Striking parallels

What is the standard procedure for explosive demolition of high-rises? Implosion, according to experts speaking on the History Channel feature Modern Marvels.

"The building implodes when the vertical support columns are removed," as the feature story says. One expert noted that an imploded building generally falls in eight or nine seconds.

Both main World Trade Center towers and a third high-rise collapsed in a manner indistinguishable from implosion. The NIST specifically said that the walls of the main towers fell inward because core columns were "shortened" (allegedly by fire heat and buckling). Though trade center collapse times are difficult to ascertain precisely, very rapid falls are seen in videos and collapse times seem to be not much faster than free fall time.

Typically, RDX explosive charges are placed at two points on a column to snap it and a third "kicker" charge blows the piece out horizontally in order to initiate rapid collapse, according to History Channel experts. The two RDX charges are placed in notches burned out of the column by a worker.

RDX is used not only by demolition firms but also by the Pentagon. It packs a detonating velocity of 26,000 feet per second and a pressure of 3 million pounds per square inch.

In some cases, the charges are timed to go off at intervals of fractions of a second. A hotel at Clearwater Beach, Fla., had charges spaced a half second apart, and some of these blasts were visible from the street. Each of the twin towers collapsed after bright, tightly timed blasts went off, but the NIST dismissed these as coincidental jet fuel blasts (jet fuel doesn't pack enough punch to sever a column, the NIST admitted).

If one looks at videos of the trade tower collapses and the various controlled implosions shown on the History Channel, one finds little if any difference, except for scale.

The segment may be purchased online from the History Channel.