Question: I have analyzed my model in Staad.pro and observed that it displays the instability warnings.
Answer: Since solid elements by their basic nature have no rotational degrees of freedom at their nodes, so there will be no rotational
stiffness at that node.
You can tackle this problem in 2 ways :
1) Have dummy beams which flare out from the base of the column to other corner nodes on the solid elements.
The moment at the base of the column will hence be transmitted to the other nodes of the solid as axial and shear forces.
The diagram below illustrates this method.
2) Have a dummy member (or members) extending into the solids. The diagram below illustrates this method.
This method has the limitation in that the bottom of those members may be unstable to twisting about their
longitudinal axis unless there is an alternative mechanism to prevent that. If there is no moment acting along that
unstable degree of freedom, you may ignore the warning.
Finally, you can check if the applied loads are in equilibrium with support reactions, you may consider the zero stiffness
warning messages harmless, and ignore them.