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Structural Engineering and Mechanics
  Volume 77, Number 5, March10 2021 , pages 661-672

Modifications to fire resistance ratings of steel frames based on structural configuration: A probabilistic-based approach
Behrouz Behnam

    In this article, the role of spans number and length in fire-resistance ratings (FRRs) of fireproofed steel frames are investigated. First, over a span-lengthening scenario, two one- and three-bay frames under the ISO834 fire are examined. It is shown that the FRRs of the frames rely highly on the changes made on their span length. Second, a building designed for three spans number of three, four, and five under natural fire is investigated. The beams are designed for two load-capacity-ratios (LCRs) of optimum and ultimate. The fire curves are determined through a probabilistic-based approach. It is shown that the structural vulnerability vastly increases while the number of spans decreases. The results show that for an optimum LCR, while the five-span frame can meet the required FRR in 87% of the fire scenarios, the four- and three-span frames can meet the required FRR in only 56%, and 50% of the fire scenarios, respectively. For an ultimate LCR, the five-, four- and three-span frames can meet the required FRR in 81%, 50%, and 37.5% of the fire scenarios, respectively. Functional solutions are then proposed to resolve the insufficiencies in the results and to rectify the application of the standard-based FRRs in the cases studied. The study here highlights how employing current standard-based FRRs can endanger structural safety if they are not connected to structural characteristics; a crucial hint specifically for the structural engineering community who may be not well familiar with the fundamentals of performance-based approaches.
Key Words
    fire-resistance rating; standard fire; natural fire; load to capacity ratio; steel structures; probabilistic-based
Behrouz Behnam: School of Civil and Environmental Engineering, Amirkabir University of Technology, 424 Hafez Street, Tehran, Iran

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