Assessment of eccentric brace effect in steel structures performance and comparison of it with moment frame system in progressive collapse phenomenon

Structure safety in the design of civil engineering projects has always been very important for engineers. One of the mechanisms that structure will fail and in recent years is much attentioned to it is progressive collapse. Progressive collapse in structures during earthquakes even in an explosion near the construction has become a major challenge and can create problems for structures and may even lead to the destruction of the entire structure. Currently the most available structures is only designing against the gravity loads and lateral loads (wind and earthquake).In fact a resistant structure against the earthquake is not resistant against the progressive collapse necessarily. Therefore designing the new and special structures against the progressive collapse is necessary. Progressive collapse is defined as extension of primary local failure from element to other element that finally collapsed all part of the structure or big part of it. Potential hazards that cause progressive collapse are fires, gas explosions, make a mistake in design of structure, accidents, bomb and even an unprincipled excavation that cause sudden removal one or more elements of structure and etc. The purpose of this paper is to investigate progressive collapse in steel structures with eccentric braced frames that also the influence of parameters such as height, bracing arrangement and type of structural system is examined In this study it is analyzed the progressive collapse due to column removal in steel eccentric braced frames that are designed seismically according to Iranians guidelines(seismic regulations of Iranian 2800 code) with using of alternate path method and nonlinear dynamic analysis. Also in the continuation of research it is analyzed the progressive collapse due to column and brace removal simultaneously in steel eccentric braced frames and analysis the progressive collapse in moment frames and comparison of it with eccentric braced frames. Also it is evaluated the influence of parameters like number of floors, location of braces and type of connections. For this intent two structures with five and ten stories with braces in middle spans, and also two structures with five and ten stories with braces in lateral spans, one structure with five story with system of moment frames and one structure with combinatorial system of moment frames with eccentric brace which is five story in ETABS program were analyzed. Then one of outside frames for analysis of progressive collapse modeled in SAP2000 program. Results showed that remove a single column only when there is not any brace beside the removed column and simultaneous removal of columns and braces only in the last floor causing progressive collapse to the structure. Results showed that the probability of progressive collapse with simultaneous removal of columns and braces will increase when the height of the frames that middle spans is braced increases and will decrease when the height of the frames that lateral spans is braced increases. Also with comparison of eccentric braced frames and moment frames is resulted that eccentric braced frames is stronger than the moment frames against the progressive collapse. But combinatorial system of moment frames and eccentric brace in comparison with the other analyzed systems is completely resistant system.