RISAFloor Concrete Floor Slabs can design be transferred into RISA-3D for lateral design using either a Rigid Diaphragm or Semi-Rigid diaphragm. In RISA-3D, the wind and seismic loads are calculated and then applied into the diaphragm level to distribute the forces to the Lateral Resisting System.
For example, looking at the plan view of a simple building the wind and seismic load is represented as a distributed load into the diaphragm. This building has three shear walls to resist the lateral forces.
The Rigid Diaphragm is considered fully rigid along the entire plane and internally the program connects all nodes at that level together rigidly. As the lateral loads are applied to the Rigid diaphragm, they are distributed to the lateral walls based on the stiffness of the walls.
The Semi-Rigid diaphragm is sub-meshed into a Finite Element Analysis (FEA) with plates that represent the stiffness of the diaphragm. As the loads are applied into a Semi-Rigid diaphragm the flow of the diaphragm deflects as shown below. This will often produce a more realistic distribution of the forces to the lateral resisting elements.