National Highways (NH) maintains approximately 1500 bridges across England’s strategic road network, plus numerous other steel structures at risk from fatigue damage. Fatigue cracking in bridges can require urgent repairs, such as at the Boston Manor Viaduct near London in the run-up to the 2012 Olympic Games. This is more complex than planned maintenance, costs more and often requires lengthy road closures, disrupting travelling across the network. 

Arup is developing toolkits to support NH in identifying and quantifying risks early. These will enable them to intervene before problems develop while helping keep costs down and reducing disruption to road users.  

Using our technical guidance, NH will be able to identify and proactively repair the bridges most at risk and carry out cost-effective maintenance before problems arise. Our work will be part of a new technical standard for the management of steel bridges, which will be applied across the road network and provide benefit to the wider industry.

Understanding the causes of fatigue in steel bridges 

Fatigue in steel bridges occurs over time when steel is repeatedly stressed, typically by thousands of vehicles crossing the same bridge day after day. In the worst situations, this can cause cracks to form in the structure. Fifty years ago, fatigue was not as well understood as it is today, and road bridges were not designed with it in mind. The initial stages of fatigue cannot usually be seen with the naked eye, but specialists can use modern inspection techniques to find small cracks that indicate fatigue issues.  

However, how can you predict which bridges will develop a problem caused by fatigue? Age is one factor. More than 50% of the bridges managed by NH were designed before the design code for fatigue, BS 5400 part 10, was first published in 1980. Due to this lack of guidance, some bridges may have components with a fatigue life shorter than their intended design life. 

Age is not the only factor, though. There are hundreds of different bridge designs, and even those that appear identical may differ to suit their location. Many are painted, so inspection of underlying components by eye is not easy. Each bridge has carried different volumes of traffic and heavier vehicles, resulting in increased stress levels. In addition, over the decades, records of some construction or design decisions have been lost.  

Throughout the work, Arup has been able to adapt to changes from the client without compromising on quality on the end product, which was evident in them being able to support National Highways with the rewriting of the Specification for Highway Works through this task.

Michael Lewis

Senior Structures Advisor, Asset Management, Safety, Engineering and Standards, National Highways

A straightforward, cost-effective approach to asset integrity management 

Working closely with NH and experts from The Welding Institute (TWI), Arup led this project, producing guidance in three steps:  

The first part aimed to devise an easy-to-use guidance for identifying steel bridges most likely at risk of fatigue. Our bridge engineers and materials specialists drew on our experience of analysing major bridges, such as the Rijkswaterstaat bridges, and worked with experts from TWI to develop a clear, risk-based evaluation process to identify which bridges are most at risk. NH can then inspect and intervene on each as required. 

The second part created guidance for contractors and maintenance teams. Our guidance helps NH’s teams select appropriate intervention strategies, ranging from modelling techniques to sampling and, in some cases, repair methods. It ensures that investigation teams explore the condition of each structure thoroughly, but without conducting unnecessary or overly time-consuming modelling. We have included guidance on sampling of material from bridges for material analysis, and on destructive and non-destructive testing techniques. This sets out which methods to use in each situation and what can be inferred from the results.   

The final part focused on providing clear advice for repairing bridges experiencing fatigue. With so many different types of bridge and structure, each operating under different conditions, choosing the most appropriate repair option is vital. For example, should existing welds be ground out or completely replaced? Or should we replace the whole structure? Our guidance distils our multidisciplinary expertise into clear advice so that NH can repair its assets while also conserving time, money and materials. 

A clear path to robust, affordable asset maintenance 

Our work, in collaboration with NH’s asset management team and technical specialists from TWI, will help prevent long unplanned closures and keep maintenance and upgrade costs down, limiting disruption to road users. With the ability to quantify and assess the risk of fatigue in steel bridges, NH can focus resources on the bridges most at risk and choose the right option – whether that is to monitor, repair or replace – at the right time.  

Local councils, who also look after some road bridges with a steel primary deck element, and other major asset owners are keen to understand how best to quantify their risk and prioritise their own maintenance programmes. Our work is informing a new design standard for advanced fatigue assessment, repair and strengthening techniques that will share this knowledge with the wider industry. 

Partners & collaborators

The Welding Institute