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Future of Bridges | Maintenance in the spotlight

Humber dehumidifaction gantry

Collapse of the Polcevera Viaduct in Genoa has thrown bridge maintenance into the spotlight, after near misses like at Hammersmith in the UK, with engineers calling for more rigorous, predictive regimes.

The catastrophic failure of the Polcevera Viaduct this summer sent shockwaves around the world and left the civil engineering sector reeling.

The dramatic scene and devastating news of the 43 people who died as a result jolted many into asking searching and difficult questions about engineering competency.

Just like the Grenfell Tower disaster a year earlier, the collapse of the bridge in Genoa, Italy, has served as a stark reminder that while the developed world – and the UK in particular – has world leading engineering expertise – complacency must never set in.

Asset management regime

A more complete understanding of the triggers that led to the Polcevera collapse will hopefully be forthcoming in the fullness of time. Already attention has focused on why this was not foreseen. Among the important questions is what monitoring and asset management was in place to ensure critical problems were anticipated and mitigated, particularly given the reported maintenance work that was being undertaken around the time of the collapse (see box overleaf).

The issue is far reaching and one area that has been neglected in the past is the whole life behaviour of bridges, engineers tell New Civil Engineer.

“So far, we have been building assets pretty much in a ‘set and forget’ mode,” says University of Cambridge Laing O’Rourke lecturer of construction engineering Ioannis Brilakis.

“Bridges have been built with little to no consideration of how to monitor their condition, how to repair them, how to replace components etcetera. Now that many bridges are reaching the state of being ‘deficient’ or ‘not fit for purpose’, lots of people are starting to think about this.”

Ej whitten bridge full

Ej whitten bridge full

EJ Whitten Bridge

The US problem is a case in point. It has a staggering 614,387 bridges, of which almost four in 10 are at least 50 years old and 56,000 are considered to be structurally deficient, as reported in the most recent study by the American Society of Civil Engineers (ASCE) via its 2017 Infrastructure Report Card.

This is not strictly news – the US problem has been discussed at the highest levels of industry for well over a decade and there has been some political will and funding in place to attempt to deal with it. However, the scale of the challenge is still huge.

“There is a growing awareness of the cost to the economy of the problem of bridge maintenance,” says Aecom vice president Barry Colford. Colford was, until 2015, bridgemaster for the Forth Road Bridge in Scotland and now leads Aecom’s complex bridge preservation practice from Philadelphia – so has a close eye on the issue.

“There is an ongoing discussion about what is meant by ‘structurally deficient’ but it is fair to say, there is a problem,” he says. “While the number of bridges that are in such poor condition as to be considered structurally deficient is decreasing, the average age of America’s bridges keeps going up and many of the nation’s bridges are approaching the end of their design life.”

Increased understanding is vital

Increasing understanding of how bridges behave in their working life and how that impacts their moving parts is vital. “One strand to the debate is the consideration needed around what the vulnerable elements of a major bridge are that might need attention,” says consultant Cowi UK executive director David MacKenzie.

“Bridge engineering is an evolving field,” he points out. Fire risk mitigation, strain on bearing deterioration, improved materials enhancing stay cable design and rail track bed systems are all areas where engineering knowledge has been increased; yet they are all areas where more learning is still needed.

“We have to change our way of thinking about inspection and monitoring,” says Colford. “We need to think more about identifying the components of the bridge that have the highest risk rating and select them for monitoring and inspect them more frequently. Conversely, low risk rated elements may only need to be inspected every 10 maybe 20 years. In this way we focus resources on where the risk is greatest.”

Ensuring that future stay cable design is as flexible as possible is one great example. Obvious immediate but already outdated learnings relating to Polcevera include ensuring that no cables are encased in concrete where deterioration can be far too hidden – an issue brought into sharp focus on the Hammersmith flyover closer to home.

Severn crossing exposing cables for inspection

Severn crossing exposing cables for inspection

Severn Crossing: exposing cables for inspection

In addition, wider deployment of multi-strand systems that enable strand by strand replacement rather than cable by cable replacement of stay cables would also be beneficial. And increasing awareness of, say, the behaviour of rail track slab, which acts as part of a bridge structure, is important, explains MacKenzie.

In the US, research work in recent years for the Federal Highways Authority with the Transport Research Board has harnessed best practice and generated guidance such as the Design Guide for Bridges for Service Life.

Colford says the work was in direct response to US transport bodies’ realisation of the importance of designing to ensure that service life is achieved. This realisation came as  the direct and indirect costs, traffic delays and disruption, were beginning to impact on the political conscience after years of neglecting the issue.

“We are getting better at designing more durable bridges, but one area that has not been looked at with as much attention is our ability to integrate inspection and maintainability at the design stage,” he warns.

MacKenzie agrees: “In the past there was no real thought about how to replace bearings, for example,” he says. “Right now, we’re in a better position than being nowhere near resolving [such issues], but we’re not far enough along the way to solving them.”

Bridges have been built with little to no consideration of how to monitor their condition, how to repair them, how to replace components

Consideration of the maintenance, operation and repair of bridges is increasingly vital in upfront design work, he says. “A big part of it is designing for access – can you walk to where any work needs to be done, or can you park up safely, for example.”

This challenge is made more taxing by the more stringent but perfectly reasonable health and safety considerations in today’s construction environment. “What we consider acceptable from 20 or 30 years ago is quite different to today,” says MacKenzie.

As a result, engineers like  MacKenzie are innovating ways to provide access to as many components as possible that may need human intervention via the deck elements. Enabling inspection work to take place without bridge closures while also removing the risk of trying to access components near moving traffic can be done, says MacKenzie, and it makes more robust maintenance and repair regimes more attractive to bridge operators because it makes it less costly and operationally challenging.

This issue is at the heart of work Cowi is undertaking as part of a £1.3bn upgrade to the 518m long EJ Whitten bridge in Melbourne, Australia.

Filling the gap between two decks

The upgrade includes adding deck in the gap between the bridge’s twin decks. Work is going beyond what is structurally necessary to upgrade the bridge. Designers have considered practical details such as ensuring there are no major physical hurdles to climb over within the deck itself, as there often are in many existing bridge decks.

Technology is also helping engineers solve the access and wider monitoring conundrum. This ranges from the obvious – such as drones that can provide footage of hard to see elements – through to increasingly small and cost-effective monitoring gadgets.

Advances in acoustic monitoring and dehumidification in the past decade, have been used to great effect on major bridges such as the Humber, Severn and Hammersmith Flyover.

Acoustic monitoring is the most effective long-term monitoring regime that can be applied to the main cables of suspension bridges suffering deterioration and lost strength due to wire cracks and breaks, says Colford. Meanwhile dehumidification is the “only effective method of preventing further deterioration and strength loss in main cables”.

“This has been shown by the results from UK suspension bridge work,” he says. “In addition, once a main cable has been dehumidified the relative humidity, temperature and flow of the air exhausting along the cable is constantly monitored.” All of which sharpens the focus on effective monitoring.

Which is not to say that it is fine for monitoring to continue to be simply a retrospective add on.

Sharing problems is something we absolutely need to do more, so we can learn from any mistakes

“The more data we have the more effective our decision making can be,” says Colford.” However, it has to be done well. “A lot of monitoring on bridges is carried out following the manifestation of a problem … We are reactive rather than proactive.”  

The reason for this is cost, he adds. “The equipment needs to be maintained and inspected; we need a long-term commitment to recording, storing and using the data in a meaningful way and perhaps most importantly, what do we decide to monitor?

And while most long span bridge builds now incorporate some monitoring: “Whether the right amount and form of monitoring is being carried out is worthy of more study,” he states.

“The way forward is through ‘digital twinning’,” says Brilakis – where a digital model resembling the real-world asset is created at the earliest opportunity. “We are approaching a point where we have the technology to build virtual replicas of the real assets and connect the real with the virtual so that problems are sensed and reported in near real time,” he says.

Feeding into the model

MacKenzie agrees, and adds that even building problems that were faced during construction into the model would provide greater knowledge of the intricacies of the living asset. “You would potentially have knowledge about challenges earlier on, added to which later inspection results can be fed into that.”

All agree there is an increasing appetite for this. MacKenzie cites an example of how a client from India’s railway bridge sector recently visited. “They were recognising the huge benefit that digital engineering and building information modelling offer to maintenance,” he explains.

This is perhaps a step change. If countries with a history of less than admirable safety records are now turning their minds to future proofing their bridges, it is high time the whole world made a concerted effort.

Highways England is certainly making efforts to appear more open to learning. Its immediate response to the Polcevera collapse was to launch a three month review of its bridge maintenance regimes as a “precautionary measure” being undertaken “simply because that’s a sensible thing to do”, in the words of chief executive Jim O’Sullivan.

The actions are wise. “Bridge engineering is an evolving field,” says MacKenzie, urging the industry to work harder to remove barriers to learning. “Sharing problems is something we absolutely need to do more, so we can learn from any mistakes rather than hiding them away.”

“If there are learnings or problems, we need to encourage people to report them to [organisations such as confidential reporting bodies] Standing Committee on Structural Safety and Confidential Reporting on Structural Safety . We have to so we can avoid another Polcevera.”

 The cost of monitoring: the Scotland example

 Structural health monitoring (SHM) installed on the Forth Road Bridge following the discovery of the truss end link fracture in 2015 has cost £3.5M.

 However, that price tag is small compared to the cost of rectifying the problem – £7.6M for temporary repairs and at least £10.3M for the permanent solution, according to bridge owner Transport Scotland.

Accessing the parts of the bridge where work had to be carried out in 2015 after the bridge was closed to traffic made up half the temporary repair cost, says Ewan Angus, major bridges director for Amey.

This was due to the faulty member and seven other similar members located some 60m above the water level and below the bridge deck.

Traditional scaffold towers were built up from the tower base to reach these members, as this was considered safer and more reliable than using suspended platforms. It was certainly a case of all hands below deck, recalls Angus: “We effectively did 18 months’ work in 21 days and had to have plans B, C, D and E on stand-by because there wasn’t time for the plan not to work.”

This further added to the repair bill. With the loss to the local economy reportedly worth £1M a day, the aim was to open the bridge to traffic as quickly and safely as possible.

Angus expects maintaining and improving the Forth Road Bridge’s SHM to cost around £1M if the structure is to be kept in a state where it can be used by HGVs. Currently, only buses, cyclists and pedestrians can use the bridge. “It’s a bit frustrating that it’s  not being used [fully], but it will at least be ready if and when it is needed.”

The cost of setting up SHM and the ongoing costs of monitoring are clearly significant, but can easily be justified against the cost of emergency repairs, potential financial loss to the economy and the risk of a catastrophic failure.

Transport Scotland roads directorate chief bridge engineer Hazel McDonald says it was an easy decision to invest in monitoring and analytics on the Forth Road Bridge and Queensferry Crossing following the 2015 event (see feature p40). “Technology which helps to give us more quantitative information in advance of works, potentially reduces works duration, or risk to inspection and maintenance personnel is to be welcomed,” she says “Another truss-end link type failure had to be avoided.”

Ruby Kitching

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