Use of Portable Tuned Mass Dampers for Vibration Control of Pedestrian Bridges

Emily Bell

Abstract


Use of high strength materials, optimized computer analysis and design, and tendency of architects to design long-span footbridges has resulted in many cases of such structures having excessive vibrations due to pedestrian movements. One method of correcting the problem is the use of tuned mass dampers (TMDs). A TMD is a device consisting of a spring, mass and a damper that is be placed on or inside a footbridge to counteract its unwanted movements. This research attempts to develop a small portable TMD (PTMDs) from commercially available components to minimize the fabrication and installation costs. This study describes the details of the PTMD and its application on a pedestrian bridge with annoying levels of vibrations created by pedestrians. It attempts to answer the main research question: “Can the developed PTMD be a viable means of correcting vibration problems in pedestrian bridges?” The developed PTMD consists of a weight box with inserted plates, springs and adjustable damper, so that the mass and damper could be easily tuned on site. The PTMD (approximately 130 lbs) was installed in a pedestrian bridge. The bridge’s resonance frequency was measured using an electrodynamic shaker, and a number of accelerometers connected to signal analyzer. The TMD was first roughly tuned to the two modes of vibration by placing steel insert plates into the weight box. It was then placed on the footbridge and after exciting the structure using the shaker, the PTMD was fine-tuned by adjusting smaller insert plates and the damper element. Once the PTMD was tuned, it was locked and the test was repeated using the electrodynamic shaker and an individual walking over the footbridge at the structure resonance frequencies. A computer model of the structure was also created to predict the effectiveness of the PTMD. From this study it was found that Portable Tuned Mass Dampers (PTMDs) can provide an effective means of alleviating the excessive movements of footbridges at a lower cost and higher convenience to users. Even though the PTMD had only a very small mass compared the total mass of the structure, it could significantly reduce the vibration generated by a shaker and a pedestrian. From this study is was also found that the changes in the natural frequency of structure due to temperature variations can present a challenge as this may cause the off-tuning of the PTMD and prevent optimal operation. It was concluded that for such cases, Multiple Portable TMDs (MPTMDs) should be used for which each PTMD can be tuned to a different frequency within the range resulted from temperature changes.

Keywords


Vibration, Portable Tuned Mass Damper, Footbridge

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