Practical Suggestions for Specifications for the Vibration Serviceability of Footbridges Based on Two Recent Long-Span Footbridges

Abstract

Human-induced vibration serviceability often governs the dynamic design of modern footbridges. Many design specifications have been issued, applying different load models, response calculation strategies and comfort limits (three key aspects), which may inevitably cause inconsistent evaluation results for the same structure. It requires the comparison of specifications, and better practical suggestions proposed, to guide the vibration serviceability of footbridges. This article performs the most comprehensive evaluation of specification performance. It includes six current specifications: the UK NA, the HiVoSS guidelines, the Sétra guidelines, the ISO standard, the American guide and the Chinese standard. After comparison of the three key aspects, practical suggestions are proposed. Evaluations are made based on two recent long-span footbridges in China. The footbridges are found to be low in damping ratios and there exist low and close natural frequencies, even closely-spaced modes. Vibration serviceability assessments show obviously inconsistent results owing to differences in the three key aspects. Furthermore, owing to contributions of closely-space modes, the total structural responses are significantly larger than the single dominating mode (often applied in response calculations according to the specifications). Thus, the article suggests considering the contributions of multiple modes in designing for the vibration serviceability of long-span footbridges. Finally, considering large vibrations, mitigation measures are applied. Instead of applying external dampers, e.g. tuned mass dampers (which may add to the economic costs of the structure), effective vibration mitigation measures are suggested by changing structural parameters. Except for the traditionally applied wind-resistant cables, a steel box girder with infilled concrete is applied to the main girder, replacing the original design. As a result, vibration amplitudes are significantly reduced. Footbridges are also made less sensitive to human-induced excitation.