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Chapter: Civil - Prestressed Concrete Structures - Design Concepts

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Difference in load deflection of under, partially and fully prestressed

Partial safety factors, are therefore used for each limit state being reached.


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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