Feasibility study of using endurance time method for seismic evaluation of self-centering buckling restrained braced frame (SC-BRC-BF)

The conventional bracing frame (CBF) systems show a limited drift capacity before buckling subjected to seismic loads. So, the induced damage in the structure reduces the strength and stiffness. In the last two decades, self-centering (SC) systems have been developed to resolve the deficiencies of the conventional seismic-resistant systems. In SC systems, the structural damage and residual drift are negligible, while they provide sufficient strength and stiffness. In these systems, prestressed elements are used to provide the initial stiffness. On the other hand, the steel plate shear wall, bracing, beam connection to the column provide energy dissipation mechanism. These elements are used as replaceable fuses after sever earthquakes. When the force applied to the structure is greater than the initial prestressing force, the gap created in the structure causes the energy dissipating elements to work. The main feature of SC systems is that they return to zero deformation after each load cycle. So, the post tensioned elements must remain elastic to be able to reduce the residual displacement. This property of the systems represents flag-shaped hysteresis lateral load-deformation curves. Among the engineering community, three methods of equivalent lateral forces (ELF), dynamic spectral analysis and dynamic time history analysis are commonly used for seismic analysis of structures. The endurance time (ET) method is a new method for seismic analysis and also for performance-based design of structures. In this method, the structure is subjected to special ET accelerations in which the dynamic response of the structure increases with time. The time needed for the structural failure index (such as the maximum drift of stories) to reach a certain level of performance or failure is defined as the structural ET. As a result, a structure that has a longer ET, has better performance against earthquakes. The main advantages of the ET method include: 1) by providing a suitable estimate of the structural response in each time history analysis, saves a lot of computational time for seismic evaluation, 2) the nonlinear properties of the structures may be considered which can be used for a variety of structures and complex behavior, 3) this method has a simple concept and principles for engineering applications, and 4) this method has a high capability for experimental work with a shake table. In the current research, the self-centering buckling restraining column braced frame (SC-BRC-BF) system was examined. This system not only increases the drift capacity, it also reduces damage and residual drift in the system. The SC-BRC-BF system consists of two rigid cores connected by buckling resistance columns (BRC) between tha adjacent floors. The BRCs are used as replaceable fuses to dissipate the input energy and to reduce the seismic responses. Vertical post tensioned cable is used to restore the system. For this purpose, a preliminary design approach was introduced for SC-BRC-BF systems with 3, 6 and 9 stories via SAP 2000 software. The simulation of structures under time history analysis and ET method was done via OpenSees software fin a 2D framework. Different seismic responses were investigated including: 1) roof drift, 2) the maximum strain of core elements, 3) drift concentration factor (DCF), and 4) Inter-story drift. The response of structures was examined at both DBE (Design Base Earthquake) and MCE (Maximum Considered Earthquake) levels. Comparing the responses from ET method and the conventional time-history method, the error rate does not exceed 10 and 15 % at the DBE and the MCE levels, respectively. The results obtained from seismic evaluation using the two mentioned approaches, corroborated the high efficiency of ET method with a few number of time history analyzes.