If you are interested in GAPs with initial openings, please look into ATENA-GiD User's, 5.3.6 Interface Material, Initial_Gap_Load_for_Volume.

If i use contact volumes respectively interface material (3d) as the contact definition between the side surfaces of two macro elements which got shell material (shell elements) assigned i got numerical problems (Analysis finished with error, no step calculated). When i use volume elements instead (i believe that's what i get if i choose solid concrete) the calculation starts (the rest of the model is the same). I understand there are some specialties about shell elements from the trouble shouting and manual because they have a fixed thickness (for example). Is it possible to create contact on the side surfaces of shell-macro elements that stand on each other via contact volumes?

I got another problem. I recognized you can create contact volumes of two surfaces (in 3d) that have identical position. But you can do it as well with surfaces between which is a distance which means you do really create a volume. Does this make a difference in calculation?

Thank you in advance!