Concentrically braced frames with X-bracing configuration (X-CBFs) are one of the most popular systems for seismic resistant design. Their ductile behaviour is provided primarily by yielding of braces in tension and to a small extent by plastic rotation in buckled braces. An improved CBF configuration by introducing bracings with modified sections was developed in research program. In order to facilitate joint detailing, it is efficient that braces intersect into a horizontal member named "splitting beam". Experimental study has shown that braces, being above and below the splitting beam, often do not buckle simultaneously. This leads to concentration of inelastic deformations within one of the brace and causes unbalanced horizontal and vertical force. Finally, it might result in reduced storey ductility and premature brace fracture. It was assessed through extensive parametric non-linear analysis that splitting beams in cooperation with columns have crucial impact on the brace buckling sequence and on the type of the global plastic mechanism. An analytical model and design procedure for determining the stiffness and strength demand of splitting beams and braced frame columns are proposed. The procedure ensures achievement of buckling of both halves of compression braces and formation of anticipated plastic mechanism. Some case studies based on static nonlinear analysis are presented.
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