REDUCING VIBRATIONS IN A FOOTBRIDGE USING A SEMI-ACTIVE TUNED MASS DAMPER

Abstract eng:
The stress-ribbon footbridge located at the campus of the Faculty of Engineering of University of Porto (FEUP) is a slender structure that connects the main buildings of the university to the students’ canteen. Its dynamic behaviour has been studied over the last few years in several areas of the system dynamics. First, the modal parameters in terms of natural frequencies, damping ratios and modal shapes were identified, and a complex non-linear numerical model of the structure that takes into account the various construction phases was calibrated. Then, a comprehensive study of the analysis of the vibration levels of the footbridge involving either regular and vandal loads was developed. Given the perceptible levels of vibration that often affect that structure, an active vibration control system was implemented for research purposes. Since 2009, a dynamic monitoring system composed of 4 accelerometers and 4 thermal sensors collets time series, enabling the Operational Modal Analysis and the Vibration-based Structural Health Monitoring of the footbridge. In 2013, a Tuned Mass Damper was installed in the context of a research project related to Smart Inertial Vibration Control Systems. At a first stage, this device worked passively, tuned close to the critical natural frequency in terms of proneness to resonant pedestrian loads. More recently, the control system was updated to a semi-active functioning with the objective of increasing the system efficiency. This was done because the footbridge has several critical vibration modes around 2 Hz and only one control device was installed. Using the Phase Control law, it is possible to adjust, in real-time, the vibrating frequency of the inertial mass to the structure frequency, enabling not only the correct tuning of the device but also the possibility of performing multimodal control. After a description about how Phase Control works, this paper describes the laboratory tests of the semi-active device, which showed that the system behaved as numerically expected. Then, the implementation of the device in the footbridge is described and all equipment and instrumentation are listed. Finally, the effect of the control system in reducing the vibration levels of the footbridge over the last few years is analysed. This was done by observing the data from the continuous dynamic monitoring system from 2010 until 2016. It was noted a tendency in the reduction of the peak accelerations of some sections of the deck after the installation of the passive TMD, which were even more attenuated after the activation of the semi-active device.

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