Jokikylä bridge spans the river Kiiminkijoki in Haukipudas, Northern Ostrobothnia and is a major milestone in Finnish bridgebuilding. Work started on building the bridge in 2022 and the bridge was opened in autumn 2023. The old bridge needed to be replaced due to damage, deficient load-bearing capacity and poor conditions for light traffic. Additionally, the new bridge provides safer conditions for pedestrians and cyclists
NYAB was the lead contractor for the project. Built by Steel Group Pohjanmaa Oy, the bridge is an excellent example of how SSAB Weathering steel can contribute to revolutionizing bridgebuilding. Made with SSAB Weathering 420 ML, the bridge is one of the first of its kind in Finland. This weather-resistant high-strength steel has turned out to be an excellent choice for bridgebuilding since it is very low maintenance and provides excellent resistance to challenging conditions.
SSAB Weathering 420 ML was chosen as the material for Jokikylä bridge because of its excellent weather resistance and low maintenance.
"Workshop time is saved if steel is not painted, especially in large projects. SSAB Weathering does not need painting, which in turn also significantly reduces maintenance costs and environmental impacts,” says CEO Henrik Kiviniemi at Steel Group Pohjanmaa Oy.
Erkki Krankkala, Technical Development Manager at SSAB, emphasizes that whereas the maintenance frequency of conventional painted steel is for example 10 years, it’s double that with SSAB Weathering 420 ML. Krankkala points out that painting conventional steel also causes major environmental damage since the old paint has to be removed and destroyed. SSAB’s weather-resistant steel is an eco-friendlier alternative as it doesn’t need painting.
According to Erkki Krankkala, 2019 saw changes to the EN10025-5 material standard for weather-resistant steels, with the addition of strength classes S420 and S460. These changes subject steel to stricter requirements. SSAB Weathering 420 ML has excellent impact toughness, especially in cold conditions. The impact toughness value of conventional structural steel is normally around either 27 J or 40 J at-20°C. The value for SSAB Weathering 420 ML is 27 J at -50°C.
“This means that SSAB Weathering 420 ML retains its toughness and better withstands impacts also in low temperatures,” Krankkala explains.
Impact toughness value measures the ability of steel to resist fracture caused by impact. This is important because it indicates the toughness of the material and its ability to absorb energy before fracture, which is particularly important in structures subjected to dynamic loads, such as bridges.
SSAB Weathering forms a dense and tightly attached oxide layer, a patina, on the surface of the steel in 2-4 years, which protects the steel from corrosion. The steel must be exposed to alternating wet and dry conditions for the protective layer to form.
“The patina is self-healing, which means that scratches and other surface damage do not result in the spread of corrosion, as a new protective layer of patina is formed on the damaged area. We have 32 years of bridge research by VTT Technical Research Centre of Finland on the corrosion resistance of steel in bridges," Krankkala says.
According to Krankkala, paint adheres considerably better to weather-resistant steel than to conventional steel. A six-year study in Sweden found that when surfaces of weather-resistant and conventional steel were scratched, only the conventional steel scratch rusted and damaged the paint surface around the damage site. On the weather-resistant steel, however, the scratch patinated and the paint remained intact.
“Many bridges built from Cor-Ten steel in the 1970s are still in good condition. They are repaired of course, but not because of the steel,” Krankkala says.
“A problem with old steel bridges is their deficient load-bearing capacity. It is precisely for this reason, for example, that old bridges have to be replaced with new ones. Demands on bridges are continuously increasing,” Kiviniemi says.
As the decks of bridges get to the stage where they need to be repaired, the load-bearing capacity of bridges is also reviewed and the bridges are updated to take account of today's traffic conditions.
Workshop time is saved if steel is not painted, especially in large projects. SSAB Weathering does not need painting, which in turn significantly reduces maintenance costs and environmental impacts.
Steel Group Pohjanmaa manufactures around ten smaller bridges and a few larger ones each year. The fact that the Jokikylä bridge was transported to the construction site in five sections gives an idea of its size. The longest section was 28 meters long, 8.15 meters wide and weighed around 46,000 kg. On-site bridge installation took place in four phases, since limited space required the bridge to be assembled one temporary support tower or support at a time.
“Timelines are usually quite tight and manufacturing times quite short. For the Jokikylä bridge, the manufacturing time was around 10 weeks. SSAB supplied the plates cut to shape. We don’t usually take steel surface treated, especially for smaller projects. However, larger items can also be surface treated. SSAB also does a lot of prefabricated bevels,” Kiviniemi says.
“Material delivery time plays an important role in keeping a bridge on schedule. Cooperation with SSAB has been excellent and since all steel deliveries arrived on time, we have also met the bridge completion timeline,” Kiviniemi enthuses.
At times, there are more difficult questions for steelmakers, especially regarding material quality and surface treatment. SSAB's experts were closely involved in the bridge project, providing information and support, especially on matters concerning welding and fillers. Naturally, experts are already involved in a project at the tender stage.
“If we don't have the answer right away, we have experts in different fields, such as beveling or welding. Our welding experts also visited Steel Group Pohjanmaa’s works, where they reviewed the welding and fillers of weather-resistant steels. Our experts are always in direct contact with the customer, so that no intermediaries are needed,” Krankkala says.
Jokikylä bridge demonstrates that SSAB Weathering steel can provide outlasting and eco-friendly solutions for bridgebuilding.
“New bridges are already promised a useful life of more than a century," Kiviniemi says. Use of SSAB Weathering 420 ML enables lighter and longer lasting structures, which require less maintenance and have longer outlast in challenging environmental conditions.
SSAB and Steel Group Pohjanmaa will continue to work together on future projects and the ambition is also make increasing use of fossil-free steel.
"The quality of the steel supplied is good and arrives on time. Cooperation with SSAB had been very smooth,” Kiviniemi sums up.
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