post tensioned concrete

huangyhg

post tensioned concrete
hi:
i am relatively new to the design of pre stressed concrete (studied way long).what i want know is, that when checking for ultimate limit state, the applied moments are specifies as "secondary moments + factored dead&live load moments".these are compared with the section moment capacity.what i want to know is that why these secondary moments/hyper static are treated as external moments?
please help!!
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the secondary prestress moments result from the externally imposed support reactions, which are in turn caused by bending of a beam due to prestress.  these reactions, and secondary moments, only occur if the beam is statically indeterminate and restrained at it's supports.  there are no secondary moments for simple span beams.  so these secondary moments do not include the internal primary prestress moments which result from the prestress force in the tendon times it's eccentricity.
the internal primary moments are not considered at the ultimate limit state (uls) since you're checking the section at it's ultimate capacity irregardless of loading or prestress.  since the sectional stresses at uls are taken at their ultimate value, maximum concrete strain and near yield in the tendon, any additional stress caused by internal prestress is irrelevant.  also in this state the concrete around the tendon is cracked, and therefore the prestress is generally considered to be overcome.  don't think about this too hard since it is a theoretical simplification.
good reply, coop...
you mean that, we consider the secondary moments as external moments as they result from external support reactions which are introduced due to support restraints?
the primary moments do not result in any support reactions and hence are not treated as external moments.right?
that's about right.