- Posted July 28, 2014 by
Engineers: Poor Fireproofing Led to World Trade Center Collapse
By Greg Mantell
Improper fireproofing was the main cause of the World Trade Center collapse during the September 11, 2001 terrorist attacks, according to newly-published comments in a leading science journal.
Engineers say the spray foam insulation on the buildings’ floor trusses was not as thick as it should have been, causing the steel to overheat and buckle. The authors add both towers would likely still be standing today if the fireproofing had been thicker.
The comments were published quietly in April of this year in a fire sciences journal and received no media attention. But the claims could have implications for skyscraper safety worldwide.
The official investigation of the World Trade Center disaster by the National Institute of Standards and Technology contends that the airplanes scraped off most of the fireproofing from the vertical columns in the twin towers, causing the steel to overheat and buckle, leading to a progressive collapse.
But two prominent engineers dispute those claims, saying it is highly unlikely the planes could have scraped off most of the fireproofing in the buildings. A “more reasonable assumption”, they say, is that the fireproofing on the floor trusses was simply not as thick as it should have been. That lack of insulation led the floor trusses to buckle and collapse.
In short, while the jets started the fires, it was “the fire protection (design) of the building” rather than the airplanes that caused the collapse of the twin towers.
The comments were published in the April 30th issue of the Journal of Fire Sciences. The letter was co-authored Forman Williams, a professor at the University of California at San Diego, who is one of the world’s leading experts on heat transfer. It was co-authored by James Quintiere, a professor emeritus at the University of Maryland.
Williams, who served on the National Skyscraper Safety Team, was the lead engineering adviser to the official NIST investigation of the World Trade Center collapse. Quintiere is a former Chief of the Fire Science and Engineering Division at NIST. He has spoken out against the official NIST report since 2002.
NIST responded to, and rejected, the claims by Williams and Quintiere in the same issue of the fire sciences journal, saying it stood by it stood by its earlier conclusions.
Gail Porter, a spokesperson for NIST, added in an email:
“The analyses showed that, for the steel members where the insulation was intact, the fires did not lead to temperatures that were sufficient to result in significant structural weakening. Logically, additional installed insulation would not have improved the structural stability. By contrast, the steel members with insulation dislodged by the aircraft and/or debris impact heated quickly, as would be expected, and contributed to the building collapse. In other words, the initial thickness of the sprayed-on insulation was not a significant factor in the probable cause of the structural collapses of the towers.“
But Williams and Quintiere challenge several key findings of the NIST investigation--the thickness of the fireproofing insulation, the amount of combustible material in the building, and whether the planes could have scraped the fireproofing off the building.
William and Quintiere said NIST overestimated the thickness of the fireproofing on the towers. NIST claimed contractors accidentally oversprayed a lot more foam insulation on the buildings steel floor trusses than the fire safety plans called for, a claim Williams and Quintiere said seems unsubstantiated and “incredulous.” The two note that photographs of the fireproofing insulation in the World Trade Center contradict NIST’s findings, showing “highly uneven coatings, with both thick patches and bare spots.”
Furthermore, the fireproofing on some of the upper floors of the North Tower was upgraded in 1995, Williams and Quintiere note. They say this was apparently done because the Port Authority realized there was a problem with the fireproofing. The rest of the towers were not retrofitted before the buildings were destroyed.
“As we understand (it) part of the differences between the two buildings was due to a phased-in upgrading of the truss insulation, which had begun when it was learned that the original 1/2 inch (1960s) coatings needed correcting to 1.5 inches (1990s),” Williams and Quintiere wrote.
The retrofitting explains why the North Tower stood twice as long as the South Tower; the fireproofing insulation on the burning floors on the North Tower was twice as thick as that on the South Tower.
“WTC 2 fell in 56 min and WTC 1 in 102 min. Having credible amounts of insulation of 3/4 and 1.5 in, respectively, is a telling coincidence,” Williams and Quintiere said.
In an interview, Williams said a ballpark estimate is that more likely two to three inches of insulation would have been required for the buildings to survive.
“It is weak to conclude that the towers most likely would have fallen even if they had had more extensive structural fire protection,” Williams and Quintiere wrote in the journal forum.
One of the main reasons NIST came to an incorrect conclusion about the collapse of the building, the authors said, is that NIST underestimated the amount, or load, of combustible materials in the building and how hot the fire got.
Williams and Quintiere maintain NIST overlooked 170 file cabinets full of 100,000 pounds of paper in the World Trade Center and also underestimated the amount of stuff in people’s offices. One tenant, in particular, was constantly being cited for having paper stacked high on the window sills.
Williams and Quintiere pointed out, “The bigger the load (of combusitble material), the longer the fire and the higher the temperatures in the steel structure.”
Quintiere said NIST missed an opportunity to improve skyscraper safety by focusing on the adhesiveness of the fireproofing rather than the thickness.
When designing the new World Trade Center, the Port Authority of New York and New Jersey, the building’s owner, appears to have focused on adhesiveness of the fireproofing rather than thickness.
A spokesperson for Skidmore, Owens, and Merrill, the architect of the new One World Trade Center, claims the “the adhesion/cohesion values for our fireproofing (on the new building) are significantly higher--on the order of seven times--than the original towers.”
Williams said it would be very hard to prove the fireproofing in the new building is seven times more adhesive.
The Port Authority, though it has publicly touted many aspects of the new building's safety features, refused to comment on the fireproofing in the new building for reasons of public safety. The Port Authority also declined to comment on the fireproofing in the original World Trade Center.