Nonlinear seismic behavior of elliptic-braced moment resisting frame using equivalent braced frame

Recently, the elliptic-braced moment resisting frame (ELBRF) which is a new lateral bracing system installed in the middle bay of the frame in the facade of buildings is introduced. This system not only prevents a solution for opening space problem in view of architectural aspects, but also improves the structural behavior. The main drawback of its using in view of numerical modeling in multistory buildings is lack of curved frame element in design and analysis software. To overcome this shortcoming, in this paper, for the first time, an equivalent element for elliptic brace is presented for ELBRF through a laboratory program and nonlinear finite element analysis, which will contribute to its accurate and easy modelling and design. To evaluate the hysteresis behavior of the equivalent element, an experimental test on a 1/2 scale was conducted for a single-story single-bay ELBRF specimen under cyclic quasi-static loading and the results were compared with those for the equivalent element in a story base model. Good agreement was observed between the experimental and analytical outcomes. The seismic demand analyses of ELBRF and frame with equivalent bracing system in 3, 5, 7, and 10 stories were carry out through different loading patterns in Nonlinear Static Pushover Analysis (NSPA) and Nonlinear Time History Analysis (NTHA) with 20 earthquake records using OpenSees software. Story drift, displacement, and story shear were evaluated. Relatively accurate estimations were achieved by NSPA in comparison with NTHA. Also, the seismic performance of the equivalent element for the ELBRF system against earthquake was examined and then response modification factor (R factor) was acquired. The values of 8.5 and 12.2 for the R factor were calculated at the ultimate and the allowable stress limit states, respectively.