"Wounded Buildings Offer Survival Lessons"
By James Glanz. New York Times, December 4, 2001
By James Glanz. New York Times, December 4, 2001
At some indefinable hour during an interminable night, the fires that had raged for nearly two days in the skyscraper at 90 West Street still puffed and flared when Derek Trelstad, a structural engineer, walked into the building and cautiously began climbing the stairs, the double snout of his respirator wheezing in the haze.
Charred wreckage dangling from the ceilings and heaped on the floors cast strange shadows in the light streaming from rescue equipment outside through punched-out windows on the building's northern facade. On Sept. 11, flaming steel debris from the south trade tower had smashed against the thick terra cotta of that facade and started the fire within.
"It was like the ultimate haunted house," said Mr. Trelstad, a senior project director at LZA/Thornton- Tomasetti in Manhattan, a firm that is helping the city evaluate the structural soundness of buildings around the disaster site.
But Mr. Trelstad soon made remarkable discoveries by looking behind the burnt wreckage, examining the building's vintage steel structure and the archaic, tile fireproofing materials that protected much of it.
He found that except for a few spots where structural columns had slightly buckled on the upper floors, the building, a 1907 landmark designed by Cass Gilbert, had battled the fire and essentially won.
The building had avoided the fate of 7 World Trade Center, the 47-story skyscraper on the northern edge of ground zero whose collapse in a fire on Sept. 11 has emerged as a mystery. Moreover, the 1907 high-rise could also be refurbished and occupied.
As engineers widened their survey of surviving but severely damaged buildings on the rim of ground zero, they realized that they had uncovered an engineering clinic of sorts, one that might provide a rare positive note within the zone of devastation.
Investigations of how and why those buildings survived could lead to improvements in the way skyscrapers and other structures are built. Experts said early indications that older buildings performed better than newer ones were still difficult to confirm, partly because the buildings had been under different stresses during the attacks.
But they said a reconsideration of some bygone building practices would be an almost certain consequence of the new information.
"Engineers can take it as an accidental experiment, or a `found experiment,' " said Dr. Henry Petroski, a professor of civil engineering at Duke University and the author of "Design Paradigms: Case Histories of Error and Judgment in Engineering" (Cambridge, 1994).
The lessons around ground zero go well beyond protecting against fire. On a purely structural level, debris from the south trade tower raked down the front of the Bankers Trust Building, a 40-story high-rise built in the 1970's, and knocked out 11 stories of a major steel column in its facade. But not even the portions of floors immediately above the gash collapsed.
Likewise, steel members from one of the towers were hurled westward like spears and lodged in one corner of the American Express tower, knocking out a structural column along three floors but producing no secondary collapse. Debris from 7 World Trade sheared off parts of two adjacent structures the Verizon Building and a City University of New York building which also remained standing.
Because the structures had not been built with such an attack in mind, engineers said, their survival was a side effect of design elements put in place for other reasons. But as studies isolate those elements, they could become part of common engineering practice and perhaps even be mandated in new municipal building codes, the engineers said.
"I think we will start seeing some of that coming into our codes, or at a minimum, engineering practice," said Richard Tomasetti, president of LZA/Thornton-Tomasetti. "We're going into a new era."
While no one advocates a return to the design principles of a century ago, the engineers who have examined the still-graceful high-rise at 90 West Street say it may carry some important lessons for the streamlined skyscrapers of today.
Built five years before Mr. Gilbert's more famous F. W. Woolworth Company Building, which happens to overlook ground zero from the northeast and was undamaged in the attacks, 90 West Street has a Gothic facade made of two layers of terra cotta totaling more than a foot in thickness.
The building originally boasted a top-floor establishment that billed itself as the highest restaurant in New York, according to the city's Landmarks Preservation Commission a curious parallel with Windows on the World in the north trade tower.
When the south tower collapsed, said Mr. Trelstad, the engineer, some of its huge columns fell and ripped out many of the steel spandrels, or cross beams between windows, on the north facade of 90 West Street. One series of spandrels, from the 11th down to the 3rd floors on the east part of the facade, was destroyed as if a giant claw had run down the front of the building.
Whether because the heavy terra cotta acted like armor or because some of the blows were glancing, none of the vertical steel columns holding up the building were damaged in the south tower's collapse. But it was not until Mr. Trelstad ventured inside that he fully appreciated the building's resilience.
He was first struck by the scale of the devastation and the eeriness of the setting. Fires set by the south tower's debris had gutted the 2nd, 3rd, 10th and 23rd floors, and much of the northern portions of the 4th, 5th, 8th and 21st floors. At least two people are believed to have died in the building, trapped in an elevator.
As if that were not sobering enough, during his second visit, several men who Mr. Trelstad did not know were in the building suddenly ran screaming down a stairway from the roof, saying they had been radioed that another nearby tower was about to collapse.
"There was that heightened sense of awareness that everywhere you look, you're afraid you're going to see something you don't want to see," Mr. Trelstad said.
Instead, what he observed soon lifted his spirits a bit. Both fire stairwells had been surrounded by four to six inches of heavy tile fireproofing and were virtually untouched by the blaze. Most of the dozens of steel columns holding up the building were encased in four-inch-thick blocks of tile much like the material of which flower pots are made.
Except for four places on the upper floors where columns had softened and bent slightly in the heat, the heavy tile had done its job and protected the steel. And even though that kind of tile can be brittle, columns in the facade that had been exposed by the impact of debris still had their fireproofing in place.
Fireproofing in the floors was still more impressive, with an archlike arrangement of tile a foot thick having stopped the flames from burning through one story to the next.
The cost and installation of such tile today would probably be prohibitive, Mr. Trelstad said, but the new evidence could lead to a reassessment of the lightweight fireproofing used in modern skyscrapers like 7 World Trade Center.
Engineers said that any direct comparison between 7 World Trade, which was also set on fire by falling debris but then collapsed, and 90 West was fraught with difficulties. For one thing, the conditions the two buildings initially faced on Sept. 11 were not clear.
For another, the recent discovery that 7 World Trade held tens of thousands of gallons of diesel fuel to power electrical generators in the event of a power failure has set off a debate among engineers over whether there may have been extraordinarily hot conditions that softened the steel and led to the collapse.
But with that debate unresolved, engineers said, the collapse remains one of the deepest mysteries their profession has faced. No other modern, steel-reinforced skyscraper except for the trade towers themselves has ever collapsed in a fire.
Moreover, 7 World Trade did not have the lightweight steel trusses holding up its floors that may have been the first elements to soften and fail in the trade towers. Instead, said Silvian Marcus, executive vice president for the Cantor Seinuk Group and a structural engineer involved in the original design of the building, the floors were supported by sturdy I- beams.
Irwin Cantor, one of the original structural engineers for the building but no longer affiliated with Cantor Seinuk, said, "There's something that doesn't compute."
"That's why up to this minute, nobody in the engineering profession is ready to say to you, `I know what made that building come down,"' added Mr. Cantor, who is now a consulting engineer and a commissioner in the Department of City Planning.
Although Mr. Cantor said he believed the diesel fuel played a role in the collapse, he said it was also likely that fireproofing in 7 World Trade, which was completed in 1987, was damaged in the impacts caused by falling debris.
That fireproofing was of a type used in virtually all modern, steel- reinforced buildings: a mélange of mineral fibers and concrete-like materials called binders that make the fireproofing sticky. Sprayed onto steel in layers that are often less than an inch thick, the material insulates well in controlled, laboratory tests, which generally involve sedate fires in large ovens.
But the material is so soft and friable that it can be removed with a putty knife even after it has been in place for years. In the chaos of any terrorist attack involving a blast or an impact, many engineers have said, the material can easily be knocked off the steel, leaving it unprotected in a fire.
A return to tile fireproofing is unlikely because of its expense and weight, said Mr. Tomasetti, the LZA/Thornton- Tomasetti president. But he said the new information would almost certainly prompt a drive to improve spray-on fireproofing with chemical additives or other measures.
A managing principal at the firm, Thomas Scarangello, said structural damage to other buildings not involving fire had reinforced an engineering lesson: create structures that are able to redistribute loads spontaneously if part of a building is suddenly destroyed. "All engineers are adding it to their experience list," Mr. Scarangello said of the investigation around the trade center.
Exceptionally robust connections between columns and beams as well as unexpectedly sturdy floor bracing allowed for that sort of rerouting when debris knocked out a structural column in the Bankers Trust Building at 130 Liberty Street, Mr. Scarangello said.
But on a recent day at ground zero, perhaps the most vivid example of that redundancy could be seen at the command center run by Kenneth Holden, commissioner of the Department of Design and Construction, the city agency leading the cleanup.
Mr. Holden's office at the command center is on the 30th floor of the American Express tower, tucked into the southeast corner of the building, a few floors above the spot where steel from the trade towers knocked out a structural column.
"When they told me where my office was," Mr. Holden said, peering out of a corner window, "I felt like a canary in a coal mine."