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Words of warning

A BRITISH engineer warned American colleagues that they should account for the risks posed by aircraft crashing into skyscrapers just six months after the World Trade Center opened 28 years ago.

James Sutherland, cofounder of British consultant Harris & Sutherland, highlighted structural engineers' blinkered attitudes to these risks in a paper for the Structural Engineers Association of California annual convention in October 1973.

The paper set out a stark warning in the wake of the 1968 Ronan Point tower block collapse, when a gas explosion caused a progressive collapse.

'What would be the effect of a Boeing 747 colliding head on with a tower block and what difference would the type of structure make?' asked Sutherland's paper.

'Just wait until the loss of life in the building is comparable with, or disproportionately greater than in the plane, and then the shouting will begin, ' said the paper.

Speaking to NCE this week, Sutherland said his paper, 'The Sequel to Ronan Point', had not attracted much attention at the time as 'the US engineering world was far more interested in potential earthquake risks'.

However, he said, the paper highlighted a lack of vital engineering thought towards the risks of progressive collapse of the type seen in the 23 storey Ronan Point tower. In addition to gas and bomb blasts, he warned of potential hazards such as aircraft collision.

With words now given new meaning, he added: 'The failure of one or more such members which led to the collapse of an office tower of say, 50 storeys, in working hours, is almost unthinkable.

We conveniently ignore such possibilities until something dramatic happens, as it did with Ronan Point.'

The paper pointed out that work had yet to examine how such structures might be designed to deflect and distort 'possibly with unacceptable permanent deflections, but above all, without collapse'.

Sutherland said that he feared many of the concerns he voiced in 1973 were still valid.

'Most engineers do not really think about progressive collapse - there is still a lack of a philosophical approach to design.'

He added that if the twin towers had really been designed to withstand a Boeing 707 impact, as is widely reported, he wanted to know what calculations and assumptions engineers had made.

Sutherland maintained that a change in design attitudes was needed above any change in codes of practice. 'There is a move towards greater robustness, ' he said. 'But we need better design thinking rather than rigid codes.'

Antony Oliver INFOPLUS www. nceplus. co. uk

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