NEW REVELATIONS about the chaotic evacuation of the World Trade Center (WTC) towers on 11 September 2001 have triggered calls for a complete rethink of tall building escape requirements.
Engineers now believe designers underestimate the time needed to evacuate skyscrapers.
They said that wider stair wells and better communications are needed to maximise the chances of emergency escape.
Last month American investigators reported that the 'average surviving occupant' of the twin towers spent 48 seconds per floor descending the emergency stairs.
They said an average of only 37 people exited the stairwell doors every minute after terrorists flew the Boeing 757 aircraft into the WTC towers.
These conclusions are published in an interim report from the US National Institute of Standards & Technology (NIST).
The time taken to descend each floor was about twice that assumed from simulated evacuations.
The exit rate was at the lower limit achieved in nonemergency simulations:
'Occupants were often unprepared for the physical challenge of full building evacuation and were unprepared to encounter transfer hallways during stairwell descent, ' says NIST report.
It adds that movement down the escape routes was also hampered by 'mobility challenged' people.
These were people slowed down by obesity, heart conditions, pregnancy and recent surgery. Up to 60 were assembled on the 12th floor of WTC1, and 20 were being assisted down the stairs when it collapsed.
Arup director and tall building emergency expert Peter Bressington said: 'It just takes a few people to become exhausted and sit down to severely restrict the flow down the staircase'.
'Anyone with mobility problems should be identified then evacuated via lifts with increased fire protection. Codes should be changed to allow this, ' he said.
BRE technical development director Professor David Purser pointed out that at the time of the disaster the WTC towers contained far fewer people than its maximum capacity.
'If they [the towers] had been near capacity and a proper evacuation alarm had been given, people would have been completely stuck on stairs and at exits.'
He added: 'This report raises the question whether the current strategy of assuming phased evacuation, usually in response to a fire alarm, is adequate.
'This approach results in narrow stairs, which couldn't cope with a simultaneous evacuation after a bomb blast or chemical attack.'
Bressington agreed, adding:
'We should be making provision for simultaneous evacuation, especially in tall buildings. But this isn't just a case of wider stairs. A lot can be achieved by proper building management, improved communications within the building and more frequent evacuation rehearsals.'