July 29, 2010
How to Model a Semi-Rigid Diaphragm
RISA-3D does not inherently have the capability to define a semi-rigid diaphragm. If you’re dealing with a complex roof system then you probably need to be able to model something that behaves accurately though.
This is the perfect application for Semi-Rigid Diaphragms. A semi-rigid diaphragm is modeled with plates, and requires you to model and submesh the plates appropriately. In order to adequately mesh your plates it is good to be familiar with Plate-Member Interaction. Plates are modeled using General materials, so the first step is to set up a material with the material properties of your diaphragm.
Link: Plate-Member Interaction
Next, model your plates using the specified material, and with an accurate thickness. Be sure to check the Plane Stress option. This makes it so that the plates only have stiffness within their own plane, and as a result won’t add any composite bending strength to the rafters. With an adequate mesh you can get realistic behavior of a roof diaphragm.