...and they start out by mentioning 3-dimensional shapes???
The reason why you see so many triangular shapes on bridges, et al, is because straight steel beams cost less than curved beams.Some of those beams are tensile links, too. If you look at the bottoms of the longest bridge spans, you will notice tha the arch is usually of a circular profile, or ellipsoidal.
Ellipsoidal tubes are stronger in one direction than in the perpendicular direction, making construction trickier. A triangulr cross-section would likewise be stronger directionally. I have never seen triangular cross-section tubes used for any cycling purpose.