FWIW, Not my comments but forwarded to me by an structural engineer who
forwarded it to me from one of his lists, Ron :
<SNIP>
A word on the structure of the WTC towers:
The WTC towers had a distinctive structural system which utilized
the exterior wall framing for lateral bracing -- a so-called lattice
framework. This allowed minimization of internal lateral bracing
and opened up the floor plans. You can see the effect of that when
the buildings collapsed, with the lattice framework crumbling and
the interior imploding. The lattice works so long as it remains
intact as a system: if a part of it goes, then the whole system
goes.
The planes punched holes in the lattice, one tower punched
on two sides, maybe the other too. Portions of the lattice of
the second tower briefly remained standing after the collapse,
then fell.
The system was considered daring at the time of construction, for
it distributed loads more efficiently than legacy column-and-beam-
supported systems. Probably the legacy systems would not have
totally collapsed due to damage at upper floors, although floors
above the damage would have come down if columns were
weakened.
----------------------------------------------------------------------------
--
{followup}
The '45 Empire State Building crash is oft studied in architectural
and structural engineering to learn why the building withstood the
hit. The plane was a B-24, I believe, but in any case a much smaller
craft than the ones which hit the WTC and the Pentagon. The '45
plane's engines did penetrate the building, shooting out the far side
and falling to the ground and killing passersby, but most of the plane
remained inside the structure for it was made of far more fragile
materials than a building. A relative small amount of damage was
done to the structure of the building though fire was devastating,
especially from flaming gasoline cascading inside.
The fireball that shot from the second WTC tower hit, opposite
where the jetliner penetrated, blew out windows and perhaps part
of the latticework exterior structure. Flaming fuel probably
cascaded down the shafts of elevators and ductwork and
stairwells whose fire-protection enclosures would have been
destroyed by the explosive crash and ballistic heavy plane
parts. These fuel flames, and fires started from them,
would have weakened interior structural support beyond
protection provided by code-required fireproofing. Once
the interior structural supports were weakened, and
the exterior lattice lost its integrity collapse was inevitable.
I modify my first evaluation to speculate that the interior
supports appear to have given way before the exterior lattice
(whose girdle of closely-space columns and thin vertical
windows between gave the buildings a unique look compared
to use of large panes of glass elsewhere) The lattice amazingly
contained the interior collapse and the whole mess dropped
vertically, almost, as newscasters report, as if executed by a
demo expert.
I did not expect the Twin Towers to collapse. To suffer terrible
fires and localized interior damage but not total collapse. The
first was unbelievable, and as I said, I thought only the portion
above the crash fell. Then the smoke cleared momentarily
to show the totality. Then the second tower, collapsing in a
near-perfect copy of the first. The sudden dropping of the
floors above the crash, that impacting load overpowering
the remaining system, and the straight drop collapse, neither
tower falling much to the side, indicated what had happened.
Close-ups of the exterior show the latticework bridging the
crash penetrations, reminding of sales pitches from the
19th Century when cast-iron manufacturers promoted
their architecture with structural components missing
with no apparent destabilization -- the load automatically
shifting to remaining components. Their prognostications
failed at the first intense fire which overheated and cracked
the cast iron, sometimes collapsing more quickly than
predecessor masonry bearing wall and wood floor system
composites
----- Original Message -----
From: "Daniel Terp" <dterp@tallcity.com>
To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
Sent: Wednesday, September 12, 2001 6:51 PM
Subject: Re: [MV] WTC building design
On Tue, 11 Sep 2001 23:45:20 -0400, you wrote:
>"After the 1993 trade center bombing, one of the engineers who worked on
the
>towers' structural design in the 1960's claimed that each one had been
built to
>withstand the impact of a fully loaded, fully fueled Boeing 707, then the
>heaviest aircraft flying."
It wasn't the impact that collapsed the building, it was the heat from
the burning fuel. They say it got to over 1600 degrees and melted the
steel frame.
If the fuel hadn't caught fire, the buildings would still be standing.
=Mil-Veh is a member-supported mailing list=
To unsubscribe, send e-mail to: <mil-veh-off@mil-veh.org>
To switch to the DIGEST mode, send e-mail to <mil-veh-digest@mil-veh.org>
To reach a human, contact <ack@mil-veh.org>
This archive was generated by hypermail 2b29 : Mon Oct 08 2001 - 10:58:59 PDT