I'm analyzing a weldment and attempting to use shell elements for a gusset. I bonded the shell surface of the gusset to the adjacent beam (the entire beam, not just the surface contacting the gusset). This is the only geometric configuration set it will let me specify. During solution I get an error message "Model is unstable. Constraint may not be adequate." When I set the gusset as a solid and specify the appropriate contact set I have no issues. I'm running 2011 Premium SP2.0. Any ideas on what is happening? Thanks in advance.