For the large deterioration of the mechanical properties of steel under high temperature, the resistance of steel members will be largely discounted. The calculations of yield strength and Young's modulus of steel under high temperature recommended by ECCS have been employed in this paper. A simple and feasible time-variant model of the resistance of steel members and a reliability index analysis method of steel structure under fire has been given. An example, which illustrates the action of ISO834 temperature rising curve on the time-variant reliability index of a steel beam have presented. The results showed that the reliability index of a steel beam decreased with the increasing of the fire action time. When the time of fire action exceeds 138 minutes, the reliability index of a steel beam is less than the minimum value stipulated by the norm.
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