CQUAD Offset

Hi,
i have used this offset option (ZOFFS input of CQUAD) once only and did not experience any errors. In your case i supposed that you have forgotted to auto constrains (AUTOSPC ON) the 6th DOFs of CQUADs, though this is the default option in SOL 101 (SESTATIC). So if the model loaded with torsional moment then the CQUAD elements may be subject to rotational moment normal to their axis. Since normally they do not have this kind of rotational stiffness, the job will fails due to the singularity of stiffness matrix. I recomment you to activate this PARAM, AUTOSPC or use the PARAM, K6ROT if you have another DOFs, that should not be constrained automatically.

regards