نوع مقاله : پژوهشی
نویسندگان
1 دانشکده فنی و مهندسی، دانشگاه اراک
2 مرکز سازه و مصالح امن، پژوهشکده عمران، دانشگاه صنعتی مالک اشتر
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
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.
کلیدواژهها [English]
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