I understand that when solving a static problem, the stiffness matrix is normally singular and that adding specific restraints is what yields a unique solution to the problem. But in dynamic problems, this issue doesn't arise even if the same concept applies, which is the system will have a rigid motion. So, why I don't get the same error when solving dynamic problem. I tried that using SAP2000 by analyzing a frame system without specifying any supports. the static analysis gave errors while the dynamic analysis didn't. So, why?