With just two years to go before the 30-year PFI concession for the A55 Anglesey extension comes to an end, the final phase of works to repair and future-proof the historic Menai Suspension Bridge in north Wales has just begun.

The exact condition of some parts of the bridge is still to be established, but UK Highways A55 director Tom Cunningham is as confident as he can be that the hand-back will happen on schedule.

One thing he will not have to worry about is unhappy motorists getting restless in traffic queues caused by lane closures, and the resulting loss of income due to lower traffic volumes.

Conventional underdeck access systems that take up valuable deck space have been ditched in favour of Spencer Bridge Engineering’s new custom-designed, lightweight, modular AeroTruss platforms.

Four of the platforms are making their debut on the bridge, and the critical-path operation of grit-blasting and repainting of the deck beams started this month. This will continue in parallel with repairs to the parapet and deck corbels, inspection of the bearings at the towers and a number of other items that make up the six packages of Spencer’s contract.

UK Highways A55 is the special purpose vehicle that was awarded the concession to build and maintain the A55 extension to Anglesey in 1998. Cunningham admits that the last six years have seen many ups and downs in the route to handover.

The Menai Suspension Bridge – built by Thomas Telford, opened in 1826 and with a rebuilt superstructure dating from 1940 – is a key transport connection that links the Welsh mainland to the island of Ynys Môn

Technical challenges

The Menai Suspension Bridge – built by Thomas Telford, opened in 1826 and with a rebuilt superstructure dating from 1940 – is one of the historic bridge assets that the special purpose vehicle manages, along with Robert Stephenson’s Britannia Bridge.

These two key transport connections link the Welsh mainland to the island of Ynys Môn and carry all the highway and rail traffic to the island and the port at Holyhead. The Britannia Bridge is prone to closure in high winds, at which time the Menai Suspension Bridge becomes the critical transport link to the island. But recently it has faced a host of technical challenges, as Cunningham relates.

With the concession company obliged to hand back the asset to the Welsh Government in an agreed condition, the 2028 deadline is driving the pressure to work quickly and efficiently. The exact details of the agreed condition have taken time to establish, Cunningham admits, given that the concession was signed almost three decades ago.

“We’re trying to iron out the grey areas and agree on what the hand-back condition will be. Various surveys have been carried out for this purpose, the first one a year or so ago, and there will be a further inspection in 12 months’ time, a year out from hand-back, to see how that work is going,” he says.

A series of repair and renovation contracts had already been planned for the bridge, starting with the replacement of footway panels in 2022. But the programme was derailed by unplanned closures and emergency works to address critical issues that came to light.

In October 2022, the bridge was closed on advice from Cowi due to concerns about brittleness in the sockets of the hangers. While the risk of a single hanger failing was low, the consequences of such a failure could be catastrophic, with the suspension system at risk of ‘unzipping’ due to lack of redundancy. Failsafe devices were rapidly designed and installed to allow the bridge to be reopened to traffic while a permanent repair was developed.

This led to the first phase of works to the bridge, in which the hangers and sockets were replaced. Spencer Bridge Engineering was the contractor for the scheme, which ran from September 2023 for almost exactly a year.

The current work – Phase 2 – is intended to be the last, but there is still some uncertainty as to the scope of repairs that will be necessary, Cunningham says. The challenge of accessing the underside of the bridge, and the fact that concrete has to be broken to make the tower bearings visible, mean that the true condition of these elements will not be fully revealed until work starts.

“We hope that any extra work, such as work on the bearings, will be fairly minor; however, there is time within the programme and also a contingency in place to cover additional costs. We still plan to hand-back on the agreed date, but of course you never know.

We’ve put an awful lot of work into ensuring that the programme is as robust as possible, for example by taking steps to address the exposure of the bridge and its susceptibility to wind.”

We’ve put an awful lot of work into ensuring that the programme is as robust as possible, for example by taking steps to address the exposure of the bridge and its susceptibility to wind

The hanger replacement contract suffered somewhere between 20% and 30% of downtime due to wind, Cunningham says. “This is double what we would normally see; we started work in October, which was also the worst period of the year for weather. One of the reasons for using Spencer’s AeroTruss on this contract is to try and reduce that downtime. It also means we can reduce traffic management, so we are giving the people who use the bridge more access to it.”

Kerry Evans is general manager of UK Highways A55 and although her remit involves the management of 40km of highway and its assets – 98 structures in total – she admits that the Menai Suspension Bridge takes up most of her time.

Understanding the engineering judgements that drove the 1930s reconstruction of the bridge helps to inform decision-making on the recent and ongoing works, she says.

“Something will always pop up that you’ve never considered, and you have to understand the rationale behind it before you try and find a solution. The engineers would have used ‘best endeavours’ in their work, but the timeline suggests a very short design period for such a massive undertaking,” Evans explains. The need to accommodate military vehicles on the bridge was the key driver, compared with today’s main aim of future-proofing the structure.

AeroTruss

Without a doubt the biggest difficulty on the bridge is access – conversations about the challenges of the work inevitably converge at the same point. This means the use of Spencer’s AeroTruss is expected to be transformative.

First developed for this very bridge, the original prototype was refined after the contract was rescheduled and was proposed in the bidding process for the Phase 2 contract. Consultant Ramboll acted as independent checker for the modular design and also signs off each individual platform configuration.

Conventional underdeck access systems that take up valuable deck space have been ditched in favour of Spencer Bridge Engineering’s new custom-designed, lightweight, modular AeroTruss platforms

Evans says: “Last year when we finished the hanger replacement, we paused the work programme to recheck our objectives. There are only two bridges over the Menai Straits, so closing this one has a big impact on traffic, not just locally also on the port at Holyhead. The question was, how could we minimise the impact and disruption?

“We had anticipated between 12 and 18 months of work with [temporary] traffic lights on the bridge, and that’s not palatable to anyone,” she continues. “Use of the AeroTruss access platform means we don’t have to have any plant on the bridge that requires a lane closure. We will have a couple of days with traffic lights and maybe a closure for specific bits of work, but very little closure overall.”

The lightweight modular system can be adapted to fit a bridge of any size or geometry, says Spencer Bridge Engineering bridges director Luke Fisher, and its development has been informed by the company’s wide experience of accessibility issues on historic bridges.

“We’ve been doing this kind of work for a long time and have used a lot of different access systems in the past, with poor results,” he says. “We wanted to address all the pitfalls of the conventional systems and make a modular system that we can adapt for any bridge. It was designed in house involving our own constructors, site managers and engineers to build on what we’ve learned from previous contracts.”

The system is lightweight, stiff and aerodynamically efficient; all components are profiled to minimise the wind load on the structure and address the fact that historic structures generally have limited dead load capacity.

There is the option for built-in encapsulation – dubbed ‘speedy curtain’ – for blasting and painting activities. As the name suggests, it can be easily deployed as well as being quickly pulled back in high wind events, maximising working time.

Trusses are made of aluminium tubular sections, linked by one of 30 bolted nodal connections that are custom made to suit various orientations. This enables the elements to be assembled in a whole range of different configurations and at different angles. As a bolted connection, it doesn’t rely on friction, so the inspection requirements are reduced, making it additionally suitable for permanent applications.

The flooring system is aluminium open grid with the option to add plywood for construction operations that require encapsulation. The platform has a trolley system that can be suspended from the runway beams on the bridge, but if there are no beams or they are not certified for use, it can be traversed along the bridge using the cross girders.

One other feature of the platform that Fisher is keen to point out is the provision of an integrated rescue davit in the stair tower, which makes it possible to easily evacuate incapacitated operatives. “These are areas that are hard to reach, not just on temporary work platforms but also permanent gantries. If there’s an emergency and someone needs to get off the platform, it’s often a challenge,” he says.

Four AeroTruss platforms are now in place on the Menai Suspension Bridge – a small 18m by 3m platform below the deck at each tower to provide access to the bearings, and two painting platforms – each 15m2 – which will work towards each other from the ends of the main span, to facilitate grit blasting, steel inspection and repairs, and painting.

Four AeroTruss platforms are now in place, a small platform below the deck at each tower to provide access to the bearings, and two painting platforms, which will work towards each other from the ends of the main span

Next steps

The first main milestone in the contract has been successfully completed, says Spencer Bridge Engineering project manager Thomas Inglis, with all four AeroTruss platforms installed on the bridge, and signed off ready for use.

While the deck repairs and repainting are firmly on the critical path – and due to begin as NCE went to press – the next important milestone is the inspection of the bearings, which will reveal what kind of work, if any, is needed.

“The parapet is being dismantled and sent away for steelwork repairs,” says Inglis. “This minimises both the amount of blasting and painting we do on the site and the interface with the other works. We learned from our work on the Union Chain Bridge to properly tag all pieces and ensure that tags are maintained throughout the whole refurbishment process.”

Repairs to the footway corbels will progress along the east and west sides of the bridge one at a time, so that the other footway can remain open. The other two work packages involve repairs to the cantilevered footway where it doglegs around the towers, and repairs to the land saddles on the anchorages.

A conventional access system would have been installed from deck level and required a lot of operatives and disruption to traffic, with scaffolding elements lifted over the side of the bridge and a high risk of dropped objects. While installation of the AeroTruss platforms was a tricky operation, being reliant on favourable tidal, wind and weather conditions, it was done without disrupting bridge users, Fisher explains.

“We built the trusses off site, undertook all the testing, then brought all four platforms in on barges and lifted them up with traffic still running and the bridge fully open.” The Phase 2 contract is due to be completed in spring 2027.

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