Simpson Gumpertz & Heger, Inc. (SGH) developed finite element models of the components, connections and subsystems of the World Trade Center (WTC) towers to study their structural performance in the fire environment that followed the aircraft impact to the towers. The results of this study were used to develop global models that captured with numerical efficiency the important failure modes and sequential failures of components and subsystems and to determine the probable sequence of structural responses that let to the global collapse initiation. The study was conducted as part of the investigation on the WTC disaster by the National Institute of Standards and Technology (NIST). The structural response to the fire environment/ was established by hand calculations and finite element analyses (FEA) for: connections including interior and exterior truss seats, knuckles, column splices, and spandrel splices; components including a section of the floor system, including concrete slab and a single truss and a single column, and subsystems including full floors and a section exterior wall. The key structural responses, failure modes, and failure loads were identified. The finite element models, developed in ANSYS, captured the nonlinear responses of the connections, components, and subsystems, including temperature-dependent material properties such as thermal expansion, plasticity and creep of metals, large deflection and the resulting instability, and failure modes of members and connections, modeled by break elements developed for this purpose. The models were subjected to gravity and thermal loads. Construction sequence was included in component models. NIST provided temperature-dependent nonlinear material properties, aircraft impact damage to structural members, and temperature time histories of structural elements for subsystems, which were used as input in this study.
Citation: NIST NCSTAR - 1-6
Pub Type: NIST Pubs
World Trade Center, high rise buildings, building collapse, disasters, fire safety, fire investigations, terrorists, terrorism, structural analysis, floors, walls, deflection, plastics, structural response, damage, stability