Brussels, 16 Sep 2002
Engineers thought about the unthinkable but it was not enough to prevent the World Trade Centre (WTC) catastrophe. One year after the event and attention is still focused on what can be done to prevent future towering infernos on the scale witnessed in New York on September 11 last year.
The events surrounding September 11 were a defining moment in the history of tall building construction. Engineers had anticipated single extreme events such as a collision by an aircraft into a tall building, but not in combination with widespread fires, as experienced at WTC. Investigations at the collapse site in New York yielded a surprising conclusion - that the contents of the offices were effectively the more flammable ingredient in the inferno than the aircraft fuel. Little could be done to control the WTC fires as the plane impact severed the water feeding system for the sprinklers and fire fighters could not reach the impact floors in time.
Stopping complete burn-out
Some of the fire security measures discussed in the wake of the WTC tragedy, explains Dr John Roberts, Director of Babtie Group and a Fellow of the Royal Academy of Engineering, include designing a building to survive the complete burn-out of its contents. "There are some quite simple things that we can do without increasing the cost of new buildings too much - and some we can do with existing buildings as well," he said. "One of the most important considerations is how to get everyone out of a very tall or large building simultaneously - in the past we've planned mainly for phased evacuations away from fire or bomb damaged areas." Measures need to be taken to ensure that passive fire protection can protect the building if the sprinkler system is knocked out. Mr Roberts also believes lifts should be made fire and impact proof to facilitate rapid evacuation, instead of the conventional wisdom of always using the stairs in an emergency. "We need to plan and site stairwells to minimise the chance of them becoming impassable." Only four people escaped from above the impact zones in either of the WTC towers because the stairs and lift shafts were damaged.