Troubled bridges

Highways England (the UK’s largest bridge owning authority) and Network Rail are responsible for maintaining around 12,000 concrete bridges, some constructed as far back as the 1920s and to out-of-date standards.

Maintaining these bridges costs millions of pounds a year, with an associated economic cost from the lane closures during any maintenance. Highways England calculated this cost as being somewhere between £10,000 and £50,000 per lane, per day.

Finding a way to increase the working life of these ageing structures could have significant economic benefit for the UK.

Strength tests

Under existing guidelines for structural assessment, many bridges built before 1972 would have been condemned because they were deemed as being designed to insufficient standards. A lack of information about older structures also means that assessment errs on the side of caution.

The team at Bath, working together with Highways England and leading engineering consultant Parsons Brinckerhoff (now WSP) looked at the shear capacity of bridges with insufficiently anchored reinforcement. They developed a simplified and pragmatic assessment procedure using a plasticity approach. The new test showed that, in many situations, the capacity of these bridges was sufficient.

They also established a shear assessment methodology for bridges constructed using pre-stressed beams, laterally post-tensioned together, widely used by the UK rail network. Our research identified that lateral pre-stress increases capacity by over 40%. This effect had been ignored in previous assessment methods.

Necessary strengthening

A strengthening method was developed for bridges that were found to have inadequate capacity. The method, termed 'deep embedment strengthening' involved fibre Reinforced polymer (FRP) rods glued into holes drilled through the beams to act as additional shear reinforcement. The technique is more efficient and ductile than alternative methods and can be applied to slabs as well as beams.

Worldwide success

Findings from the project have led to the researchers being commissioned to write various design and maintenance guidance documents that are routinely used by infrastructure owners and consulting engineers around the world.

The research has allowed the life of concrete structures to be extended through developing proper methods for assessing existing capacity as well as the means to increase capacity where necessary. This has prevented buildings and bridges from being condemned as unfit for purpose, ensuring that our ageing structures remain in service for the present and future.

This has led to vast vast savings in reconstruction costs for the UK economy and the prevention of disruption to millions of infrastructure users.

Bath team

The team from Bath included Professor Tim Ibell, Dr Antony Darby and colleagues from the Department of Architecture and Civil Engineering.

REF submission

This research was part of our REF 2014 submission for Architecture, Built Environment and Planning.