Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

No.24: 9/11 attacks on New York

9_11_4977

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.”

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.