Why The WTC Buildings Collapsed: Two Engineering Articles [View All]

[Bolo Boffin]

http://www.tms.org/pubs/journals/JOM/0112/Eagar/Eagar-0112.html

It is known that structural steel begins to soften around 425°C and loses about half of its strength at 650°C.4 This is why steel is stress relieved in this temperature range. But even a 50% loss of strength is still insufficient, by itself, to explain the WTC collapse. It was noted above that the wind load controlled the design allowables. 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 650°C fire.

The additional problem was distortion of the steel in the fire. 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 along the 18 m long joists was certainly not uniform. Given the thermal expansion of steel, a 150°C temperature difference from one location to another will produce yield-level residual stresses. This produced distortions in the slender 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 fireproofing was knocked off in places and left on in others. Fire affected the two areas quite differently, with 150°C temperature difference easily obtained between the two areas. The fire burned hard in some areas and wasn't present in others, another source of non-uniform temperatures that were affecting the structural steel.

Also:

www.gostructural.com/V3N7/WTC.pdf

It's a great .pdf, chock full of good information. The words "modulus of elasticity" are used. The words "controlled demolition" are not.

The NIST site is always worth checking out as well:

http://wtc.nist.gov