Characterization of Polyester-Rope Suspended Footbridges

Abstract

In rural parts of the world, lack of access to roads that are useable year-round significantly contributes to poverty. Suspended footbridges can improve access at locations that require medium span crossings (15 m to 64 m). This dissertation challenges the idea that modern bridges of this type must use steel rope, a well-established material for this application. Polyester rope, an unconventional bridge material, is investigated as an alternative to steel rope for rural suspended footbridges. The specific goals of this research are to: (i) characterize the static and dynamic behavior of polyester-rope bridges and (ii) determine which design criteria and system parameters will influence future design guidelines for these structures.

Numerical and analytical, static, natural frequency, and pedestrian excitation computations are performed to investigate the influence of polyester rope’s material stiffness on the static and dynamic response of polyester-rope suspended footbridges. Polyester rope’s low stiffness leads to larger static bridge deflections than occur for steel-rope structures. These deflections are accompanied by a nonlinear increase in a bridge’s geometric stiffness and lead to high levels of safety against overloading. Polyester rope’s low stiffness also requires that these bridges be prestressed to meet static and dynamic serviceability (pedestrian comfort) limits specified in footbridge guidelines. The damping ratios that are utilized in the pedestrian excitation analyses follow from the first set of full-scale physical tests that have been performed on a medium-span polyester-rope bridge.

Multi-objective optimization is utilized to find minimum volume polyester-rope and steel-rope suspended footbridge designs across the medium span range when subject to in-plane static and dynamic strength and serviceability constraints. The optimization problems are evaluated with a novel methodology that combines a genetic algorithm with static, natural frequency, and pedestrian excitation analyses. The impact of cross-sectional area, material stiffness, prestress, damping, mass, and stiffening stay elements on rope volume requirements for these bridges are investigated. Minimum volume results are presented graphically as functions of span to provide visual design aids that can be included in future bridge guidelines to facilitate comparisons between different systems under a range of constraint combinations.