I fail to understand why anyone would anticipate that simply crashing two huge jetliners into NYC's largest buildings (and the Pentagon) would not create the desired effect (whatever that was).

Planes have crashed in high rises before and yet have never had the effect that occured on 9/11. It has been argued that no plane the size of 767s even did so, but the WTC towers were specifically designed to withstand crashes of planes of this size (707s are around the same size as 757s). Frank D. Martini, who was the construction manager for the twin towers at the time of the collapse and died during the collapse, had this to say earlier in 2001 concerning the possibility of jet impacts to the buildings:
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The building was designed to have a fully loaded 707 crash into it. That was the largest plane at the time. I believe that the building probably could sustain multiple impacts of jetliners because this structure is like the mosquito netting on your screen door. This intense grid * and the jet-plane is just a pencil puncturing that screen netting. It really does nothing to the screen netting.
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http://www.911eyewitness.com/samples/demartini.php