# Effect of Vinyl Flooring on the Modal Properties of a Steel Footbridge
## 1 Introduction  
Modern footbridges are prone to vibrate under pedestrian-induced loading, due to two main factors: their first natural frequencies usually lie inside the frequency ranges which characterize the rhythmic human activities [1] and they usually exhibit low damping ratios [2]. Several examples have been reported in the literature about slender footbridges which have suffered from pedestrian-induced vibrations [3, 4]. Two methods are usually used to control the vibratory response of slender structures under pedestrian action [2]: either (i) to modify the natural frequencies of the structure; or (ii) to increase the damping ratios of the structure (or both). The first method may be applied successfully during the design phase of the footbridge [5] or after its construction when the structural modifications are compatible with the aesthetic and functionality of the footbridge [6]. The second method is usually applied to existing structures or it can be also applied during the design phase if the slenderness of the structure makes impossible to control its response via the modification of either its stiffness or its mass [7]. The two usual options to increase the damping of a footbridge are [2]: (i) to take advantage of the damping introduced by non-structural elements; or (ii) to install external damping devices [8; 9]. While the second option has been studied, analyzed and applied in numerous cases [10], the first option has received much less attention according to the best knowledge of the authors. In this sense, the main motivation of this work is to shed some light on the use of non-structural elements as passive damping systems in order to reduce the pedestrian-induced vibrations in slender steel footbridges. The effective implementation of this alternative has two main advantages: (i) a smaller cost, when compared to the conventional external damping devices; and (ii) a better integration with the aesthetics of the footbridge. This study was initially motivated by the unexpected high experimental damping ratios of a real footbridge, the Jorge Manrique footbridge (Murcia, Spain). This footbridge, a bowstring arch structure, suffered from comfort problems (falls of the users and pedestrian-induced vibrations) since its opening [11; 12]. In order to improve the adherence of its deck and thus solve the problems of pedestrian falls, a vinyl flooring was installed on the structure. Unexpectedly, the placement of the vinyl flooring not only avoided falls among users but also reduced the pedestrian-induced vibrations. This last fact motivated the study of the effect of the vinyl flooring on the modal parameters of the structure. The modal parameters of the structure were identified experimentally and it was checked that the experimental damping ratios of the footbridge were unexpectedly higher than the values usually recommended by the most advanced design guidelines to assess the vibration serviceability of footbridges [13; 14]. Due to the unexpected experimental damping ratios of this footbridge, the high vibration damping properties associated with vinyl polymers [15] and the growing practice of using vinyl flooring as pavements of footbridges [16], the authors decided to conduct a more detailed study on the effect of vinyl flooring on the damping ratios of steel footbridges. In order to address this study and as it was not possible to remove the vinyl flooring of the Jorge Manrique footbridge due to service requirements, a laboratory steel footbridge with vinyl flooring was designed and built. A comparative method, successfully proposed by other authors to estimate the effect of non-structural partitions on the modal properties of floors [17], has been adopted herein to estimate the damping ratios associated with this non-structural element. The estimated damping ratios of the laboratory footbridge increased from an average value of 0.328% to a value of 2.397% after the vinyl flooring was installed.