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Chapter: Civil - Prestressed Concrete Structures - Theory and Behaviour

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Concrete Structures: Stages of loading

The member is under pre-stress but is not subjected to any superimposed external loads.

Stages of loading

 

Initial stage

The member is under pre-stress but is not subjected to any superimposed external loads. Further

subdivision of this stage is possible.

 

1. Before pre-stressing: Concrete is weak in carrying loads. Yielding of supports must be prevented.

2. During pre-stress:

a. Steel: This stage is critical for the strength of tendons. Often the maximum stress to which the wires will be subjected throughout their life may occur at this stage.

b. Concrete: As concrete has not aged at this stage, crushing of concrete at anchorages is possible, if its quality is inferior or the concrete is honeycombed. Order of pre-stressing is important to avoid overstress in the concrete.

3. At transfer of pre-stress: For pre-tensioned members, where transfer is within a short period, and for post-tensioned members where transfer may be gradual, there are no external loads on the member except its own weight. n

4. De-shuttering: The removal of form-work must be done after due consideration

Thus the initial pre-stress with little loss imposes a serious condition n the concrete and often controls the design of the member.

 

Final stage

This is the stage when actual working loads come on the structure. The designer must consider various combinations of live loads on different parts of the structure with lateral loads such as wind and earthquake forces and strain loads produced by settlement of supports and temperature. The major loads in this stage are:

 

1. Sustained load: It is often desirable to limit the deflection under sustained loads sue to its own weight and dead loads.

2. Working load: The member must be designed for the working load. Check for excessive stress and deflection must be made. But this design may not guarantee sufficient strength to carry overloads.

3. Cracking load: Cracking in a pre-stress member signifies a sudden change in bond and shearing stresses. This stage is also important

4. Ultimate load: This strength denotes the maximum load the member can carry before collapse.


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