How to deal with unilateral constraint , when performing random vibration analysis with FEM

Dear friends

I have confronted unilateral constraint when I perform modal analysis or Random vibrationanalysis by finite element method. For instance, the attached figure shows a situation that a cover is bolted with the box while the central part of the box, the partition,isn’t connected with the cover. In this case, how should I deal with the connection between them?As far as I know, ANSYS or ABAQUS simply determines that it is bonded or separate by the initial status of contact. Actually none of these treatment corresponds to the fact. This problem has been puzzlingme for quite a long time. Are there anyrelated approaches or theories proposed?

Any help or advice would be appreciated.