The Söderström railway bridge was originally built from carbon steel in the mid 1950s. Today, it is an essential link to carry Stockholm Metro commuters across one of the city’s main waterways. It is one of Sweden’s most heavily trafficked railway bridges and carries 340,000 passengers per day. Four tracks carry north and southbound traffic for the red and green lines. The bridge also has a fifth track for maintenance and service trains, which has had less use over the years.
Faster to install new superstructure
A survey in 2013 by Trafikverket, Sweden’s transport authority, found that although the bridge foundations were in good condition, the carbon steel superstructure was extensively corroded in some areas. In addition, the survey found that the bridge had already exceeded its planned fatigue life.
Ideally, coastal carbon steel bridge structures should be protected by regular painting. However, there are no alternative routes to carry traffic on the Metro’s red and green lines. This limits the possibility for closures and the amount of maintenance that can be carried out.
SL estimated that it could replace the carbon steel superstructures faster than carrying out repairs. Therefore, it was a logical for SL to order new superstructures for the four metro tracks. It devised a program to replace the tracks individually, using the spare track to keep metro trains running without interruptions.
Life Cycle Cost analysis
Trafikverket carried out a Life Cycle Cost (LCC) analysis to find the solution that provided Stockholm’s taxpayers with the best long-term value. LCCs cover all of the costs associated with infrastructure over a lifetime. They include the cost of initial design and engineering, purchase and fabrication of material, logistics, transportation and installation. They also cover the cost of inspection, maintenance and repair, as well as end-of-life decommissioning and scrap value of the raw materials.
At €2 million, the purchase price of a carbon steel superstructure was significantly lower than €4 million for a duplex stainless steel alternative. However, extensive painting during its life would cost many tens of millions of Euros. In addition, maintenance would require extensive closure of lines and put teams at risk by working on an electrified railway over water. Therefore, duplex stainless steel was the least cost option.
The LCC analysis identified Outokumpu’s Forta LDX 2404 as the preferred material. This is a ‘lean duplex’ grade of stainless steel, named because it contains lower levels of high-value alloy materials compared with other duplex grades of stainless steel.
It is strong and resistant to corrosion from the brackish water in Stockholm. As a result, it enabled Trafikverket’s main contractor Stål & Rormontage to create a structure capable of a -120-year lifetime in this environment.
Prefabricated material kits
Stål & Rormontage fabricated the bridges in sections of 16-23 meters long. It sourced prefabricated material kits from Outokumpu’s Nordic Plate Service Centre (PSC) at the Degerfors steel mill in central Sweden.
Mladen Perkovic, Head of the PSC, said: “Because stainless steel has not yet been widely adopted for structural applications, standard sized I-beams are not produced by steel mills. That frees up engineers to specify the exact beam cross section that they need.
“We can deliver beams that have top and bottom flanges of different thickness or widths, a web that tapers or any design that you can imagine. This avoids over-engineering and allows for lightweight structures that also provide the highest levels of corrosion resistance.”
In the case of the Söderström bridge, the PSC produced I-beams of 6-14 meters long and around 1000 mm high, and also carried out bending of plates to form components for truss structures. In total, the PSC provided material for 48 bridge spans with a total length of almost 800 meters. It’s probably the world’s longest stainless steel railway bridge.
Twelve-day installation for each bridge
Stål & Rormontage took delivery of the material kits at its workshop in Sölvesborg to produce 12 bridge spans for each track with a total length of almost 800 meters and weighing up to 18 tonnes.
The fabricator then transported the bridge sections to site by truck and barge so that they were ready for installation with the support of a 220-tonne mobile crane on a barge. Each bridge required a traffic shut-down over three summers up to 2020.
Welding of bridge sections and bearings was carried out during the day. However, all demolition and assembly tasks had to be carried out within a four-hour window overnight when the bridge was free of traffic.
Inspiration for structures around the world
The Söderström bridge is just one example of how stainless steel can be highly competitive for heavily trafficked bridges, as well as bridges over tracks or highways that are difficult to inspect and maintain. Another example is the Añorga bridge in northern Spain, which was built in 2011 from Forta LDX 2101 for a 130-year lifetime.