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This is a good image illustrating the span and truss construction of the original WTC. If I remember correctly a thin layer of concrete would then be poured over the metal deck, this would create a flooring and bond the entire floor into a lightweight stiff cohesive unit. As you can see though those flooring (spans) are only attached and supported at each end. When the structure was intensely heated for long enough the spans buckled and finally gave way at the attach points there was nothing to stop the momentum and the building ‘pancaked’ down.

I look at that design and I get the willys that they built a 1400' building like that.

The 9-11 conspirators try to make an issue that the jet fuel did not burn hot enough to cause this failure, but they ignoe that one the fires started creating their own convection currents it became like a wind tunnel or blast furnace and the temperatures could easily become hot and intense enough.

Smarter people than you and me disagree with your assessment:

The World Trade Center was not defectively designed. No designer of the WTC anticipated, nor should have anticipated, a 90,000 L Molotov cocktail on one of the building floors. Skyscrapers are designed to support themselves for three hours in a fire even if the sprinkler system fails to operate. This time should be long enough to evacuate the occupants. The WTC towers lasted for one to two hours—less than the design life, but only because the fire fuel load was so large.

A basic engineering assessment of the design of the World Trade Center dispels many of the myths about its collapse. First, the perimeter tube design of the towers protected them from failing upon impact. The outer columns were engineered to stiffen the towers in heavy wind, and they protected the inner core, which held the gravity load. Removal of some of the outer columns alone could not bring the building down. Furthermore, because of the stiffness of the perimeter design, it was impossible for the aircraft impact to topple the building. However, the building was not able to withstand the intense heat of the jet fuel fire. While it was impossible for the fuel-rich, diffuse-flame fire to burn at a temperature high enough to melt the steel, its quick ignition and intense heat caused the steel to lose at least half its strength and to deform, causing buckling or crippling. This weakening and deformation caused a few floors to fall, while the weight of the stories above them crushed the floors below, initiating a domino collapse. It would be impractical to design buildings to withstand the fuel load induced by a burning commercial airliner.

MIT

Thanks.

Very informative.

So to my untrained eye, one of the major differences is that the span and truss design, while having sturdy enough vertical columns, lacks the heavy horizontal girders of true steel frame construction?

Excuse my terminology....