Canoa Bull

7/26/2019 Canoa Bull

1/40

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

54


2/40

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

55


3/40

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

56


4/40

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-

PCIJOURNAUJuly-August

1986

57


5/40

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

58


6/40

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


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


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

59


7/40

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

60


8/40

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-

PCIJOURNAUJuly-August

1986

61


9/40

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

62


10/40

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.

PCIJOURNAUJuly-August 1986

63


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


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

65


13/40

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V l^OO

R1O

hoop

A

'

y

^ r

R12

SECTIQN

3

2-7.S

strar

R6

stirr

* >

225

3-12-

stran

SEC

83

mm

ids

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