Adequately Rigid Cross Beams for Bridge Tower Designs

AbstractIn bridge tower designs, the cross beam provides the connection for a special lean-on bracing, which effectively enhances the lateral load transfer mechanism of the bridge tower. This study investigated the adequately rigid cross beam for a bridge tower based on elastic buckling and second-order analyses of a symmetrical portal frame. To obtain exact closed-form solutions, a classical matrix structural analysis approach considering a second-order effect was applied. First, the global structural stiffness matrix was formulated by combining the element stability stiffness matrices after coordinate transformation. Then the buckling axial load was formulated in the elastic buckling analysis. The lateral stiffness and bending moment of the bridge tower were formulated in the second-order analysis. These structural behaviors were then plotted in relationships with the cross-beam rigidity for given column inclinations. The tower lateral stiffness and the column base bending moment owing to the increase in the cross-beam rigidity had an asymptotic trend approaching the behaviors associated with an infinitely rigid cross beam. Considering this asymptotic trend, the adequately rigid cross beam requirements for the stiffness and strength behaviors were formulated and recommended for bridge tower designs. A detailed design procedure and an elementary design example are presented. Design implications for multiple-cross-beam bridge towers and bridge piers are also noted.