Parametric Demonstration of Load-Dependent Critical Temperatures for Flexural Fire Resistance of Composite W-Shape Floor Beams

Abstract

Parametric numerical modeling was performed for three composite floor beam configurations (with different W-shape sections and one-way span lengths supported by shear connections) under exposure to one standard fire and three natural fire temperature-time histories. The parametric matrix included three levels of passive fire protection, four combinations of axial and rotational restraint at the beam ends, and three levels of applied flexural loading. A previously validated lumped mass heat transfer modeling approach was used to calculate steel temperatures for each flange and the web, and a one-dimensional finite element (FE) heat transfer modeling approach was used to calculate the temperature gradient through the structural thickness of the floor slab. A previously validated fiber-beam FE structural modeling approach was then used to model the flexural response of the one-way composite beam under fire. The results of parametric analysis showed that the loss of flexural resistance under any fire exposure can be conservatively predicted using a critical bottom flange temperature based on AISC 360-22, Table A-4.2.4, which is expressed as a function of the applied flexural utilization ratio, M/M_n. The bottom flange temperature at which flexural failure would occur was relatively consistent regardless of variations in beam end restraint as well as the level of applied fire protection. For composite beams that survived natural fire exposure through burnout, the bottom flange temperature always remained below the load-dependent critical value. In those cases, variations in beam end restraint significantly impacted the magnitude of residual tensile reaction forces that develop at the ends of the beam during cooling.