19 Dec 2002, 15:04, MIKE ROSS (1:167/133), wrote to Joe Nicholson:

Hi MIKE.

 >> BTW the buildings falling straight down is a known failure
 >> mechanism called pan-cake or accordion mode. The weight of
 >> the upper floor falling on the one below it causes it to fail
 >> instantly and the whole thing cascades floor after floor all
 >> the way to the ground.

 JN>> Even 3- and 4-story buildings are designed that way today. That
 JN>> is the theme of a few engineering stories before and after WTC,
 JN>> written mainly for first responders to emergency situations.

 MR> Well, what I think Charles was refering to in the case of 
 MR> demolition is the older style structural steel frame cage type 
 MR> construction. As you rightly point out new buildings aren't made like 
 MR> that anymore.

Hit the nail on the head!  The structural component of the WTC was the
central core of steel.  Ordinary old buildings that demolitions people
normally work with have basically "four legs", and if the timing
is off allowing any one leg to 'fail' too soon, the building falls
sideways.

The central core is like dropping donuts on a horseshoe pole, they can only
go straight down.


     Good luck...  M.

--- Msged/386 TE 06 (pre)