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# Taking account frictional forces in Static Structural Analysis (Optistruct)

## Taking account frictional forces in Static Structural Analysis (Optistruct)

(OP)
Hi. I would like to learn how to take into account frictional forces effects in structural analysis. It can be linear or non-linear. For my case, there is a rack model one on the top of the other like attached file. Rack's legs are connected to the ground with SPC constraint element which has only free translation dofs on ground plane. Connection between racks (marked locations in attached image) are connected with RBE2 elements which has free translations dofs parallel to contact plane.. When I add contact between two racks in linear static analysis I get very different results and I can't see sliding between two racks in linear static analysis. It moves like contacts are like freeze contact. I tried to perform small displacement non-linear static analysis but model didn't converged.. I would like to know how to prepare models for this kind of analysis.

### RE: Taking account frictional forces in Static Structural Analysis (Optistruct)

Contacts in linear static analyses are always frozen - their states can't change because this would invalidate the linear assumption. You have to use a nonlinear analysis if you want to account for the relative motion (sliding or separation) of the surfaces in contact. Convergence issues may arise there because of so-called rigid body motions - you have to make sure that boundary conditions and contacts prevent the model from becoming a mechanism, which is not allowed in static analyses.

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