ORTHBROOK, Ill., Aug. 25 - For more than two years, federal investigators have been struggling to resolve a critical and contentious question concerning the collapse of the World Trade Center towers: was the spray-on fireproofing initially placed on the twin towers' innovative lightweight floors sufficient to protect them in a major fire?Now, a series of federally sponsored tests that ended here today has produced a provocative but complex finding: the fireproofing, as it was installed during the construction of the trade center in the 1960's, met the standards of the day.
But, in a conclusion that may have ramifications for understanding other tall buildings and future structures, investigators from the National Institute of Standards and Technology found that the test used to determine fireproofing sufficiency, then and now, may itself be flawed - unable to predict accurately what will be required in a real-life fire. As a result, the towers indeed may been more vulnerable to a fire than anyone could have known.
The questions about the fireproofing in the towers have become part of an emotional debate over whether the Port Authority of New York and New Jersey, which oversaw the building of the trade center, and the structural engineer involved in its design deserve part of the blame for the towers' collapse. Doubts have been raised not only about whether the original fireproofing was sufficient, but also about whether the Port Authority did enough to make sure that the lightweight, spray-on material did not fall off as the years passed, as inspections conducted at one point suggested might have been happening.
The debate over the sufficiency of the fireproofing on the World Trade Center's lightweight floors - essentially metal and concrete decks supported underneath by a series of inch-thick zigzagging rods - intensified in May 2003 when federal investigators concluded that the Port Authority, back in the late 1960's, apparently never performed the formal laboratory fire test on the design.
That meant there was no way for the Port Authority to say for sure that the towers' floors would hold up against an extremely intense two-hour fire, as was required then under the New York City building code. The Port Authority, when the towers were built, said it was committed to meeting or exceeding the city code, even though as an agency created by the two states, it was not required to do so.
For federal investigators, then, determining just how well protected the building was with fireproofing - the questions of whether it met basic standards and whether the standards were based on good science - took on critical importance.
The investigators set up a testing program at the nation's most famous fire-testing site, the Underwriters Laboratories, which owns a giant oven into which sections of floor can be placed and burned. The investigators had to decide first what thickness of fireproofing to test, since the original Port Authority plans called for half-inch amounts while the buildings, as actually constructed, ended up with three-quarters of an inch on the floor trusses. They decided to conduct it both ways.
They also decided to take the unusual step of testing longer sections of floor than normally required, to reflect more accurately their lengths as they existed in the trade center buildings. In each of the tests, the pieces of floor would be subjected to flames reaching 2,000 degrees and monitored to see if they maintained their ability to support a large amount of weight and prevent the spread of intense heat for at least two hours. The two-hour measure is the standard required in the city's building code at the time.
The piece of flooring with the half-inch coating of fireproofing failed. That result meant that the trade center's design, despite contentions by the Port Authority, would not have met the city's code. But the shorter piece of flooring covered with the three-quarter inch layer of fireproofing - equivalent to what was in the towers when they were completed - did last the two hours. As a result, this would allow the Port Authority to claim that the towers, as built, would have met the city's standard, even if the thicker fireproofing might have been a quirk that resulted from the way the contractors sprayed it on nearly 40 years ago.It was the different results that surfaced when the longer pieces of floor - the ones that more accurately reflected floor sections used in the trade center - were tested that have provoked concerns about the legitimacy of the widely accepted furnace tests. One of the larger pieces of floor - the one that was set up to simulate the restraint applied by a real-life building - failed in a fire test.These results left investigators with a disturbing reality: in the test in which they used the equivalent of a scale-model toy car, the results suggested that the fireproofing was sufficient. But when they used what would have been the equivalent of a real car, the fireproofing failed.
"We want a link between the performance in the lab test and the performance in a real-scale, live situation," said William Grosshandler, chief of the fire research division of the standards institute. "We need to understand two tests came out differently."
Steve Coleman, a Port Authority spokesman, said the agency would have no comment on the test results until it had a chance to review them. Monica Gabrielle, who had traveled to the Underwriters Laboratories to witness the final test on behalf the Skyscraper Safety Campaign, said she ended the day simply with confusion.
"This is supposed to be science," she said, referring to the differing test outcomes. "I am not quite sure what the tests revealed."
From the time questions were first raised about the fireproofing in the buildings, the Port Authority has argued that the damage done by the two giant airplanes flying into the towers at extraordinary speeds caused such damage that the fireproofing was almost irrelevant.
The federal investigators have always acknowledged the uniqueness of the damage done, and the challenges posed for the buildings. S. Shyam Sunder, the lead investigator at the standards institute, said as much before the fireproofing tests began. The attack on Sept. 11 created fires in both buildings that were far larger and more sudden than ever could be expected to start all at once in a traditional office fire. The impacts of the planes also knocked off at least some of the fireproofing. And it destroyed a swath of exterior and core columns in the buildings, structural elements that were actually responsible for holding up the towers.
Further complicating the picture is the fact that on the day of the attack, the fireproofing on the floors in the two towers differed in thickness. The south tower, which fell in 56 minutes, had only three-quarters of an inch of fireproofing on its upper floors, the same as when it was built. But the north tower, meanwhile, which stood for 102 minutes, had 2.5 inches of fireproofing on the same floors because the Port Authority had in 1995 decided to gradually upgrade the fireproofing after apparently questioning whether the original thickness was sufficient.
The fireproofing, then, is one of only many variables that may have played a role in the speed of the collapse; others such as the angles, speed and height at which the planes hit are perhaps even more significant. But the investigators still believe that the floors may have played a role in initiating the collapse, perhaps simply because the various tests show that the floors sagged a great deal during intense fires, which may have been enough to undermine already weakened outer columns.
New York City officials have already, at least temporarily, banned the use of the floor design that was featured in the twin towers, concerned that the combination of lightweight materials and spray-on fireproofing is unwise at best.
But city officials have not anticipated that in their final report, which is expected in December, the federal authorities might recommend revising the basic system used nationwide to determine if floors sections, columns or other structural elements that are slated to be used in new construction projects can sustain the stress and heat of an intense fire, an outcome that the tests here suggest is possible.