WTC construction slideshow
There is a link to Leslie Robertson's website, a structural engineer who worked on the original design.
http://www.lera.com/sep11.htm (interestingly the page mentions WTC7)
"We designed the towers to resist the accidental impact of a Boeing 707, perhaps lost in the fog while seeking to land. The impact of the Boeing 767s, commandeered by the terrorists, even though larger and flying much faster, was still unable to bring down the towers. The fire-resistive systems, however, did not and could not have contemplated the subsequent fire fueled by thousands of gallons of jet fuel."
There is a contact e-mail address at the bottom.