Laurence Clark Crossen
Compared to earlier towers, the WTC structures were unique in that the outer columns were also tremendously strong, to the extent that they were probably capable of supporting the entire weight of the building by themselves.
To judge from the photo Charlie provided above, it does not seem like there was any outer columns at all comparable to the very obvious supports in the middle. Charlie's page on the core structure agrees with Jerry that, "These exterior walls bore most of the weight of the building,"- according to the 9/11 report. However, it also finds the 9/11 report to have been mistaken about the central core structure, making this claim questionable.
This was in contrast to other traditional skyscrapers like the Empire State Building, which had columns and beams on regular spacing forming a rectangular grid. Thus, the floor spaces inside were interrupted at regular intervals by huge posts.
Charlie's page says, "the outside of each tower was covered by a frame of 14-inch-wide steel columns; the centers of the steel columns were 40 inches apart." Look at the upper left corner of the building and one can see no column at all any larger than these. I suggest that the absence of support columns between the center and the outer columns could be enough to explain the rapidity and thoroughness of the collapse.
"...the core columns were connected to each other at each floor by large square girders and I-beams about two feet deep." "...the base of a core column, whose dimensions, minus the four flanges, are apparently 52 by 22 inches," -from Charlie's link above= http://911research.wtc7.net/wtc/arch/core.html