How the twin towers could have survived

Minutes after the two planes crashed into the twin towers of the World Trade Centre on September 11, fires fed with thousands of litres of aviation fuel generated temperatures several times greater than a nuclear power station, paralysing the building's water supply system and making firefighters' hoses useless, according to a leaked draft of an official investigation into the tragedy.

It was the effect on the buildings' structure of fires burning out of control rather than the impact of the hijacked airliners themselves that caused the towers to collapse, the report says.

Had the flames been quelled in the manner intended by the designers of the centre's fire-suppression system, the damaged buildings might have remained standing, except in the event of an "earthquake or a windstorm".

The investigation, commissioned by the federal emergency management agency and the American Society of Civil Engineers, was not due to be released until the end of next month, but the New York Times obtained a draft copy. The findings have troubling implications for other tall buildings beset by fire on their upper floors.

The huge fireballs endlessly shown on television that day caused little structural damage, the report says. Indeed, they quickly disposed of a third of the 10,000 gallons (37,850 litres) of aviation fuel carried by each aircraft.

But the remaining fuel ignited inside the building in the following minutes, aided by a vast supply of furniture, paper and computer equipment, and the temperature quickly reached 1,100C. Television pictures show a cascade of molten metal plummeting down the side of the south tower.

Flying debris almost certainly sheared the pipes carrying water to the sprinkler systems and cut the standpipes used by firefighters, the investigators report. The hoses which the doomed emergency workers hauled up the stairs would have been completely ineffective.

The findings may also shed light on why the south tower fell before the north tower, despite being struck after it. Fireproofing material around the steel standpipes had recently been thickened on all the floors of the north tower where the plane struck, but on only one of the floors hit in the south tower. The investigators suggest that the material may have been dislodged by the impact of the jets, causing the pipes to melt until they were soft and vulnerable, a fate which may have affected the less insulated ones to a greater degree.

The difference in how long the towers remained standing - the south tower for 56 minutes and 10 seconds, the north for 102 minutes and 5 seconds - may also be due to the greater speed of the plane that hit the south tower and the fact that it hit 10 floors lower.

Even in the uncontrolled blazes, the two towers showed a remarkable resilience that certainly saved many lives, the report concludes. "The ability of the two towers to withstand aircraft impact without immediate collapse was a direct function of their design and construction characteristics."

The steel columns lining the exterior were tied together with metal plates that distributed the stress of the impact away from severed columns to intact ones.

The system was so effective that columns only 6 metres from the impact points were under almost no more stress than usual. But as the temperature rose, the web of steel columns surrounding the building proved no match for the raging fires inside.