Hello folks!
I’m trying to join the two closed-polysurfaces(the base and the shell), but the Boolean Union doesn’t work.
Could anybody help me out? 
Ohne Titel.3dm (1.2 MB)
When you have complex concurrent (overlapping) surfaces, Rhino has a hard time finding the intersection to unify them. In your case, if you run the _Intersect command, you will see that Rhino finds 9 separate intersections. there are many ways to solve this. One quick (and dirty) way would be to 2D scale your bottom platform about its approximate center (using _Scale2D) a tiny bit - by say a factor of 1.01 - and then re-run your Boolean union.
–Mitch
Guess that there are non planar surfaces that occupy the same space so boolean fails. For this the better way to do it is to split/trim the inside wall of your object which I did by extracting that surface and making it slightly larger.
Once you have trimmed the inside wall then trim away the excess from the plane and join up.
Because the bottom of you object is planar the cap command will then close the object solid and you are done. IMO Boolean union was the wrong tool for this job. HTH
LOL or just do what Mitch wrote 
Well, I said it was “quick and dirty”… 
@sochin your method is cleaner, I just wanted to avoid writing out a whole procedure…
–Mitch
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