Helsinki Central Library Oodi was a builder’s welding challenge

The Helsinki Central Library, Oodi, which was completed a year ago, has received a lot of incense from its architecture. The magnificent building looks like wood, glass, and stone, but, beneath the surfaces, it is a very steep welded steel structure. Linde supplied welding gases to Normek, who made the library’s steel structures, throughout the project.

The building received the European Steel Structure Award last year.

Helsinki Central Library: a welding wonder

“Ode is a unique building. It is half a house and half a bridge. For us as a supplier of steel structures, it was a unique job in terms of the amount of welding, quality requirements and the size of the parts to be processed”, says Pasi Parkkinen, CEO of Normek, who acted as the project manager for the project.

The backbone of the ode is formed by two steel arches over a hundred meters, which stand on large base elements. The arcs are tilted away from each other. At the end, the distance between them is more than five meters. On the ceiling, the distance between the arches is already more than twenty meters. The arches carry the weight of the entire building. On the trusses built between them rose the upper floors of the building than the decks of a large road bridge. The exceptional structure allowed for an almost full-length open lobby space without pillars.

28,000 tons of steel

The steel structures were manufactured by Normek Oy, a member of the German Donges group of companies, in Oulu, Naarajärvi, and Alavu. As the plot in the center of Helsinki was very cramped, the structures were manufactured as far as possible in machine shops, and the car was lifted off the platform for direct installation.

Normek supplies 28,000 tons of steel structures per year. Deliveries to Oodi were not even ten percent of this, but Parkkinen still considers it an exceptionally large job. “Not so much in tons as in hours.”

The amount of welding increased due to the exceptional material thicknesses. Up to 120 mm plate was used in the lower parts of the arches. “Such thicknesses are not used even then,” says Parkkinen. The heaviest structures such as arches and arch end housings were manufactured at Normek’s Oulu workshop. The arches were transported to the site in fourteen blocks – seven in each arch. A total of 800 tons of steel was used for the arches.

Under the arches, 63 square meters of steel base elements, each weighing one hundred tons, were built at each end.

According to welding coordinator Tomi Nykänen, the manufacture of the base structure involved a lot of welding. The 400 mm thick cell element has vertical stiffeners every few tens of centimeters. Therefore, the elements had to be made as narrow grooves welded together from both their surface and base plates. The last welds of the base elements were made at their final location. Taking them as a whole, transporting them from Oulu to the cramped construction site in the heart of Helsinki would have been an undone place.

“The last joint was difficult because the large welds made on one side twist the whole element into a bend,” Nykänen says. “Before the last joints, welding tests were carried out at the workshop to predict the magnitude of the deformation. Based on the results, the elements were placed on the site at an angle to each other with the help of jacks that the stress caused by welding straightened the lines”.

The calculations had to be done carefully because the ends of the arches had to be brought to the right place with an accuracy of 10 mm.

According to Tomi Nykänen, the company’s goal was to weld as mechanically as possible not only in machine shops but also on a construction site. Conveyors moving along magnetically mounted rails were also used in the assembly of the arches. The welding current, additive, and shielding gas were fed to a conveyor that did the work itself. It was up to the welder to monitor and operate the remote control.

Normek often uses installation companies on construction sites, but in the case of Oodi, only one’s installers and work management could be relied upon.

The welders were hired from a partner, and they came for a week of training at the Naarajärvi plant before starting the installations. During the training, the welders, among other things, performed the necessary proficiency tests and became acquainted with the use of welding machines and conveyors coming to the construction site.

The installation of the arches began in February 2017, when Helsinki was also in full winter. Under no circumstances could the thick plate used in the arches be welded without thorough preheating. It was cold at the Oodi site, and at its best, there were six 60 kW resistance heaters in use overnight. Had it not been, the next day would not have been welded at all. However, the welders focused well because the arc blocks in the installation shift were heated to 120 degrees against the weld.

Construction of the ode began in 2016 and was opened to the public on December 5, 2018. The limit of two million visitors was already broken in August of the following year. Oodi has won several awards and has been selected e.g. become the world’s best new public library in 2019.