The document discusses composite construction using precast prestressed concrete beams and cast-in-situ concrete. It describes how the two elements act compositely after the in-situ concrete hardens. Composite beams can be constructed as either propped or unpropped. Propped construction involves supporting the precast beam during casting to relieve it of the wet concrete weight, while unpropped construction allows stresses to develop under self-weight. Design and analysis of composite beams involves calculating stresses and deflections considering composite action. Differential shrinkage between precast and in-situ concrete also induces stresses.

1. Composite Construction

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• A composite beam is one whose cross-section consists of two or
more elements of different materials, acting together while
carrying some or all of the loads
• Composite prestressed concrete consists of precast prestressed
beams and cast insitu concrete.
• The insitu portion is not usually prestressed and therefore,
often consists of lower grade concrete provided with ordinary
reinforcement.
• After the insitu concrete has hardened, the two elements
perform as one.
• Depending on the stiffness, the precast member can be
designed to carry the weight of the in situ concrete or can be
propped, so that it carries only its self-weight during casting.
Composite beam

3. • In latter case, the props are removed when the concrete has
hardened and the weight of insitu topping is then carried by
the composite action.
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5. Advantages….
• Economical
• Less time consuming
• Reduction in the false work and shoring cost
• No need of formwork and scaffoldings
• CIPC slab provides continuity at the ends of elements over
adjacent spans
• Provides stability to girders
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6. Types of composite
construction
 Propped Construction
 The dead load stress developed in the precast prestressed
units can be minimized by propping them while casting the
concrete in situ.
 Unpropped Construction
 If the precast units are not propped while placing the in situ
concrete, stresses are developed in the unit due to the self-
weight of the member and the dead weight of the in situ
concrete.
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8. Analysis and Design…….
• Analysis Problem
– Given data
• Sectional properties
• Length of the member
• Load conditions
– Result
• Internal stresses or forces
• Design problem
– Given data
• Length of the member
• Load conditions
• Forces or stresses
– Result
• Sections to be checked for its adequacy
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9. Analysis of composite sections for
concrete stresses
• The flexural stresses are calculated using elastic analysis.
• Let the flexural stress be denoted as f, the bending moment M
and section modulus or elastic modulus Z, then
• Bending moment is computed from the self weight of the
precast unit and the weight of wet cast in situ slab concrete in
case of unpropped construction.
• On the other hand, in propped construction, the weight of
concrete is not considered as the propping of the beam relieves
the weight of the wet concrete in the situ slab.
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Z
M
f ±=

10. Problem-1
• A precast pretensioned beam of rectangular section has a
breadth of 100 mm and a depth of 200 mm. the beam with an
effective span of 5 m is prestressed by tendons with their
centroids coinciding with the bottom kern. The initial force in
the tendons is 150 kN. The loss of prestress may be assumed to
be 15 percent. The beam is incorporated in a composite T –
beam by casting a top flange of breadth 400 mm and thickness
40 mm. if the composite beam supports a live load of 8 kN/m2
.
Calculate the resultant stresses developed in the precast and
insitu concrete assuming the pretensioned beam as: (a)
Unpropped, (b) propped during the casting of the slab. Assume
the same modulus of elasticity for concrete in precast beam
and insitu cast slab.
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15. Differential Shrinkage
• In composite construction, the precast prestressed beams are resisting
the applied loads along with the cast In situ slab.
• Precast elements were placed earlier, creep and shrinkage would
have already taken place.
• The wet concrete of slab is laid over the precast unit, and the
shrinkage and creep continues.
• The magnitude of the tensile force is computed as
Nsh=ξcs Ec Ai
where Nsh = Magnitude
ξcs = Shrinkage strain
Ec = Modulus of elasticity of insitu concrete
Ai = Cross sectional area
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16. • This tensile force is balanced by a compressive force applied at
the centriod of equal magnitude.
• The force applied at the cast in situ slab causes a direct force
acting at the centriod of the composite section together with a
bending moment.
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17. • The formulae to compute the stresses due to differential
shrinkage are as follows
Stress at the top fibre of the slab
Stress at the bottom fibre of the slab
Stress at the top fibre of the beam
Stress at the bottom fibre of the beam
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f
Z
M
A
P
tc
s
−+=
f
Z
M
A
P
bc
s
−+=
Jc
s
Z
M
A
P
+=
bc
s
Z
M
A
P
−=

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sP M
f
tZbZ JZ
cA
Direct compressive force and bending
moment
section moduli at top, junction and bottom
of the precast beam
Area of composite beam
Uniform tensile stress at centre of the
slab
Where,

19. Problem-2
A composite beam of rectangular section is made of inverted T-beam
having a slab thickness of 150 mm and width of 1000 mm. the rib size
in 150 mm x 850 mm. The in situ concrete slab has EC = 30kN/m2
and
the thickness of cast in situ slab is 1000 mm. If the differential
shrinkage in 100 x 10-6
units, estimate the shrinkage stress developed
in the precast and cast in situ units.
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21. Deflections of composite
beams
• The estimation of deflections at service loads essentially
depends on two factors i.e, the stages of loading and the
difference in modulus of elasticity of precast concrete unit
and the cast in situ slab.
• Unpropped Construction- Deflections are computed using
sectional properties and modulus of elasticity of the precast
unit. Loads are due to prestress, self weight of beam and the
weight of the cast in situ slab.
• Propped Construction- The composite beam section properties
are used to determine the deflections due to live load and self
weight of the cast in situ slab.
Note- If different grades of concrete are used in cast in situ slab
and precast beam, then equivalent second moment of area is to
be used in computations.
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22. Design of composite beams of precast
unit and cast in situ slab
 In design of composite beams, the moment of resistance of
precast prestressed unit is increased by the addition of cast in
situ slab.
 Two possibilities are
 A prestressed beam of known size is to be added with a cast in situ
slab.
 A prestressed section is to be designed for a composite section of
known size.
Section modulus of composite action
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( ) ( )





−−−
≥
min
1
MMffZ
MZ
Z
twctb
b
bc
ηη

23. Section modulus of composite action
Where,
Zbc= section modulus at the bottom fiber of composite section
Zb= section modulus at the bottom fiber of precast beam
M1= moment acting on composite beam
M= moment acting on precast beam
Mmin= minimum moment
η= loss ratio
fct,ftw= compressive stress at transfer and tensile stress at
working load.
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( ) ( )





−−−
≥
min
1
MMffZ
MZ
Z
twctb
b
bc
ηη

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





++≥
bcb
tw
Z
M
Z
Mf
f
ηηη
inf 





−≥
t
tt
Z
M
ff min
sup
The minimum prestressing force P is computed from the following expression
( )
( )bt
tb
ZZ
ZfZfA
P
+
+
=
supint
and the eccentricity of the prestressing force is
( )
( )tb
bt
ZfZfA
ffZZ
e
supint
supint
+
−
=
finf
,fsup
=stress in concrete at the
bottom and top of section
respectively
Zt
,Zb
= section moduli at the top
and bottom of section
respectively
Also

25. Problem-3
Design a composite slab for the bridge deck using a standard invested T
section. The top flange is 250 mm wide and 100 mm thick. The bottom
flange is 500 mm wide and 250 mm thick. The web thickness is 100
mm and the overall depth of the inverted T-section is 655 mm. The
bridge deck has to support a characteristic imposed load of 50
kN/m2
over an effective span of 12 m. Grade 40 concrete is specified for
the precast pretensioned T section with a compressive strength at
transfer of 36 N/mm2
. Concrete of grade 30 is used for the insitu part.
Determine the minimum prestress necessary and check for safety
under serviceability limit state. Section properties: Area = 180500
mm2
, position of centroid = 220 mm from the soffit.I = 81.1 x 108
mm4
,
Zt = 18.7 x 106
mm3
, Zb = 37 x 106
mm3
. Loss ratio = 0.8, Mmin = 0.
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29. References
• Prestressed concrete-K.U.Muthu, Azmi Ibrahim,
Maganti Janardhana and M.Vijayanad (Based on IS
1343-2012)
• Design of prestressed concrete structures- T.Y.Lin
and NED.H.Burns.
• Fundamentals of Prestressed Concrete –N.C.Sinha and
S.K.Roy
• Prestressed concrete –N.Rajagopalan
• Prestressed Concrete- N.Krishna Raju
• Reinforced concrete –Limit State Design-Ashok K Jain
• IS 1343-2012-Prestressed Concrete Code of Practice
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30. Thanks for listening
All the best
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