Fire-Induced Progressive Collapse Analyses of High-Rise Towers

Isobe and his collaborators are seeking for the true cause of the total collapse of the WTC towers back in 2001 by conducting several numerical simulations, including the full model aircraft impact analysis of the WTC tower. Isobe suggested in the past papers that the spring-back phenomenon due to rapid unloading caused by aircraft impact possibly have destructed member joints in the core structure, which might have caused the towers to become brittle and unstable. However, it does not give answers to the total collapse of WTC tower 7, which collapsed, reportedly, only by fire. In this study, the investigation is made from a different point of view and the influence of fire and heat is taken into account. The main objective of this study is to conduct some fire-induced collapse analyses to investigate how fire patterns, structural weakness of member joints, and outrigger trusses on roof tops give influence on the collapse behaviors of high-rise towers. The analytical results obtained using the ASI-Gauss finite element code show a clear difference between each fire pattern, between the models with strong and weak member-joint strengths, and between the models with and without the outrigger trusses. In general, the strongly designed models withstand the total collapse, whereas the weak models initiate their collapse when the temperature reaches to the highest, and ends, eventually in a total collapse. The models with outrigger trusses tend to withstand longer in time by catenary action only if their load paths are protected