عنوان مقاله [English]
Natural disasters and their harmful impacts have always been one of the most challenging problems all over the world. As such phenomena are inevitable since the distant past mankind has been trying to predict them spatially and temporally and evaluate their loss. Earthquake, as a natural disaster, could not be predicted in time. However, its magnitude and location are predictable to some extent and so is the corresponding loss. Theoretical and computational advances in civil engineering lead to a precise understanding of structures’ behavior and earthquakes. Therefore, in the recent decades, nonlinear behavior and performance-based method have been introduced in the seismic evaluation of structures. Many studies have been carried out in this field by research centers and agencies like FEMA and ATC, resulted in useful guidelines. Eccentrically braced frames have high stiffness and suitable energy damping against the lateral forces like the earthquake. In this bracing system, the required stiffness and formability of the frame is provided by the link beam, and are dependent on the details and characteristics of the link beam. In recent years, Eccentrically Braced Frames (EBF) has been utilized as a resistant system against the earthquake lateral forces. The research has shown that the EBF have the ability to combine a high stiffness in the elastic range as well as an excellent ductility and energy dissipation in the inelastic range. Currently, seismic design provisions of most building codes are based on strength or force (base shear) considerations. These building codes are generally regarding the seismic effects as equivalent static forces with a height wise distribution, which is consistent with the first vibration mode shape. However, the design basis is being shifted from strength to deformation in modern performance-based design codes. Determining the shear story and overturning moment under earthquake excitation is an important problem in the seismic design of structures. There are several approaches in order to estimate an acceptable accurate response for the shear story and overturning moment of the structure in the nonlinear region. Both ATC and FEMA approaches are good ideas to evaluate the seismic performance, but more simplified approaches should be applied in seismic design codes. Most of the seismic design codes suggest a very simple relation for estimating the shear story in design base earthquake. In this study, some criterions of the mentioned guidelines are studied, which are about seismic evaluation of the eccentric braced frame (EBF) systems, then the suggestions are offered. In this research, a comparative study has been done to analyze the behavior of regular steel building structures of 4, 8, 12 and 16 stories, located in zones of high seismic hazard and soil type 2. Three-dimensional building systems composed of steel frame system with Intermediate Link Beam (EBF) have been selected for investigation. These 3D building structures have been considered with 4, 8, 12 and 16 stories. Then, the performance level of all regular structures is evaluated in one hazard level (with the return period of 475 years). In order to evaluate the performance level of the aforementioned structures, they were modeled three dimensionally using SAP V14.00 software for both nonlinear static and dynamic analysis. The criteria for predicting the target location guidelines ATC-40, FEMA-356 and modified methods in FEMA-440 were used. The loading pattern design for nonlinear static analysis of single-mode and modal pushover (MPA) was used. For nonlinear time history dynamic analysis out of nine coupled ground motion accelerations from the strong motion database of PEER, with a minimum of 20 km and maximum 45 km from the source and magnitude range of 6 to 7.5 were selected. The performed procedures in FEMA-356 and proposed plastic hinges in this guideline are utilized for performing the static nonlinear analysis. The soil type II was considered having the shear wave limit between 375 to 750 m/sec. The result and the accuracy of pushover analysis has been compared with the nonlinear time history analysis. This indicates that the results obtained by FEMA-440, are closer to the results of the nonlinear time history dynamic analysis. It is also concluded by the investigating of the shear story and overturning moment of the mentioned models that these parameters are dependent greatly on the length of the link beam and inadequacy of push-over analysis in demonstrating tall buildings performance are other results of this study.
آییننامه طراحی ساختمانها در برابر زلزله- استاندارد 2800 ویرایش سوم، 1384، کمیته دائمی بازنگری آییننامه طراحی مدلها در برابر زلزله، مرکز تحقیقات راه و مسکن.
مبحث ششم مقررات ملی ساختمان، بارهای وارد بر ساختمان، 1385، دفتر تدوین و ترویج مقررات ملی، وزارت مسکن و شهرسازی، ایران.