The Longest Duplex Stainless Steel Railway Bridge in Sweden

Stockholm, the Swedish capital, like many settlements on the water, could not function without its network of bridges. More than 340,000 people cross the Soderstrom Railway Bridges every day. But after 60 years of heavy use, they were in need of a good repair. In 2013, the local transport department determined that while the foundations were in good condition, the carbon steel support structures beneath the tracks were severely corroded, and the bridges had also exceeded their specified fatigue life. So, in 2017, work began on replacing all four aging carbon steel structures with 2404 duplex stainless steel. Each Soderstrom Bridge is 192 meters long and consists of 12 prefabricated sections. In terms of total length, the new railway bridge is the longest stainless steel bridge ever built.  

Duplex stainless steel structures: more than just aesthetics

When duplex stainless steel bridge structures first appeared, they were praised for their good looks, and novel designs such as the Helix Bridge in Singapore emerged. Today, designers are increasingly looking for more than just aesthetics in duplex stainless steel, using its strength and corrosion resistance in many applications, even hidden ones, to create structures that are lighter than carbon steel in the past, using less material and requiring less maintenance. All duplex grades are at least twice as strong as 316 stainless steel and have good welding properties, making them suitable for a wide range of applications. Duplex stainless steel bridge structures are becoming increasingly popular around the world due to their longer service life and lower maintenance requirements.

Although structural duplex stainless steel has been incorporated into design specifications around the world, carbon steel still dominates the bridge construction market. Carbon steel bridges are considered to be less expensive than stainless steel bridges, and carbon steel bridges have been in existence for more than a century and are very familiar to the bridge construction and maintenance industry. In contrast, the use of duplex stainless steel to build steel bridges began in the early 2000s and is widely considered expensive.

Soderstrom Railway Bridge, surrounded by water, is a key commuter node in central Stockholm, with a train passing every 3 minutes, about 500 trains a day ©Rolf52 | Dreamstime.com

The real cost of a “low-cost solution”  

The Stockholm archipelago is located in a marine environment and is subject to corrosion from the brackish water of the estuary and de-icing salts in winter. Carbon steel bridges do not have the corrosion resistance benefits provided by chromium, molybdenum and other alloying elements, so they must be painted or otherwise protected to withstand the harsh conditions. At first glance, the four substructures built from carbon steel appear to cost less at €2 million, while the duplex stainless steel costs twice as much at €4 million. But these figures are misleading because they only take into account the initial manufacturing costs. In fact, the overall cost of the carbon steel structures over their life cycle is much higher because they need to be repainted every 30 years, three times in their 120-year design life. The cost of repainting and repairs (excluding indirect costs of the railway closure) is estimated to be €60 million over this period, 30 times the original investment in materials. Looking at the whole life cycle costs, the cost can be significantly reduced with duplex stainless steel.

While all steel is 100% recyclable, the duplex stainless steel option has a lower environmental footprint. If the stainless steel is selected correctly, the bridge structure will not deteriorate over time. If the code requirements are followed and the maintenance is correct, stainless steel can achieve a nearly unlimited life expectancy. It also does not require any environmentally harmful surface treatment to prevent corrosion. Such coatings or paints deteriorate over time and can pollute the environment if they peel off. Painted bridges need to be inspected regularly, touched up every few years, and eventually completely removed and repainted every 25-30 years. This sounds simple, but it means closing the bridge, installing scaffolding, completely enclosing the bridge, preparing the surface before applying the new coating, capturing the removed paint chips, and properly discarding the old paint. Such maintenance work also carries a certain risk of injury to the people involved. Duplex stainless steels largely avoid these potential health and environmental costs.

The Swedish Transport Administration has also estimated that over a 120-year period, the indirect costs to the public due to temporary loss of infrastructure convenience during repair and maintenance of carbon steel structures will increase by a further €3 billion. This highlights the region’s reliance on these bridges and the huge disruption caused by closures for maintenance and repair. 

The construction and installation 

The Söderstrom Bridge was made of 2404 duplex stainless steel, the first bridge designed with this grade of stainless steel. This grade contains 1.6% molybdenum, which contributes significantly to its resistance to crevice corrosion and pitting corrosion. In fact, 2404 duplex stainless steel not only has twice the yield strength of 316 stainless steel, but also has improved atmospheric corrosion resistance.

Because the use of stainless steel in large structural applications is a relatively new practice, steel mills do not usually produce I-beams in standard sizes like carbon steel products. However, I-beams that are mass-produced often use more material than is necessary. Customization allows engineers and designers to specify the exact section size required, saving unnecessary metal. As a result, each section on the Söderstrom Bridge was tailored to the environmental conditions of the archipelago’s brackish waters.

The one-meter-high I-beams were fabricated at Outokumpu’s Plate Service Center. The pieces were cut from the plates using a water jet, submerged arc welded and then pickled. Stal & Rormontage then welded the prefabricated I-beam sections manually using TIG/MIG welding. The 48 sections, ranging in length from 16 to 24 meters and weighing up to 18 tons, were transported to the site by barge and installed. To minimize disruption, the bridges were replaced one by one, with commuters being relocated to alternate maintenance tracks. Replacing each of the 12 sections of the bridge took two weeks, with the old structure removed at night and the new welded during the day. In total, an impressive 600 tons of 2404 duplex stainless steel was used in the new sections.

Prefabricated bridge sections are welded manually in the workshop © Stål& Rörmontage AB

Offshore crane lifts bridge sections from barge and places them in place © Stål & Rörmontage AB

New track installed just metres from the moving train © Stål & Rörmontage AB 

With replacement work completed in 2021, activity along Stockholm’s busiest railway line has returned to normal. This huge Swedish restoration project is not unique, as many bridges around the world are reaching the end of their service life. The forward-thinking duplex design of the Soderstrom Bridge highlights the benefits of using structural stainless steel for bridges in coastal areas or where de-icing salts are used. Molybdenum-containing duplex stainless steels will play an increasing role in much-needed infrastructure upgrades and new construction projects around the world, for the benefit of the environment and the people who rely on them.