Fire Brought Down 7 WTC: Report

At a technical briefing this week, Shyam Sundar, lead investigator for the federal building and fire investigation of the World Trade Center disaster conducted by the National Institute of Standards and Technology, revealed in detail how fire brought down the 7 World Trade Center building. The reports conclusions were first announced last week. The collapse had been the source of much speculation as no planes hit the building, which appeared to collapse much the same way one would have had charges been set to bring it down in a controlled fashion. According to NIST, “There was damage to the building from the collapse of World Trade Center Tower 1, which was about 370 feet to the south.” And the report concluded that the debris started fires o”n at least 10 floors of the building. The fires burned out of control on six of these ten floors for about seven hours.” As a water main had been cut by the collapse of the twin towers, the sprinklers in building 7 failed to operate in “much of the bottom half of the building.” However, tall buildings have burned before without collapse, so the investigators knew from the outset that they weren’t dealing “with a textbook case,” the report advised. What they did not find, however, was any evidence of explosive charges. Nor was the collapse the result of fire from the diesel fuel stored in the building.What they did find: The uncontrolled building fires caused an extraordinary event, the collapse of World Trade Center 7. “This is the first time that we are aware of, that a building taller than about 15 stories has collapsed primarily due to fires,” according to NIST research. Apparently the Achilles heel that caused the collapse, suggest investigators, might have been thermal expansion of long-span floor systems located in the east side of the building. Fires on floors 7 through 9 and 11 through 13 were particularly severe. Long-span steel beams on the lower floors of the east side of the building, expanded significantly due to these fires, damaging the floor framing on multiple floors. Eventually, a girder on Floor 13 lost its connection to a critical interior column that provided support for the long span floors on the east side of the building. The displaced girder, and other local fire-induced damage, caused Floor 13 to collapse. This began a cascading chain of failures of eight additional floors—many of which already had been at least partially weakened by the fires in the vicinity of the critical column. With the support of these floors gone, column 79 buckled, which initiated the fire-induced progressive collapse of the building. In quick succession, the remaining interior columns failed from east to west across WTC 7, until the entire core began moving downward. Finally, the remaining outer shell or façade of the building fell. While the study found that tall buildings are generally safe, the new phenomenon of thermal expansion has led to suggestions for design and construction. If thermal effects concerns are raised by the evaluation of a building design, NIST recommends possible retrofits including strengthening connections, strengthening floor framing, increasing structural redundancy, and adding additional fireproofing to vulnerable areas. The report also urges the strengthening of building codes, standards, and practices. Specifically, codes should be altered to ensure the prevention of structural collapse in building fires, where sprinklers do not function, do not exist, or are overwhelmed by fire.