Study of Using steel angle to Upgrade the Seismic Behavior of Centrically Braced Frames

Abstract: Seismic Behavior of Concentric brace frames has been one of popular topics in earthquake engineering. Relatively low cost and the ease and speed of implementation has led to the widespread use of these braced frames but past earthquakes experiences show inappropriate behavior, inability to dissipate high energy and the lack of ductility. This paper presents a new mechanism by combining the steel angle and slot in brace member to improve the seismic performance and postponing the buckle. Stresses and displacement of structures using nonlinear static and dynamic analysis by finite element software "ABAQUS" are evaluated. At first In order to verify the results and ensure the implementation details and parameters used in the numerical model, results of laboratory test under cyclic loading were compared. Evaluation hysteresis loop obtained from numerical analysis and experimental results show suitable match. then, for nonlinear static analysis, seven diagonal brace model were created and tested. In the first specimen, the normal braces with no particular change, modeled and evaluated. in the second to fourth models, slot were created near the gusset plate with various dimension to reduce the axial load capacity of brace to less than it's buckling load. In the fifth model to evaluate the performance of steel angles in cyclic loading, brace section was perfectly cut and double angle used to connect two parts of brace for transfering axial load. Finally, in the sixth and seventh models, in the hope that achieving suitable seismic behavior, combination of steel angle and reduced brace section were used. Results indicate improved seismic performance and ductility of CBF systems. Concentration of inelastic response in steel angle results in high energy dissipation and prevents from nonlinear behavior in other elements. In addition, comparing the hysteresis loop of proposed model with that of normal braces shows symmetric and stable rational behavior where strength and stiffness degredation is not seen in the displacement up to about 2 cm while the normal brace buckles in about 1 cm. After that, In order to investigate the behavior of the system under seismic loading, dynamic time history analyses using the horizontal component accelerograms of the Imperial Valley, Loma Perieta and Kobe earthquakes were performed. According to the results of the nonlinear static analysis, proposed sample was chosen as a specimen with acceptable behavior and suitable ductility. Therefore, in this section regarding the long time duration on dynamic analysis, its seismic behavior was compared with the normal brace. Comparison of results obtained from three seismic records, demonstrates less input energy and base shear and appropriate seismic behavior of proposed model due to sensible stiffness reduction of proposed brace. It should be noted that due to the appropriate results obtained in numerical analysis, specimen fabrication and experimental work to verify the results in the next stage of research should be on the agenda. Keywords: Slit brace, steel angles, nonlinear static and dynamic analysis, energy dissipation, ductility. Keywords: Slit brace, steel angles, nonlinear static and dynamic analysis, energy dissipation, ductility. Keywords: Slit brace, steel angles, nonlinear static and dynamic analysis, energy dissipation, ductility.