THE first emergency broadcast to the workers in the twin towers of the World
Trade Center told them to stay at their desks until requested to evacuate. It’s
standard practice when fire breaks out in tall buildings: clear people down from
floors in the immediate vicinity, then firefighters can attack the blaze from
the floor beneath. Everyone else is supposed to be safer staying put until the
blaze is under control.
That’s the theory. But it wasn’t a day for following standard procedures.
“It’s a difficult point to make, but it’s important,” says Ed Galea, director of
fire safety engineering at the University of Greenwich. “On the day, people
didn’t do what they were supposed to do, and probably the ones who didn’t are
the ones that survived.” Clearly, the world’s most devastating terrorist attack
will lead to a rethink of skyscraper design.
It’s not the first attempt to destroy the buildings that symbolised the
financial heart of America. In 1993, terrorists parked an explosive-laden truck
in the garage beneath the twin towers and detonated it. Six people were killed
and over a thousand injured. The subsequent inquiry revealed that most people
took between 1 and 3 hours to get out of the 415-metre-tall structures. After
the investigation, escape procedures were sharpened up to include better
lighting and signs indicating escape routes. And security staff constantly
patrolled emergency staircases.
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But these measures, although undoubtedly useful, assumed what fire engineers
regard as a “regular fire”. Strikes by two fully fuelled 767s were immeasurably
more devastating, and the scale of the subsequent fires dramatically dictated
the outcome of events.
The buildings were designed to resist an accidental collision with a small
plane—not a direct hit by a large jet laden with nearly 90,000 litres of
aviation fuel. Only nuclear facilities are built to withstand this kind of
attack—it’s the standard doomsday scenario. The twin towers were
essentially hollow steel tubes with holes punched in the sides for windows. A
central steel core, which supported service risers, lift shafts and fire
escapes, was linked to the outer shell by steel floors.
The impact would have knocked fireproof cladding off the structural steel
members supporting the buildings. “Having thousands of gallons of aviation fuel
going up in what is essentially a hydrocarbon explosion would generate
temperatures well in excess of 1000 °C very quickly,” says Galea. Without
cladding, the raw steel columns would have rapidly heated up, softened and begun
to sag. Essentially, the buildings lost their supports. The hollow tube design
contained much of the debris as the towers collapsed.
Crucially, the evacuation procedures assumed—in accordance with
regulations—that the buildings wouldn’t fall in such a short space of
time. Whole companies stayed at their work stations. Firefighters headed up the
stairs towards the fire. Their brave response undoubtedly added to the death
toll but the emergency services had no way of knowing the buildings were about
to collapse.
Information on the integrity of a building is just not available to the
fireman on the street, says Jeremy Hodge of the Fire and Risk Sciences Division
at Britain’s Building Research Institute in Garston. But the solution is
relatively simple. Cheap load sensors attached to structural members could
record the forces being borne, and warn of an impending structural failure, he
says.
Even if the emergency services could have predicted the collapse, there’s
little they could have done to get people out more quickly. “If you put in the
number of staircases you’d need to get everyone down at once, there wouldn’t be
any room for offices,” Hodge says. So firefighters have to change tack in
skyscrapers. They attempt a phased evacuation—moving out people on floors
above and below the fire, while putting the rest on alert.
Fire doors, sprinkler systems and inter-floor fire barriers should contain
the fire until the firefighters can bring it under control. But this was a
blazing inferno stoked by jet fuel. Only foam sprinklers can snuff out such a
fire, and only aircraft hangars have them.
Fire engineers have to look for alternative ways of getting people out
rapidly, says Hodge. Fire-safe lifts and spiralling escape chutes are
possibilities. Galea thinks lifts could evacuate people rapidly, but the idea is
fraught with problems. “What if the lift opens on the fire floor, the mechanism
gets damaged, or you’ve got fire in the lift shaft?” he asks. Spiralling escape
chutes would undoubtedly lead to injuries and congestion, warns Hodge.
Redesigning high-risk buildings looks inevitable, says John MacArthur of
construction engineers Arup. “It’ll take some time and we’ve got to think very
hard about it, but we can’t just let it go.” However, massively strengthening
prestigious buildings won’t necessarily solve the problem, says Galea.
“Terrorists will just go for those buildings that didn’t quite qualify.”
Ultimately, the disaster will mean that evacuation procedures and building
design will join aviation security under intense scrutiny. “We need to study
what happened and figure out what worked and what didn’t work,” says Galea.
“It’s absolutely vital we learn from this. If we don’t, it’s a wasted tragedy.”
