Structural engineers in the UK wereamazed that the World Trade Center towers managed to stay up for so long. Steve Turner reports.
Performance of the structures American Society of Civil Engineers manager for technical activities James Rossberg insisted that the assault on the World Trade Center towers was way above what could be considered a design condition.
'Given what the buildings were exposed to they performed extremely well and were a tremendous testament to the engineers that built them.'
It is believed that the aircraft took out a substantial number of columns and floors in the impact.
This left columns unpropped and spanning beyond design capabilities.
Heather Stanley, managing director of consultant Yolles Partnership, said that the tube design of the towers - in which the outer skin of the building resisted most torsion forces - would have been very effective at resisting the initial impact.
She explained that the design had redundancy and that the structure's multiple load paths allowed holes from missing columns to be bridged.
Engineers suspect that the initial impact would also have disrupted the fire protection on much of the remaining steelwork leaving it exposed to enormous temperatures as the aviation fuel exploded in the towers.
Resistance to fire Corus fire, environment and corrosion manager John Dowling has overseen extensive fire testing at the Cardington test facility in Bedfordshire. He confirmed that it was uncommon for office fires to reach temperatures likely to damage the building structure.
A 1991 fire at Mercantile Credit Group's 17 storey headquarters in Basingstoke (NCE 25 April 1991) took fire fighters over four hours to control but caused no structural damage, said Dowling.
Ysrael Seinuk, president of skyscraper design specialist Cantor Seinuk, confirmed that the unexpectedly high temperatures effectively 'cooked' the towers. He added that the relatively narrow windows would not have allowed heat to escape, compared to more modern glazing that would have blown out.
Only the offshore industry invests in fire protection to prevent collapse at such high temperatures, Professor Dougal Drysdale of the University of Edinburgh explained. Critical members on many oil and gas platforms have cladding to insulate steel up to 1,000degreesC, plus extensive water drenching systems.
Collapse resistance Once the exterior columns and central steel core were weakened by the heat, the weight of the floors above caused them to buckle and collapse. The force from the floor above induced a progressive collapse.
But Heather Stanley dismissed the idea that this collapse could have been designed out.
'The load on the members would have been astronomical, ' she said. 'Would you design members to be able to support this load?' she asked. 'What would be reasonable provision? Ten storeys? Fifty storeys?'
At the corner of the building walls were interconnected to transmit shear so that together with the floor of the building, they formed a torsionally rigid tube structure which was fixed to the foundations and transmitted all wind loads.