EVALUATION ON FAILURE PROBABILITY OF A PROTECTED STEEL BEAM THAT SATISFIES REQUIRED FIRE RESISTANT PERFORMANCE BASED ON A FIRE RESISTANT PERFORMANCE-EVALUATION TEST

Abstract

Regarding practical fire resistant designs for steel structures in Japan, the performance-based design represented by the AIJ Recommendations for Fire Resistant Design of Steel Structures is rarely used, on the other hand, the specification-based design code (route A) on the Building Standard Low of Japan is widely applied to determine specification of fire proofing material used for the steel member, which has been verified by the fire resistant performance-evaluation test (hereinafter referred to as fire test). The fire test condition differs from the actual condition of fully-developed compartment fire occurred in a building, in particular, regarding the fire temperature and time relationships, the boundary and loading conditions for the heated steel member. Furthermore, the fire loads in the compartment room, mechanical properties of steels at elevated temperatures, and dead and live loads applied to the member possess the large dispersion. To evaluate the safety side performance of protected steel beam based on the specification-based design code, the large loading values at the fire test are used in comparison with the actual condition, however, that probabilistic fire safety at the fully developed compartment fire has not been clarified. To improve the fire resistant safety of the protected steel members applied for many steel buildings, the reliability evaluation on the specification-based design code is of importance.

The main purpose of this study is to evaluate the failure probability of the protected steel beam based on the specification-based design code in a case when the fully-developed compartment fire occurred, by using the theoretical probabilistic calculation model which has been proposed by the authors. To determine the thickness of fire proofing material that satisfies each required performance (1, 2, or 3 hours fire resistance performance) at the fire test, the numerical simulation using the Monte Carlo method considering the dispersions of steel strength and possessing fire resistance time is conducted. The failure probability of protected steel beam with the above required thickness arranged in the fire compartment room are evaluated by the parametric calculations considering the dispersions of fire loads, steel strength, and dead and live loads, respectively.

The main conclusions of this paper are summarized as follows:

1) The failure probability of protected steel beam based on the specification-design code strongly depends on the intended use of compartment room with the changeable fire load values and the applied vertical loads. The selection of specification of fire proofing material based on the specification-based design code, that is, the classification of 1, 2, or 3 hours fire resistance performance based on the Building Standard Law of Japan, is not reasonable to decrease the failure probability at the building fire.

2) The improved specification-based design considering the effects on both intended use of compartment room and loads applied to the steel beam was proposed. By using the proposed design, the failure probability can be rationally decreased in comparison with the current specification-based design code.