An SSAB Weathering steel bridge – built to last

A sustainable bridge that’s built to last, blends in harmoniously with its natural surroundings, and offers a solution to the triple constraints of time, quality and cost. This is what the City of Vantaa was looking for when it commissioned a footbridge for a national park at Lake Kuusijärvi, just north of Helsinki in Finland.

The fabrication and installation of the steel structures for this weathering steel bridge was done by Normek, a leading steel building construction and façade solutions company in Finland. As a specialist in the design, engineering, construction and on-site installation of steel structures for bridges and buildings, it brought its experience in other weathering steel bridges to the project. The delivery included steel girders, center columns and railings, all made in weathering steel.

Sami Torvi, project engineer at Normek, juggled time schedules, material purchases, manufacturing plans and quality documents for the project. He emphasizes the need to take a flexible approach to manufacturing:

“No two bridges are alike,” Torvi says. “We’ve got to be very flexible in our manufacturing processes because it’s very unlikely that we will manufacture two similar bridges. Every product is unique, and we need to think carefully all the way from material purchase through to final installation.”

Welding SSAB Weathering steel is relatively simple and is similar to the process for common low-carbon steels if WPQR is in place. However, the filler metal will differ depending on various factors like the code, job specification requirements, location of the weld, whether it will be painted and whether color matching is needed. Torvi says: "We didn't even have to preheat before welding, and that also contributed to savings in welding and fabrication time."

WSP, one of the top ten design and engineering firms in the world, chose SSAB Weathering 355 plate for the bridge and its main components including girders and supporting columns. According to Sami Niemelä, business area director at WSP Finland, the decision was based on a myriad of factors to meet the client’s and end-user needs: safety, structural performance, maintenance, construction and life-cycle costs, and aesthetics.

When properly detailed, designed and situated in the right environment, weathering steel bridges make a popular choice that can tick all those boxes. Rusting is problematic and even dangerous in other steel structures, and corrosion can threaten the function and integrity of steel bridges in the long run. But the “useful corrosion” of SSAB Weathering steel acts as protection against further deterioration. Rust forms in the steel’s outer layer only and then turns into a barrier that helps resist any future corrosion.

Another key aspect of Vantaa’s bridge design was that the bridge should blend in with its surroundings – a well-known national park that offers smoke saunas, ice swimming, and cross-country skiing right over the bridge. The City of Vantaa also wanted to minimize the bridge’s environmental impact.

“Weathering steel is aesthetically suitable to the natural environment,” says Niemelä from WSP. “We also liked the idea that the color of the steel changes a bit from light brown to darker brown over time.”

Sustainable construction is emerging as a key consideration in bridge design, construction and lifelong maintenance. Weathering steel bridges like this one have impressively long lives – potentially over 100 years – which decreases the need for replacement. And at the end of its useful life, the Vantaa bridge is 100% recyclable.

The bridge thus grabbed the attention of the Finnish Construction Steelwork Association, who gave an honorary award to the WSP Group for Steel Structure of the Year 2020.

When the City of Vantaa and WSP chose SSAB Weathering steel for the bridge construction, they got a material requiring very little maintenance. This will not only minimize disturbance to the site but will also save money in the long run. And since any repairs tend to be less complicated, weathering steel bridges cost less over their lifetime than those made in other materials.

As a bonus, the bridge doesn’t need paint for either protection or aesthetic reasons. The rust patina of the steel is attractive on its own and helps avoid emissions of harmful volatile organic compounds (VOC) from paint coatings.

Sami Torvi from Normek points out other benefits of the no-paint approach.

“Surface treatment costs are one part of the total price of the product, so painting is left out of both the workshop and the construction site,” he says. “Repair of paint damage is omitted, as is maintenance painting. So, overall, this shortens production lead time – a huge advantage – and lowers production costs.”