Modal testing of a 34m catenary footbridge

There is an increasing need for modal testing of full-scale footbridge structures which reside in open-space environments and are in operation. Very short duration of modal testing and demand for high quality of estimates of the modal properties are the usual, but obviously conflicting, requirements. In circumstances like these and considering the plethora of modal testing methods, it is advantageous to know which is likely to perform satisfactory and quickly produce desirable results. This paper demonstrates a modal testing procedure which produced good quality results in less than four hours of field work. The procedure is based on FRF estimates from a transient chirp excitation having energy between 1 and 39 Hz which was applied by an electrodynamic shaker during the first 5 s of a 25 s long data acquisition block. Multi-channel data acquisition coupled with MDOF parameter estimation performed in-situ proved to be crucial for the success of the exercise. Somewhat surprisingly, it was shown that otherwise lively catenary footbridge tested had no lateral modes excitable by normal walking. The only relevant modes were pure vertical and lateral-torsional modes where the lateral movement was almost negligible. All relevant modes of vibration were possible to be excited by a vertical shaker excitation, and, apart from taking precious testing time, the lateral shaker excitation did not yield any additional relevant information about vibration behaviour of this structure.