مهندسی عمران شریف (Nov 2019)
A PERFORMANCE-BASED METHOD FOR CALCULATION OF MEMBER CAPACITY MODIFICATION FACTORS OF STEEL MOMENT FRAME UNDER IMPULSIVE LOADING
Abstract
Design of structures under impulsive loads is usually accomplished through nonlinear inelastic dynamic analysis followed by implementing acceptance criteria of the nonlinear analysis specified in design codes. Nonlinear dynamic analyses inherently consist of convergence and computational effort problems. 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 impulsive loading. Capacity modification factors are proposed for different loading conditions that provide a design procedure to perform the linear dynamic analysis, instead of the time-consuming nonlinear dynamic analysis. Herein, an algorithm is proposed to calculate the capacity modification factors as an inverse problem. Firstly, a designed structure is nonlinearly analyzed under impulsive load; then, the structure is checked whether the acceptance criteria are satisfied. For a steel frame structure, story drifts should be restricted to 1/14 of story height; a 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; otherwise, the structure should be redesigned to reach the desirable condition. Results of the linear analysis are checked by ASCE41-13 acceptance criteria for the linear analyses. These acceptance criteria control demand and capacity moments for a beam, demand and capacity axial force, and moment interaction of columns such that the capacity modification factors are involved in 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). Three loading conditions are defined to mobilize three deformation modes consisting of lateral, gravity, and lateral-gravity modes. The first mode includes laterally distributed and concentrated loads on the left column and downward loads on the beam; the second mode only includes downward loads on the beam; the third mode only includes laterally distributed and concentrated loads on the left column. 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 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 impulsive loads are not ductile as much as internal columns. In other words, internal columns can go beyond the linear limits more than external ones. The roof beams have lower ductility than the internal beams. The reason is the directly imposed impulsive load on their span. Therefore, the calculated factors can be used for new acceptance criteria that need linear dynamic analysis. The proposed procedure leads to avoidance of performing the complicated nonlinear analysis under impulsive loading, while acceptance criteria of nonlinear analyses are satisfied by employing linear dynamic analysis. In addition, this development reduces computational efforts and can be extended for future design codes.
Keywords
