[John Jamieson]

Huh, the arches on the WTC are not structural, but decorative. Main structure is 18" square tubes on 40" centers, with 22" wide windows.

[ItCanHappenToYou]

The arches may be more decorative than structural in the WTC, but they certainly don't hurt the building's strength. The ubiquitness of the arch that I mentioned refers to buildings made before the advent of the steel beam and glass skin construction of modern skyscrapers.

[supercat]

Huh, the arches on the WTC are not structural, but decorative. Main structure is 18" square tubes on 40" centers, with 22" wide windows.

I don't know why you say they're only decorative. They sure look structural to me. Only 1/3 of the upper vertical columns have a support column directly beneath them. The arches serve to support the other 2/3 of the upper vertical columns. While the exact shape of the arches may have been motivated by decorative concerns, either those arches were structural or there was a lot of wasted material in that building.

BTW, looking at the structure I would guess that there must have been a horizontal connection closing the top of each arch. Even though such connection isn't readily visible, does anyone know what form it would have taken?