In structural engineering, a diaphragm is a horizontal structural element — usually a floor slab or roof deck —
diapharm transfers lateral loads (like wind or earthquake forces) to the vertical resisting elements such as shear walls, braced frames, or moment-resisting frames.
Types of diaphragm
Rigid
diaphragm with infinite stiffness
In ETABS we usually go with rigid diaphragm to reduce computation time
In ETABS horizontal forces will get applied at center of mass
Semi Rigid
diaphragm with finite stiffness
With semi-rigid diaphragm we need to provide stiffness but in reality it provides similar results to rigid diapharms