- © 2015 by the Seismological Society of America
The collapses of the World Trade Center north tower (WTC1) and south tower (WTC2), struck by terrorist attacks on 11 September 2001, produced ground tremors discernable at nearby seismic stations (Kim et al., 2001) and, as will be shown, other stations throughout the northeastern United States. Although seismic signals produced by building collapses are not unusual (Holzer et al., 1996), the mass of the falling World Trade Center (WTC) towers and traumatic circumstances were extraordinary. This study determined the collapse durations of the towers with unique precision from the precise timing of seismic station records. Collapse duration is a pivotal indicator of the collapse resistance of the towers (Bažant and Verdure, 2007) and, as such, might inform future building design.
WTC1 was the first struck, at 12:46:30 UTC (8:46:30 a.m. local time) at its 96th floor by a hijacked airliner, but remained standing longer than WTC2, struck ∼16 min later, at 13:02:59 UTC at its lower 81st floor. The burning WTC2 abruptly collapsed ∼56 min after being struck. WTC1 collapsed similarly ∼102 min after it was struck (Shyam‐Sunder, 2005).
The detailed structural analysis of the collapsing towers (Hamburger et al., 2002; Bažant and Verdure, 2007) revealed the top floors of each tower, above the strike/combustion zone, descended as a unit, successively crushing floors beneath while pushing ahead an accumulating zone of compacted material. During this progressive collapse, peeled structural fragments fell loosely ahead of the more slowly descending upper‐tower segments, which were mostly obscured by opaque dust, smoke, and debris (Hamburger et al., 2002). Small‐magnitude ground shocks were likely produced by the loosely falling fragments, as well as high‐velocity elastic impulses transmitted into the remaining structure beneath the descending upper …