At the September 11, 2001, terrorist attacks on WTC1 and 2, extensive structural damage, including localized collapse, occurred at the several floor levels directly impacted by the aircraft. Despite this massive localized damage, each structure remained standing for approximately one hour or one hour and a half. Although the damage of the beams and columns in outer framed tube of each tower are clarified at the published FEMA/ASCE report, the damage of the floor system and inner core columns are not estimated. The purpose of this study is to know the reason why the buildings remained standing at the impact through the several analytical studies, such as the impact analyses using the simplified model, that estimates the overall damage, and the rigorous finite element model, that estimates the local damage, and stress analyses after some structural members are lost.
