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An international group of researchers has proposed a new vibration absorbing technology to mitigate the unexpected effects of “vibrational resonance” that can cause bridges to become structurally unsound and collapse.
The research, published in this month’s issue of Physics World, suggests the installation of lightweight resonators at load-bearing points on the underside of a bridge.
The resonators, which consist of heavy, concentrated cubic masses, would be linked to the bridge by a series of bars and appear under the bridge “like a set of hanging baskets”. The resonators would be fine-tuned to the specific vibrations that make a bridge susceptible to collapsing, redirecting the vibrations into the resonators to make the bridge safer.
“Even though the Volga and Millennium bridges were designed using industry-standard packages, the fact that problems arose shows that it is all too easy with large and complicated structures to overlook vibrations that may cause structural problems under practical conditions,” the researchers write.
The vibrational resonance problem most recently occurred with the Volga Bridge in the Russian city of Volgograd in May 2010 and the Millenium Bridge in London in 2000. The dramatic bending and motion of the 7.1km Volga Bridge, which can be seen in the video below, was blamed on high winds. The bridge was closed while a series of five tonne hydraulic “mass dampeners” were installed at a series of points on the roadway of the bridge.
The excessive movement of the Millenium Bridge across the River Thames was caused by pedestrians stepping in time with the swaying movement of the bridge. The problem was corrected by installing fluid-viscous dampers and mass dampers to control movement at a cost of £5m over a period of 5 years.
The “wave bypass” technique devised by the researchers for bridges has many similarities to those being used by engineers trying to create Harry Potter-style invisibility cloaks, which exploit man-made materials known as metamaterials to bend light around objects.
The concept of a wave bypass is also found widely in nature, specifically in certain moths and butterflies, which have remarkable nanostructures that create diffracting mirrors. The mirrors funnel light of a desired colour into a vivid optical display on the organism’s body.
“It is, to us, remarkable that the principles lying behind structured mirrors, waveguides and bypass structures can be used not just by the humble sea mouse for its iridescence but also by engineers in the quest for more resilient and robust bridges,” the researchers write.
The proposed solution was developed by Alexander Movchan from the University of LIverpool, Ian Jones from Liverpool John Moores University, Michele Brun from Caligri University, Canada and Ross McPhedran from the University of Sydney, Australia.