Finite Element Analysis of Steel-Concrete Composite Floor Systems under Traveling Fires

Abstract

Traveling fires occur in large open-plan compartment and have been observed in many fire accidents including the First Interstate Bank fire in Los Angeles in 1988, the One Meridian Plaza fire in Philadelphia in 1991, and the World Trade Center Building 7 fire in New York City in 2001. Despite the significant structural damage observed in these incidents, existing fire safety codes do not have regulations dedicated to ensuring the fire safety of large open-plan compartments, nor are traveling fires explicitly considered in the fire design process. To address this deficiency, the dissertation presents a computational study aimed at better understanding the thermal and structural response of steel-concrete composite (SCC) floor systems exposed to traveling fires. Improvements to the finite element modeling of SCC floor systems were developed as part of the dissertation work. Specifically, a formal macro-modeling approach for SCC floor systems was presented, which addresses a modeling error that has remained largely unreported in the research literature. Using this modeling approach, a numerical analysis of an axially-restrained SCC beam was performed. The results showed that failure of a restrained SCC beam is heavily influenced by its span length: a composite beam with a short span tends to fail in the compressive beam-column stage, while a composite beam with a longer span tends to fail in the tensile catenary stage. Additionally, conditions which are favorable for the mobilization of tensile catenary action were determined, which provides structural engineers with the information required to improve the fire resistance of SCC beams. A fomulation for an elevated-temperature tension stiffening model for use in the finite element modeling of SCC floor systems was also developed. Surprisingly, no elevated-temperature tension stiffening model existed in the research literature, despite the established role that tension stiffening plays in the modeling of reinforced concrete members at ambient temperature. First, the energy-based stress-strain model of plain concrete developed by Bazant and Oh (1983) was extended to the elevated-temperature domain by developing an analytical formulation for the temperature-dependence of the fracture energy. Then, the elevated-temperature model was developed based on the modification of the proposed elevated-temperature tension softening model. The applicability and validation of the proposed tension stiffening model was then presented through the numerical analysis of several experimental tests of SCC floor systems exposed to fire. Using a sequentially-coupled thermal-structural analysis procedure, the thermal and structural response of two code compliant SCC floor systems were then examined under various fire types, including a family of traveling fires, two post-flashover fires, and a standard fire exposure. The results of the investigation showed that fire insulations derived from prescriptive approaches might not provide adequate safety under traveling fires. Failure times derived using a critical temperature criterion and a critical displacement criterion both showed that SCC floor systems perform poorly under traveling fires, which was not the case under the two post-flashover fires. The findings demonstrate a large vulnerability with prescriptive fire codes, and strengthens the case for the use of performance-based design in engineering practice.