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Cracking the code

Bestriding the M5 in Devon is a new landmark bridge - the first in the UK to be fully designed and constructed using Eurocodes. Declan Lynch finds out how it was built.

With its impressive twin bowstring arch structure, drivers on the M5 can hardly fail to spot the Redhayes bridge. But what they won’t notice is that it’s the first bridge in the UK wholly designed and constructed using Eurocodes.

Designers Parsons Brinckerhoff (PB) decided to use the bridge as test case for using Eurocodes - which is now the national standard - and to provide valuable experience for the team creating the new crossing.

And there is good news for those who view Eurocodes with suspicion: “There is nothing to be scared of,” says PB team leader and project manager for the project Vyv Pike.

Three spans

Redhayes Bridge is a 100m long pedestrian and cycle crossing, spanning the M5 north of Junction 29. The bridge splits into three spans: a main span 68m long with two side spans of 16m sitting 17m above the M5 and weighing 250t. The road width is 4.5m in the middle and expands to 7m at each abutment.

Client Devon County Council funded the scheme via the previous government’s Community Infrastructure Find (CIF) designed to promote sustainable transportation linked to growth.

“Funding came through in 2008 and it will be open in just two and half years later,” says Devon County Council head of waste, engineering and transport services David Whitton.

“We took on financial risks prior to securing funding but the grant made it happen.”

“Of course it was a challenge using the new suite of standards, but our design team rose to that challenge brilliantly”

Steve Denton, Parsons Brinckerhoff

Devon secured £5.5M funding from CIF, of which £3.7M is being spent on Redhayes Bridge. It is part of the Exeter and East Devon growth point, which began 15 years ago to help bring high-end jobs to the region.

The bridge’s location is designed to make best use of the existing country lane network and will link the proposed Exeter Science Park to a radial route to Exeter’s city centre.

When PB came on board, it decided to use the project as a trial to fully design and construct using Eurocodes. “It was something PB suggested and we we’re happy to go along with it,” says Whitton.

Pike, who with senior engineer Andy Cook headed up the bridge design, says: “The key thing is that the fundamental design and construction doesn’t change.”

Latest technology

Because Eurocodes are much newer than the last revision of British Standards, PB engineers could apply the latest developments in technology. In particular, the team could take advantage of the more detailed coverage of buckling analysis and pedestrian induced vibrations - both essential when designing the bridge.

Pike describes the codes as less prescriptive but more encompassing. “For a designer, using a new code is always a challenge, but it’s a credit to our team that we managed to deliver it on time and on budget.”

Designs went through peer review checks using PB’s in-house resources and involve staff from four different offices. “It allowed us to update our design tools,” adds Pike.

Using Eurocodes for construction meant that main contractor Carillion, steel fabricator Rowecord and anyone else involved in the project needed to be aware and implement the changes.

Specification changes

Carillion needed to be aware any changes to specification with the Eurocodes execution (construction) standards.

“Our concrete standards were rewritten to come in line with Eurocodes,” says Carillion site agent Neil Shannon.
Carillion began work on site in June last year. It was a particularly constrained site, not only working beside the M5, but also on the flight path to Exeter airport.

“We used our rail experience in planning possessions but we needed to think carefully it,” adds Shannon.

Work began by excavating 2,000m2 of soil from the embankments to create the concrete abutments either side of the M5 cutting for the bridge to sit on. Carillion installed hard shoulder screens on each side so no debris would fall onto the roadway, using just two hard shoulder possessions. The excavator had to sit on the edge of the embankment and dig down.

“It took about 16 weeks to complete this - the ground conditions were sandstone, so relatively straight forward,” says Shannon. The bank side abutment was constructed using reinforced concrete, with the concrete poured using a machine sitting on the top of the bank.

Rowecord fabricated the impressive twin arch structures. Due to the size of the bridge, it was partly created at their yard in Newport, South Wales, and then assembled at the roadside.

“The longest piece transported was 42m x 7m - it’s big and required out of hours transport but Rowecord has plenty of experience of this,” says Shannon.

Each arch was brought to site as four legs and one crown piece, which were then put together by being placed on two trestles mid-way in from each end of the arch. Arch sections were guided in place using jacks and a crane.

Once in position, the arches pieces were butt-welded together, flushed then coated with protective paint. The top of the trestles served as welding stations for the arches.

A further two welding stations were created on the deck to stitch the three decks on the main span together before the deck could be suspended from the arches.

Rowecord began on site in November, with the east side of the site being turned into a temporary works centre.

Once fabrication was complete then the arch needed manoeuvring into position. Along with a motorway possession, engineers needed seek clearance with Exeter Airport.

“The height of the jib meant it was a potential hazard to planes approaching Exeter airport,” says Shannon.
Crane supplier Sarans used a 1,200t crane, the UK’s biggest, to lift the bridge in position. The team was ready to make the lift on the first weekend in February, but the one element it could not control was the weather.

“The height of the job meant it was a potential hazard to planes. Luckily we had a good relationship with the airport”

Neil Shannon, Carillion

“It was too windy and we couldn’t lift it. Luckily, our good relationship with the airport meant we could roll it back one day. There were some frantic phone calls but we got the green light and lifted that night.”

Spring pads

The bridge was positioned onto four spring pads connected via temporary brackets. Engineers are finishing welds to the permanent connection - with the bridge’s thermal expansion being compensated by eight bearings underneath the roadway deck.

Overall, all parties are pleased with the bridge and the knowledge gained using Eurocodes.

“Of course it was a challenge using the new suite of design and execution standards, but the design team rose to that challenge brilliantly,” says PB director of engineering Steve Denton.

“We put in place special processes to support the team including comprehensive training, peer review of design method statements by our in-house Eurocodes experts and regular technical review meetings.”
It may be the first bridge to be designed using Eurocodes in the UK, but definitely not the last.

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