Analysis and design of composite beams.applications of composite beams, precast elements, cast in place elements, prestress composite beams

1. UNIT-IV
COMPOSITE CONSTRUCTION
Mr.P.SELVAKUMAR
Assistant Professor
Department of Civil Engineering
Knowledge Institute of Technology, Salem

2. Composite Sections
• A composite section in context of prestressed concrete
members refers to a section with a precast member and cast-
in-place (CIP) concrete. There can be several types of
innovative composite sections.
• The advantages of composite construction are as follows.
– Savings in form work
– Fast-track construction
– Easy to connect the members and achieve continuity.

3. • The precast member can be first pre-
tensioned or post-tensioned at the casting
site.
• After the cast-in-place (cast-in-situ) concrete
achieves strength, the section is further post-
tensioned.
• The grades of concrete for the precast
member and the cast-in-place portion may be
different.
• In such a case, a transformed section is used
to analyse the composite section.

4. Analysis for stresses
• The analysis of a composite section depends upon the type of
composite section, the stages of prestressing, the type of
construction and the loads.
• The type of construction refers to whether the precast
member is propped or unpropped during the casting of the
CIP portion.
• If the precast member is supported by props along its length
during the casting, it is considered to be propped.
• Else, if the precast member is supported only at the ends
during the casting, it is considered to be unpropped.

6. • Stress profiles for the precast web
• At service (after the section behaves like a composite section) the
following are the stress profiles for the full depth of the composite
section.

7. Estimation of Deflection
• Deflections are computed by taking into account the different
stages of loading as well as the differences in the modulus of
elasticity of concrete in the prestressed unit and the in situ cast
elements.
• The initial deflection due to prestress, self-weight of the beam and
the weight of in situ cast concrete.
• If the beam is not propped is computed on the basis of the section
and modulus of elasticity of the precast unit.
• If the precast beam is propped during constructions, the deflection
due to the dead weight of in situ concrete is also computed on the
basis of the composite sections

8. Flexural strength of composite
members
• The ultimate strength of composite prestressed sections in flexure
is governed by the same principles used for ordinary prestressed
concrete.
• The percentage of tensioned reinforcement is less than that in most
simple beams, so that the section is invariably under-reinforced.
• The compression zone generally contains entirely of in situ concrete
of lower compressive strength and the value of the cube strength of
concrete to be used in flexural strength equations will obviously be
that of in situ cast concrete.
• If the compressive zone contains a part of precast elements, the
average compressive strength computed by considering the cross-
sectional areas of in situ and precast concrete is used in the
computation of compressive forces.

9. Shear strength of composite members
• The support sections of composite members where
web shear cracks are likely to develop should be
cracked under service loads for safety against cracking
in shear.
• The ultimate shear strength of composite sections with
web-shear or flexural shear cracks is computed using
the empirical expressions suggested in British,
American and Indian standard codes.
• If the shear at the section under design ultimate loads
exceeds the shear strength, suitable shear
reinforcements are designed according to design code
provisions.

10. • Design ultimate horizontal shear stress at interface
• When links are provided, their cross sectional area should be at
least 0.15 per cent of the contact area. The space of links in T-Beam
ribs with composite flanges should exceed neither four times the
minimum thickness of the in situ concrete nor 600mm.

11. • The amount of steel required Ah is obtained from the
equation
• The ultimate limit state is computed by the expression

12. • The permissible value of the horizontal shear stress for
different types of contact surfaces is specified as
– 0.6 N/mm2, when ties are not provided and the contact
surface of the precast elements is free of laitance and
intentionally roughened to an amplitude of 5mm.
– 0.6 N/mm2, when minimum vertical ties, according to
section 8.3.3 are provided and the contact surface is not
roughened.
– 2.5 N/mm2, when minimum vertical ties are provided and
the contact surface is roughened to an amplitude of 5mm.

13. • When shear stress exceeds 2.5 N/mm2 then shear friction
reinforcement is to be designed and the required area of
reinforcements is given by

14. Differential shrinkage
• In composite members using prestressed units in situ cast concrete,
a considerable proportion of the total shrinkage will have already
taken place in the precast prestressed beam before the casting and
hardening of the in situ concrete.
• The magnitude of differential shrinkage is influenced by the
composition of concrete and the environmental conditions to which
the composite member is exposed.
• A reasonable estimation of stresses developed due to differential
shrinkage may be made using the following assumptions
– The shrinkage is uniform over the in situ part of the section
– Effect of creep and increase in modulus of elasticity with age and the
component of shrinkage, which is common to both the units is
negligible.

15. • The uniform tensile stress induced in the in situ concrete is εcs Ec
and the magnitute of the tensile force is computed as
Nsh = εcs Ec Ai
Ai= Area of the in situ concrete section
Ec= Modulus of elasticity of the in situ concrete

16. Stresses due to differential shrinkage

17. Shrinkage induced stresses
• The maximum permissible stresses in the precast prestressed
concrete and the in situ cast concrete are mainly governed by
the compressive strength of concrete in the respective
elements.
• The higher value of compressive stress is permissible only in
composite sections with the stipulation that the failure of the
section is due to excessive elongation of steel.
• This requirement is to safeguard against the explosive
compressive failure of the concrete at the limit state of
collapse.

18. • These stresses may be increased by up to 50 %, provided that
the allowable tensile stress for the prestressed unit is reduced
by the same amount.
• The higher values of flexural tensile stresses are permitted
since it has been proved by experiments that the
development of cracking.
• Which is prevented by the uncracked prestressed concrete
which is bonded to the in situ concrete.

19. Design of composite sections
• The dimensioning of composite sections involves determining
the required size of the composite section using a standard
precast prestressed beam of known section properties in
order to support the required design service loads.
• The critical stress condition generally occurs at the soffit of
the precast prestressed element under minimum and
maximum moments.
• Hence, at the stage of transfer, when the minimum moment is
acting on the precast prestressed beam, the stress condition is
given by the expression.

20. • We then have the stress condition at the soffit of the composite
section
• The prestress required at the bottom and top fibres of the precast
prestressed beam is computed under the following equations

21. Thank You