You should be able to close up the gaps with triangular planes at the very least. Probably the easiest way would be to use the roof tool, though as said, don't have ArchiCAD 15 yet, so I'm not aware of what the shell tool is like. Just draw your roofs between the collumns in 3d view.
If thickness isn't an issue, you could also use mesh tool.
If your shape is simple (like my circular example), you can copy a lot of things around and it should be relatively fast. If you want a more free flowing shape like the one you posted, it'll likely be a lot of work with most triangle being unique shapes.
Erwin Edel, Project Lead, Leloup Architecten
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