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No spare capacity


In accordance with colleagues, I express my horror over the destruction of the World Trade Center (NCE 20 September).

As practising engineers we follow codes and standards so projects are realised economically at the time. How often do we ask ourselves individually what would be the result of catastrophies such as that of the twin towers - what do we have to do to design out the impact of a jumbo jet?

The twin towers were hit at different times and levels and angles and both failed in similar fashion - suggesting little spare capacity.

It appears that the floors hit were all subject to intense heat exposure, and redistribution of load in these led to the collapse of the upper tower section. The dynamic impact of the upper on to the lower led to the complete failure of the lower tower. This is where I take some issue.

It seems to me that the vertical drop loading should have been resisted by the lower structure through stress and overstress up to the yield point.

But it didn't, thus indicating that a suitable factor of safety was either non-existent or was too easily exceeded.

I shall be disappointed, but not surprised, if the post mortem on the design and details of the towers discovers such weaknesses. We must now review the capacity of towers worldwide to withstand adversity and take appropriate remedial measures.

Carlo Dinardo (F), Mirren Court, 119 Kenfrew Road, Daisley DA3 4EA

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