Behavior and Design of Composite Beams for Fire Loading

Abstract

Current U.S. design methods for structures in fire rely upon prescriptive approaches that focus on providing fire protection to keep material temperatures below a given threshold. While these methods are typically conservative, they do not take into consideration the structural performance of such members. In order to move towards a performance-based design approach for fire, it is necessary to quantify not only the thermal response but also the mechanical performance of structural components at elevated temperatures. A numerical model was developed and benchmarked to calculate the flexural capacity of partially composite beams at elevated temperatures. The model accounts for the failure modes associated with composite beam strength including: full yielding of the steel beam, failure of the concrete slab in compression, or shear stud fracture. An extensive parametric study was conducted to determine the influence of variations in cross-section geometry, level of composite action, and temperature distribution. A simple design method for determining the nominal moment capacity of a composite beam at elevated temperatures was developed based upon the parametric study results. This method uses a temperature-dependent retention factor along with the ambient nominal moment capacity to calculate the moment capacity of the composite beam at elevated temperatures