INVESTIGATION OF THE BEHAVIOR AND PROGRESSIVE COLLAPSE OF MID - RISE STEEL MOMENT-RESISTING FRAMES EXPOSED TO FIRE

This paper investigates the behavior of mid-rise steel moment-resisting frames (MRF) subjected to compartment fire and progressive collapse scenarios due to floor drop and column removal. In this study, initially, a typical 15-story building with a moment- resisting frame system and a 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. Moreover, for performing thermal analysis, a standard fire curve (ISO834) is used. Results of this study indicate that beams do not deform significantly until approximately 400\circC; however, after that, vertical displacements of beams increase significantly due to degrading mechanical properties of steel. Moreover; beams deform and collapse at about 500\circC to 650\circC. In addition, the beam heating initially causes axial forces in the beams due to thermal expansion restraint. Furthermore, Demand to Capacity Ratios of beams increase from early stages of fire and the most increase of DCRnom occurs at about 350\circC to 400\circC. Demand to Capacity Ratios of columns increase from early stages of fire, causing the columns to reach their linear limit state in early stages. This behavior results from the thermal expansion of heated floor, which induce large horizontal displacement and, consequently, cause additional bending moment in the outer column. Moreover, by one story floor drop, columns survive up to 500\circC. However, at higher temperatures (about 600\circC to 750\circC), these heated columns lose their strength and buckle. In column removal scenarios, in the first and 7th stories, where beams lose their strength at about 200\circC and 400\circC, respectively, more damage is observed compared to floor drop scenarios. Fire in lower stories of structures is more critical than that in upper stories since they carry more gravity loads and start to behave nonlinearly at lower temperatures. Moreover, fire in the middle spans produces a more critical situation than that in the end spans, and interior columns are more critical than corner columns.