Vibrational performance of pedestrian bridges due to human-induced loads
The vibrational performance of footbridges due to human-induced loads has been investigated, based on modal and pedestrian tests carried out on three prototype footbridges. Analyses using calibrated finite element models of these structures were also conducted. All test structures presented natural frequencies within the range of excitations produced by pedestrians and were therefore suitable for investigating the applicability of some current guidelines for vibration performance. In addition, the inclusion of a footbridge made of glass reinforced plastic in the test programme enabled the performance of this new type of footbridge construction to be investigated. The techniques of ambient excitation, impulse response using an instrumented hammer, and free-vibration decay were employed to obtain the modal properties of the test structures. The practicalities of using these techniques are discussed and improvements in their application are suggested. Very good agreement was obtained between the experimental and the numerical results. The calibrated numerical models were employed to investigate ways of removing the natural frequencies of the structures from the common range of pedestrian excitation, thereby improving their vibration performance. The handrails were identified as a potential way to increase the stiffness and thus the natural frequencies of a structure. In addition, use of a catenary shape or pre-camber in combination with horizontal restraint at the bearings were also shown to be useful for increasing natural frequencies since beneficial axial effects are introduced. In the case of the glass reinforced plastic footbridge, it was shown that a selective distribution of mass that could be conveniently added within the cells of the deck was the best strategy for frequency tuning. Guidelines for vibration performance are suggested, focusing on the definition of the pedestrian load and frequency ranges of interest, acceptability limits to vibration, treatment of multi-frequency component vibrations and vandal loading.