Re: Structural failure of NY's WTC

[Larry Cartwright <exit60@IA4U.NET>]

To better understand what happened, you might want to study up on how
buildings are demolished intentionally and legally.  It is a fascinating
industry and an artful use of applied physics.  I have been casually
following developments in the demolition business since 1972, when the
infamous Pruitt-Igoe housing project was imploded in St. Louis where I
was living at the time.

One of the most prominent demolition companies is the Loizeaux family's
Controlled Demolition Inc headquartered in Maryland.  CDI holds most of
the world records in demolition, the largest legally demolished building
being the 439 ft tall Hudson's department store here in Michigan they
brought down 3 years ago.  The Hudson's demo was an immensely complex
undertaking.  The Detroit Free Press has videos of the finale at
<http://www.freep.com/news/hudsons/video.htm>.

The CDI website is at <http://www.controlled-demolition.com/index.html>,
and it will be greatly improved in the near future by the addition of a
series of videos they have been getting ready.  At their site, I
recommend you scroll down to "Table of Contents" and click on "Press
Release Page", which is a gateway to descriptions and photos of their
most recent works.  I was amazed at the 1/4 mile tall tower they brought
essentially straight down.

The collapse of the upper stories of each of the WTC towers looks very
much like the results that the demolition industry works so hard to get
in controlled collapses, commencing at one lower corner and progressing
laterally across the structure.  The structure of each building is
selectively weakened by cutting and explosives, but the essential
structural integrity and "weight" of the building is relied upon to pull
itself straight down once the process is started.  The Murrah Building
in Oklahoma City was so severely damaged by the terrorist explosion that
parts of the building actually had to be reconstructed before the whole
thing could be demolished by this implosion technique.

The "pancaking" effect of bringing a building down by starting at the
top and using the impact of upper floors to demolish lower floors is not
uncommon in conventional demolitions which do not use explosives.  You
don't want to start demolishing the lower part of a building and have it
fall over on your equipment.  For example see the photos of conventional
demolition at the University of Texas this summer at
<http://www.utmb.edu/iutmbevents/demogallery.htm>.  You will note the
devastation by "pancaking" which occurred between the 4:18pm and 4:38pm
photos, and that's only roughly a 10-story structure.

There are many building-demolition videos on the Internet that you can
find with your favorite search engine.  I think Las Vegas holds the
record for sheer number of detonated-implosion demolitions by a single
metro area.  Also, Tom Harris has a presentation suitable for kids on
Marshal Brain's "How Stuff Works" site at
<http://www.howstuffworks.com/building-implosion.htm>.

Best wishes and God Bless America,

Larry

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Larry Cartwright
Retired (June 2001) Physics Teacher
Charlotte MI 48813 USA   <exit60@ia4u.net>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Tucker Hiatt wrote:
> Moral astonishment aside, I was completely taken by surprise upon
> observing the physics of collapse of the World Trade Center
> buildings.   Reading reports such as
> <http://abcnews.go.com/sections/scitech/DailyNews/WTC_towers_structure010911.html>
> has helped very little.
>
> Would any PHYS-Ler care to speculate regarding why a structural
> failure (due to intense heat) at or above mid-section would lead to a
> domino-like structural failure all the way down?  Clearly, many
> experts in structural integrity were surprised when the collapses
> occurred -- some of those experts were rescue workers toiling
> underneath the doomed buildings.