I have seen several Al frames with large "triangular" tubes, all of which have a fairly decent (1/4" to 3/8"?) radius on the edges. What impresses me are some of the extrusions I see - I think Colnago has some - that are essentially square cross section with semicircular depressions in each side. Come to think, they may be trying to replicate their logo in the tube cross section.

Cheers...Gary