. Method of
debonding potential
plstic hinge regin of Unit 3.
A ll
units were designed usingthe
N ew
Zealand concrete design code
5
with
the
addition
of the
suggestedsup-
plementary seismic design recommen-
dationswhere necessary
as
discussed
in
the
previous section. The strength re-
duction
factorswere takenas
= 1 in
all
calculations and the
overstrength factor
for
the longitudinal beam reinforce-
ment, used forthe
calculation
of the de-
sign
shearforces,w astakenas
1.25.
DetailsofTest Units
Unit
1 was
detailed using code provi-
sions
fo r
seismic loading, with
a
poten-
tial
plstic
hinge reginin thebeamad-
jacentto thecolumnface.Unit2 was not
detailed
for
seismic loading, being
de-
signed
using code provisions
for
gravity
loading
only.Unit
3 was
detailed using
code provisionsfo rseismic loading and
w as
identical
to
Unit
1 in all respects
except
that
the interface between the
precast
U-beam
and
cast-in-place
con-
crete
core
in the
potential plstic hinge
regin wasdeliberately debonded in an
attempt
to improve the plstic hinge be-
havior. Thedetails of thereinforcement
in all units is shown in Figs. 9 and 10.
The
interior
surfaces
of all
precast
U-beams had been roughened with an
amplitude typically of 3 mm(0.12in.).
This
surfaceroughnessw asachieved by
chemical retarding
of the
interior
sur-
face
after
initial
set and
thenremoval
of
th e
surface
cement paste
from
around
the aggregate by washing with water
and wire
brushing.
The in-place concrete of the units was
castin the same orientation as for apro-
totype
structure and according to antici-
pated site practice. There were
two
poursof
in-place concrete
fo r
each unit.
First,
the
lowercolumn
was
cast
up to
the height where the precast U-beam
would be seated on it. The precast U-
beamw asplacedon theedgeof the top
surface
ofthis column pour when
con-
crete strength was gained (see
Fig.
7). In
PCIJOURNALVJuly-August
1986
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13/40
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