Improvement of dynamic performances of suspension footbridges by modifying the hanger systems

Light and slender suspension footbridge structures with little damping are vulnerable to dynamic loads. Hanger systems can affect some dynamic performances. The hangers of suspension bridges can be placed in two forms: vertical or inclined form. To improve aerodynamic stability, inclined hangers can be used instead of vertical ones. Inclined hangers are more liable to fatigue. Some inclined hangers show considerable signs of distress and some of them show slackness due to their location. In this paper, a suspension footbridge with vertical hangers has been studied as a case study. For investigating the influence of hangers on the behavior of the footbridge such as natural modes and their resonance probability, vertical hangers replaced by inclined ones. Then by analyzing two models (with vertical and inclined hangers), maximum accelerations and deflections of them have been compared in order to vertical, lateral, torsional and coupled lateral, and torsional modes. To reduce internal forces, fatigue and slackness in hangers, horizontal cables have been added to inclined hangers by the present authors. The added horizontal cables transfer the tensile load from overstressed hangers to adjacent slacked hangers. In addition, these cables increase stiffness of the suspension footbridge. In this paper, some of dynamic characteristics of footbridges with inclined and modified hangers such as natural modes and frequencies, as well as vertical and lateral accelerations and deflections of the deck to compare comfort level have been investigated. Also hanger parameters such as the force, slackness, overstressing, and fatigue of inclined and modified hangers have been compared. Results showed that the modified hanger system had been improved considerably in comparison with inclined hangers.