Modelling and testing of a historic steel suspension footbridge in Ireland

Daly’s Bridge is a historic steel suspension footbridge in Ireland, known locally as the ‘Shaky Bridge’ for its noticeable movement under pedestrian loading. Although there is concern regarding the performance of the structure, testing or modelling has not been carried out to date and inadequate information exists in relation to carrying out such analyses. In this paper, Daly’s Bridge is instrumented and tested for the first time and a model of the bridge is established and improved in the process. Apart from ambient vibration, excitation from traversing pedestrians and cyclists is considered. Video analysis of dynamic deflection, a wavelet-packet-based technique using acceleration responses and dynamic measurements from a cheap smartphone accelerometer application are used to identify and compare the natural frequency of the bridge. The work contributes to the evidence base of full-scale measurements from instrumenting and analysing responses of aging pedestrian bridges, highlighting the complexity, challenges, opportunities and limitations related to the varied levels of information available from disparate sources. The study also highlights the need to investigate to what extent cheap sensors can be successfully used as compared to their more expensive and sophisticated counterparts.