Investigation of the behavior of mid – rise steel moment resisting frame with long spans exposed to fire and progressive collapse

This paper investigates the behavior of a steel moment frame system with long spans subjected to compartment fires and progressive collapse scenarios due to girder drop and column removal. In this study, initially, a typical 15 – story building with moment frame system with long spans and story height 3.2 (m) was designed using relevant chapters of national building code of Iran for conventional gravity and lateral loads. In order to perform thermal analyses, the most critical frame of this structure is modelled in finite element software OpenSees. Then the nonlinear behavior of the frame is studied at elevated temperatures under different fire scenarios and progressive collapse Scenarios due to girder drop and column removal. In this analyses, the structure is subjected to both gravity and thermal loading simultaneously. Also for performing thermal analyses, nonlinear analyses and standard fire curve (ISO 834) are used.
Results of this study indicate that beams do not deform significantly until approximately 400°C, but after that, vertical displacements of beams increase significantly due to degrading mechanical properties of steel. So beams deform and collapse at about 500°C to 600°C. Also heating the beams of structure, initially causes the axial force in the beams due to thermal expansion restraint. So Demand to Capacity Ratios of beams increase from early stages of fire and the most increase in DCRnom occurs at about 350°C to 400°C. Also by one story girder drop, columns survive to 500°C. But at higher temperatures (about 600°C to 800°C), these heated columns lose their strength and buckle. In column removal scenarios in first and 7th story, where beams have lost their strength under effect of gravity loads and at about 400°C respectively, more damage is observed compared to girder drop scenarios.