I am getting zero stiffness/instability warnings at nodes where I have beams connecting to solid elements. What could be the reason?
Solid elements do not have any rotational degrees of freedom (DOF) at their nodes or in other words there is no rotational stiffness at the nodes of the solid elements. So if you have a beam element (or a plate element) connecting to a node of a solid, without there being any other element connected, the moments from the beam( or plate) at that node cannot be transmitted to the solid. A typical example would be a column modeled with beam elements that connects to a pedestal or a foundation that has been modeled with solid elements. If the beam element is connected to a single solid node, the moment at the base of the column has no load path to the support. Hence the software generates the zero stiffness/instability warnings with regards to the rotational DOFs. There are a couple of methods to handle the issue
First Method:
Create dummy beams which flare out from the base of the column to other corner nodes of the solid elements as shown next. The moment at the base of the column will then be transmitted to the other nodes of the solid as axial and shear forces.
Second Method:
Create a dummy member (or members) extending into the solids as shown in the picture below
This method has a limitation in that the bottom of those members may be unstable to twisting about their longitudinal axis (Global MY moment in this case) unless there is an alternative mechanism to prevent that. So if you define the dummy members this way, you would still end up getting some warnings with regards to this twisting DOF. However if you do not have any moment acting along this unstable degree of freedom, you may ignore the warning.
Last but not the least, whenever you get any zero stiffness/instability warning, it is advisable to ensure that the statics check is satisfied and displacements are reasonable. More details on zero stiffness and instabilities can be found in the wiki below
(31) STAAD.Pro Instability And Zero Stiffness [FAQ] - RAM | STAAD Wiki - RAM | STAAD - Bentley Communities