| Until recently, structural steel members exposed to fire were designed using prescriptive approaches that do not account for actual loading conditions and real fire scenarios. The last decade has seen the promotion of performance-based codes which allow more rational engineering approaches for the design of steel members exposed to fire. For example, Appendix 4 of the 2005 AISC Specifications (referred to hereafter as "AISC Specifications") now allows steel members to be designed against fire using room temperature design specifications and reduced material properties. The AISC Specifications allow using the same capacity reduction factors for fire design as those used for room temperature design. For example, a capacity reduction factor of 0.9 is suggested for steel beams and columns. Most other codes suggest that a capacity reduction factor of 1.0 be used. This recommendation is based on arguments that the probability of fire occurrence and the strength falling below the design value simultaneously is very small, and that fire design is based on the most likely expected strength. The Commentary to the AISC Specifications also states that the fire load may be reduced by up to 60 percent if a sprinkler system is installed in the building. Automatic sprinklers reduce the probability of occurrence of a severe fire. The reduction in fire load should be based on proper reliability analysis that includes the effect of sprinklers on the occurrence of a severe fire, and correspondingly on the probability of failure of structural steel members. However, limited work has been done to develop capacity reduction and fire load factors based on reliability analysis.;In this study, a general reliability-based methodology is proposed for developing capacity reduction and fire load factors for design of steel members exposed to fire. The effect of active fire protection systems (e.g., sprinklers, fire brigade, etc.) in reducing the probability of occurrence of a severe fire is included. The design parameters that significantly affect the fire design of steel members such as fire load, opening factor, thermal absorptivity of compartment boundaries, thickness, density and thermal conductivity of insulation are taken as random variables. Raw experimental data published in the literature was analyzed to obtain the statistics of parameters for which no statistical information was available in the literature. Model errors associated with the thermal analysis models are also characterized based on experimental data. It is found that uncertainty associated with the fire design parameters is significantly higher than that of room temperature design parameters.;To illustrate the proposed methodology, capacity reduction and fire load factors are developed for simply supported steel beams and axially loaded steel columns in U.S. office buildings. It is found that the capacity reduction and fire load factors should not be constant for all design situations as suggested in current design specifications, but should vary depending on the presence of active fire protection systems in a building.;In addition, a simplified method is proposed for computing inelastic deflections of simply supported steel beams exposed to fire, and it is shown that the fire design of most beams is governed by the strength limit state. |
