عنوان مقاله [English]
TThis paper investigates the behavior of mid – rise steel moment resisting frames (MRF) subjected to compartment fire and progressive collapse scenarios due to subsequent floor drop and column removal. In this study, initially, a typical 15 – story building with moment resisting frame system and story height of 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 using OpenSees software. Then the nonlinear behavior of the frame is studied at elevated temperatures under different scenarios. In these analyses, the structure is subjected to both gravity and thermal loading simultaneously. Also for performing thermal analysis, standard fire curve (ISO 834) is 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 650°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. Demand to Capacity Ratios of columns increase from early stages of fire, which cause the columns reach their linear limit state at early stages. This behavior is due to thermal expansion of heated floor, which induce large horizontal displacement and consequently cause additional bending moment in outer column. Also by one story floor drop, columns survive to 500°C. But at higher temperatures (about 600°C to 750°C), these heated columns lose their strength and buckle. In column removal scenarios in first and 7th story, where beams have lost their strength at about 200°C and 400°C respectively, more damage is observed compared to floor drop scenarios. Also, fire in lower stories of structures is more critical than upper stories since they carry more gravity loads and start to behave nonlinearly at lower temperatures. Also fire in middle spans produces more critical situation than fire in end spans and interior columns are more critical than corner columns.