Clearing a vertical path to safety

Fire safety and egress experts debate evacuation standards for high-rise buildings in the aftermath of 9/11
August 11, 2010

No one will ever know exactly how many people died immediately from the impact of the plane crashes into the Twin Towers. But what happened in, and to, the stairwells will be looked at for quite some time. For the lucky 18 who escaped from the floors above the crash location in the South Tower, a detour in the vertical path of Stairwell A was the reason they had a clear escape path.

Contrary to initial reports, experts now know that the common office furnishings and equipment found on those floors fueled the fires, not the jet fuel that was consumed in about nine minutes. And with the other five stairwells destroyed and the standpipes severed, hundreds of people trapped on those floors died from the burning interiors and the resultant collapses.

A picture taken on September 11 shows a firefighter ascending a North Tower stairwell. While he ultimately made a U-turn and got out alive, the photo depicts another problem that firefighters and human factors experts have known for decades: that the standard width of most building staircases is inadequate for evacuation and firefighting.

"People actually have to stop and twist to the side to let somebody go by, particularly someone loaded down," says Jake Pauls, an architecturally trained, Silver Spring, Md.-based expert on human factors, egress, stairwell design and building codes. "When a firefighter walks in a standard stairwell, you lose about three-quarters of its capacity."

Egress capacities assume people can walk side by side. Other photos taken that same day show a staggered set of people walking down the stairs. "It's a misconception that 44 inches allows two files of people to go down shoulder to shoulder," says Pauls. "It's only when you get to the 56-in. stairwell that you get two abreast."

 
As occupants evacuate the North Tower, a New York City fireman climbs stairwell to reach raging fire on the floors above.


Codes differ on stair-width standards

Code battles and how they ultimately shake out will determine what width is required where. The newly released NFPA 5000, the Building Construction and Safety Code issued by the National Fire Protection Association and supported by fire officials and plumbers, is currently competing for adoption against the upcoming edition of the International Code Council ICC code, supported by architects and building owners, which will become available early next year.

With NFPA 5000, the standard rule is 0.3 in. of stair width per person. With code requiring a minimum of two 44-in.-wide stairwells, that allows a floor capacity of 293 people (88 ÷ 0.3 = 293). ICC code boosts this allowance to 0.2 in. per person, which translates to 440 persons on a floor.

Another stair-width issue being debated in ICC committees is increasing egress capacity in lower floors. Pauls and others suggest that when the total upper-floor occupancy exceeds 2,000, either add another staircase or widen the two required staircases from 44 in. to 56 in. On a typical 20-story office building, occupancy of 2,000 would be reached at roughly the seventh story. With that added 12 in. of width, "the bottom line is that it would have affected about 1/1000 of the area of the seven floors," says Pauls. But wider stairs on those floors would allow owners to increase the occupancy substantially. "The formula is not proportional. With a 27% width increase, you could add up to 37% capacity," he adds. But because of the perception that space is being lost, a code change that Pauls says was being pushed through was stopped by appeal.

Another tactic for increasing capacity that may catch on more after September 11 is retrofitting buildings with fire escapes. "The fire department likes them as a way to attack, "says Glenn Corbett, an associate professor of fire science at John Jay College of Criminal Justice in New York and a fire captain. Corbett says he recently consulted with an owner of an existing building who wanted to add a cafeteria with an occupancy of 600 on the eighth floor of the building, which would have required a third stairwell. Ultimately, as often happens, the project got cancelled because of aesthetics.

 
External fire escapes are one way to provide greater stairway capacity on the lower floors of a building.


Drywall can fold under pressure

Eyewitness reports on that fateful day say that drywall-enclosed stairwells failed to withstand the impact and peeled off in huge sections. Firefighters add that stairwells protected by concrete and steel would have resulted in fewer casualties. Vincent Dunn, a retired deputy chief of the New York City Fire Department and author of the book Collapse of Burning Buildings: A Guide to Fireground Safety, asserts that the National Institute of Standards and Technology (NIST) should evaluate the substitution of drywall for concrete blocks when enclosing stairways and elevator shafts in high-rise buildings. "Powerful hose streams collapse [the drywall]. They didn't do that with the concrete."

Some experts suggest reconfiguring the building core as a cost- and space-effective approach, such as creating a combined refuge area, stairwell and firefighter service elevator in a pressurized concrete shaft. For instance, the Petronas Towers in Kuala Lumpur, Malaysia, use a central core of high-strength concrete to help protect some of the elevators and stairs.

Phased evacuation

Stairwell codes address evacuation needs by looking at a building's most densely occupied floor. In a standard office building, 100 sq. ft. of space per person is allocated. What codes don't do is address the evacuation of an entire building, relying instead on a limited evacuation to non-fire floors until the emergency subsides, often known as the "defend-in-place" strategy, which some fire experts don't support.

"Modern high-rise buildings will not contain the fire from floor to floor," says Dunn. He and others say another reason the strategy is no longer valid is because occupants may doubt the instructions of fire officials.

In the 9/11 attacks, estimates are that both buildings were less than half full, so the buildings' stair capacity was never fully tested. And as many have pointed out, using elevators, which is against standard fire evacuation procedures, may have ultimately saved a lot of lives. "Avoiding the stairwell accelerated passage before and even after the planes struck," says Jack Murphy, president of the Fire Safety Directors of Greater New York.

Further complicating the issue is the common firefighting strategy of designating stairways for attack, evacuation and ventilation. "People go down one, and firefighters go up the other. If lots of people use them, it complicates things for putting out the fire," says Corbett.

Human factors

Currently, Pauls sees the need for big changes in the high-rise development industry. "The whole office development industry doesn't understand that there may be a sea change out there. People may only be willing to rent if they have a certain comfort level based on the building being prepared for a rapid evacuation."

Pauls is co-leading the World Trade Center Evacuation Study Initiative, an international group of about 100 people aiming to improve knowledge of evacuation. So far the group has received funding from the Centers for Disease Control for future interviews with 9/11 survivors.

"We have to ask: 'If you were in a 50-story building now, and there was a fire, what would be your motivation to stay? What would be your motivation to go and how quickly?' That goes right to the heart of the defend-in-place philosophy behind the code, because we're still not designing buildings for total evacuation," says Pauls.

While on Aug. 21 NIST launched a two-year, $16 million investigation that will include egress studies (see p. 49), Pauls' estimate to survey or interview survivors is about $10 million. With $3 million in the study dedicated for evacuation and human behavior, "the NIST funding for these issues is paltry. There's a lot of urgency that they still haven't recognized. They focus more on steel now, which can sit there for years, while people's perceptions and recollections decay much more quickly."

But after seeing this type of apparent inertia and apathy for decades, Pauls knows what it will take to change priorities: "Another 9/11."

         
 

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