Q: I have defined an envelope type load combination and reinforcement computed is very high. What could be the reason behind that?
A: You need to understand that ETABS/SAP/SAFE will do the design for each design combination separately and after that enveloping of max positive and max negative reinforcement will be done. So there is no need to define an envelope type combination by the user for doing the design and in fact it is incorrect to do that. In envelope type combination it is possible that max moment, shear, axial forces and torsion may not be from the same combination and in such a case the forces can’t develop concurrently and hence design will not be consistent with actual response.