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WTC ten years on: How the twin towers collapsed

The specific circumstances of the 9/11 attacks and the construction of the Twin Towers dictated the precise manner in which the buildings collapsed.

WTC1 and WTC2 were struck at different angles and sustained slightly different damage. In both towers, the 100t fuel laden wide bodied jet airliner struck at 800km/h created gaping holes in the “perimeter tube” structure.

Despite this, there were enough alternative load paths left intact for the structures to remain stable. It was the shredding, disintegrating remains of the aircraft plunging further into the towers that dealt the fatal structural blows, while the fires finished the towers off.

In both towers, Nist said, the aircraft wreckage penetrated the building’s core, severed core columns, blocked escape stairs, cut the single water supply line for the sprinkler system and dislodged spray applied fire protection that had been used to coat all the structural steelwork.

Without fuel in the tanks, the thin skinned planes would have been shredded by the towers’ high strength steel perimeter columns.

Jet fuel soaked the towers’ upper floors and cores and fuelled a fire around the now unprotected columns and floor trusses. This fire was more severe in WTC2, Nist said, because it had a larger “debris pile” fuelling it, and more available oxygen due to a greater number of broken windows in that tower. This, combined with greater damage to core columns in the first place, explains why WTC2 was the first tower to collapse.

Nist said it believed the floor truss and column joints that had not taken direct impact remained intact, but heat softened perimeter columns were pulled inwards due to column shortening and truss sagging.

This triggered the progressive buckling and structural failure seen in footage of the collapse. The massive kinetic energy of the upper storeys’ downward plunge simply pulverised the lower floors to dust.

The Nist report

On 30 June 2005 the National Institute of Standards & Technology (Nist) published its 10,000 page draft final report into the collapses of the Twin Towers.

The investigation highlighted two crucial factors: the role of fire protection applied to the steelwork, and the role of the fuel carried by the two aeroplanes.

The steelwork’s spray-applied fire protection was identified by Nist as a key factor, because it is believed to have been scoured away from the
steelwork by debris and shockwaves, leaving the steel unprotected.

However, this theory was controversial, and many engineers questioned its validity. The Nist report also brought to light the complex role of the fuel carried in the aircrafts’ tanks in the microseconds following the their impact. Without fuel in the tanks, the thin skinned planes would have been shredded by the towers’ high strength steel perimeter columns.

But the 38,000l of jet fuel that was inside the inner wing sections and fuselage fuel tanks effectively turned the planes into sledgehammers.

In addition, the engines and steel landing gear were hardier, and could have caused serious damage to perimeter columns and taken out core columns even without fuel. The impact also ruptured the fuel tanks, creating fireballs that engulfed the structures.

The 38,000l of jet fuel that was inside the inner wing sections and fuselage fuel tanks effectively turned the planes into sledgehammers

Nist’s structural failure investigation was one of the most comprehensive in history, and its recommendations spanned a wide range of topics.

On the subject of increasing structural integrity — especially for the prevention of progressive collapse — Nist said future design codes should be changed to anticipate bomb blasts and gas explosions above the ground floor. But it said it did not believe buildings should be designed specifically for aircraft impact.

  • NCE has just published more on this and other stories from its archive on 9/11 from 2001-2005. To view them click here.

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