[backhoe]

Thanks for using that click the pic graphic... I had to hunt back thru several hundred replies, but I finally found "Aw, Jeez!"

Been a welder since the 1960's, and I can tell you when steel is heated a few hundred ( let alone a couple of thousand ) degrees, it loses much of its strength. Fire pancaked those buildings- nothing more to it.

[Calpernia]

Thank you for adding that backhoe. I know nothing about explosives. Would be nice to hear why things can't happen then a bunch of pile on butt kickings that occur.

[concretebob]

BINGO...we have a winner in the "How Come those Buidings Fell" category.
I was also a welder. Then became a commercial concrete worker.
Now I'm a building inpsector..oh wait...a "damn" building inspector..have to keep those titles straight..
I inspect, among other things, the spray-applied fireproofing for commercial buildings..it's a nasty looking, gray colored, cementitious, (sea-men-ti-shus) material, which looks like elephant snot with hair..
but it will protect the structural steel framing of a building for 3 hours, at 2200 degrees Farenheit...I doubt it will protect a building from a commercial airliner loaded with fuel, though.
What amazed me at the time, was that Congress had to convene a special hearing to find out what I, or backhoe, or any construction worker who has built them, could have explained in 5 minutes, without visual aids.

[concretebob]

Almost forgot the most important factor...
the concrete floor slabs..
assuming the floors were lightweight concrete..(a contradiction in terms for some, I know) the per cubic foot weight of LW concrete ranges between 114 pcf to 118 pcf..(as opposed to "hardrock" concrete, which is 147 pcf to 152 pcf..)
Thats JUST the concrete..I don't remember the exact square footage of the floors, but I'm sure someone knows..(assume 20,000 sq feet per floor,)
at 4 inches thick, every yard of concrete theoretically yields 81 square feet..a yard of concrete is 27 cubic feet. 116 pcf x 27 = 3,132lbs..so for every 81 square feet, you have 3,000 lbs..theoretically, of course..
now just to make the math easy, lets say 100 square feet per yard..200 units of floor space, 20,000 divided by 100=200..thats 600,000 lbs per floor or 300 tons...PER FLOOR..just the concrete..theoretically, of course.

I had also heard or read somewhere, that these building had the structural support members on the outside of the building..a new type, (in 1963) of construction technique..self-supporting floor slabs, tied to the structural elements..