grade is usdl for casting the flange at the site. 4. Write the advantages of composite construction in PSC. (MAY/JUNE 20
PRESTRESSED CONCRETE STRUCTURES COMPOSITE CONSTRUCTION
UhiIT-IV 2MARKS 1.
Deline composite construction. (Nov/DEC
2009)
.
Composite construction ofproviding monolithic action between i) prefabrioated units like steel beams, precast reinforced or prestressed concrete beams and
ii) Cast-ins-situ
concrete. This
is found to provide a greater structural efficiency compared with the conventional methods of constnrction. The resulting structure formed hy two or more materials is called method
conqmsite c onstr uctio rr 2.
Ilifiereutiate ordinary construction and composite construction. The conventional me*rod of the steel beam and slab construction, there is no composite
action between the two and each component carries the entire load transmitted by the slab. It can be rtalized that
if
sufficient shear comection are provided between the prefabricated beam and
the cas-in-situ slab, the tw'o
will
acts as one unit and the load will be resisted by composite
actioo3.
Dtfue the concept of composite in PSC, The composite beam essentially consists of a prestressed prccast stem and a cast-in-situ
flmge. The flmge is cast at the site while the prestressed precast stem is made in the factory. While a high raage grade concrete is used for the precast stem, concrete of a relatively lower grade is 4.
usdl for casting the flange at the site.
Write the advantages of composite construction in PSC. (MAY/JUNE
r'
Appreciable saving
in the cost of
in a
steel
2009,
Noy/DEc 2009)
composite memberfcompared with a
reinforced or prestressed conciete member.
/
Sizes ofprecast preskessed units can be reduced due to the effect of member.
r'
Low ratio of size of the precast unit to that of the whole composite member.
/
In many
cases. Precast prestressed units serve as supports and dispense
with the form
work for placemrent of in-situ concrete.
r'
Composite metubers are ideally suit€d disruption of normal traffrc.
for
constructing bridge decks without the
l
the
t:
mcdium
rf material in a composite section in which .:1 foroes while the highcompressive rcsists construution s€ogfi cmcrete of in-situ
suq$
pcsuessetl units resist the tensile forces'
oE-;;*osite
cast-in-situ clncrete is achieved action between the precas and
is often combined systems prestressed concrete
%'#L*
The
with other materials such
resulting
as
formed by
economy and efiiciency' rcidorced concrete fo' U'" 'uft" of tmormorematerialsiscalledcompositeconstrtrction.sothecombinationoftwoormore as a combination of two materials actiug the So construction' materials is called composite
action io ptestressed concrete strucflrres' monolithic action called ccimposite 2008' 2009' on a composite member' (APR/MAY effect of differential shrinkage
(8) nxndnthe
"'0"' ,n"
.
in-situ concrete in a composite
b"T * "i'"l*''elv
and lower grade (M-15 or M-20)
precast prestressed ration' On the other hand' the water-cement higher has correspondingly
concreteisofhighergradeandmostofitsshrinkagehasalreadyoccurr;dbeforetheplacement ofin-situconcrete.Consequently,thein.situconcreteshrinksmorethantheprecastconcrelc. to propped construction with references and unpr.opped by you meaa do z?) lvt at
t./ MAY/JUNE 2012' 2013) concrete struetures? (APR'/MAY 2009' prestressed -composite ---'the following are the stress * ,"*r.. (after the section behaves like a composite section) developed in the section' i) The dead loads stress profiles for the firll depth of the composite the consete in situ' 'precast by propping them while casting prestressed units can be minimized propped This method of construction is term edas
co*':":U':'
--
-L:^^^-^?a
ii)Iftheprecastunitsarenotproppedwhileplacingtheinsituconcrete,strdssesaredeveloped
intheruritduetotheselfweightofthememberandthedeadweighloftheinsituconcrete.This constraclion' method of construction is referred to x unpropped
S.Writetheagsumptioninanalysisofultimatestressesincompositeconstructions. (APR/MAY 201l)
following assumptions' The analysis at ultimate is simplified by the
l'Thesmallstraindiscontinuityattheintei..faceoftheprecastarrdClPportionsisignored. is also ignored' 2. The stress discontinuiry at the interface
3.IftheClPportionisoflowgpdeconcrete,theweakerClPconcreteisusedforcalculatingthe stress block.
I
f
rcc&
{''t** fg
bars +hrooyA hO@ fn
Trnqasy
upt
l>sc detDe/*D
I0. Write the analysis of composite construction sections. The analysis of a composite section depends upon the type of composite section, the stages of preshessing, the types of construction and the loads. The types of construction refer to
whether the precast member is propped or unpropped during the casting of the CIP portion. t1re precast member
If
is supportcd by props along its length during the casting, it is considered to
be propped.
11. Explain the effect of deflection on a composite member. The deflections at different stages in the precracking range of.the composite beam may be
determined with reasonably accuacy using the linear elastic theory. The deflection depends upon the method ofconstruction (propped or unpropped) and the stage ofloading. In a composite
beam forward by the combination of precast preshessed concrete and in-situ concrete,
t.he
moment of inertia of only the precast component should be taken into account in computing the
initial and final camber due to prestress. 12. Explain the effect of shear strength on a composite member.
A
composite beam comprising prestressed concrete and plain or reinforced concrete may
fail in one of the following ways; it may fail in vertical
shear
like a conventional non-composite
in this manner. It should be verified that the design ultimate shear does not exceed the ultimate shear strength in accordance with the code beam. In order to prevent shear failure
provisions.
+
13, Write the construction techniques of composite constructioDs in prestressed coDcrete.
Composite constructions comprising prestrusscd concrete lurd conoTete may
differ from
l. Proportion,
each other in respect of the
plain or
reinforced
following:
geometry and position of the prestressed concrete elements in the composite :
construction.
2. Proportion, geometry and position of the ptain or reinforced concrete elements, in the composite construction. 3. Sequence of construction
^nd
stages
of loading.\
4. Marurer of connection between constituent materials.
14. Sketch some typical cross sections of composite bridge decks with precast prestressed elements.
Termane-nb ./fonnNore-
t
res*essed b aur.u
?reus
P
o"7*' Psc e-lemetlt 15. Explain
with neat
sketches, the stresses developed due
to differential shrinkage iu
structural elements comprising precast prestressed and cast in-situ concrete elements.
A
reasonably estimation of stresses developed due to
differentially shrinkage may be made using
the following assumptions:
.
L The shrinkage is uniforn over the in-situ part ofthe section and 2. Effect of creep and increase in modulus of elasticity with age and the component
shrinkage, which is common to both the units is negligible.
,
of
I
I
16. How do you compute the shrinkage and resultant stresses in composite members?
The resultant stresses stresses developed in the precast beam and. in-situ cast slab are shown in the figure. The effect of using concrete with different moduli of elasticity in beam and
slab results
in an increase in the compressive
corresponding decrease
in the compressive
Deflection due to
Prcstress selfweight
at the top fibre of the beam with
stresses developed
a) Unpropped construction and b) Propped
Deflection due to
stress
:
in
t]1e
in-situ cast slab'
a
These
conltructiol'
l-iav
r
@
I as+ez
I
18.'Briefly outline the method of computing the ultimate flexural strength of comPosite sections,
Theultimatestrengthofcompositeprestressedsectionsinflexureisgovemedbythe same principles used
for ord.inary prestressed sections'
most composite sections, the percentage oftensioned reinforcement is less that that in zone generally simple beams, so that the section is invariably under-reinforced. The compression vale of the cube consists entirely of in-situ concrete of lower compressive strenglh and the of in situ cast stength of concrete of lower compressive strength equations will obviously be that average However, if,the compression zone contains a part of the precast element, the
In
case of
concrete.
compressive strength computed by considering the c,/s the computation
of in situ
and precast concrete is used in
of of compressive force. The method of estimating the ultimate flexural stren$h
composite sections using formulae is mentioned below:
I
7
9'rRENorTtl
FLE{URA L
*l*re. {P-
=
chara t*cr h
. dp. drea o!
trc
OF
CoMPoSr'rE
X-u" N",4 d
Sbenc?Ek o
Pre.strel nt-
"Pft^
a. DlPtr gl
f
voncre^be *e-ali
or,-
19. Briefly outline the method of, eomputing the ultimate shear strength of composite sections.
The ultimate shear strength
of
composite sections
with web-shear or flexure-shear cracks is
computed using the empirical expressions. If the shear at the section under design ultimate loads exceeds the shear strength suitable shear reinforcements are designed according to the design code provisions. The composite action of the integral
unit is mainly dependent upon an effective
shear connection at the contact surface between the precast and in situ cast elements.
l_
looo
t7 V, t A. ---
bfr_
/u
o.rl k Y! --- l/u
6fs
20. Specifu the various
,t"p. irrot
"dTTil"fri-Eilmposite
sections.
The dimensioning of composite sections involves determining the required size of the composite section using a standard precast prestressed beam ofknown section propelties in order to support
the required design service ioads. Alternatively,
it
may become necessary to determine the
section modulus of the precast prestressed section for a composite slab of given depth. In either
cast, formulae relating the section moduli
of the precast prestressed and composite section,
loading on the member, permissible stresses in the concrete and loss ratio, may be developed by considering various stages of loading. The critical stress condition generally occurs at the soffit of the precast prestressed elenient under
minimum and maximum moments. Hence, at the stage of transfer, when the minimum moment (self weight ofprecast beam) in acting on the precast prestressed beam, the stress condition is
[Finr- (Mn
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