SSAB Weathering steel defies marine conditions on the Huikku Bridge in Hailuoto

“They are impressive structures, not just matchsticks,” laughs Henrik Kiviniemi, Business Director at Steel Group Pohjanmaa Oy, the company which is responsible for the engineering of the bridge steel structures.

SSAB Weathering 460ML steel plates are delivered to Steel Group's workshop as prefabricated products from SSAB Europe's mill in Raahe. "There the shape is cut as required for the flange or groove, and the bevels for welding are partially finished," explains Kiviniemi.

At the Steel Group’s workshop, the pieces are prefabricated into sections, which are then transferred to the bridge construction site for installation.

"The thermomechanically rolled weathering steel plates have good weldability and do not usually require preheating. The material is easy to work with in every way," says Kiviniemi. He also praises the fact that the materials arrived at the workshop on time and that the schedule has been kept. Long-term cooperation and experience have only increased trust.

“We have also been working with SSAB’s welding experts, and able to go through the welding parameters and other details, right down to the choice of wire,” says Kiviniemi.

The massive bridge sections are over 4 meters high, 6 meters wide, and 26–32 meters long. In addition, SSAB Europe’s Pulkkila factory has supplied around 150 tonnes of weather-resistant SSAB Weathering Tube 355WH for the project. The tubes connect the sections together.

SSAB Weathering is suitable for demanding conditions

The Hailuoto Causeway project is being built by the alliance organisation. Tuomo Järvenpää, chief designer of the Huikku Bridge and head of design at AINS Group, says that planning for the project began in 2022.

“During the development phase, various options for the Huikku Bridge were explored, including the materials. After comparing the risks and costs, it was decided that a composite girder bridge was the way forward.”

In addition to the appearance and overall structure of the bridge, the water permit specified a light blue color so that the bridge would blend into the landscape rather than stand out. The span-to-rise ratios of the bridge were optimized to achieve a more balanced design in relation to the road plan.

“When selecting materials, weather-resistant steel emerged as a good option for the low-salt marine environment and for a bridge location where maintenance painting and servicing above the water is difficult and expensive. Painting costs are reduced, and the bridge span doesn’t need to be painted,” says Järvenpää. The number of maintenance breaks affecting bridge use will also be reduced.

During the construction planning phase, 460MPa was selected as the steel strength, which reduced steel consumption and lightened the weight of the girders to be installed without compromising strength.

Maintenance-free patina surface protects against corrosion

Due to its good corrosion resistance and low maintenance requirements, SSAB Weathering steel is a suitable choice for bridge structures on the coast of the Bothnian Bay. It can also withstand northern conditions: weather-resistant steel retains its impact toughness even at temperatures as low as –50 degrees Celsius, says SSAB's Technical Development Manager Erkki Krankkala.

“Weather resistance is achieved through the chemical composition. Over time, it forms an oxygen-impermeable patina layer that protects against corrosion without the need for separate surface treatment,” Krankkala continues.

“However, if the steel is painted, as was done on the exterior surfaces of the Huikku Bridge sections for aesthetic reasons, the paint will adhere better than on ordinary carbon steel.”

Krankkala points out that scratches are not a problem for the weather-resistant steel's patina surface. “The patina repairs itself".

FACT BOX:
- SSAB Products used: SSAB Weathering 460ML, SSAB Weathering Tube 355WH
- The Hailuoto Causeway project is built by the alliance organisation: The Finnish Transport Infrastructure Agency, GRK Finland Oy, AFRY, and AINS Group
- Fabricator: Steel Group Pohjanmaa Oy