More WTC True Facts from Dave Farber's IP list: From: John Young Subject: New York Report Date: Tue, 11 Sep 2001 11:14:39 -0700 [...] A word on the structure of the WTC towers: The WTC towers had a distinctive structural system which utilized the exterior wall framing for lateral bracing -- a so-called lattice framework. This allowed minimization of internal lateral bracing and opened up the floor plans. You can see the effect of that when the buildings collapsed, with the lattice framework crumbling and the interior imploding. The lattice works so long as it remains intact as a system: if a part of it goes, then the whole system goes. The planes punched holes in the lattice, one tower punched on two sides, maybe the other too. Portions of the lattice of the second tower briefly remained standing after the collapse, then fell. The system was considered daring at the time of construction, for it distributed loads more efficiently than legacy column-and-beam- supported systems. Probably the legacy systems would not have totally collapsed due to damage at upper floors, although floors above the damage would have come down if columns were weakened.