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No.24: 9/11 attacks on New York


The 11 September 2001 terrorist attacks on the World Trade Center’s twin towers in New York sent shock waves through the structural engineering community. Immediately after the attacks many questioned whether tall buildings would ever be built on such a scale again.

Terrorists flew Boeing passenger jets into the Twin Towers striking one just above the 80th storey and the other just above the 60th. Thousands died in the resulting progressive collapses of the 415m and 418m tall towers (NCE 13 September 2001).

The towers were struck at different angles and sustained slightly different damage. In both, the 100t fuel laden wide bodied jet airliners struck at 800km/h creating gaping holes in the steel “perimeter tube” structure. Despite this, enough alternative load paths were left intact for the structures to remain stable. But it was the damage caused by the aircraft plunging further into the towers that dealt the fatal structural blows, while the resulting aviation fuel fires finished the towers off.

In both towers the aircraft wreckage penetrated the steel cores, severing core columns, blocking escape stairs, cutting the only water supply line for the sprinkler system and dislodging spray applied fire protection which coated the structural steelwork.

Jet fuel soaked the towers’ upper floors and cores and fuelled a blazing inferno.

Heat softened perimeter columns were pulled inwards due to column shortening and truss sagging and this triggered the progressive buckling and structural failure which can be seen in footage of the collapse. The massive kinetic energy of the upper storeys’ downward plunge pulverised the lower floors to dust.

Four years after the attacks, the United States National Institute of Standards and Technology (Nist) released damning findings from its building and fire safety investigation of the disaster.

Its 30 recommendations identified key areas of US building and fire codes, standards, and practices that warranted revision. Many have yet to be acted on, but the more technical recommendations have been embraced.

Improving fire protection, recognising the importance of structural redundancy and providing sufficient means to escape buildings are the three key areas of change. But the need to prevent progressive collapse is something that still has to be tackled in the US.

Recommendation 1 of the Nist report was clear: “Nist recommends that progressive collapse be prevented in buildings through the development and nationwide adoption of consensus standards and code provisions, along with the tools and guidelines needed for their use in practice.”

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