Before reading this, be sure to check out the article on X-Bracing. In RISA-3D, you can set a member to be tension-only by double-clicking on it and selecting Tension Only or Euler Buckling from the Advanced Member Properties.

In the example below, if the braces are set to Tension-Only, the model will register as unstable:

Under gravity loading the elastic shortening of the columns will cause some elastic shortening of the braces. This means that each brace is slightly compressed (with about 50 lbs of axial force). When the braces are set to Tension Only they are not allowed to take any compression at all, so they are both ignored during the solution, resulting in an unstable frame.

By setting the braces to be Euler Buckling instead of Tension Only, the program will allow these braces to take compression only up to their Euler Buckling limit, which is enough to compensate for the elastic shortening. If any significant compression enters the brace it will still be ignored during solution.

Tags: Instabilities, RISA-3D