C Thurston of San Francisco, CA wrote:
Thanks for Dave Heller's well-written piece on the science of the WTC building 'collapses.' I would like to try to clarify one or two points.

From the available video of the collapse of WTC7 it is clear that this was a true demolition-style IM-plosion, meaning that the building folded in on itself and the falling material landed almost entirely within the original building footprint, leaving the neighboring structures essentially undamaged. The entire building begins falling simultaneously, almost as if it's going down an elevator into the ground. This can only be accomplished, of course, by removing all of the vertical structural support at the column bases with simultaneously detonated explosive charges. This was obviously not a 'pancaking' collapse because all floors begin falling at the same moment.

The behavior of WTC1 and WTC2 is actually quite different. These two buildings are detonated from the top down, almost like pulling down a zipper. This creates the illusory plausibility of a floor-to-floor, top-down pancaking sequence. From a look at any of the videos, however, it is abundantly clear that these two buildings are EX-ploding as they come down, raining heavy steel on the surrounding buildings and causing fatal destruction to the other WTC buildings in the complex (except for WTC7, which was too far away). In combination with the enormous churning dust clouds, almost the entire mass of each tower landed OUTSIDE of its original footprint. While this again can only be accomplished with carefully timed and placed explosives, it is NOT a demolition-style implosion (engineered to protect nearby property), but rather a highly unusual sequence of top-down explosions, timed to occur at the rate of the free-falling material from above so that the explosive events would be mostly hidden within the growing clouds of dust and falling material. (Sections of heavy steel perimeter columns were ejected up to 500 feet in all directions!) Here again, though, as you have pointed out, the rate of 'collapse' is the big give-away - a 'natural' collapse could never occur at the rate of free-fall, if at all. The inertia and structural resistance of the not-yet-pancaked portion of the building would slow down the collapse, and probably arrest the process long before reaching the ground.