Considering Catenary Action in Designing End-Restrained Steel Beams in Fire

When a building compartment is subjected to fire attack, there are complex interactions between the fire affected members with the surrounding members. The behaviour of the steel frame members in fire can be drastically different from that of its member in isolation. This paper studies the behaviour of steel beams with the increase of temperature from beam action phase to catenary action phase and until failure. The load bearing mechanism in the catenary action phase is discussed and the failure criterion is defined. A new ultimate limit state based on 15% maximum strain of steel material at elevated temperature is proposed to determine the ultimate load capacity of beams failed in the catenary action phase. Wide ranges of beam parameters including various beam sizes and span lengths with different degrees of end restraints are studied. Comparison of results with those obtained from nonlinear finite element analysis shows that the proposed design method could enhance the critical temperature of steel beams by over 200 °C if proper attention is given to the integrity of connections to resist the catenary force. In this respect, methods to estimate the catenary force and beam deflection are provided.