Published: Sunday, August 14, 2011, 7:07 PM Updated: Monday, September 12, 2011, 11:50 AM
In the anguished weeks after terrorists toppled the World Trade Center towers, architects and engineers pondered the unthinkable: the end of the skyscraper era.
From the 1880s, when the
first high-rises poked above
Chicago and
New York skylines, tall buildings had defined the nation's ingenuity, economic clout and can-do optimism. They were capitalism's exclamation points, steel fingers pointing the way upward.
Now, though, the structural and safety innovations that had made the world's tallest structures possible had failed in catastrophic, unprecedented ways. Skyscrapers suddenly felt like targets, like deathtraps.
On Sept. 10, 2001, New York architect
Carl Galioto's firm had just wrapped up the design phase for a new 50-story Stock Exchange building. Two weeks after the attacks, when Galioto was able to return to his Wall Street office, the project was dead.
"I remember those conversations," he said recently, recalling his peers' gloom. "Will tall buildings ever be built again? Will people want to occupy them?"
A decade later, the answer is yes.
Following an exhaustive
federal probe of what caused the towers' downfall, and years of debate and strife over how to respond, the building and fire codes that dictate safety features in new high-rises finally are reflecting the disaster's harsh lessons. They call for hardier structural systems,
more-reliable fireproofing, and better exit stairs, among other things.
In Asia and the Middle East, skyscrapers are vying for the
world's height record. And at lower Manhattan's Ground Zero,
new towers bristling with enhancements that go beyond the code requirements are rising from the scarred earth.
Some recommended code changes remain unmade, however. And the standards for existing tall buildings like those in Cleveland are largely unchanged. That means older skyscrapers' components are not substantially safer than before the jet strikes, although evacuation planning and drilling has gotten better.
The post-9/11 enhancements won't make high-rises or other buildings terrorism-proof, experts stress. A determined assailant, armed with enough firepower, can overwhelm the sturdiest structure. The changes are meant to give occupants a greater chance of getting out safely after an attack or other calamity -- particularly a large fire -- and to lessen the odds of a full-scale collapse.
"The [high-rises] that are being built now and in the future, I think we can feel have better resistance to whatever happens," said structural engineer
Gene Corley, one of the world's foremost authorities on building collapse. "I'm troubled by our slowness in adopting some of these recommendations, which have extremely low cost and would do a lot of good. But I do think we have made some progress."
An unprecedented collapse
No American high-rise had succumbed to a total collapse before 9/11, although there had been warning signs. Several skyscraper fires, including a raging 19-hour blaze in Philadelphia's 38-story
One Meridian Plaza in 1991, had caused significant structural damage and raised serious questions about the adequacy of safety systems and firefighters' ability to cope with the challenges that high-rises posed.
The 44-inch stairwell widths dictated by building codes, which helped determine how many people could evacuate at a time in an emergency,
hadn't changed since the early 1900s. They were based on stationary measurements of pre-World War I soldiers and didn't reflect how people move on stairs, much less the jump in body size and obesity rates that had occurred in the following century.
"We've known for decades that the width of stairways is not adequate," said
Norman Groner, a John Jay College researcher who studies emergency behavior, and who has pushed for width increases as well as
elevator safeguards that would allow occupants and firefighters to use them during emergencies. But "the wider you make the stairwells, the more you subtract from the leasable space and the less the building is worth."
Advances in lightweight construction methods and materials made it possible to build
super-tall buildings 100 stories and higher, with huge floor spaces unobstructed by support columns. That meant many more occupants and far more combustibles – 40,000 people in the Trade Center twin towers, and about 60 tons of paper, furniture and other flammable material on each nearly acre-sized floor.
Spray-on fireproofing material had replaced the heavy masonry and concrete encasements that protected the structural steel frames of older buildings.
The fluffy insulation was known to flake off when an elevator cab rumbled by, or when ventilation fans kicked on.
Automatic sprinkler systems were helpful, but had capacity limits. In the twin towers, the sprinklers could deliver enough water to snuff a fire covering about one-tenth of a floor's 40,000 square-foot area. Firefighters' hoses could cover at most another one-quarter.
As a result, firefighters in New York and elsewhere reluctantly adopted a
"defend-in-place" strategy with large high-rise fires. They were allowed to burn out, while firefighters concentrated on evacuating people from the affected floor and those immediately above and below who were at greatest risk. Other occupants were told to stay put, since stairwells couldn't handle a rapid full-building exodus.
For the strategy to work, though, smoke and fire had to be kept to one floor – a tough prospect, given numerous ventilation ducts and other routes of spread.
Another critical necessity: a high-rise's supports and their fire-proofing had to be able to withstand high temperatures until the fire exhausted its fuel.
Otherwise, the building and its occupants would be at risk from progressive collapse, where a buckling column or slumping floor launched a cascade of structural failures that brought the entire skyscraper crashing down.
The twin towers' fire-proofing was supposed to provide protection for 3/4 of an hour to four hours, depending on the component.
But when Islamic extremists exploded a truck bomb in the Trade Center's underground garage in 1993, forcing a complete evacuation, "it took us five hours to make sure everybody was out," said retired New York City deputy fire chief
Vincent Dunn. "And we had floors that were [fire-] rated for two hours. There's a lot of problems with that."
Federal studies lead to recommendations
Several government agencies and engineering groups launched investigations in the wake of the twin towers' collapse. The most detailed was a three-year probe by the federal National Institute of Standards and Technology.
NIST researchers
determined that, even though the jet impacts caused heavy structural damage on multiple floors,
the skyscrapers would have remained standing if the jets hadn't knocked off fire-proofing coating columns and floor trusses.
Without that protection, steel supports weakened under the relentless fires, which burned unchecked because the impacts severed the sprinklers' supply pipes. The jet strikes also destroyed all but one escape route, a single set of stairs in Tower2, trapping those in the upper floors
Still, 87 percent of the twin towers' estimated 17,400 occupants that morning got out', either because they were below the impact zones or, in the case of Tower 2, had begun to leave before the second plane hit. More than 2,700 people, a mix of tenants and rescuers, were still inside when the columns and floors finally gave way and the skyscrapers pancaked to the ground.
Researchers
interviewed Trade Center survivors, to learn more about how people behave in emergencies, especially in tall buildings. Although occupants largely didn't panic and helped others, the scientists found that tenants were generally unfamiliar with the towers' design and safety features. Only 10 percent had ever entered a stairwell as part of an emergency drill. After the jets struck, occupants took an average of 6 minutes to begin leaving, spending the time collecting belongings, shutting down computers, making phone calls or waiting for instructions.
"In general, people [in skyscrapers] react in the same way as in other settings," said occupational health researcher
Robyn Gershon, who led Columbia University's
survivor study. "That's not the right way. In a high-rise, you don't appreciate the scale. You think you're living in the horizontal world." Trade center occupants "had almost no sense of how long it would take to move through that many stairs."
Although the twin towers were disaster investigators' primary focus because of their sheer size and the human toll, engineers were keenly interested in the Trade Center complex's
Building 7, which also collapsed on 9/11.
That 47-story skyscraper had a more conventional structural frame than the twin towers, and it wasn't hit by jets or doused with fuel. But when debris from Tower 1 set Building 7 afire – a blaze that burned for 7 hours because the sprinklers were disabled – it fell too. Fortunately, all occupants had evacuated and emergency teams were ordered out.
Building 7 was the first total high-rise collapse strictly due to fire, and further highlighted the need for improved fire-resistance.
NIST investigators made 31
recommendations based on their reviews of what happened in the twin towers and Building 7. The proposals called for substantial changes in how new high-rises are designed, built and maintained, and in evacuation and emergency response, though the researchers urged owners of existing buildings to also consider acting on the suggestions.
Turning the NIST recommendations into mandatory building and fire safety codes took several years. The voluminous "model" building and fire safety codes from which state and local governments pattern their own binding standards first had to be updated.
That process involves debate not just about the technical basis for the changes, but their applicability and cost, too. Most tall buildings probably are not terrorism targets. Some developers objected to the requirements, saying they might make skyscrapers prohibitively expensive.
"We're always trying to make a balance between cost and safety," said
Shyam Sunder, who led the NIST investigation. "And we're always trying to make a trade-up in how safe is safe enough."
"The question is what I call the bookends," said Robert Solomon of the
National Fire Protection Association, a non-profit group whose model fire codes are used by many cities. "Do we say [the Trade Center attack] is such an unusual occurrence that we're not going to do anything? Or do we really up the ante and go with all kinds of redundant systems and features?"
Significant changes in building codes
In the end, the National Fire Protection Association and the
International Code Council, whose model building and fire codes are the blueprint for most U.S. communities, followed most of the 9/11 investigators' recommendations. They made significant changes in the 2009 and the upcoming 2012 codes, which apply to new high-rises. Ohio is preparing this fall to adopt its version of the 2009 codes, mirroring the changes.
The national code improvements include glow-in-the-dark exit markings in stairways; a third or fourth stairway depending on the building's height; greater separation between those stairways to lessen the chance of a single calamity disabling all of them; stickier, more robust fire-proofing, with inspections to ensure its proper application; backup water supplies for sprinklers; impact-resistant walls around elevator and stairwell shafts; fortified elevators that firefighters and, in some cases, occupants can use in an emergency; stricter and more consistent fire-resistance standards for skyscrapers' structural components; radio amplifiers that help rescuers better communicate inside buildings; and improved emergency evacuation plans and disaster drills.
Even before the code changes, the designers of the replacement World Trade Center buildings voluntarily were incorporating advanced safety features into the new towers, including some – like blast-hardening and extra backup systems – that went beyond the anticipated requirements.
"We felt we had something to prove," said architect Galioto, whose former firm, Skidmore, Owings & Merrill, designed the new
1 World Trade Center and
7 World Trade Center towers. Galioto is now the managing principal of the New York office of
HOK, one of the world's largest architecture firms. "We felt we had to develop a tall building in downtown Manhattan that people would be comfortable coming to work in."
Likewise, newer high-profile federal buildings such as courthouses have protective designs and features that exceed local and state building codes, even though the structures technically are exempt. The heightened security focus pre-dates 9/11, stemming from the 1995 truck bombing of the
Murrah Federal Building in Oklahoma City.
The building design details are classified, but "the level of capacity, the redundancy of [safety] systems, the willingness to assume risk, has changed dramatically for our clients," said Cleveland architect
Paul Westlake, whose firm works extensively on federal projects.Among its efforts: a prototype perimeter security system of planters, benches and other functional, visually pleasing vehicle barriers that safeguard historic buildings such as the
Federal Reserve Bank of Cleveland.
While high-rise safety is improving, more changes are needed, according to the
Skyscraper Safety Campaign, an advocacy group founded by New Yorker Sally Regenhard, whose son
Christian was among the
343 firefighters killed in the twin towers' collapse.
Regenhard and two of the group's advisers, safety expert
Jake Pauls and fire protection engineer and John Jay College professor
Glenn Corbett, say high-rises should be required to have stairwell video cameras and other monitoring technology to give rescuers better "situation awareness" of fire, smoke, evacuation progress and structural dangers.
That information would help commanders make better decisions about where, how and whether to deploy their personnel. "Had the fire services had situation awareness [in the twin towers], they would have saved a lot of their people by not entering the buildings, because there was no way they could do anything except die," Pauls said.
Cost and privacy concerns so far have stymied attempts to add video monitoring to building or fire codes, Pauls said.
The minimum widths for stairwells in new skyscrapers also remain unchanged. "This was the frustration," Pauls said. "We couldn't even get that adopted . . . when the needs had obviously grown, with people's larger body size, larger girth and reduced fitness."
High-rise occupants and building managers bear some responsibility for being prepared for emergencies. That includes reacting promptly to alarms, knowing how to get out, and even something as simple as keeping comfortable shoes around for long stair descents.
"When you get on an airplane, the first thing they tell you to do is look around and make sure where the exits are," said Henry Green, president of the
National Institute of Building Sciences. "We don't do that when we go into buildings, even now. How many folks have actually looked down the stairwell when they see an exit sign?"
In Ohio's tallest skyscraper, the 57-floor
Key Tower in Cleveland, there are yearly evacuation drills where tenants must completely exit the building, even if it means walking down dozens of stair flights. Floor wardens check offices and direct traffic. Tenants get a printed report of how long it took their personnel to get out.
Mega-law firm
Squire Sanders occupies eight of the tower's uppermost floors, with 40-mile views and window ledges that double as falcon perches. Managing partner
David Goodman remembers the hasty exit on that terrible morning a decade ago. With enhanced evacuation planning and practice, he feels safe.
"We believe we've done the right things, but we know it's easy to become complacent a year or 10 years after some incident that prompted new procedures," Goodman said. "We have strived not to."
