Re: Structural failure of NY's WTC

["John S. Denker" <jsd@MONMOUTH.COM>]

> Tucker Hiatt asked about:
>
> > the physics of collapse of the World Trade Center buildings.

Then at 01:58 PM 9/16/01 -0400, Ludwik Kowalski wrote:
> Without reading anything my answer would be "under the
> weight of what was falling down."

Yeah.

At the next level of detail:

1) The outer walls were the primary load-bearing structure.  (There was
also an inner ring of load-bearing vertical members, but it's the same
idea.)  This is unlike the typical building where many of the internal
walls are load-bearing.

2) The load-bearing wall was, obviously, strong enough to support
everything above it.

3) In contrast, the floor of the Nth floor was only strong enough to
support the weight of what's supposed to be on that floor, plus a modest
safety factor.

4) The floors were made of reinforced concrete.  This is a wonderful
material.  It resists compression because of the concrete.  It resists
tension because of the reinforcing bars, which are steel.

5) Burning kerosene is very hot.

6) Steel softens when it gets hot.  The rebar didn't immediately soften,
because it was protected by being inside the reinforced concrete, but this
protection lasts only a limited time.

So the scenario is this:

A) The rebar in floor N softens.  The concrete crumbles onto floor
N-1.  This is more than it can handle, so it collapses onto floor
N-2.  Proof by induction.

B) After this has gone on for a few floors, there is quite a lot of falling
mass.  It has also reached a floor which is still pretty strong, and pretty
firmly attached to the outer wall.  When this floor fails, it pulls the
wall strongly inward.  So the wall fails, too.

(Aside:  If the floors had been less-firmly attached to the walls, they
might have all pancaked down, leaving a 110-story hollow shell.  Hardly an
improvement.)