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Why is reinforcement design so robust when I define an envelope-type load combination
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A: You need to understand that ETABS/SAP/SAFE will do the design for each design combination separately and after that enveloping of max +ve and max --ve 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 Answer: An envelope-type load combination should not be defined for design because ETABS, SAFE, and SAP2000 consider each load combination separately, generate a design for each load combination, then envelope this resultant set of design specifications to provide maximum positive and negative reinforcement. When an envelope combination is defined manually, the maximum 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, and torsional response from all load combinations are superimposed on a single design process which generates an over-designed system with capacity for unrealistically extreme loading.