If the deflected shape and mode shape was as shown below, does this mean that there is something wrong while modeling the diaphragm or while meshing it or it is ok that the deflected shapes to be like this, and if not can you provide me any suggestion to fix my model ?
Alos I have these warnings and i wonder if they are related to this behavior
The gap in the mesh is minor. It could probably be fixed with some adjustments to the deck edges. The large deflections I assume occur because the out-of-plane stiffness of the deck is ignored (or it is very small) so a little lateral overturning results in a lot of deck displacement out-of-plane. It's not a problem per se, but increasing the out-of-plane stiffness can usually get it under control. See also: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/3348/ram-frame---semirigid-diaphragms-faq