Detailed Dynamic analysis of large-span footbridges to optimize structural control measures

Footbridges in urban areas are more and more meant to set architectural highlights and not just to connect two sides. According to that and the conference’s principles, the design becomes more and more spectacular by applying the attributes taller, longer and lighter. Despite the extreme lightness, bridge structures span more than 30 meters, cable stayed structures more than 100 meters while maintaining a filigree support structure. These complex structures display several natural frequen- cies distinctively below 5 Hz so they can be easily excited to perturbing vibrations by pedestrians or joggers due to low structural damping. Since the modal parameters of the bridge can’t get arbitrarily adapted, the application of structural control devices such as Tuned Mass Dampers (TMDs) is a common measure to ensure comfort and safety of footbridges. The effectiveness of the applied sys- tems hereby strongly depends on the specification and tuning of the TMDs.

The application of TMDs is already state of the art and has been realized at many projects, for ex- ample the Millennium Bridge in London. So this contribution will focus on the challenge to apply TMD systems to be effective for several modes that are identified to be susceptible by broadening the frequency range in which the system is effective without necessarily increasing the effective mass of the TMDs to avoid too much additional loading. This approach requires a detailed dynamic analysis to assess the dynamic response of footbridges under realistic loading according to defined comfort criteria. It also requires to modify the commonly known optimization criteria by analyzing the TMD effect for a more realistic assumption of human induced vibrations and for interaction effects.

The following paper describes calculation approaches for detailed dynamic analyses of footbridges to assess relevant susceptible vibration modes considering realistic loading group loading and syn- chronization effects. It also shows examples for adapted Tuned Mass Damper System specifications which are optimized to provide sufficient damping for several susceptible modes by abandoning the optimum specification for just ine single susceptible mode. In addition project examples will be introduced for which the preliminary theoretical analysis and experimental results of measurements will be presented.