According to an interim report by the National Institute of Standards and Technology released Wednesday, documents show that builders in 1969 directed contractors to coat the floor supports with a half-inch of fluffy, spray-on fireproofing. Though the fireproofing was later upgraded, no tests were apparently conducted to see how it handled intense flames.
Arden Bement, NIST director, said the interim report does not draw conclusions or make recommendations. ``That's for later, when we have a much more complete picture,'' he said.
In 1999, the building's owners, the Port Authority of New York and New Jersey, issued guidelines to upgrade the fireproofing to a thickness of 1 1/2 inches. As recently as 2000, officials were still redoing the fireproofing, noting in one property assessment that some areas could withstand only one hour in a fire.
By Sept. 11, 2001, the NIST report found, fireproofing had been upgraded on 29 floors in the areas of the towers where the hijacked planes struck _ 19 floors in the north tower and 10 in the south tower.
The north tower fell about an hour and 45 minutes after terrorists attacked with the first hijacked jetliner. The south tower collapsed about an hour after it was struck. Nearly 2,800 people were killed.
When the trade center was built, the Port Authority _ as an interstate agency _ was not bound by New York City building codes, or any other for that matter.
In 1963, the agency instructed engineers and architects to comply with the local building code, but it was not until 30 years later that an agreement was established to allow fire and other inspections.
In the 1960s, the architects of the complex chose to build according to fire resistance standards that required floors to last for two hours and columns to hold for three, NIST found.
A higher level, which required three hours for floors and four hours for columns, was not chosen because there was ``no economic advantage,'' said a 1987 memorandum by a chief structural engineer for the trade center.
The towers were built with an innovative floor system, and investigators said they have not found any evidence that it was tested with fireproofing to meet the two-hour standard.
Glenn Corbett, a fire science expert who has testified before Congress about evacuation and other safety issues in the trade center, said the system should have been tested.
``I don't know how you could make an assumption about a new material,'' said Corbett, who also serves on a team of experts advising NIFT.
Still, the fast-moving fires that raged in the towers on Sept. 11, 2001, may have been far more severe than the typical conditions assumed in the fire resistance test, Corbett said.
Sally Regenhard, whose son was among the 343 firefighters killed, said Wednesday she was ``horrified to know we could build such a fragile building with a minimum amount of fire protection.''
In addition to the fireproofing, investigators are looking closely at a photograph taken of the south tower's east face, about 12 minutes before the building crumbled. In the photo, part of the 83rd floor is sagging at an angle of about 45 degrees.
The evidence lends weight to the theory that the buildings fell because the floors sagged first, pulling the towers' side columns inward. Another theory says that the columns failed, and that brought down the buildings.
``We have not yet taken a position on which of the two is the most probable mechanism, but we are going to be looking at each of these mechanisms in great detail,'' lead investigator Shyam Sunder said.
Other studies have found that the collapse of the 110-story twin towers was caused by flaming jet fuel softening the buildings' steel.
The NIST's two-year investigation was launched last year to investigate the collapse and evacuation of the trade center in an effort to make buildings safer.
___
On the Net:
The report: http://www.nist.gov/public_affairs/releases/wtc05-07-03.htm
