During the analysis phase, Staad considers any member as an uniform homogeneous entity. It does not consider that across its section, few of its portion is of steel and rest of them composed of concrete and hence Staad reports the stress of the entire member considering a single material property. The analysis is performed considering the member behaves purely elastic which means the stress-strain curve across either side of the neutral axis remains linear. Now, regarding the stress developed in the concrete and the reinforcement bars, it is more of the design phase. Staad takes the bending moment and then computes the total internal compressive force thus generated by the beam section following the stress strain curve (usually non-linear considering the LSD) from the respective design code. The program determines the tensile force it requires to resist that compression. These calculations are internally performed by the program. In the similar way, in case of column design, it follows the bi-axial moment interaction curve to determine the design rebar. So, program only reports the design forces like axial force, moment or shear but you cannot see the stress thus developed in the individual entity across its section.
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