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Collapse mechanisms

Fire engineers are still debating the exact sequence of events that caused the catastrophic collapse of the iconic Twin Towers (NCE 30 June).

Conspiracy theorists still pour over every detail of every report into the disaster, looking for apparent anomalies and contradictions. But most engineers have long since accepted the basic scenario as established both by the NIST investigation and the earlier analysis by the American Society of Civil Engineers and the Federal Emergency Management Agency.

Put simply, WTC1 and WTC2 collapsed primarily because terrorists flew fuel laden wide bodied jet airliners into them at high speed. Each tower was struck at a different angle and sustained slightly different damage. Had it been 300t Boeing 747s hitting the towers square on instead of 100t 757s, collapse would have been virtually instantaneous, as too many structural members would have been severed in the impacts.

Instead, the very strong 'perimeter tube' structure of the towers took massive damage - but there were enough alternative load paths around the gaping holes left by the Boeings for the structures to remain stable. However, as the shredding, disintegrating remains of the aircraft plunged further into the towers, they dealt three more blows that were to prove fatal.

In both towers the wreckage penetrated the core, damaging and severing core columns and blocking escape stairs.

And in both cases the impact also severed the single water supply line feeding the sprinkler system. Worst of all was the effect on the spray applied fire protection that coated all the structural steelwork.

How much of this was dislodged by the initial impacts will never be known. In its simulations NIST chose to assume that the steel only lost protection by debris scour.

Jet fuel remaining after the initial fireballs splashed through the upper floors of the towers and poured down the cores. It burned only for a few minutes, but this was long enough to ignite the office contents on many floors. Flames from these fires began to heat up the columns and floor trusses that were now naked and unprotected.

In the immediate aftermath of the 9/11 tragedy, many eminent engineers voiced their suspicions about the vulnerability of the floor truss/column joints to fire. NIST believes these joints in fact remained intact except where they had taken direct impact. NIST also believes that a combination of core column shortening and floor truss sagging eventually pulled heat softened perimeter columns inwards, triggering progressive buckling and structural failure.

These opinions are based partly on an exhaustive study of nearly 7,000 videos and much the same number of still photographs and the examination of 236 fragments of steel from the towers, partly on large scale fire testing of the floor trusses, and partly on computer simulations of the effects of both aircraft impact and fire spread. Once the upper storeys began their downward plunge, the effects were brutal.

Entire office floors including all contents were literally pulverised to dust by the massive kinetic energy of the falling towers.

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