Robustness of a partially restrained, partially composite steel floor beam to natural fire exposure: Novel validation and parametric analysis

Abstract

This study investigates the structural response of a partially restrained, partially composite steel floor beam to a realistic or “natural” fire, which includes a rapid ramp-up to peak intensity followed by a decay phase to burnout. Thermo-structural finite element models are validated against a pair of experiments that were performed on the same unprotected specimen, which remained in place and undisturbed following the first test for 8 years until the second test. The tested specimen showed minimal damage after having been subjected to two natural fire curves and provides a novel validation opportunity for predicting the residual post-fire state of these assemblies. The validated models are then used to evaluate the prototype assembly (with and without fire protection) for its resistance to the ASTM E119 standard fire curve as well as its robustness to several natural fires with varying intensity and duration. Progressive levels of structural damage based on peak and residual response to natural fire exposure are proposed as a means to quantitatively and qualitatively evaluate the effectiveness of various fire protection strategies. These damage levels can also be used as the initial condition for determining the resilience of composite floor systems to fire (i.e. to evaluate the time and cost of recovery and repair based on the fire-induced loss of functionality). To complement this resilience-based approach, a standard fire resistance rating would essentially represent a collapse prevention benchmark and could be used to validate performance-based structural-fire calculations if the level of superimposed loading and the times needed to reach both the thermal and deflection limit criteria were reported.