This report uses the term "fire-induced collapse" to indicate the failure of a structure, or significant portion of a structure, that could be attributable directly to a fire event in the building. In some cases, the building may have been under construction or in process of renovation, or it may have experienced significant damage prior to the fire caused by a blast, impact, or an earthquake. A subsequent report on the collapse of the WTC towers (NIST NCSTAR 1, September 2005) found that WTC 1 and WTC 2 collapsed due to aircraft impact damage to the structure and fireproofing as well as to fire. Both effects (damage and fire) were equally important. In the absence of structural and insulation damage, a fire substantially similar to or less intense than the fires encountered on September 11, 2001, likely would not have led to the collapse of a WTC tower. On the other hand, it was concluded in NIST NCSTAR 1A (October, 2008) that WTC 7 would have collapsed from fires having the same characteristics as those experienced on September 11, 2001, even without the initial structural damage to the building initiated by the collapse of WTC 1. The Building Performance Study conducted by the Federal Emergency Management Agency (FEMA 403, May 2002) refers to the partial collapse of WTC 5 as "fire-induced." No analysis was conducted to determine the relative roles of the initial structural damage and the subsequent fires that led to the fate of WTC 5.
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