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Braces systems have been commonly used as load-bearing systems in steel structures due to their easy and rapid construction and erection, reduced amount of materials used for the lateral load-bearing system (as compared to the shear wall system), and higher conformity with structural deformations at upper stories Buildings in seismic regions are prone to severe damage due to large deformations and low capacity to absorb energy. X-bracing is effectively employed in maintaining the structure stability when earthquakes occur or extreme lateral loads act on the structure. The widespread use of X-bracing systems in steel structures reveals the importance of understanding mid-connections in this kind of braces. When applying a lateral force on X-braces, one structural member comes under tension while the second one is under stress. Buckling occurs to the member subjected to stress and when effective length factor reduces, the member handles a more critical load. X-brace effective length factor is not only influenced by end connection forces but it also depends on mid-connection and its implementation. If the stiffness of tensile bracing is considerable enough, the mid-connection can provide the interaction between the two braces. In fact, should the mid-connection have enough bond, stress brace buckling will be in the second mode. In this state, when the mid-connection slightly changes into a lateral sway, it stops and the stress bracing deforms as double-curved. Nevertheless, in its ideal state, the mid-connection lateral displacement is considered zero. This study aims to design based on Abaqus 3D software the two new mid-connections, flanged joint, and yielding ductile shear panel. Their buckling and post-buckling were also analyzed using nonlinear geometry and materials. In flanged joint, one member is bonded and two other ones are connected to this bonded member. Interior stiffener is used to prevent this bonded member from crushing. Furthermore, in new X-bracing systems instead of mid-connection sheets a yielding ductile shear panel is used. This sample was examined in two modes with and without stiffener. The results indicate that flanged joint increases the critical load and consequently decreases X-bracing effective length factor. This joint also leads to brace buckling coordinate or as it is known as X-bracing second mode buckling. This kind of buckling increases the ultimate strength and decreases displacement outside of bracing member sheet compared to that of the common joint. Furthermore, yielding ductile shear panel connection increases the X-bracing critical load as well as decreases X-brace effective length factor. The using of stiffener in this kind of connection improves loading and reduces X-brace effective length factor. The connection along with the stiffener comes with a 32 percent increase of the X-bracing ultimate strength due to the yielding of the shear panel and the lack of brace buckling. Also, this connection brings about a 23 percent increase in energy adoption by bracing frame. It is important to note that both connections increase the X-bracing behavior factor compared to the common connection. Yielding ductile shear panel connection not only shows the best stress performance among the connections but also with the lowest costs the X-brace can be repaired by changing mid-connection after the earthquake.

Keywords: X-Brace, Effective length factor, Buckling and Post-Buckling, Flanged joint, BDSP

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