Some of the connection problems discussed above result in mechanisms (structures that have moving parts), but there are other ways to create mechanisms in a VisualAnalysis model. Some of these are obvious once you "see" the problem. Others are more subtle mathematical instabilities that would not happen in a "real" structure, but cause problems in the matrix analysis.

Plane Frames or Trusses?

You can change the structure type in Project Manager to a Plane Frame, Otherwise, to stabilize a planar model in a Space Frame structure type, simply Shift+Click on a node and fix all of them in DZ (or your out-of-plane direction).

Correcting 3D Instability

When trying to analyze models in 3D it is common to get instability because of either (a) forgetting about the out-of-plane direction, (b) forgetting about rotations, or (c) having simply too many end-releases and/or not enough support. You can debug your model in this fashion:

1. Do you have sufficient support at the base of the model to prevent both translation and rotation?
2. Look for duplicate nodes (perhaps increase the default nodal tolerance first)
3. Look for overlapping or crossing elements that are not really connected.
4. Check that each member's Connect Crossing setting is set to Yes (default), except for X-braces.
5. Remove ALL the member end releases (shift+click a member), except perhaps simple beams. 
6. Is your model stable for analysis under both dead and lateral load? reasonable displacements?
7. If so, add end-releases carefully, only add releases where you need them. (If you don't have weak axis bending, you don't need weak axis releases!)
8. Repeat step 5, as necessary to eliminate moments you do not want or expect.

Animate the 'Failure' Mode Shape

VisualAnalysis can help you locate mechanisms in your model by performing a modal analysis. If a modal result is the only result available, it means your structure was unstable. You can animate the result view to get an idea of how or why the model is not stable.