Finding out Forces/Moments for reinforcement design for slabs modeled using solid elements
First of all, there is no direct way to get moments, axial forces and shear forces for solids in STAAD.Pro. For solids STAAD.Pro directly reports the normal, shear stresses and principal/Von-mises stresses. It is difficult to accurately back calculate the forces from the stresses and so when reinforcement design is required, I would suggest modeling the slab using plates whenever possible. I am not aware of any accurate way of calculating forces/moments for solid elements but since I have been asked these question quite a few times by users, I thought of putting together this approximate procedure that one may consider using
Finding out the Moments and axial forces from solid stress
One approximate way is to find out the maximum and the minimum normal stress in the cross section at the two extreme edges of the cross section. These normal stresses are SXX or SYY or SZZ depending on the face being considered. The maximum normal stress = P/A + M/Z and minimum normal stress = P/A – M/Z. Solving these equations one can get P and M. The values of A ( cross sectional area) and Z (section modulus) can be calculated based on the solid geometry.
Finding out the Shear Stress from Solid Stress
Find out the average shear stress on a face by adding the shear stress for the individual elements and dividing the summation thus obtained by the number of elements on the face. These are the SXY or SYZ or SZX stresses depending on the face being considered. Multiply the average shear stress by the area of the cross section to get the total shear force on the cross section.