Stages of loading
member is under pre-stress but is not subjected to any superimposed external
of this stage is possible.
Before pre-stressing: Concrete is weak in carrying loads. Yielding of supports
must be prevented.
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.
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
De-shuttering: The removal of form-work must be done after due consideration
the initial pre-stress with little loss imposes a serious condition n the
concrete and often controls the design of the member.
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:
Sustained load: It is often desirable to limit the deflection under sustained
loads sue to its own weight and dead loads.
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.
Cracking load: Cracking in a pre-stress member signifies a sudden change in
bond and shearing stresses. This stage is also important
Ultimate load: This strength denotes the maximum load the member can carry