September 14, 2002
9/11 Department
Evacuation Engineering.
In one my September 11th anniversary postings I mentioned my initial back-of-the-envelope calculations on the World Trade Center death toll. My first thought had been that the World Trade Center holds 50,000 people and I might have just seen them all die. My brain eventually started working and I realized that people must have been streaming down the emergency stairs the whole time, even if the video didn't show them. My new figure of 20,000 dead was still pretty scary. Below, I mentioned how relieved I was that the death toll was so much smaller, and I didn't care why.
It turns out I cared enough to look it up after all. Here's my original calculation: I guessed that south tower had stood about an hour and the north for 90 minutes. I had read somewhere that here in Chicago fire stairs are supposed to permit 45 people per minute to exit. I had no idea how many stairwells the towers had, but I guessed four in each tower. Multiply that out and 45*60*4=10800 escaped from the south tower and 45*90*4=16200 escaped from the north tower, totaling 10800+16200=27000 escapees, leaving 50000-27000=23000 still in the towers.
I was wrong on every count. The bad news was that the towers only had three stairwells each. The good news was that the population figure for the World Trade Center, which turned out to be 58000, was for the entire complex of buildings. The towers only hold about 10000 people each and at 8:46 in the morning, many of those desks were empty, leaving each tower with an estimated population of no more than 7000 people. An added bonus is that that the towers lasted 73 and 103 minutes. Do the math again, and 45*73*3=9855 could have escaped from the south tower and 45*103*3=16200 could have escaped from the north tower.
In other words, everyone was able to escape except for those killed by the plane crashes or trapped above the crash floors. In the south tower, many people above the crash floor got out because they started leaving when the other tower was hit. A handful managed to escape from above the crash site. In the north tower, the 91st floor was the dividing line. Everyone on it and most of those below escaped and survived. No one above it survived.
The evacuation from the twin towers is one of the biggest success stories of 9/11. When the World Trade Center was first attacked by terrorists in 1993, it took four hours to evacuate the towers. This was judged unacceptable, and the buildings were given $90 million worth of safety improvements. A backup generator was installed, along with emergency stairwell lighting and better exit marking signs. Disabled workers were provided with special chairs that could be carried down the stairs by two volunteers. Fire wardens were appointed on each floor and regular escape drills were conducted. The terrorists who attacked the towers in 1993 probably spurred improvements that saved thousands of lives.
A good description of the evacuation is in Dennis Cauchon's
"For
many on Sept. 11, survival was no accident"
in USA Today. A
detailed engineering description can be found in the House Committee on
Science's World
Trade Center Building Performance Study. The Executive
Summary is a worthwhile overview. Chapter 1 gives a good overview
of the events of the day at the World Trade Center in engineering terms.
Chapter 2 discusses the engineering
and collapse of the twin towers. Other chapters discuss the other damaged
buildings at the site, including buildings 5 and 7, which suffered collapses
apparently due to fires, something which had never before happened to protected
steel frame buildings anywhere in the world. Finally, chapter 8 contains a list
of conclusions
and recommendations. Nothing like this has ever happened before, so this is
the first chance engineers have had to study these kinds of events outside of
computer models. Many of the recommendations are for additional studies of still
poorly understood events on that day.
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