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Americans warned about aircraft risk in 1973

News : Cover story

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, co-founder 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.

Last week the twin World Trade Center towers both suffered a catastrophic progressive collapse after each was struck by a hijacked airliner. The structures are thought to have been weakened by heat from buring aviation fuel (NCE last week).

'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.

'We can read of a crashing plane demolishing a house or two with reasonable equanimity, and even the one that embedded itself in the Empire State Building did not hold the headlines for long.

'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.

'Aircraft are getting larger and more numerous, and at the same time more lives in big cities are becoming dependent upon fewer but stronger supporting members, ' said his paper.

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'.

Following last week's catastrophic World Trade Center collapse, Sutherland said that he feared many of the concerns he voiced in 1973 were still valid.

'I don't think that there has been a great deal of effort within design offices to really understand progressive collapse, ' he said. 'Most engineers do not really think about it - there is still a lack of a philosophical approach to design.'

He added that if the 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 this week 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.'

His 1973 paper also called for a radical change in the attitude towards codes and regulations which he feared had started to replace engineering judgement.

'Even with all the new requirements which have followed the Ronan Point collapse, it is still possible in Britain - indeed easy - to build unstable structures 'within the Code', ' he wrote. 'Designers should be continually querying assumptions and asking themselves 'what happens if. . .' and the 'but what would happen if instead'.'

The paper concludes that designers must be encouraged to use their skills rather than rely on formulaic codes of practice.

The World Trade Center had two towers. The north - tower 1 - was 417m high and the south - tower 2 - was 415m high.

Each was 63.5m by 63.5m square on plan and had a 24m by 42m core. Each storey was 3.66m with a ceiling height of 2.62m.

The complex was designed by architect Minuru Yamasaki & Associates with Emery Roth & Sons for client New York Port Authority. Seattle-based structural engineer Worthington, Skilling, Helle & Jackson designed the structures. Project management was by Tishman Realty & Construction Co Construction began in 1966, with the towers opened in April 1973. They were the world's tallest buildings for only a few months until Chicago's Sears Tower took over in 1974, just 30m higher.

Each of the 110 floors of both towers provided 2,900m 2of office space and total effective floor areas of 319,000m 2.

External columns are of constant overall cross section of 450mm x 450mm.

Wall thickness and grade of steel in the external columns varied in successive steps in the upward direction. Wall thickness decreased from 12.5mm to 7.5mm, yield point of the steel from 70.0 to 29.5 kg/mm 2.

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