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Danish and British engineers have completed a maintenance project on the third largest suspension bridge in the world.
The project to retro-fit dehumidification equipment to the cables of the East Bridge in Denmark is the largest such project undertaken, and will remain so because modern suspension bridges are now built with dehumidification technology pre-installed.
The East Bridge is part of the 18km long 'Great Belt Fixed Link', which links the Danish islands of Zealand and Funen. The complete Link also includes a low-level Western Bridge and an undersea bored tunnel for the railway. It took a decade to build and was completed in 1998.
The East Bridge's towers are 254m high and it spans 1.6km
Danish sub-contractor Davia and UK firm Spencer Group partnered on the project over the spring and summer of 2014 and 2015. The perilous work was done from gantries and involved walking along the cables of the bridge, for as far as 800m at a time, climbing for up to 190m at gradients of up to 28 degrees.
Spencer Group's “Cable Crawler” gantries were fabricated specifically for the Great Belt project and enabled the negotiation of the cables without returning the gantries to the bridge deck, eliminating the need for lane closures after initial installation.
Andy Macdonald, project manager for Spencer, paid tribute to project team for their technical expertise and physical endurance while working in such extreme conditions.
“Every bridge project has its own unique set of challenges, but this was a major step up and on a different scale from any of those we have previously undertaken.
“Jobs like this are not for people of a weak disposition. The guys are pretty tough – they have to be. After a few days you just get used to working so high up, with just the bridge deck below you. On a sunny, calm day it was the best place in the world to work. The views were stunning – you could see for miles.”
Weather was a major challenge for the project. Staff could not work from the gantries in high winds while, in wet conditions, the wrapping process could not be undertaken, nor could personnel walk on the cables.
Despite these restrictions, almost 6km of cables on the East Bridge were sealed in an elastomeric wrap known as Cableguard. As each section was wrapped, thermal blankets were used to heat the wrap and create a continuous airtight seal around the main cables before dry air was pumped into the lengths of wrapped cable.
The dehumidified air absorbs moisture inside the cables, carrying it out to the atmosphere at an exhaust point further along the cable, thus preventing corrosion and extending the life of the bridge.
The superstructure of the bridge is a fully welded closed box girder
Some 105,000m of wrap was used to encase the main cables, each of which is 83cm in diameter, around 3km long and comprises of 18,647 steel wire strands bound together tightly.
Eight Spencer staff worked in Denmark for the duration of the £8 million project, with a 16-strong team from Davai and Danish sub-contractor Belvent, which provided the dehumidifiers, pipework and injection and exhaust sleeves.
The Spencer and Davai teams had to walk the cables at least twice a day, seven days a week, to start work and after lunch to access the four gantries. Conditions were made more comfortable by the installation of incinerating toilet systems at the top of each of the towers – making these the highest lavatories in Denmark.
The project culminated in a month-long comprehensive programme of testing to ensure the wrapping had provided an air-tight seal across all the main cables.
Lars Fuhr Pedersen, technical director, of the Danish owner / operator of the East Bridge, A/S Storebælt, said: “This is a very important piece of work in respect of our ongoing maintenance of the East Bridge and in safeguarding its lifespan of at least 100 years. We have enjoyed an excellent working relationship and appreciate the importance that Spencer has assigned to safety, which means that the project has been completed with no accidents.”