Rigid Diaphragms can be added using the Diaphragm spreadsheet. Just enter the node label and then choose the horizontal plane.

Applying wind loads to structures with rigid diaphragms can be done automatically in RISA-3D. In the image below we have a structure that has rigid diaphragms and the wind load has already been applied at each diaphragm level.

In RISAFloor, the deck edge (also known as Slab Edge) is selected as Rigid or Flexible. In a building that needs to be designed for both, start with Flexible.

When using rigid diaphragm analysis in RISA-3D it can be useful to evaluate the story shear in each frame. This is a perfect application for the Internal Force Summation Tool.

Flexible diaphragm behavior on sloped roofs is assumed to act through a “ceiling” diaphragm. This means that load attribution is only performed in the horizontal plane.