Floating bridge concept at the Bjornafjord - coutesy of Statens Vegvesen.

Floating Bridge – World’s Longest – Proposed at Bjørnafjorden

January 3, 2020
by Nick Rodda
7 min read
100 Comments

There are construction projects, and then there are mega construction projects.  Norway’s ambitious, and to be honest, awe inspiring E39 Coastal Highway project would, by anyone’s definition, be considered in the latter category.  The project, the largest in Norwegian history, seeks to build or improve over 1,100 kilometers of road between Kristiansand and Trondheim along Norway’s west coast.  The aim is to improve travel times by as much as 50% and provide safer, more reliable transportation links for both people and commerce.

This sounds great, but if you’ve ever been to Norway you’ll know that nearly the entire coastline of this small Scandinavian country is carved up by many fjords.  These narrow inlets, with their steep sides and deep waters, are proving to be a significant technical challenge to the engineers working on the project’s seven major water crossings.

A trek along the E39 today involves many such water crossings and unless you own an Amphicar you’ll probably need to take a ferry to get across.  Factor in wait times and the crossings themselves and you’ll soon understand why the journey from Kristiansand to Trondheim takes 21 hours.  That’s an average speed of about 35 miles per hour!  And that doesn’t make allowances for bad weather or high traffic volumes.  There is nothing worse than a canceled run or not making the boat you’ve been waiting for.  But imagine taking a trip along the E39 where there are no ferries to wait for – that is one of the major goals of this project.  Imagine making this journey in a mere 10 hours! 

But why didn’t they just build bridges back when the E39 was originally built, you may be asking?  Fair question.  The answer may or may not surprise you – the technology just wasn’t there.  These fjord crossings are long and deep, which separately may not pose much of a challenge, but together they put this project in a whole other universe.

One of the biggest challenges is the mighty Bjørnafjorden crossing, located south of Bergen.  At 5 kilometers across, it is the longest crossing being considered on the project.  With little protection from the harsh North Sea and with 600 meter deep waters to cross, any bridge built here will be a major undertaking. 

At the helm of such a project is the Statens Vegvesen or the Norwegian Public Roads Administration (NPRA).  Together with nearly 40 different consultants and contractors, researchers and others, the project team has been working diligently to consider many different alternatives for the crossings.  NPRA holds an annual conference called the Technology Days, with an entire day’s agenda devoted entirely to the E39 project.  I was lucky enough to be invited to present on Washington State’s floating bridges at Technology Days 2018, where I also participated in a panel discussion on marine structures in Norway.

Years of research and option vetting has culminated in the selection of the preferred design concept for the Bjørnafjorden crossing.  Among the more novel solutions under consideration have been floating suspension bridges, two different types of submerged floating tunnels and end anchored discrete pontoon floating bridges.  In September 2019, NPRA announced that it had selected the single curved discrete pontoon option.  This was not unexpected.  Norway has two similar floating bridges, so the concept is familiar, although the proposed bridge would dwarf the Nordhordland bridge length by over 3 times.

To permit marine traffic to pass the bridge, a 300 meter long cable stayed transition span will be utilized at the south end with 45 meters of vertical clearance.  Discretely spaced (not connected) steel pontoons, 40 all told spaced at 100 meters, will be used on the crossing.  Think of these as floating foundations, rather like stepping stones across a stream.  In this type of design, the superstructure does the work.  It is not clear what type of superstructure will be utilized here, but if Norway’s other floating bridges are any indication – expect either a tubular truss or an orthotropic box girder.

Both Nordhordland and Bergsøysund are end anchored only, that is they do not use anchor cables to hold them in place.  There is a practical reason for such a design – the water depths.  For Bjørnafjorden, due to its extreme length, the proposed bridge will have to have anchor cables, likely on more than one pontoon, but probably not on all of them.  Whether these will be splayed out at an angle or tethered vertically remains to be seen.

Another novel aspect to how Norway approaches floating bridge design is their curvature.  Bjørnafjorden will likely be curved inland, like an arch.  Since the majority of the bridge will be untethered, the arch is necessary to resist the lateral forces on the bridge.  Allow me to explain.

On a straight crossing, such as any of the four Washington State floating bridges, you have anchor cables to resist the applied lateral loads – wind, waves and current.  Pretty straight forward, right?  As the load hits the bridge, it wants to move out of alignment.  The windward side cables see increased tension while the leeward side slacks off. 

Where you lack cables for resisting this lateral force, you need some other method.  That is where the curve comes in.  An arch transmits applied load to its foundations as thrust, which puts the members of an arch into compression.  The more load applied, the more compression you get.  For Norway’s floating bridges, lay that arch on its side.  Lateral loads applied from wind and wave action act on the bridge and resolve themselves as compressive loads in the superstructure.  Large struts at each end transmit the huge thrust into the bridge anchorages.  The inherent compression in the system acts to maintain the bridge geometry.

It still remains to be seen how certain other aspects will be dealt with.  Both Nordhordland and Bergsøysund were hinged at each end to allow for tide changes and to release the end moments caused by both lateral and vertically applied loads.  Early proposals have the ends fixed at Bjørnafjorden as it was felt the movements that would be created would be too great for the expansion joints.  While feasible, this will likely mean very large end anchorages.

So what will all of this cost?  Estimated construction costs for Bjørnafjorden are around $1.3 Billion according to early estimates.  Assuming four lanes, a 10 foot shared use path and some other assumptions based on US road standards that gets us to about an 80 foot bridge deck width.  That brings the square foot bridge cost to about $1,000.  The recently completed Evergreen Point Bridge in Seattle was built for around $600, as a comparison.  While they are different styles of bridge, it must be pointed out that Bjørnafjorden will feature a cable stayed transition and be made mainly from steel – both are likely reasons for the slightly higher cost, all other things being equal.

I look forward to visiting Norway in the coming years to see this project under construction.  Look for more information on this project as it develops, as well as an upcoming blog post on my visit to Trondheim in 2018 and my tour of the Bergsøysund floating bridge.

When completed, the Bjørnafjorden floating bridge will almost certainly surpass the Evergreen Point Bridge as the world’s longest by a factor of two.  Competitive bidding is set to open in 2022 for Bjørnafjorden, and assuming a few years for construction we will likely have a new world record holder before this next decade is out.

The largest transportation project in Norwegian history – definitely.  But to my mind, it may also be one of the most ambitious transportation projects in the world.  As a bridge engineer, I look at this and wish I could be a part of it.  This is once in a lifetime stuff.  For some currently working on the project, it may be a career project for them.  But what a project!

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About the author

Nick Rodda

I am an aspiring blogger with an interest in all things bridge related. This blog is dedicated to informing readers about the latest developments in bridge engineering. Look for new posts periodically!

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