Making bridges last

University experts have studied one of Britain’s most prominent landmarks in a bid to discover how the latest satellite technology could make bridges last longer and remain safe for motorists, pedestrians and cyclists.

The team from The University of Nottingham’s Institute of Engineering Surveying and Space Geodesy (IESSG) and Brunel University spent 48 hours on the Forth Road Bridge, which links the Lothians to Fife in Scotland, to find out how the bridge’s movements were affected by both heavy traffic and other environmental factors like the wind.

Their findings could enable engineers to assess whether bridges that were built decades ago are still safe to be used by traffic and if they need to be modified to prolong their lifespan.

The structure of a bridge is designed to move fractionally when a load, such as the weight of traffic or a gust of wind, is applied. Engineers can use precise measurements and calculations to build a computer model showing how bridges should be moving under safe and normal circumstances.

However, over the last few decades traffic on Britain’s road has increased dramatically, putting a strain on bridges that were never designed to cope with such heavy loads. This loading increase could cause bridges to move significantly more, which could have potentially serious consequences for the safety of the structure.

The collaborative project is aimed at giving the Forth Estuary Transport Authority, which maintains the Forth Road Bridge, a clearer picture of how the bridge is coping with these increased loads by accurately measuring how much it is moving by using small GPS receivers. The techniques and technology are capable of measuring the 3-D positions of the GPS antennas to an accuracy of a few millimetres and at a rate of 10 Hertz.

A network of seven receivers were placed at specific points on the bridge, along with two off-bridge reference receivers, relative to which the bridge receivers were positioned. Using satellite signals, the academics were able to continually plot the position of the receivers, which allowed them to build up a detailed picture of how the bridge was moving.

Dr. Gethin Roberts, of the IESSG, said: “At about 1am on the second day we were there, we stopped the traffic and had two 40 tonne lorries drive over the bridge in different formations to see how the bridge behaved under their load. Interestingly, the biggest movement from the bridge came not from the lorry but from the build up of traffic waiting to cross behind it."

“Luckily, and completely coincidentally, on separate occasions we were also able to observe a 100 tonne lorry crossing the bridge and how it behaved in gusts of winds up to 60 mph.”

When the Forth Road Bridge opened in 1964 it was the longest suspension bridge in the world outside of the USA. Traffic has risen steadily from four million vehicles to more than 23 million by 2002. In 1964, the heaviest commercial vehicles weighed just 24 tonnes but, due to the increasing weight and number of heavy goods vehicles on UK roads, the traffic loading experienced on the bridge is double that expected by engineers who designed the structure at the end of the 1950s.

Consequently, a large capital programme to strengthen the bridge to take these increased loads is continuing. Dr. Chris Brown from the School of Engineering and Design at Brunel University said: “The GPS network on the bridge has enabled us to obtain really good quality data over prolonged periods."

"Working in collaboration with consulting engineers at WA Fairhurst and Partners in Glasgow, we have already been able to establish from their estimate of the performance of the structure and our measurements that the bridge is moving roughly in line with expectations. We will now also be able to carry out more detailed calculations to show how wind and other environmental factors affect the behaviour of this particular bridge.”

The methods outlined by the study could offer a more permanent monitoring system that would allow engineers to assess the way in which bridges are moving over a long period of time under different loads and to identify even small changes in performance. This could help engineers to decide what maintenance works are needed to ensure they remain safe.

The research follows on from a larger project entitled A Remote Bridge Health Monitoring System Using Computational Simulation and GPS Sensor Data, funded with £310,000 from the Engineering and Physical Sciences Research Council. As well as the data collected from the Forth Road Bridge, the Nottingham academics carried out similar investigations at the Wilford Suspension Bridge in Nottingham and collaborated with Brunel University on the Millennium Bridge in London and the Humber Bridge in Hull.

The IESSG is a European Union-funded Marie Curie Training Centre, the only one in Europe in the field of satellite positioning technology. The data from the research project will also be used by research students visiting the institute from other parts of Europe.