Professor Newmark provides a link to a useful rebuttal of this claim, which is not new, in Popular Mechanics. (Suggestion: go to the printable format and scroll.) There's a bit more in Henry Petroski's Pushing the Limits, soon to be a book review on these pages, with this simple explanation at p. 174.
The collapse of the lower floors of the towers under the falling weight of the upper floors occurred for the same reason that a stack of books supported on a coffee table can break that same table if dropped on it from a sufficient height.Do not attempt this experiment with Grandma's keepsake table. Professor Petroski continues,
Within days of the collapse of the towers, failure analyses appeared on the Internet and in engineering classrooms. Perhaps the most widely circulated were the mechanics-based analysis of Zdenek Bazant of Northwestern University and the energy approach of Thomas Mackin at the University of Illinois at Urbana-Champaign. Each of these estimated that the falling upper structure of a World Trade Center tower exerted on the lower structure a force some thirty times what it had once supported. [You'd thus need a pretty tall ladder to simulate that effect on a table with a stack of books.] Charles Clifton, a New Zealand structural engineer, argued that the fire was not the principal cause of the collapse. He believed that it was the damaged core rather than the exterior tube columns that succumbed first to the enormous load from above. Once the core support was lost on the impacted floors, there was no stopping the progressive collapse, which was largely channeled by the structural tube to occur in a vertical direction.Much as a controlled implosion would do.
What makes this story particularly sad is that Professor Reynolds taught me price theory, using Alchian and Allen's Exchange and Production and Stigler's Theory of Price, works not usually associated with Wisconsin economics; it is in his class that I learned the "shipping the good apples out" and other classics rather than yet another variation on the Viner-Wong diagram. A commenter at Little Green Footballs (via J. C. A. Bambenek, who has a number of other useful links) notes,
I was hoping this would slide under the radar until Morgan came to his senses. The reason: the good professor is a relative. Age, stress, and a possible grudge against the government have combined to cloud his judgment and cause him to make a public spectacle of himself.I hope his mind will clear. It would be a shame for the man who summed up the case against socialism during the debates over Hillary-care as "it's boring" to be best known for this.