Good vibrations for Forth Road Bridge

A team from the Cambridge Centre for Smart Infrastructure and Construction (CSIC) have designed vibration energy harvesters which convert ambient vibrations into electricity, eliminating the need for batteries and making remote monitoring of the long-span suspension bridge’s health much easier. A trial deployment of the harvesters will take place early next year.

The 2.5 kilometre-long Forth Road Bridge, which connects Edinburgh and Fife, now carries far more traffic than it was originally designed for. About 25 million vehicles now cross the bridge each year, nearly ten times the number it carried when it opened in 1964.

The increased  strain that the additional traffic load has had on the structure became apparent during a routine inspection of the bridge’s cables in 2004, when extensive corrosion was discovered in the strands making up the main suspension cables.

After the damage was identified, the decision was made to build a new road bridge alongside the Forth Road Bridge and Forth Rail Bridge. The Queensferry Crossing is due to open in 2016, and traffic on the Forth Road Bridge will then be limited to buses, taxis, cyclists and pedestrians. Although traffic load will be reduced significantly when the new bridge opens, there will still be a requirement for ongoing monitoring of the older structure.

Professor Campbell Middleton and a CSIC bridges team believe the work they are undertaking to develop and demonstrate wireless sensor network (WSN) technology may have a role to play in monitoring various key structural elements on the bridge. However, one of the problems holding back the adoption of WSN, particularly in difficult-to-reach areas like underneath a busy bridge, is the need to change batteries on a regular basis. The issue is not the cost of the batteries themselves, but rather the cost of the human power to replace the batteries.

In a bid to solve this issue, Dr Yu Jia and Dr Ashwin Seshia have developed a new vibration energy harvester based on a phenomenon known as parametric resonance, which amplifies vibrations. The device has the potential to harvest significantly more energy from ambient vibrations than previous designs, and vibration data collected from the Forth Road Bridge during a field investigation is now being used by Dr Jia to optimise the harvester for a trial deployment at this Scottish landmark early next year.

“As vibration energy harvesting improves and the amount of energy available to power sensors increases, new radio technologies are emerging with lower power requirements,” said Professor Middleton. “We may be approaching the point at which a vibration powered wireless sensor network, with no need to change batteries, becomes a reality. This would be a world first.”

The low cost, wireless, battery-free sensors will enable CSIC to measure the behaviour of key structural elements on this critical piece of infrastructure, giving its owners a far greater understanding of the actual capacity and level of safety of the bridge.

Professor Middleton continued: “The Forth Road Bridge offers a fantastic opportunity to test this innovative technology which will provide key information to the bridge owners and managers, leading to knowledge and reassurance of its on-going safety performance, which could see the Forth Road Bridge surviving a further 50 years or more.”