How Do I Avoid Instability Warnings in a RISA-3D Model?

Have you ever received an instability warning when running a three dimensional RISA-3D model? This is because RISA-3D cannot build the stiffness matrix with the configuration you have modeled. In some cases, your model is truly unstable and in others it’s a matter of correctly modeling your structure for a 3D finite element analysis program. Below are a couple of tips to avoid instabilities in your RISA-3D models.

• Every joint in your model should have at least one fixed Member End Release framing into it. A good example of this is a truss. For more information on modeling boundary conditions for a 2D truss within RISA-3D see the Support article listed below.

Link: Tips & Tricks: Truss Boundary Conditions

When only one connecting member end releases is modeled as fixed (all others are modeled as pinned), no moment can be transferred at this joint. However, it allows the program to assign rotational stiffness to this joint which decreases your chances of getting an instability warning.

• Ensure that members framing into Boundary Conditions also follow Rule 1. This means that a column with a pinned Boundary Condition should have a fixed end release. Again, this ensures the program can assign rotational stiffness to the joint without changing the load path of the structure.

• Look closely at any Tension Only or Compression Only members. If you are running a Load Combination that has only gravity load then all of your braces will be inactive as they are not in tension. Therefore, your model may have no lateral stability.