The easiest way to set up this problem is to merge all of the volumes into a single solid using the boolean operation is Onshape - then you do not need to use any contacts at all in SimScale. I took a quick look - and the model merged into 2 parts (the blocks are simply hidden). It seems there are gaps in the model, thus I could not get the full merge.
For applying the boundary conditions in SimScale, you can remove the blocks and apply a fixed BC directly at the location where the truss member and block intersect - on both ends
To apply the load in SimScale, you again can remove the box and apply the load directly to the model by applying a force (using m*a)
- In Onshape, create single volume model of the bridge using the Boolean operation
- In Onshape, Use the Split face operation to create the faces for the fixation and load on the model
- Delete the blocks from the model
- Upload only the single volume bridge to SimScale - you will need no contacts in this case
- Perform meshing and apply fixation and force BC's in SimScale.
Hope this helps