Diaphragm action in building

in staad rigid here are some explaination regarding your query:

1. Rigid Floor Diaphragm which assumes that the floor is very rigid to experience
any in-plane and out-of-plane deformation. The rigid diaphragm action of floors
assumes that the floor is stiff enough to undergo rigid body movement.
2. Semi Rigid diaphragm which assumes that the floor is very rigid to experience
any in-plane deformation but no out-of-plane deformation.
3. Flexible diaphragms which assumes that the floor has no rigidity to resist lateral
loads.
The rigid floor diaphragm assumption may not be appropriate if a relatively narrow
building has closely spaced shear walls (i.e. the shear walls are stiffer than the floor
diaphragm). In the case of a low rise building, the floor diaphragms may be flexible
compared to the shear walls as in light wood framed construction. For long narrow
buildings with deep beams the rigid floor diaphragm assumption has to be evaluated
carefully. The presence of a slab opening for elevators or stairs can weaken the floor
diaphragm action. Wood and metal decks without concrete fills may not be modeled as
rigid diaphragms unless the floor system is braced properly. Hence, the use of these
options in STAAD.pro requires good engineering decision making based upon the actual
site conditions.