Structural Responses of World Trade Center under Aircraft Attacks

At the September 11, 2001, terrorist attacks on the New York World Trade Center (WTC) Towers, extensive structural damage, including localized collapse, occurred at several floor levels directly impacted by the aircraft. Despite this massive localized damage, each structure remained standing for approximately 1 h or 1 h 30 min. Although the damage to the beams and columns in the perimeter tube of each tower were clarified in the published ASCE/FEMA report, the damage to the floor system and inner core columns were not estimated. The purpose of this study is to determine why the towers remained standing after impact through several analytical studies, including impact analyses using a simplified model to estimate the overall damage, a rigorous finite element model to estimate the local damage, and stress analyses after some structural members are lost. The results of the stress analyses show why both buildings did not collapse immediately after impact, and WTC2 collapsed sooner than WTC1.