NEW YORK, May 8 (UPI) -- Federal investigators examining the World Trade Center collapse said Wednesday only 30 of the 110-story twin towers had fireproofing material upgraded from one-half inch to 1 1/2 inches.
"We do not know if it was insufficient, but we are not sure that it was sufficient," Shyam Sunder, investigator for the National Institute of Standards and Technology, said at a news conference in New York City.
Arden Bement Jr., NIST director, said the 24-month investigation is still accumulating data, building design drawings and records, adding it is impossible at this stage to reach conclusions.
The plans for the twin towers began in the early 1960s and were under the compliance of the 1963 New York City building code, a revision of the 1936 building code, according to Bement.
"Later the plans were revised to fit the 1968 building code and in 1993 there was a memorandum to formalize compliance," Bement said. "In 1968, either a 1A classification that required three hours for floors and four hours for columns to hold in fire or 1B for floors to last for two hours and columns to hold for three could be chosen -- 1B was chosen for economic reasons."
By 1999, the Port Authority of New York and New Jersey, which owned the towers at the time, required an upgrade of fireproofing material from a minimum of one-half inch to 1 1/2 inches.
At the time of the Sept. 11, 2001, terror attacks, 20 floors in the north tower and 10 floors in the south tower had been upgraded with the sprayed insulation material, according to the NIST report.
"We found no thickness requirement for fireproofing but we found a letter that said one-half inch should be used," Bement said.
An asbestos-based fireproofing spray had been used on the first 34 floors of the World Trade Center, but the remaining floors used a non-asbestos-based spray, said NIST.
Several theories outside of the NIST investigation have been put forward -- the floors collapsed pulling columns inward or the columns failed.
Michel Bruneau, Ph.D., professor in the Department of Civil, Structural and Environmental Engineering at the University at Buffalo is an expert on steel structures and co-author of "Ductile Design of Steel Structures."
"The World Trade Center is built like a tube with holes, the holes being the windows. So you can cut part of the tube, and it will still remain standing," Bruneau told United Press International. "But steel loses strength when its temperature reaches 1,200 degrees Fahrenheit."
The towers collapsed less than two hours after being struck. Nearly 2,800 people were killed. The investigation is still attempting to collect relevant design drawings, photographs and videotape to trace the spread of the fires as well.
The investigation expects to analyze that information to validate generally applicable methodologies for use in fire safety design and retrofit of structures, and to evaluate the performance of fireproofing materials and connections used in steel structures, said Bement.
The results of such an investigation could lead to major changes in both U.S. building and fire codes and in engineering practice, despite the unique design features or circumstances under which the buildings collapsed.
