There was more inflammable material on a single floor of the World Trade Centre than in the fuel tanks of the airliners that crashed into them. Furniture, paper and other combustibles fed a massive fire initially started by the fuel that quickly brought the buildings down.
A year to the day after the attacks on the World Trade Centre, the British Association Festival of Science organised a session on the lessons to be learned from the collapse of the buildings. In a session entitled "Staring Disaster in the Face:Engineers on the Front Line", researchers gave details of studies showing how existing buildings can be changed to reduce loss of life in the event of an attack.
"We now have to think the unthinkable at the design stage in a way we have never had to do before," stated Dr John Roberts, director of the Babtie Group and fellow of the Royal Academy of Engineering. It was important that people now took stock of the situation of tall buildings and applied what changes they could to make them safer.
"Buildings are vulnerable to deliberate wartime attack, but there are things that can be done," he said.
He described what had happened inside the two towers that caused them to fail structurally. Each floor of the towers offered 20,000 square feet of space, much of which was covered with inflammable materials. The "fire load" across each of these floors was greater than the fuel carried by either of the planes, he said.
The impacts caused the source of fire to be spread over a very large area, so great as to overwhelm fire protection systems such as sprinklers. The intense heat of these fires caused steel components holding the floors to fail, allowing the floors to fall in a "progressive collapse".
Each tower housed between 20,000 and 22,000 people at peak capacity, he said, and at full occupation would have taken almost four hours to evacuate, according to research.
"These buildings had not been looked at for rapid evacuation," he said. "One needs to think again about how to plan for rapid evacuation."
He argued that owners of tall buildings should be made to licence their structures in the same way as football grounds.
Ground owners had to undergo safety certification for their facilities every year, he said. They also had to show that grounds could be fully evacuated in eight minutes or less, otherwise their safety certificates would be withheld.
"Amazingly, there is no minimum evacuation time in the UK building codes," he said.
The House of Commons in London last week published a report on safety in tall buildings prepared by its select committee for transport, local government and the regions. It called for safety licensing and made recommendations about increasing safety for occupants. It also made two key recommendations on "reducing the likelihood of progressive or total collapse in extreme events" and on enhancing the ability of people to escape, Roberts said. While new structures might benefit from enhanced safety measures post-September 11th, there were improvements that could be made in existing buildings, he suggested.
For example, relatively inexpensive changes to lift systems to make them fireproof would speed evacuation. For decades, the thinking was that lifts should never be used in the event of fire, but tests had shown that properly controlled lifts allowed much faster evacuation than reliance on stairwells only.
"I think that [prohibition on lift use] is going to become a thing of the past," Roberts said.