Dear Sir
I have modeled 3D concrete beam with and without pre-stressing force and i want to make comparison between them and study the effect of the pre-stressing force.i made 3 load cases Lc1-force at mid span,LC2-supports, Lc3-pre-stressing when i made analysis steps 1st step include load cases 1,3 and the following steps  include load cases 1,2 upto failure. when i analysed it the following message appear:zero pivot,numerical factorization this is the first problem.
note :i made it as :Example Manual (INTB)

the second problem i try to replace the load cases as follow Lc1: supports, LC2 :pre-stressing, LC3 :force at mid span.1st step include load cases 1,2 and the following steps  include load cases 1,3 up to failure. the model was analyzed(what is the difference in the last problem) but the capacity of the beam didn't change at the final step of load case1,3.