Please watch the three videos at the beginning of this discussion:
https://forum.ansys.com/discussion/1217/bolt-pretension-clamps-two-plates-together
Answers:
Use mid-surface for sheet metal parts most of the time.
Use Frictional Contact with Shell Thickness Effect set to Yes to "fill" the gap between the surfaces.
There are many ways to model this. One approach is to use solid elements on the rail and clip. Add Frictional contact between them and between the rail and the sheet metal bracket and between the clip and the sheet metal bracket. Add bolts to clamp the clip to the sheet-metal and rail.
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