One way to connect those profiles is to use an Automatic Node Merge.  I used a 7 mm Tolerance Value and turned on Face/Face.

This is the result:

Note that one of the profiles didn't get any nodes merged at the 7 mm setting, and some of the elements are getting distorted during the merge.

A much better way is to create a Named Selection of all the faces of the U-channels on the top and second Named Selection of all the faces of the I-beams that they rest on and make a Contact Definition between two Named Selections. When I did that, I found that some face normals are not in the same direction as their neighbors, so that needs to be accommodated to use this method.

I suppressed your line bodies. I suppressed Fixierte Lagerung. I changed your springs from Body-Body to Body-Ground, I suppressed Körperwechselwirkung (Body Interactions) since you don't want extra complexity during the first attempt to solve.

Hope this helps. I don't have 18.1 available at this time, I was doing this in 18.2.