Treet: the tallest timber-framed building in the world

Structural system for the 14-storey building will consist of meter- thick glulam columns in a stacked modular design

Due for completion in November this year, a 14-storey luxury apartment block in central Bergen, Norway, will be the world’s tallest timber-framed multi-family project at 49 meters (160 feet) breaking the current record holder – Melbourne’s Forté building – which stands at 32 meters tall. The building is called ‘Treet’ or ‘The Tree’ and is the brainchild of the Bergen and Omegn Building Society (BOB), which aims to be at the forefront of developing homes for the future, with a strong focus on energy consumption, sustainable development and communal outdoor spaces.

The idea of building a high-rise building by the Puddefjord Bridge was originally suggested by the architect Geir Brekke of Lund & Partnere in 2005, when the site was being zoned. The project 
was subsequently developed by BOB with significant inputs from Rune B. Abrahamsen of Sweco
AS, who designed the load-bearing structure and the use of modules in collaboration with Artec Prosjekt Team. Further, Trefokus AS, the Norwegian Institute of Wood Technology and NTNU have provided expert advice on the development of the project with additional support also coming from Innovation Norway.

During the planning of the project, BOB considered various systems for high-rise timber buildings, including the use of 
solid cross-laminated timber. This system model has been used both in Sweden (Växsjö), London (Murray Grove) and a variety of places in Austria. However, the developers arrived at the conclusion that combining prefabricated building modules with a glulam structure was the best way to successfully realize their vision. Designed by architects Artec and engineered by Sweco Norway, the structure comprises a mix of cross-laminated timber and glulam, built on concrete ground floor.

The tower will consist of a glulam load-bearing structure
and prefabricated modular flats, made from engineered timber manufactured by Moelven Limitre using only Norwegian wood. The concept involves the modules being stacked four storeys high, with
 two platforms (above on the 4th and 9th floors) being anchored to the glulam frame. These platforms are supported and reinforced by 3m-high glulam lattice beams. Another four storeys of modules are then stacked on top of each platform, thereby enabling the developers to build 14 storeys in total.

The building will offer a total of 62 apartments – 11 one-bedroom apartments (43 sqm) and and 51 two-bedroom apartments (64-66 sqm). The apartments on the 5th and 10th floors will have to accommodate the load-bearing structure, which will make them slightly smaller. However, most of the flats will have their own balconies. There will also be a terrace at the top of the building, on the top of the 13th and 14th floors. The 9th floor will include a communal gym that offers beautiful views over the city and the fjord. In addition, the 
side facing the sea will feature
an esplanade and marina; while towards Damsgårdsveien, residents will have access to a park and recreation area.

To protect the glulam structure, the building will have glassed-
in balconies on two sides. This
 will give the building a unique appearance, with the glulam structural elements being visible through the glass facade. The gable walls will be lined and insulated, enabling the project to meet passive house standards, further enhancing its environmental credentials. The project has received a grant from Enova to help it meet those standards.

“A key challenge in building a 14-storey high timber building
 is preventing it from swaying in strong winds. As such, the glulam frame has been reinforced with diagonal glulam braces whilst concrete elements have been installed on the top of the 5th and 10th floors, and on the roof. The concrete elements are not a part of the structural system but have been installed to add weight in order
to reduce movement within the building,” says Ole Herbrand Kleppe, Chief Project Manager at BOB.

All the main load bearing structures are wooden. In addition, two internal decks as well as the top deck are made out of concrete. Further, Tricoya® wood, corten 
steel and glass is used in the façades. The building’s apartment modules have been designed 
to comply with the Passivhaus sustainability standard and have been constructed in a factory 
in Estonia and then shipped to Bergen. Despite the initial cost being somewhat higher than that of a steel and/or concrete structure, the erection time is significantly shorter with the developers able to erect 4 storeys of modules in only
 3 days.

To protect the glulam construction against the weather
in Bergen, two of the sides have balconies and a curtain wall façade. The two other sides are made with extra insulation (total of 430 mm of insulation), and then covered with rusty metal plates, which require no future maintenance. Inside the main staircase, all of the glulam constructions will be fully visible, together with the CLT walls in the corridors.

According to Kleppe, the building is a pilot project to demonstrate that it is possible to build modern city residences out of materials that meet tomorrow’s standards for sustainability and greenhouse gas emissions, while still achieving a high plot ratio. Using timber instead of non-renewable construction materials represents an important step towards reducing global warming. The role of forests as ‘carbon sinks’, whereby the wood stores carbon as long as the tree is alive or is used in a structure, is expected to become increasingly important in the future.

Studies indicate that one cubic meter of structural lumber stores 0.9 tonnes of CO2, which the tree has absorbed from the air. In addition to directly storing CO2,
 the glulam in the structural frame replaces materials such as concrete and steel. This is even more important than the CO2 directly stored by the wood as research shows that the combination of stored CO2 and the replacement of non-renewable materials avoids approximately two tonnes of CO2 emissions per cubic meter of structural lumber.

Kleppe estimates that the building will probably use about 9,500 cubic meters of lumber in
 its load-bearing structures thereby avoiding approximately 18,000 tonnes of CO2 emissions. This is equivalent to avoiding driving a good 105 million kilometers in a petrol 
car that consumes 7.5 liters per kilometer or avoiding more than 210 million crossings of the Puddefjord bridge in Bergen. In addition, there is also the CO2 stored in the wood in the prefabricated building modules. Overall the building will avoid
 more than 21,000 tonnes of CO2 emissions.

“We wanted the project to be a pilot project for the industrialization of the construction process. The aim is therefore to use a high degree of prefabrication, both of the building modules and glulam load-bearing structures. Floor and wall linings in corridors, stairwells, balconies and lift shafts will also be prefabricated, in order to reduce the on-site construction time. The quality of the individual products will be higher, as they will be manufactured indoors 
in a factory, in a dry, controlled environment. Overall, we expect this to improve the final results quality,” concludes Kleppe.

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