What 9/11 taught us about fire engineering, and the lessons that got lost
From fireproofing to alarms to stairway width, building codes cost lives at the Twin Towers. But in the aftermath, not that much has changed.Listen 13:42
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An enduring photo from the North Tower of the World Trade Center symbolizes the heroism of the first responders on Sept. 11, 2001. It’s a tight shot of a firefighter climbing up the stairs in full gear. Sweat covers his face, and he’s actually looking right at the camera — the flash reflects off his helmet and his eyes.
When Glenn Corbett looks at this photo, he’s looking past the firefighter, focusing on the people all around him, trying to get out.
“You’ll notice that the people going down, the people evacuating are … stopped in most photographs,” Corbett said. “And in some cases, they’re sideways because of the limitations, the width of 44 inches.”
That stairwell in the North Tower was only 44 inches wide.
“So this became a major bone of contention after 9/11,” said Corbett, an associate professor of fire engineering at John Jay College of Criminal Justice in New York.
Corbett was one of the advisers to the federal team that looked into the 9/11 attacks from an engineering and fire safety perspective — what led to the collapses happening when they did, and what kept more people from escaping while the towers still stood.
The stairs were the main way people escaped the high rises after the planes hit.
“So that day of 9/11 you had two stairwells, one 44 inches, one 56, to serve literally over 10,000 people in [the] buildings,” Corbett said.
That number, 44 inches, wasn’t arbitrary, but it was outdated. Corbett said it went all the way back to World War I. The average shoulder width of a soldier was 22 inches then, so national building code standards doubled that to 44 inches. The idea was that it would be wide enough for one person to go down the stairs, while another went up. But there was a problem with that, said Corbett.
“So when you walk, you don’t walk like straight down,” he said.
The shoulders actually sway side to side, taking up more space. And the average American takes up more space now, too — we’re much heavier than we were 100 years ago.
So first off, Corbett said, the stairwells should have been wider — but there also should have been more of them. A loophole in the national building code allowed the World Trade Center towers to have three stairwells when other buildings their size would have required four.
Many of the towers’ fatal flaws came from the code itself, the rules by which the buildings were constructed.
“On 9/11, those building codes treated all high-rises the same, like technically a high-rise is typically defined as a building over 75 feet over the highest occupied level — basically if a building is over 75, it’s a high-rise,” Corbett said.
Just to put that in perspective, a 6-story building was treated the same as the Twin Towers, the tallest buildings in the world when they were completed — 110 stories.
You might think high-rise buildings are designed with total evacuation in mind, to get everyone out. But they’re not.
“We do not design for full building evacuations. We don’t do it,” Corbett said.
High-rises are designed, at most, to evacuate the three largest floors at once. It’s a bit counterintuitive, but as buildings get taller, pointing straight up into the sky, engineering a quick way to evacuate the whole building becomes something close to impossible.
“It’s because, here’s the problem: If we increase the stairwell width to accommodate more and more people, you’ll find out that, of course, the building becomes all staircases and no occupied floors, right?” Corbett said.
Even if the World Trade Center towers had had additional staircases, there still wouldn’t have been enough if the planes had struck later in the day, when more people were at work.
“Several thousand people probably would have died in the collapse only because they literally were trapped in the stairwells moving at a snail’s pace, trying to make their way out of the building,” Corbett said.
It takes a while to climb down dozens of flights of stairs. It takes longer, of course, to climb up — which hindered the work of first responders.
“Both of these fires are at least 80 floors above ground level. So just simply getting up there was a Herculean task,” Corbett said.
“We know that, for example, Battalion Chief Orio Palmer certainly got to the area of impact in the South Tower literally a minute or two before [it] collapsed,” Corbett said.
There’s a documentary about all this, with tapes that shocked listeners upon release.
Why building standards didn’t change much
With horrific stories such as that, Corbett thought something like wider staircases would be an easy addition to the building code, and become an obvious “must have” in any new skyscrapers.
“I mean, what we thought were no-brainers, right? We thought would be sort of a simple upgrade that we really felt it was needed,” Corbett said. ”But it ended up being a real fight for a lot of this stuff.”
The General Services Administration, which manages federal properties, pushed back hard against the recommendations of the 9/11 engineering and fire safety team. In the end, the code mainly changed only for buildings above 420 feet, so 30 stories and higher, roughly speaking.
“When you get above that, you’re in a different class of building,” Corbett said.
You need to have more stairs, and they need to be wider. The code also allows these buildings to use specially designed fire-safe elevators as evacuation tools.
It’s a step in the right direction, Corbett said, but not nearly enough. So far, only a handful of new buildings in the United States have these new fire elevators. Reporting for this story turned up only two that work automatically for evacuations.
And Corbett calls the 420-feet cutoff a magic number. He thinks the stricter requirements should apply to shorter buildings as well.
The main obstacle is money. Safety is an investment that may never show a return.
“We’re fighting dollars and cents. It’s all about money, right,” Corbett said. “It’s about literally trading off extra stairway width for less … rentable office space or usable office space.”
The conventional thinking now in high-rise regulations focuses on ways to keep the building standing as long as possible. That means proper fire insulation on support structures; it could mean concrete cores. But as Corbett sees it, there’s still a gaping hole in the code.
“There’s no, like, for example, someone thought, ‘Oh gee, there must be something in our building codes talking about how long it takes to evacuate,” Corbett said. “It’s not in there.”
The math part of it is not that hard. It’s entirely possible to accurately calculate evacuation times.
“A lot of the model is physics,” said Lynn Hulse, who is part of an interdisciplinary fire safety engineering group at the University of Greenwich in London. “Then you can look at it in terms of the human behavior, that’s the slightly more complex thing.”
Hulse is a psychologist, so her part in this work is trying to figure out what people will do when a fire alarm goes off — or when a plane smashes into the building next door.
When trying to understand human behavior in these situations, strike one word from your vocabulary, she said: panic.
“It’s very unhelpful,” Hulse said. “There have been a lot of studies discrediting it.”
The truth is people behave quite rationally even in extreme situations, she said. That was true on 9/11 — Hulse was part of a team that interviewed hundreds of survivors.
One thing that stuck with her is just how readily they listened to their bosses and managers, sometimes to their peril.
“They were told by authority figures that they couldn’t leave because, you know, they were in the middle of a meeting and it was an important meeting,” Hulse said. “The fact is that some people accepted those instructions and they stayed behind, unfortunately for some of them, to fatal result.”
People also spent a lot of time trying to just figure out what was going on — going over to the windows to try and get a look, asking colleagues what was happening.
All that “information seeking,” as Hulse calls it, was time that could and should have been spent leaving the building.
The problem, Hulse said, is with the nature of alarms. An alarm going off only says there’s a problem, but not what kind. No details.
“An alarm in itself isn’t enough,” Hulse said. “The challenge then is to work at what kind of information to give and not in the sense of, well, ‘We can’t tell them what’s really happening because they might panic,’ but quite the opposite. [It’s about] ensuring that it provides some information about where the threat is, to give people the chance to make a decision about, ‘OK, well, which exit should I pick? Should I leave now?”
To her knowledge, Hulse said, no such system has been actually built. Like Corbett, she sees the lessons of 9/11 as only partly learned.
For proof of that, said Corbett, you don’t have to go any farther than the site of the attacks themselves.
“Even the 9/11 Memorial itself, the museum is seven stories below grade,” Corbett said. “So there’s a big question of when you’ve got over a thousand people in the museum, how do you quickly move those people out of a dangerous environment?”
His team was never happy about it.
“We didn’t think it should be below ground in the first place,” Corbett said. “And we were very vocal about that, and we told them that, and they went ahead and did it anyway.”
I went to the 9/11 Memorial and Museum in New York to see what Corbett was talking about. The entrance to the museum is above ground, but that’s about it.
You go down a couple escalators right after entering. There are salvaged sections of metal building skeleton. You barely notice you’re going underground — the floor slopes down in corkscrew-fashion.
It’s dark, and people keep pretty quiet as they walk past preserved bits of the wreckage: a destroyed fire engine; what used to be part of a building’s antenna.
There’s an overlook, and more museum space that looks as if it’s two or three stories down. It’s a bit like the inside of a cave.
Finally, you come to the footprint of one of the towers. It was smaller than I thought it would be, this playground-sized square that used to be the building’s borders.
I asked the staff how far down we were, here at the bottom: seven stories. The way up, without stopping to look at displays was tougher.
I found myself grateful for the escalators, wondering if they’d still work in the event of a fire or attack. At some point, I noticed I was out of breath, though perhaps that was because I had to wear a mask.
Finally there it was, sunlight. I was out.
How long would it take me to get out of there in a real emergency? If everyone was going up at the same time? Ten minutes, 15 minutes — I don’t know.
I could see what Corbett was saying. It’s easy to imagine that memorial, when you’re way down there, becoming a tomb.
I wanted to see an example of a building that works, though, one built with the lessons of the Twin Towers in mind, and I found it, practically next door.
Building Seven of the World Trade Center was destroyed by the collapse of the North Tower. Corbett said it was rebuilt to be as resilient as can be, it checked off all his boxes.
Bill Dacunto, executive vice president in charge of operations at Silverstein Properties, the company behind the building, showed me around, starting in the lobby.
“This beautiful art wall is actually also a blast wall, and this wall will bend and shatter, but not shoot glass shards around,” he said, pointing to the wall above the reception area. “So anybody gets behind that would be protected.”
It looks like your run-of-the-mill corporate facade. But near the top, it’s hung by cables instead of attached, meant to move with a blast rather than buckle.
“We have a fire command station here at the front desk, but we also have another fire command station in the building, but I’m not going to disclose the location of that,” he said.
Dacunto couldn’t show me all the building’s safety features, of course, security and all, but he could show me the pride and joy — the stairs.
“This core is a 2 ½-foot-thick reinforced concrete wall. So the safety inside here is second to none,” Dacunto said.
The stairway is huge, I could imagine four or five people walking down in a row — 20% wider than the post-9/11 code requires. You could hear your voice echo.
“You are now within a concrete core which is like a bunker,” Dacunto said.
There is a space at each landing, to put injured people or to maneuver those with disabilities.
“We have air filtration in here,” he said. “That’s positive pressure to keep smoke from getting into this core system there.”
There is redundancy after redundancy. A fiber cable just for first responder communication, a cable for internal comms, independent sprinkler systems. The lighting system is on a generator and a battery backup; there are photo-luminescent stripes.
The stairways here are perhaps unlike any anywhere else. If the building goes down, these stairs will be the last part.
“You couldn’t take these down,” Dacunto said.
The building has two main stairways that become four, each leading to different exits with LED lighting systems directing traffic to the safest option, text displaying what’s going on. Another lighting system is as simple as red means stop, green means go,
You follow lights through this corridor or that, down this staircase or another until you reach the ground level.
The thing that struck me about this system is just how much space it takes up, especially in New York City, land of shoebox-size apartments that cost a fortune.
I asked Dacunto how much space is just unused in the building 99.9% of the time.
His answer was short and simple: “You always need a Plan B.”
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