Analysis of Structural Response of WTC 7 to Fire and Sequential Failures Leading to Collapse

This paper presents the structural analysis approach used and results obtained during the investigation conducted by the National Institute of Standards and Technology (NIST) to model the sequence of fire-induced damage and failures leading to the global collapse of World Trade Center 7 (WTC 7). The structural analysis required a two-phased approach to address both the gradual response of the structure to fire before collapse initiation (approximately 4 h) and the rapid response of the structure during the collapse process (approximately 15 s). This paper emphasizes the first phase, a pseudostatic (implicit) analysis that simulated the response of structural elements to fires that spread and grew over several hours and presents key aspects of the second phase, a dynamic (explicit) analysis that used the first-phase damage as initial conditions and simulated the progression of structural failures that resulted in global collapse. The analyses accounted for (1) geometric nonlinearities; (2) temperature-dependent nonlinear materials behavior for both members and connections (including thermal expansion, degradation of stiffness, yield and ultimate strength, and creep); and (3) sequential failure of structural framing and connections. Analysis uncertainty was addressed by determining rational bounds on the complex set of input conditions and by running several multiphase analyses within those bounds. The structural response from each analysis was compared to the observed collapse behavior. This approach allowed evaluation of fire-induced damage, sequential component failures, and progression of component and subsystem failures through global collapse of WTC 7.