Text 20596, 110 rader
Skriven 2005-12-31 11:50:20 av Roy Witt (1:1/22)
  Kommentar till text 20527 av Peter Knapper (3:772/1.0)
Ärende: WTC fire         (was; Lack of Gratitude
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31 Dec 05 16:19, Peter Knapper wrote to Roy Witt:

 PK>> I doubt that anyone expected the
 PK>> structures to collapse as a result of a direct hit (at least at the
 PK>> design stage the structure was expected to withstand a direct hit by
 PK>> an aircraft of the type flown at that time),

 RW>> The engineer who designed it forgot to include what would happen
 RW>> when the fuel carried by the aircraft ignited. That's what made the
 RW>> buildings collapse, not the hit.

 PK> Well they must have considered Fire as a threat even back when it was
 PK> built, because the original building included fire-proofing over the
 PK> steel structure.

Yes, they considered that.

A basic engineering assessment of the design of the World Trade Center
dispels many of the myths about its collapse. First, the perimeter tube
design of the towers protected them from failing upon impact. The outer
columns were engineered to stiffen the towers in heavy wind, and they
protected the inner core, which held the gravity load. Removal of some of
the outer columns alone could not bring the building down. Furthermore,
because of the stiffness of the perimeter design, it was impossible for
the aircraft impact to topple the building.

However, the building was not able to withstand the intense heat of the
jet fuel fire. While it was impossible for the fuel-rich, diffuse-flame
fire to burn at a temperature high enough to melt the steel, its quick
ignition and intense heat caused the steel to lose at least half its
strength and to deform, causing buckling or crippling. This weakening and
deformation caused a few floors to fall, while the weight of the stories
above them crushed the floors below, initiating a domino collapse.

 PK> However in over the 40 something years since they were built, the
 PK> fire proofing in the two towers had eroded (decayed???) to the point
 PK> that much of it was peeled off by the heat of the flames, removing
 PK> any "prevention" capability it had.

When push came to shove, that retardent was useless in the face of
how these buildings were attacked.

Typically, diffuse flames are fuel rich, meaning that the excess fuel
molecules, which are unburned, must also be heated. It is known that most
diffuse fires are fuel rich because blowing on a campfire or using a
blacksmith's bellows increases the rate of combustion by adding more
oxygen. This fuel-rich diffuse flame can drop the temperature by up to a
factor of two again. This is why the temperatures in a residential fire
are usually in the 500øC to 650øC range.

It is known that the WTC fire was a fuel-rich, diffuse flame as evidenced
by all of the black smoke you see in the photos. Soot is generated by
incompletely burned fuel; so the WTC fire was fuel rich (hardly surprising
with 95,000 gallons of jet fuel available). Factors such as flame volume
and quantity of soot decrease the radiative heat loss in the fire, moving
the temperature closer to the maximum of 1,000øC. However, it is highly
unlikely that the steel at the WTC experienced temperatures above the
750-800øC range.

The WTC, on this low-wind day, was likely not stressed more than a third
of the design allowable, which is roughly one-fifth of the yield strength
of the steel. Even with its strength halved, the steel could still support
two to three times the stresses imposed by a 650øC fire.

 PK> I understand the issue was over the "glue" that was used to bind it
 PK> to the steel, rather than the Fire Resistant Foam itself.

It had nothing to do with the glue. As far as I know, the fire retardant
didn't come off the steel, unless it was scraped off by the impact.

You have to keep in mind that the temperature of the fire was not uniform
everywhere, and the temperature on the outside of the box columns was
clearly lower than on the side facing the fire. The temperature on the 18m
long joists was certainly not uniform. Given the thermal expansion of
steel, a 150øC temperature difference from one location to another will
produce yield-level residual stresses. This produced distortions in the
slender outer structural steel, which resulted in buckling failures. Thus,
the failure of the steel was due to two factors: loss of strength due to
the temperature of the fire, and loss of structural integrity due to
distortion of the steel from the non-uniform temperatures in the fire.

The perimeter tube design of the WTC was highly redundant. It survived the
loss of several exterior columns due to aircraft impact, but the ensuing
fire led to other steel failures. Many structural engineers believe that
the weak points (the limiting factors on design allowables) were the angle
clips that held the floor joists between the columns on the perimeter wall
and the core structure. With a 700 Pa floor design allowable, each floor
should have been able to support approximately 1,300 tons beyond its own
weight. The total weight of each tower was about 500,000 tons.

 PK> In all that time it had never been replaced, and there is talk that
 PK> it had never inspected, but this has been disputed by the buildings
 PK> owners. I am not sure what the outcome of this was...

That's possible...I don't think the New York City building inspections
were lax in their duty though. Fire retardant wasn't meant to stave off a
fire of this magnitude in the short time that it burned. Fire retardant is
useful where the fire spreads like a normal fire, over a period of hours
rather than minutes.

 PK> As a result of 911 I understand that the fire proofing on Steel
 PK> structures is now being checked and replaced more frequently than
 PK> previously expected.

I pretty sure they're inspected on a regular basis.


Roy
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