Hello everyone!
I'm using RFEM6 6.16 and the "Building Model," "Modal Analysis," and "Response Spectrum Analysis" add-ons to perform a seismic analysis, and comparing my results with a calculation already carried out in ETABS (same model, loads, support conditions, etc.).
When comparing the results, I noticed that my "rigid diaphragm" assumption is not comparable to the "rigid diaphragm" in ETABS. My displacements under an equivalent load at the top of the building are always greater, and my first three natural periods are also longer (x, y, and phi_z). I think I could get closer by applying certain stiffness modifications. My question is:
What changes in stiffness result from assuming a rigid versus a flexible diaphragm? If possible, for each component of the stiffness matrix (D11 ... D88).
I understand that in ETABS, with a rigid diaphragm, only the in-plane membrane stiffnesses are set to "infinite," but I suspect that in RFEM6 the out-of-plane stiffnesses are modified.
Any information is welcome :)