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For as large as WTC 1 & 2 were, the structures were not as robust (lack redundency) as skyscrapers built using more traditional box frame contruction, such as the Empire State building.
Box frame construction is basically using a grid of steel with numerous columns spread throughout to support a building.
Curtain wall construction, such as in WTC 1 & 2, has no columns, but instead the load is carried by the outside wall frame and an internal core with no columns between - the floors are solely supported by the outside wall and internal core.
Problem with curtain wall construction is that's NOT robust (lacks redundency) ... if floor connections fail, the entire floor can give away and fall to the floor beneath it ... and under the right conditions, that can start a domino effect of floors cascading into the floor below and hence building collapse.
To intentionally demolish the Empire State building would require planting explosives into numerous columns and a lot of explosives as well as excellent timing - not something easily hidden.
Contrast that with the WTC 1 & 2 structures ... all one would need to do is plant (little to no drilling required) explosives aside the critical floor connections and/or somewhere in the middle of the floor slab (causing it to buckle and pull away from its connections) on as little as one floor - the floor connections were rated to hold the weight of around 3 floors, but under heating from the fire, the steel was significantly weakened meaning that little as one floor could have been enough to cause catastrophic building collapse.
While the above scenerio is unlikely, the point is that purposely bringing down the WTC 1 & 2 with explosives would be much easier due to their lack of redundency unlike the Empire State building, which was built like a tank containing over twice the amount of steel (and probably more concrete too) then each of the WTC towers alone despite them being much larger.
Ron
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