Simplified Analysis for Preliminary Design of Towers in Suspension Bridges

Abstract

This paper proposes an equivalent suspension bridge model for the preliminary design of support towers of suspension bridges. In this simplified model, the bridge is composed of its support towers, with equivalent springs replacing suspension cables and girders. An explicit formula for the spring stiffness was derived for the horizontal stiffness at the end of an inclined main cable and compared with Ernst's formula as well as finite-element analysis results. For the analysis of the model, dead and live loads acting on the girders of each span were replaced with horizontal and vertical forces calculated using the deflection theory and placed on the tops of the support towers. Equilibrium equations were derived from free-body diagrams for each tower and their solutions were obtained from boundary conditions at the ends of the towers. This resulted in the formulation of nonlinear load-displacement equations in matrix form for the simplified bridge model. An example four-span suspension bridge was analyzed for displacements at the tops of the towers and moment reactions at the bottoms of the towers under six live-load cases. A comparison of the model predictions with a FEM analysis showed excellent agreement. The results prove that the proposed analysis method can be a useful alternative for the preliminary analysis and design of suspension bridge towers.