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In the present paper, the behavior of eccentric braced steel frames with thin infill plates is investigated. The main purpose is to provide a new form of eccentric bracing, which improves the seismic behavior by adding a thin steel plate under the link beam. In the proposed model, due to the increase in frame stiffness, flexural stiffness and shear of the beam in the bracing frame will not respond to the forces. Therefore, in order to provide the required stiffness and ductility of the frame, it is suggested to use a steel plate under the link beam connected to the bracing connection plates. In the proposed models, two groups of models with steel plate under the link beam (middle steel plate) have been studied. In the first and second groups, all analysis are of static type, taking into account the geometric nonlinear effects in an eccentric frame with infill plates, the height of the plate under the link beam (middle steel plate) is 460 and 742 mm, respectively. The studied parameters include the height of the middle plate, the thickness of the middle plate and the effect of the arrangement of stiffeners on the performance of the frame. For numerical analysis of the models, the finite element method using Abaqus finite element software with increasing load has been used. Extraction results in the models include force-displacement curve, stiffness decay, amount of wall out of plane displacement due to buckling, inelastic dissipation energy and stress distribution in the structure (stress contour). According to the results of numerical models, the middle steel plate is very important in providing ductility and increasing the strength of the structure. Also, with increasing the height of the middle plate, the development of the diagonal tensile field into the infill plate increases, so local buckling can be converted into general buckling in the infill plates. Among the arrangements of stiffeners, it was observed that the stiffener under the middle plate has the least effect on increasing the force-displacement response of the structure. By evaluation of models with a middle plate with a height of 460 mm and two vertical stiffeners, compared to the model with four stiffeners, the structural capacity (force-displacement) has increased by about 31%. By evaluating the models with a middle plate height of 760 mm, it can be found that the use of stiffeners with different geometric arrangements do not have a major effect on increasing the stiffness in the elastic and inelastic stages, prevent the sudden decrease in stiffness in models without stiffeners due to buckling observed at the junction of the brace to the middle plate. Also, the free edge stiffener of the middle plate has practically no effect on the sudden decrease in stiffness, but on the other hand, vertical stiffeners or a combination of horizontal and vertical stiffeners have performed well in terms of preventing a sudden decrease in structural stiffness. The thickness of the steel plate has a significant effect on increasing the strength and reducing the local buckling in the middle plate. Local buckling was observed at the junction of the middle plate to the brace, which is recommended to use stiffeners for the middle steel to reduce the effects of local buckling of the plate and to prevent a sudden decrease in strength and stiffness. In steel plates with shear behavior that do not allow the complete formation of diagonal tensile fields, stiffeners prevent a sudden decrease in strength but do not have a significant effect on increasing the overall stiffness of the structure.

Keywords: Steel Frame, Eccentric braced, Steel infill plate, Finite element, Posh over loading, Ductility

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