Problem with Booelan Union and Boolean Difference on Structure

Hello maybe someone can help with my problem i have created a file to better explain my problem
i have a structure and basically want to boolean out its “massiveness” by using the same structure and boolean difference away the bottom part. I think the problem has to do with the way the structure is unified, the model my buddy created in Rhino 5 was select the grouped polysurfaces; meshToNurb->Boolean Union,
i get errors when i try to do the same in rhino 6. Please refer to the file (extract with 7zip or winrar) to get an idea of the problem
structure.rar (11.2 MB)

Please don’t double-post…