Proceedings of ISMA 2012 International Conference on Noise and Vibration Engineering pages:1145-1158
ISMA - International Conference on Noise and Vibration Engineering edition:25 location:Leuven, Belgium date:17-19 September 2012
This paper presents the vibration serviceability assessment of the Lamot footbridge. This steel footbridge was built very recently (March 2011) and is located in Mechelen, Belgium. The innovative design results in a very slender en light structure. The dynamic behavior is predicted at design stage by means of a FE model. Current codes of practice provide methodologies to assess the vibration serviceability. This resulted in a prediction of high vibration levels for normal and high pedestrian densities, indicating the structure to be lively and prone to human induced vibrations. Since the bridge is located in an urban region where dense traffic conditions are to be expected, a tuned mass damper (TMD) was included in the design of the footbridge to ensure the vibration comfort of the pedestrians.
A measurement campaign was carried out to obtain the operational modal characteristics of the footbridge. Output-only system identification is performed based on ambient vibrations (mainly due to wind), using a reference-based data-driven stochastic subspace identification. The numerical model (FE model) is calibrated by updating the model parameters such that an optimal correspondence is found between measured and calculated modal characteristics, in order to ensure the accuracy of numerical predictions of the response to pedestrian loading.
In situ vibration measurements were performed with different types of human loading (walking, running and bobbing) with groups of 1 – 5 – 10 – 34 persons. For each loading condition, the tests were performed with the TMD in operational and deactivated condition, showing that the TMD leads to a reduction by 30% of the acceleration level at mid span of the bridge. The vibration serviceability was reassessed based on the design guides, taking into account the effect of the TMD by increasing the modal damping ratio.
The prediction of modal characteristics at design stage remains a challenge because of modeling uncertainties, e.g. boundary conditions and damping values. It is the responsibility of the designer to take into account these uncertainties when assessing the vibration serviceability of a structure. In this case, the guidelines were able to alert potential lively behahiour. Thanks to the combined effect of a high modal damping and the installation of a tuned mass damper, the vibration comfort of the pedestrians seems to be ensured for the Lamot footbridge.