Optimal bracing system design for funicular twin arches against out-of-plane buckling

Twin arch bridges are often designed with a bracing member system that can significantly enhance the out-ofplane stability. This study presents an exact matrix stiffness method (MSM) for the out-of-plane buckling analysis of funicular twin arch bridges and investigates the optimal bracing system design. A novel 14 x 14 s-order element stiffness matrix of three-dimensional beam-columns was used in the MSM and the out-of-plane buckling analysis was performed after assembling the exact global structural stability stiffness matrix. The influences of the bracing location, number, length, and flexural rigidity of both the transverse bracing and X-bracing systems on the buckling capacity of the twin arch system were investigated. The in-plane stiffness of the bracing system (i.e., the horizontal flexural rigidity in a transverse bracing system) was found to be the critical factor in suppressing the out-of-plane buckling of the twin arch system. In addition, the twin arch system achieved the maximum out-of-plane buckling capacity with the bracing location at S/4, where S is the total arc length. For optimal bracing system design of funicular twin arches, it is recommended that one uses the more economical Xbracing system or a transverse bracing system with adequately rigid bracing members.