SSAB Weathering steel in pedestrian bridge design: Staying strong and lasting long

Photo: Pekka Vuola & Finnish constructional steelwork association

SSAB product used: SSAB Weathering 355
Total length: 300 m (984 ft.)
Fabricator: Steel Group Pohjanmaa Oy
Designer: Sweco Infra & Rail Oy
Owner: City of Hyvinkää, Finland

A pedestrian bridge that spans old and new

The first mention of the Hyvinkää region dates all the way back to 1495. The development of the current region, located about an hour’s train ride from the capital of Helsinki, was transformed by the railway line running through it which was completed in 1862. Today, the new Old Church Bridge (“Vanhankirkonsilta” in Finnish) in weathering steel has been built above the tracks, linking the Finnish Railway Museum and historic Old Church area with a new settlement.

Sweco Infra & Rail Oy won the tender for the bridge’s detailed design. The company is part of the Sweco Group, Europe’s leading architecture and engineering consultancy. Besides the main design, they were also tasked with project management, structural, geotechnical, electrical and street design, risk evaluations and expert assistance during construction.

BIM calculations for the steel main span were provided by CÉH, a leading building and bridge design firm in Hungary. All other spans and fitting were done by Sweco Infra & Rail.

Tied arch bridge made in weathering steel

The Old Church Bridge is designed partly as a tied arch bridge (also known as a bowstring arch bridge). When a load is placed on the deck, its force is eventually transferred outward along the curve of the arch, via inclined hangers, to the supports at each end.

The entire bridge consists of three superstructures: two composite-structure access bridges at each end plus the tier arch bridge section in between which spans 55 meters (100 ft.) over the railway line. The total length of the bridge is 330 m (1,083 ft.). With long spiral ramps at the ends, this pedestrian bridge is also bicycle and wheelchair accessible.

Why weathering steel in bridge design

The natural, rustic look of Hyvinkää’s pedestrian bridge makes a nice fit in its historically significant surroundings. But equally important benefits of SSAB’s Weathering steel in a pedestrian bridge design like this include improved atmospheric corrosion resistance, lower maintenance, longer service life and more rapid construction.

Unlike the rusting process of conventional structural steels, weathering steels form a stable, less porous rust patina under alternating wet-dry periods that provides a protective barrier over time and reduces the corrosion rate. They can be used in a wide range of bridge superstructure and substructure components under the right conditions.

Because tied arch bridges don’t depend on horizontal compression forces for their integrity, they can be built off-site and transported into place. The bridge manufacturer Steel Group Pohjanmaa (formerly Normek) produced the components in their workshop in autumn 2020 and began on-site installation in 2021.

Sami Torvi, project engineer at Steel Group, explains this benefit:

“With the prefabricated weathering steel structures, there’s minimal disruption to the site,” Torvi says. “We did the final installation by lifting during the night to avoid disturbing the train traffic, instead of pushing and jacking the bridge up above the rails with temporary supports.”

No paint for less maintenance, shorter lead times and environmental gains

Torvi also liked the fact that SSAB Weathering steel doesn’t require painting and thus helps to cut project lead times. Ordinary carbon steel requires coating after fabrication.
“It’s one process step less in the workshop, and there’s no need to manage any painting on site, which is always a bit tricky when you’re working in changing weather conditions. And it’s much more environmentally friendly when you don’t have to use industrial paints. Plus, you don’t need to do any maintenance painting, which usually has to be done after 5–10 years.”

The future beyond S355 steel strengths

The Old Church Bridge was built in SSAB Weathering 355. Weathering steels were, until recently, available in grades with strengths comparable to S355 steels. But after the latest revision to European standard 10025-5, the strength grades are higher and include S420 and S460.

SSAB accordingly launched its own Weathering 420 ML and 460 ML steels – something that Janne Wuorenjuuri, head of bridges at Sweco Infra & Rail in Finland, is keeping an eye on. This was his first bridge project of this type using weathering steel. But he said he’d be “interested in hearing more” about SSAB’s higher grades of thermomechanically rolled structural steels. These steels provide improved weldability and strength-to-weight advantages that enable the use of thinner, lighter gauges. The results: less steel used, reduced embodied carbon and cost savings.