Robustness assessment of a generic steel fire-protected moment-resisting frame under travelling fire

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Due to their vulnerability to high temperatures, steel structures are typically protected against fire by insulation materials, as recommended by fire codes. The fire resistance rating of a protected member is determined based on a standard fire test, which simulates post-flashover fire conditions. Empirical evidence has shown that fires in large open-plan compartments do not burn uniformly, and a method called “traveling fire” has been introduced in previous research to simulate the effects of spreading fires. An investigation is performed here to examine the robustness of a generic four-story moment-resisting steel structure with one-hour fire resistance rating subjected to travelling fire. Various fire sizes are considered: 12.5, 25, 50 and 100% of the floor area. The results show that while no collapse occurs during the 12.5, 50 and 100% fire sizes, the structure collapses under the 25% fire size at 85 min due to the inability of the system to transfer the load after one of the columns buckles. This seems to be in contradiction with traditional belief that a fire covering a larger portion of the floor plan would reduce the safety margin. The investigation performed underlines that the fire protection of structures based on the standard time-temperature curve does not necessarily provide adequate resistance under travelling fires. With the lack of adequate fire regulations codified for large compartments, more research is required before arriving at a better understanding of the application of travelling fire to large compartments.