Load Combinations for Buildings Exposed to Fires

Load requirements in ANSI Standard A58.1.1982 appearing in the LRFD Specification for Structural Steel Buildings are selected, whenever possible, using probabilistic load combination analysis and structural reliability theory. The occurrence of a fire with the potential to cause severe structural damage is a rare event in comparison to events giving rise to design-basis load effects. Although probabilistic load modeling techniques reveal that a structure likely is loaded to only a fraction of its design load when a fire occurs, steel structural assemblies normally are fire tested while loaded to the full nominal design load. This paper presents an improved methodology for determining loads and load combinations for use in fire-resistant structural design. The basic methodology is illustrated by determining load combinations involving fire and live loads for limit states design and for use with the standard ASTM E119 fire test. A reduced nominal live load combined with the structural action due to fire apparently provides a load combination that is risk-consistent with other currently accepted load combinations. The method is consistent with the limit states design approaches under development or in use in Europe that recognize the influence of load on fire resistance of steel structures, and is compatible with the LRFD philosophy.