RAM Elements - Huge Displacements

Question

I'm getting a "Huge Displacements" warning on a simple RAM elements model. All all accounts, the model appears stable and the maximum nodal displacements are very very small, less than .1". Any suggestions on what might be causing this warning?

Karl G · Accepted Answer

Stephani, 
 
Thank you for posting your file. 
 
Some of the columns (look at Members 9, 11, 12, 17, 27, 32, and 62) are pinned at the base (TX, TY, TZ restraint) and have only pinned beams framing into them. These column are free to twist about their own axes without restraint. This rotation is the cause of the large displacement. Fixing RY (torsion) at the base of these columns will resolve the problem. You will find that there will be no torsional moment reaction at the base. 
 
You can omit the RY restraint at the columns that have a horizontal brace framing into them, since the horizontal brace provides some torsional resistance. 
 
Karl

Seth Guthrie · Answer

Your model is set to run as a 3D frame, but it's a 1D beam and there no RX restraint so it can spin. If you look at the analysis results you might see large RX rotations. You can restrain RX at one end to easily avoid the issue (or both ends, or use an RX spring). Alternatively you can limit the analysis DOF to 2D frame, but don't use that approach where you are applying twist forces as you have here.