عنوان مقاله [English]
In this research, the capacity modification factors of steel moment resisting frames’ members are calculated in order to simplify the design of the structures subjected to blast loading. Capacity modification factors are proposed for different loading conditions which provide a design procedure to perform the linear dynamic analysis, instead of the time-consuming nonlinear dynamic analysis. An algorithm is herein proposed to calculate the capacity modification factors as an inverse problem. Firstly, a designed structure is nonlinearly analyzed under blast load, and then the structure is checked whether or not the acceptance criteria of UFC 3-340-02-3-340-02 are satisfied. For a steel frame structure, story drifts should be restricted to 1/14 of story height; and chord rotation of the members should be restricted to 2 degree. Secondly, if the acceptance criteria are satisfied with minimal tolerance, the structure with the accepted properties is linearly analyzed, and otherwise the structure should be redesigned to reach the desirable condition. Results of the linear analysis is checked by ASCE41-13 acceptance criteria for the linear analyses. These acceptance criteria controls demand and capacity moments for a beam, and demand and capacity axial force and moment interaction for columns such that the capacity modification factors are involved in the both of them as unknown variables. Thirdly, the capacity modification factors are calculated for each member using the formulations presented for the acceptance criteria of the linear analyses. Here, a portal frame is used as a representative of entire moment resisting frame to evaluate different types of loading (magnitude and condition). Finally, many capacity modification factors are attained for every member of a steel moment resisting frame. These data should be processed by statistical relations to obtain a firm results for the main members of the structure. The capacity modification factors are herein calculated for four member groups including roof beams, internal beams, external columns and internal columns. Results demonstrate that external columns exposed to direct blast loads are not ductile as much as internal columns. In other words, internal columns can go beyond the linear limits more that external ones. The roof beams have lower ductility than the internal beams, the reason is the directly imposed blast load on their span. Therefore, the calculated factors can be used for new acceptance criteria which needs linear dynamic analysis.