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Since 1813, bridge structures built on exactly the same location have been spanning the Drammen River connecting the areas of Strømsø and Bragernes in central Drammen, in the south-eastern part of Norway.

The original timber structure was replaced in 1936 by the country’s first reinforced concrete bridge. “It had an expected lifespan of 50 years, but it lasted 86 years,” says Tommy Iversen, head of project for Drammen Municipality.

Since last October, around 10,000 people a day use the new 286m-long Drammen City Bridge. This time the structure is steel-concrete composite and is 4.4m wider, offering more space for cyclists, pedestrians and buses. It is also better integrated with the Drammen Railway Station in Strømsø, with rail users able to access platforms directly from the bridge, and the old square in Bragernes – where a new public space was created along the river as part of the project.

Iversen explains that the construction of the new bridge was deemed essential by both the municipality and the Norwegian national rail administration Bane NOR.

The municipality found that the condition of the existing structure had deteriorated considerably, while Bane NOR wanted to upgrade the local station situated on the southern end of the bridge. The organisation’s plan was to introduce a new double track between Drammen and Kobbervikdalen, and improve the flood resilience of its assets.

Senior engineer at civil and structural engineering firm Degree of Freedom Mario Rando explains: “The rail tracks needed to be raised by 0.5m and there wouldn’t have been enough clearance under the former bridge.”

Bane NOR funded the part of the bridge above the railway, with the Drammen Municipality providing the funding for the rest. The two clients awarded separate contracts for the detailed design and delivery of the bridge, which added to project’s complexity. However, the work that was done ahead of those contract awards facilitated the process.

Design

A competition began in 2018 for the bridge design, even though political decisions on the project’s future were still pending. “Three different teams had the assignment to come up with three different concepts for a new bridge,” Iversen explains.

While it was known that the works would be split, the design had to cover the whole bridge. Iversen says there was a lot of coordination between the two clients from the earliest stages about parameters such as geometry, structural interfaces, concrete paving etc.

The final three designs were quite different in terms of concept and materials. Bridge experts from the Norwegian Public Roads Administration Statens vegvesen looked into the three designs and estimated the cost of each bridge, with the differences found to be minimal.

The public selected its favourite design – by attending dedicated city hall events and through a poll run by the local newspaper – which Iversen admits was also the municipality’s favourite. It was created by Oslo-based architects Saaha and UK’s Knight Architects, Degree of Freedom and landscape designer Asplan Viak.

The two clients then assigned different teams to carry out the detailed design, with the original team – as well as engineering firm Norconsult, lighting experts Light Bureau and land surveying specialists Scan Survey – awarded the detailed design contract by Drammen Municipality.

“[Having two clients and two project teams] added complexity to the project but everything ran smoothly because there was only one design and everybody had to stick to it,” says Rando. “There were details that needed solving but there was a good dialogue between the engineers.

“We developed finite element model of the whole bridge. We compared results until we had an agreement. The geometry was completely controlled because we shared the same BIM model.”

Knight Architects director Bart Halaczek says the decision to design a steel-concrete composite bridge was taken early considering the need to shorten the construction programme above the river – as working on the river is subject to limitations for environmental protection purposes.

Saaha partner Thor Olav Solbjør adds that very early on the design team opted not to have a structure above deck to avoid strong visual impact on the cityscape.

“The Drammen River is a very dark river, on a sunny day it’s like a mirror, so we decided to go for a light-coloured bridge,” he adds. “A light structure stands out against the water and is reflected in it doubling the structures curves and geometry.”

The bridge features a 19.4m-wide deck, comprising a 7m-wide bus lane, 2.2m-wide cycling ways on both sides, and two pedestrian walkways with widths of 4.8m on the upstream side and 2.8m downstream.

The structure extends 84m over the railway station, with the five spans – ranging from 14.5m to 19.5m – crossing six tracks and four platforms. The superstructure of this section consists of a reinforced concrete deck with a vaulted soffit, varying in thickness from 300mm to 650mm. It is supported by five transverse V-shaped steel piers, ranging in height from 3.9m to 7.2m, which also serve as support for the access stairways leading to the railway platforms.

The bridge then spans the 170m-wide Drammen River, with three spans – looking northwards – measuring 46m, 86m and 42m. The north abutment in Bragernes is around 8m further north from the river compared to that of the previous bridge.

“Moving the abutment improved the clearance under the bridge and allowed the bridge to land on the square itself rather than in front of it,” Solbjør explains. This enabled the creation of the public space underneath the bridge. Halaczek says that the previous structure featured “a 15m-long uncomfortable, narrow passage”.

The section crossing the river is where the structure transitions to a composite one. The concrete slab remains similar to the one above the railway but is now connected to two longitudinal steel box girders, spaced 9.7m apart. These girders converge with two steel arches, inclined at 63°, which connect to two piers in the river.

These 8.5m-high piers are made of reinforced concrete with hydrodynamic round edges and their lower sections feature pyramid-shaped plinths.

The original design featured three piers in the river, but Degree of Freedom’s analysis showed they could be reduced to two. “It is beneficial because the geotechnical conditions are complex, therefore the smaller number of supports in the river, the better,” says Rando.

Designing the bridge entailed some challenges. Rando gives an example: “We had to land very low at both ends and we had to limit the maximum percentage of the slope. That made it difficult to find the right alignment and profile.”

Halaczek adds: “The highest point of the bridge is not in the centre but further south at the transition from rail crossing to river crossing. This means the river crossing is asymmetric.”

The diminishing height of the bridge as it crosses the river in combination with the piers makes it resemble a skipping stone, according to the architects.

“The two main beams are inclined inwards to minimise the footprint in the water. But as we have different heights of deck above the water, if we had the same footprint the shape should be warped,” Halaczek says.

“So instead, we decided to make it clear that we got a height different. One pier is wider than the other and we kept the angle of the river crossing constant.”

When it came to the architectural details, the municipality wanted the design to incorporate local materials so that it blends better with the surroundings and to support the local economy. Solbjør says granite from a nearby quarry was used for the walkways.

Foundations

“This bridge is 10m high, but the piles go down 70m. This bridge had a high cost, but a lot of that money was spent below the deck,” Solbjør quips.

The foundation design and construction were challenging due to poor ground conditions and the presence of a “forest” of timber piles from the former bridge. “The bridge was smaller in size and lighter, so they decided to fix these timber piles from above, but their maximum length is 20m,” says Rando.

The depth of what was considered good soil varied along the cross section of the bridge. “The good soil was in between 40 to 70m below the water and we need to get to that depth in order to fix the piles,” says Rando.

The upper soil layer consisted of soft to medium stiff silty clay, with clay becoming more sensitive and quick clay registered towards Bragernes. In the gap between the clay and the bedrock, the soil comprises silty moraine, with increasing thickness towards the south.

To transfer the large vertical forces, a system of steel core piles was used. The piles were drilled into bedrock to take the downward loads in the permanent phase, and the tension loads caused by buoyancy during the construction phase.

Due to the long spans over the river and the prevalence of large ice loads during the winter period, additional measures were introduced to obtain sufficient horizontal capacity and stiffness. The steel core piles were inclined, to take part of the horizontal forces.

The moment capacity and horizontal stiffness of the steel core pile were increased by an outer steel pile, concreted and reinforced.

The piles were installed with a floating barge and then a sheet pile cofferdam was built to create the pile cap. Rando says: “The largest one is about 14m by 14m with a height of 4m. A big box of reinforced concrete with piles down there to 70m. It was time-consuming and costly, but essential to provide the necessary vertical and horizontal stiffness to the system.”

On the other pier [closer to Bragernes] spherical bearings were installed “which means that longitudinally there is only one point which is fixed to the bridge, and the rest are free to move. Transversally, it is restrained at every point. It was delicate work to find the right dimension of the steel boxes at every point to have the best possible appeal in geometry.”

Smart construction decisions

Extensive coordination between the project teams was also necessary during the construction phase. “The section over the railway has much smaller spans than the section over the river so they behave in a dissimilar way. It was critical to build the two sections separately and connect them only when they were almost finished to avoid transferring forces from one to the other. We avoided the need of the expansion joint, which would have made maintenance more costly,” says Rando.

PNC Norge, part of Porr Group, was awarded the main construction contract by Drammen Municipality in May 2022. Works began with the demolition of the old bridge and the installation of a temporary bridge to minimise the disruption in people’s movements during the works.

One way that the project team facilitated the construction of the new Drammen City Bridge was by making use of the existing piers. “It’s not easy to put temporary elements in the river with such bad soil conditions, so we thought to use these supports,” PNC Norge project manager for the steelworks Georg Ebner explains.

However, this proved trickier than expected because the old concrete piers had no reinforcement. Ebner says: “The whole concept of the installation was based on supporting the bridge on these existing piers during the process. Due to their bad condition we had to strengthen them. We had to pre-stress them, add concrete, add some anchors to basically hold them together with a steel frame.”

Despite the challenges, Ebner believes the solution chosen was more advantageous than other alternatives.

The bridge’s steel elements were manufactured at Porr’s new production facility in Germany and were transported via pontoons.

A special steel grade was used, the S355ML – a thermomechanically treated steel that Ebner describes as “very ductile at low temperatures”. “If the designer does not choose the appropriate mechanical properties of the steel, you could have brittle failure due to low temperatures when you have fatigue loads on the bridge,” he emphasises.

The original plan dictated that the bridge elements were to arrive in smaller pieces and be welded together on a nearby island where the site establishment had been set up. However, due to space limitations on the island, the decision was made for some elements to be pre-assembled at the factory before being transported to site.

Space limitations were also a factor on site. In order to facilitate welding and painting, PNC created platforms on top of the cofferdams, which also acted as storage spaces for material.

“It was very good that we had these big platforms because we could be a bit more independent from the river at some point. Otherwise, we would have needed more pontoons and more equipment,” says Ebner.

The bridge elements, with weights up to 105t, were lifted into position by a crane. Installation of the segments started in June 2024 and was completed by October of the same year.

With the steel elements exposed to harsh weather conditions and located near the water level – making them susceptible to tidal changes and splashes as well as abrasion due to floating ice – a strong corrosion-protection system was needed. The project team opted for one used in the offshore industry.

The corrosion-protection system comprises: 100 microns of thermal spary zinc, 300 microns two-component epoxy with aluminium, 300 microns of two-component epoxy with high abrasion resistance and 60 microns of two-component glossy polyurethane enamel.

Urban regeneration

The new bridge has a 100-year design life and therefore will be an iconic feature of Drammen City for generations to come. Since its opening last October, it has become part of the daily journeys of thousands of people.

“It’s a beautiful bridge and it’s also a practical bridge that connects the city a lot better than the old one,” says Iversen.

Solbjør says the creation of the public space in Bragernes turned this scheme into an urban regeneration project by creating a meeting place for local residents to spend their leisure time. Statues from the old bridge which represent the unification of Drammen have been installed there, creating a link between the new infrastructure and the history of the city.

It has also provided greater engagement between people and the bridge itself. “I see on social media lots of people are taking pictures of the bridge and along the bridge, and I see people just hanging out in this new area to enjoy the view. So there was definitely a placemaking aspect to the project.

“One of the most important things in our city is the connection to the river and now we have done it in a much better way,” says Iversen.

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