The typical application of lateral loads on Rigid diaphragms occurs at the center of mass for that diaphragm. This requires that joints are created at this location and lateral loads are applied at these joints. It is important to add these joints at the same elevation as the node you’ve defined for the Rigid diaphragm.

If the diaphragm is defined as a Membrane diaphragm it is infinitely rigid in-plane but has no rigidity out of plane. This is perfect for lateral load distribution. However, it makes these joints unstable for out of plane loads. This means the program will produce an instability warning for these joints in the vertical direction.

There are two ways to handle this condition. If no vertical loads have been applied to these joints, you can ignore the vertical instability warning.

The second option, which RISA recommends, is to add a vertical boundary condition at these joints. This will eliminate the instability warning but will not modify the behavior of your structure.

Tags: Diaphragms, RISA-3D