An innovative double facade using a Cor-Ten steel framework and panels has been completed on a new building in Helsinki.
The Finnish capital is a gracious granite city with relatively few tall buildings and the new Itamerentori Building, by architect Helin & Co, soars above the modest skyline.
But the building is notable not only for its height but also for the contrasting use of materials.
The cladding consists of shiny stainless steel panels, contrasting with the warm matt colour of corrugated Cor-Ten steel panels and double-glazed units.
The principal elevations are additionally clad with a separate outer glass skin supported by a framework of Cor-Ten steel. Steel is also used for the structure.
The building has a five-storey podium, flanked by a slender 16-storey tower that acts as a landmark for the new business area of the city and the western approach road. It sits in the heart of Ruoholahti, once an industrial suburb that is now rapidly becoming a new business 'centre of learning', with streets lined with sleek new, but fairly low-rise, office blocks.
One side of the five-storey podium faces Itamerentori Square, the main pedestrian hub, the other curves to follow the line of the road, the main artery into the city from the west. The podium, containing open-plan and cellular office floors, is set around a gallery of nearly 1,000m 2with a barrel-vaulted glass roof. Inner office spaces have glazed walls that give views out over it. The gallery, containing a supermarket and a cafe/restaurant, is open during office hours and can be used by the public as a shortcut to the nearby metro station.
The barrel-vaulted glass roof of the gallery spans 30m and is formed of curved steel tubular members braced with a delicate lattice of steel tie-rods. The north facade of the gallery is fully glazed and its light steel structure is stiffened against wind load by steel frames that also act as supports for footbridges connecting various parts of the building.
Additionally, the tower is sculpted to give it a mast-like form when seen from the sea.
Originally intended to bring together Finnish science and technology organisations, the building houses the Finnish National Council for Research and Development (SITRA) in the tower, the top floor of which contains conference facilities and a sauna. The Finnish headquarters of auditor PriceWaterhouseCoopers is in the fivestorey podium.
Structure and vertical service runs are designed to give minimal interference with office spaces, and partitions can be adapted in various configurations. Vertical louvred slots beside the windows, and external venetian blinds, allow office workers to control light and ventilation.
The structure is of steel columns and beams with precast-concrete floor slabs. The choice of cladding material was dictated partly by the requirements of the local master plan for 'metal and glass' facades, and partly by the client, SITRA, which wanted a material which was both innovative and environmentally sustainable. Cor-Ten fulfilled these objectives.
In addition, its dark terracotta colour makes a subtle reference to the past - to the massive red brick warehouses and power stations that were built in old industrial Helsinki, which surrounds the new 'silicon valley' of Ruoholahti.
The principal facades of double-glazed fixed lights in deep birch frames and spandrels of horizontally profiled Cor-Ten panels are enclosed in an outer glass skin supported by a delicate framework of CorTen members.
The architect has designed the spandrel panels with adequate gaps around the edges for ventilation and with profiles which allow rainwater to run off cleanly.
So innovative are these details that the manufacturer plans to publish them as guidance for future Cor-Ten buildings.