City of Helsinki : Oodi to steel arc – installation begins

The construction has lasted for a year and a half, and Helsinki Central Library Oodi (Ode) is now beginning to take shape. Next week, YIT will start assembling the building's two longitudinal steel arcs that have a length of about 100 metres. The aim is to complete the arc installation work by the end of March.

The steel arc parts will be transported to the site during February-March. Transports arrive at the site once a week and are so massive-one part of a single arc weighs more than 85,000 kilos-that they are carried out at night when there is little traffic. The arcs are transported from the plant of Normek Oy, the arc manufacturer, from Oulu. Normek has also implemented the pedestrian bridge between Kalasatama and Mustikkamaa in Helsinki. The experienced installation technicians who built the Mustikkamaa bridge are also participating in the erection of Oodi's arcs.

'It will take at least a week to weld a single arc seam with two welders working 12 hours a day. The arcs are welded from both the inside and outside. The temperature inside the arc reaches as much as 120 degrees,' says Markku Roininen, head steel structure supervisor at Normek Oy. For installing the arcs, a 350-ton tracked lattice boom crane from Havator Oy is in use at the site, a machine fairly seldom seen at building construction sites.

Helsinki Central Library is made up of a combination of a rare steel-framed bridge and building. Steel trusses, which will support the frame of the upper floors, will later be supported by the steel arcs now being erected.

'Due to the frame's arc solution, specific clearance must be taken into consideration in the structures. When structural load is imposed on the arcs, they are pushed downward. For this reason, we are not making any binding structures before all the designed structural load has been placed on the arcs,' says head supervisor Tero Seppänen, explaining the arcs and the actual construction work.

There will be a clearance of some dozens of millimetres in the complete building. The yield resulting from this is so small that visitors will not notice it.

Helsinki Central Library, with a gross floor area of around 17,000 square metres, is one of the projects launched to mark the centenary of Finland's independence. Arkkitehtitoimisto ALA Oy is the chief architect for the project. The library will be opened in December 2018, and its construction costs total EUR 98 million.

Layout: YIT