This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
As the U.S. marks the seventh anniversary of the September 11 terrorist attacks on the World Trade Center and the Pentagon, a group of UK researchers have completed a study designed to lead to improved skyscraper safety. The report, to be presented at next year's International Conference in Pedestrian and Evacuation Dynamics (PED) at Germany's University of Wuppertal, is based on interviews with 271 WTC survivors who provided first-hand accounts of what it was like for victims as they tried to leave the burning buildings.
Researchers from the Universities of Greenwich, Ulster and Liverpool concluded that more than half of the survivors delayed evacuating because they wanted to gather information about what was happening; those intent on getting more info about the attacks before exiting took between 1.5 and 2.6 times longer to begin evacuating than others; and congestion in stairways was the main cause of delay in getting out, even though the towers were less than one-third occupied that day.
The interviewees took between one and eight minutes to get moving after receiving word that that towers were in flames, leading the researchers to conclude that tenants who received accurate info the fastest were the ones who recognized the urgency and fled without hesitation.
Based on the findings, study project director and University of Greenwich math professor Ed Galea says that to improve survival chances in any future attacks, workers should know exits in advance, leave without delay, don't stop to call family and friends (to reassure them) along the way, don't discard shoes on the stairs and run enough drills to know the time it will take to get out.
Overall, 82 percent of those surveyed said they stopped at least once during their descent from the burning towers, and some stopped more than 20 times, because of backups in the stairwells (which were clogged by office workers on their way down as well as rescue workers on their way up). Less than 10 percent of evacuees interviewed stopped to rest, and 3.5 percent were delayed because of debris, smoke, heat and water on the steps.
The researchers, funded with a $2.9 million (£1.6 million) grant from the UK Engineering and Physical Sciences Research Council (EPSRC), recorded thousands of details gleaned from interviews in a repository they call the High-rise Evacuation Evaluation Database (HEED), which they used to create computer models of conditions in the WTC following the attacks. The researchers plan to make the HEED available to engineers, architects and other researchers.
The scientists conducted computer simulations of the evacuation of the North Tower that indicate, if the building had been fully occupied at the time of the strikes, more than 7,500 people would have died in that building alone, compared with the 1,462 actual number of fatalities. They concluded that after high-rise buildings reach a certain height, stairs alone are not sufficient to safely evacuate building occupants. Buildings as tall as the twin towers need some type of specially designed lifts or elevators that can operate in an emergency.
The US government's National Institute of Standards and Technology (NIST) conducted its own three-year building and fire safety investigation of factors contributing to the probable causes of post-impact collapse of the WTC Towers (WTC 1 and 2) and WTC 7. A wealth of information can be found on NIST's site, including research pointing to areas of high-priority need such as prevention of progressive construction collapse, fire resistance design and retrofit of structures, and fire resistive coatings for structural steel.
It remains to be seen whether the hard lessons in building emergency preparation and management learned on 9/11 will be put to use in the proposed 1,776-foot (541-meter) Freedom Tower under construction in the WTC's former footprint.
(Images courtesy of NIST)
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