Nowadays, the using of steel moment frames has been considered as an effective lateral load-resisting system against seismic forces. In this research, the effect of plastic hinges modelling of fully restrained connections has been evaluated on the strength of steel moment frame structures against progressive collapse. In order to assessment of the progressive collapse performance of sample structures, two structural models of 3 and 5-story buildings were designed and then the definition and

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... lling of plastic hinges have been done in two different modes. In the first mode, plastic hinges were modelled only for the beam and column members. In the second mode, plastic hinges were considered for the connections in addition to the beam and column members. The Five different types of investigated connections included WFP, WBH, WCPinWUF, WUF-B, Welded top and bottom haunch. The progressive collapse resistance of these buildings against sudden removal of internal and external column was separately studied using nonlinear static alternate path method presented in the UFC standard. The results of analysis indicated that if the plastic hinges of connections are separately defined in addition to the plastic hinges of beams and columns, the structure will have lower progressive collapse strength, and the resistance of the structure tends to its actual value. Simultaneous modelling of plastic hinges for connections and members at least 20% and maximum 37% affects the reduction of structural strength against progreive collapse. Therefore, the modelling of plastic hinges for connections and members simultaneously should be considered as a important regulation. According to the comparative results, the WCPinWUF and WUF-B connection respectively demonstrated that the most desirable behaviour and the weakest performance against the progressive collapses.