Difference in load deflection of under prestressed, partially prestressed and
fully prestressed
Partial safety factors, are therefore used for each limit
state being reached.
The values of partial
safety loads recommended in the British,
Indian and American codes.
IS code:
Load combination Limit
state of collapse Limit state of serviceability
DL LL WL DL LL WL
DL+LL 1.5 1.5 - 1.0 1.0 -
DL+WL 1.5 - 1.5 1.0 - 1.0
DL+LL+WL 1.2 1.2 1.2 1.0 0.8 0.8
The load deflection characteristics of a
typical prestressed concrete members and discussed below:
If
the beam is sufficient loaded, tensile stresses is develop in the soffit and
when this exceed the tensile strength of concrete, cracks are likely to develop
in the member.
The load deflection
curve is approximately linear upto the stage of visible cracking, but beyond
this stage the deflection increase at a faster rate due to the reduced
stiffness of the beam.
In the port- cracking
of the beam of beam is parallel to that of reinforced concrete member.
The deflection of cracked structural
member, may be estimated by the unilinear or bilinear method recommended by the
ECC.
In the unilinear
method, the deflection will be, a= L2M/? Ec Ir
where a = Max
deflection L = Effective span M = Max moment
Ec = Modulus of elasticity of concrete Ir = IInd
commend of area.
In the bilinear method, the moment curvature is
approximately by second straight line.
The instantaneous deflection in the post
cracking stage is obtained as the sum of deflection upto cracking load based on
gross section and beyond the cracking load considering the cracked section.
Hence
deflection are estimated by
a=L2 {(M?cr/ EcIc)+((M-Mc)/0.85Ecfck)}
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