Reply #53: wrong again, not massive at all

[Snivi Yllom]

First, we were talking about WTC7 not WTC 1 and 2 which have completely different structural systems. WTC 1 and 2 had comapratively lightwight and unique structural system necessary to build towers that tall and resist the wind forces, without a field of columns filling a typical floor plan:

http://www.civil.usyd.edu.au/latest/wtc.php

"The structural system, deriving from the I.B.M. Building in Seattle, is impressively simple. The 208-foot wide facade is, in effect, a prefabricated steel lattice, with columns on 39-inch centers acting as wind bracing to resist all overturning forces; the central core takes only the gravity loads of the building. A very light, economical structure results by keeping the wind bracing in the most efficient place, the outside surface of the building, thus not transferring the forces through the floor membrane to the core, as in most curtain-wall structures. Office spaces will have no interior columns. In the upper floors there is as much as 40,000 square feet of office space per floor. The floor construction is of prefabricated trussed steel, only 33 inches in depth, that spans the full 60 feet to the core, and also acts as a diaphragm to stiffen the outside wall against lateral buckling forces from wind-load pressures."

Here are your "massive" floor trusses" (actually pretty skimpy bar joists)


WTC7 was a more typical modern steel frame curtainwall clad building. The major difference with WTC7 and other typical office buildings was the series of transfer beams at the lower floors providing a column free span over the Con Edison substation. This was the building's vulnerable point as loss of a transfer beam in the fires likely caused an entire row of columns to become unsupported with no capability for load redistribution. That's what brough the building down so quickly.

The Hiroshima Building was a mix of heavy masonry, heavy over engineered steel and heavy reinforced concrete.