The Forth Road Bridge is a long span suspension bridge which, when opened in 1964, was the largest in the world outside the United States. In total, the structure is over 2.5 km long and was constructed using 39,000 tonnes of steel and 125,000 cubic metres of concrete. The traffic loading on the bridge is now significantly higher than anticipated by the engineers who designed the structure in the 1950’s. This is due to the sheer volume of traffic and, specifically, the increasing weight and number of heavy goods vehicles on the roads in the UK.
Having identified structural elements on the bridge that would benefit from continual monitoring in order to successfully determine their behaviour, ITM was contracted in the Autumn of 2017 to supply, install and commission a structural health monitoring system on this iconic structure.
One of the main areas of interest is the connection between the bridge side towers and the suspended side span at the Northern end of the bridge. Periodic monitoring has indicated that certain elements would benefit from this additional monitoring, as the behaviour of these elements is variable and is influenced by traffic, wind and temperature effects.
The monitoring system recommended by ITM comprises 32 spot weldable strain gauges, temperature sensors for the strain gauges, displacement transducers and 2 tiltmeters, all of which are required to have a working life of at least 10 years and function in all weather conditions. During installation, the ITM team was required to work in close proximity to the road, with access to the sensor locations via staging board where possible. For the harder to reach installation sites, roped access was required.
Tom Scott, ITM Project Manager for the Forth Road Bridge contract, commented: "The instrumentation installation phase of this project was complex, due to our team having to work in close proximity to the traffic using the bridge, which remained in use throughout. Our roped access installation capability was an important factor in ensuring we could reach the otherwise inaccessible areas of the bridge requiring sensors. This is an incredibly exciting project for ITM to be involved in as, unlike other projects ITM has worked on – where data is usually collected every 30 minutes to an hour – all of the data being recorded on the Forth Road Bridge is collected simultaneously every second, at a rate of 1Hz. We look forward to seeing how the monitoring system provides intelligence relating to the behaviour of this structure, and the highly variable effects of traffic loading, wind loading and temperature."