re: steel was apparently encased
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... you're referring to concrete encasement fire protection.
This concrete is NOT structural - this is one form of fire resisting material that is required to be applied to all exposed structural steel.
Impact likely did jar some of the coating off -- in the proximity of the crash.
However, even if this were to cause localized failure - a proposal to which I would take exception for reasons I've noted - how would this explain the failure, shearing off and projectile removal of all built-up plate steel core columns, for all of the dozens of floors below plane impact?
Very simply, and at the very least, the skeleton of core steel columns should have remained standing -- with the collapse of the light gauge steel truss floor pans collapsed and pancaked around the still-standing core skeleton.
.. and not to repeat myself, but the fire was relatively cool, and certainly short in duration when compared to other structural steel building fires, which have lasted for days - some with loss of fire resistant coatings.
Structural steel requires consistent, high furnace, longer term temperatures (than what we evidenced on 9/11) to lose its structural viability for specified loads -- then add to that the over design that is integral to sizing calculations.
I would again emphasize that, aside from WTT1&2&7 assertions, no other steel building has ever collapsed from fire. Many steel buildings, as well, have less durable fireproof coatings that are indeed rather delicate and easily frangible -- rendering the WTT1&2 superior, and potentially more resilient against loss of fire coatings -- than other steel buildings that have caught fire and survived. |
