12 years on from 9/11

Our hearts and thoughts are with the innocent victims and the family and friends they left behind.
We honour the fire fighters and rescue workers who gave their lives.
We salute their colleagues who continue to put themselves in harm’s way.
Let their sacrifice be our spur to making the world a safer place.

Ed Galea
12 Sept 2001

Twelve years after the collapse of the WTC towers, the far-reaching impact of the attacks is still being felt when it comes to the design of new high-rise buildings across the world.

FSEG research is still on-going and the data we have collected, both on the mechanics of large-scale evacuation, and on the human behaviour issues, is being shared across the world, as a valuable international resource.

Using our buildingEXODUS evacuation software, we analysed the evacuation dynamics of the events of 9/11 and also explored what may have happened if the buildings had been fully occupied. From this work, we concluded that, for buildings above a critical population and height, stairs alone were not sufficient for safely evacuating the entire population.

Following this work, FSEG went on to explore the use of lifts for evacuation in high-rise buildings.  We wanted to better understand the choices people make in deciding to use a lift/elevator as part of their evacuation route in an emergency.  Using the knowledge gained from this research, we developed advanced human behaviour models, which simulate human behaviour in selecting to use a lift or stairs to evacuate, and if electing to use a lift, how long they would be prepared to wait for the lift before using the stairs.   This model has been incorporated into the buildingEXODUS evacuation simulation software which now has the capability to explore the impact of lifts on high-rise building evacuation — but not only the mechanical aspects of using lifts, most importantly the human dynamics aspects.

Using the buildingEXODUS software, FSEG simulated a series of high-rise building evacuation scenarios in which the agents could elect to use lifts or stairs or a combination of both.  This research suggests that combined use of stairs and lifts produce better evacuation times than simply using lifts alone.  What’s more, if complex human behaviour is included in the evacuation analysis, the improved evacuation efficiency promised by lifts is not fully realised.  This is because some people will not want to utilise lifts and opt for stairs, while other people who attempt to utilise lifts are not prepared to wait more than a few minutes.   We know stairs alone are not sufficient for full building evacuations in large high-rise buildings.  Since 9/11 there has been a trend to use specially designed elevators for evacuation in large high-rise buildings. But elevators, even fire safe elevators, raise the complex issue of human behaviour, and we know from our studies that many people do not trust using them, or will simply not wait for them, in an emergency.  This is why it is important to have the capacity to utilise both lifts and stairs.

It is vital to take a holistic view of evacuation when designing new high-rise buildings.  When it comes to elevators, this means not just the mechanical issues of using elevators to evacuate people, but the whole issue of human behaviour, and this is what we have built into the buildingEXODUS evacuation model.  However, this is by no means the end of the story.  More work needs to be done to understand the complex human dynamics issues associated with high-rise building evacuation dynamics.

Our analysis of stair travel speeds of people in the WTC evacuation suggested that people were not walking as fast as engineers may have expected.  While our analysis suggested that the lower than expected travel speeds could be explained by the high crowd densities experienced on the stairs, it did bring into question whether the data that engineers have been using to characterise stair speed was out dated and inappropriate given the changes in population demographics in the 50 years since the data was first collected.  As a result, FSEG have started to collect human performance data for people ascending and descending stairs.  While still in the early stages of this work, some of the data that has already been collected suggests that stair walking speeds have indeed changed, at least for the younger demographic.

Twelve years on from 9/11, people need to guard against complacency. Evacuation drills and training always need to be taken extremely seriously, as successful evacuation depends in part on how quickly people respond. We found in our research that some people took many minutes to decide to evacuate the towers, while others didn’t know where the stairs were, for example. The attacks have also highlighted the need for better information systems in buildings, with proper instructions in an emergency, rather than just an alarm going off.

We hope that the insight we have gained into complex human behaviour issues in high-rise building evacuation since 9/11 will contribute to improving building design and evacuation procedures and so contribute to saving lives.


“The UK WTC 9/11 evacuation study: An overview of findings derived from first-hand interview data and computer modelling”, Galea, E.R., Hulse, L., Day, R. Siddiqui, A., and Sharp. G. Fire and Materials, Vol 36, pp501-521, 2012, DOI: 10.1002/fam.1070

“Investigating evacuation lift dispatch strategies using computer modelling”, Kinsey, M.J., Galea, E.R., and Lawrence, P.J.,  Fire and Materials, Vol 36, pp399-415, 2012, http://dx.doi.org/10.1002/fam.1086

“Human Factors Associated with the Selection of Lifts/Elevators or Stairs in Emergency and Normal Usage Conditions”, Kinsey, M.J., Galea, E.R., and Lawrence, P.J.  Fire Technology, 48, pp2-26, 2012, DOI: 10.1007/s10694-010-0176-7.

“Individual stair ascent and descent walk speeds measured in a Korean High-Rise Building”, Choi, Jun-Ho,  Galea, E.R.,  and Hong, Won-Hwa, To Appear in the Journal of Fire Protection Engineering 2013, published online 11 July 2013, http://dx.doi.org/10.1177/1042391513492738

“The UK 9/11 evacuation study: Analysis of survivors’ recognition and response phase in WTC1”. McConnell, N.C.,  Boyce, K.E. Shields, J., Galea, E.R., Day, R.C. and Hulse. L.M.. Fire Safety Journal 45, pp 21—34, 2010, http://dx.doi.org/10.1016/j.firesaf.2009.09.001.

“Investigating the Representation of Merging Behavior at the Floor—Stair Interface in Computer Simulations of Multi-Floor Building Evacuations”, Galea, E.R., Sharp, G., and Lawrence, P., Journal of Fire Protection Engineering, Vol. 18, No. 4, 291-316, 2008 http://dx.doi.org/10.1177/1042391508095092.

“Approximating the Evacuation of the World Trade Center North Tower using Computer Simulation”, Galea, E.R, Sharp, G., Lawrence, P.J., Holden, R., Journal of Fire Protection Engineering, Vol 18 (2), 85-115, 2008. DOI:http://dx.doi.org/10.1177/1042391507079343

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