Keywords: dynamic identification, footbridge, structural reinforcement, resonance, operational modal analysis.

This paper presents a numerical-experimental study of the Postiguet footbridge in Alicante, Spain. The walkway has a unique geometry and a high degree of degradation as a result of the proximity of the bridge to the Mediterranen Sea. In fact after a few years of construction, in 1993, it began to show a widespread degradation as a result of the continuous exposure to chlorides of the marine environment. In 2011 it was restored with fibre glass composite materials to improve its appearance and the response to dynamic loads.

A previous study of the walkway determined the frequencies of the structure when subject to excitation produced by pedestrians walking over the bridge. It was shown that at low excitation frequencies the structural frequencies were higher than those recommended by the Spanish code.

In this paper the dynamic behaviour of the structure under conditions prior to and following the structural reinforcement were investigated. This enabled the identification of the characteristic parameters of the dynamic behaviour of the footbridge. The values of the natural frequencies of vibration and the damping coefficients of the structure have been determined for the different modes of vibration for the two structural conditions. Operational modal analysis (OMA) techniques for the modal identification of the pedestrian bridge have been utilised by mean of the ARTEMIS 2011 software. Then a numerical model has been built using the SAP 2000 software. The analysis and upgrading of the model led to the same values of the dynamic parameters. It has been identified that the fundamental period of the structure subject to to the vertical vibrations, is 3.65 Hz, and the corresponding damping factor of 1.7%. After the reinforcement of the bridge these parameters assumed the values of 3.06 Hz for the frequency of the first mode of vertical vibration and the damping factor of 1.92%.

This shows that structural changes to the pedestrian walkway and the new materials used for the coating have changed its dynamic response: there was an increase in the damping coefficient and a decrease in the values of the natural frequencies of the free vibrations modes. This means that the Postiguet footbridge is more resistant to vertical vibrations and therefore has a better behaviour with respect to the dynamic stresses of the structure.

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