Determining the Middle Tower Stiffness Value in an In-Plane Double-Cable Triple-Tower Suspension Bridge

Abstract

Determining the stiffness of the middle tower is a key problem in designing multispan suspension bridges. In this study, the expressions for the upper and lower limits of the middle tower stiffness of double-cable triple-tower suspension bridges were deduced by implementing the antiskid safety factor and the deflection-to-span ratio of the main cable as the control indexes in an analytical algorithm to determine the appropriate stiffness of the middle tower. The formulas obtained were verified by finite-element models, and the results showed good agreement. Furthermore, the formulas can be used to quickly select the type of middle tower for a double-cable multispan suspension bridge at the initial design phase. The value ranges of the middle tower stiffness were compared between the double-cable and conventional multispan suspension bridges, and the results showed a large difference between the upper and lower limits of the middle tower stiffness in a double-cable system; the middle tower stiffness of the double-cable system was more likely to simultaneously meet the requirements of structural deformation and skid resistance of the main cable, and the value range of the middle tower stiffness can be expanded by increasing the sag-to-span ratio of the bottom cable or decreasing the ratio of the top cable. The lower limit of the middle tower stiffness of the double-cable multispan suspension bridge is smaller than that of the conventional bridge, which allows the flexible tower to be incorporated into the design of a double-cable system.