Treet or "The Tree" in Norwegian proves that sustainable solutions for buildings in an urban context can indeed be achieved by working across different sectors to create a cost-efficient, modular high-rise building using a prefabrication process. Finding inspiration in the long history of wood construction in Norway and, in particular timber bridge building, the decision to build a tall wood high-rise in Bergen was definitely appropriate. Along the way, Treet has also set a new record for being one of the tallest timber buildings to date.

The building was constructed in modules. Each module complies with Passive House standards. The modules have been stacked together on site. Every fourth level is covered by a load-bearing framework structure (Power Story) composed of glulam truss work (beams, columns and diagonals). After this level, there is an independent prefabricated module which has a platform at the top made of a reinforced concrete deck. These concrete slabs were needed on two different levels in order to give the structure the needed weight to prevent it from swaying in windy conditions.

The building was then clad in metal and glass to protect the wood elements from the weather conditions.

Regarding the energy concept, the modules were further improved in order to achieve the Passive House standard as well as attention to detail concerning the ventilation system.

Location

Damsgårdsveien 99, Årstad, 5058 Bergen, Hordaland, Norway

Project team

Project management and owner: BOB BBL (Norwegian housing association)

Architect design: Artec AS

Structural engineering: Sweco Norge AS

CLT and glulam manufacturer: Moelven Limtre AB

Wood modules manufacturer: Kodumaja AS

Consultants: NTNU (Norwegian university of science and technology), Treteknisk (Norwegian institute of wood technology) and Trefokus AS.

Development and innovation support: Innovasjon Norge (public institution for innovation and development in Norway)

Time schedule

Master plan: 2005-2009

Engineering design and building permits: 2010-2013

Construction: 2014-2015

Resident occupation started in December 2015

Building use and area

Total Constructed Floor Area: 7,140 m2

Total Constructed Net Floor Area: 5,830 m2

Total Treated Floor Area – TFA: 3,780 m2

Treet is a 14 storey residential building which is 52.8m tall (Concrete garage floor + 14 Wooden floors + Tower of stairs and elevators). The maximum vertical distance between the lowest and highest points of the timber components is approximately 49 m.

There are a total of 62 apartments – 11 one-bedroom apartments (43 m2) and and 51 two-bedroom apartments (64 - 66 m2). The apartments on the 5th and 10th floors incorporate the concrete deck platform, so they are slightly smaller. However, most of the flats have their own balconies. There is also a terrace at the top of the building, on the top of the 13th and 14th floors. The 9th floor includes a communal gym that offers views of the city and fjords.

Construction costs

Total project cost:

22 M € including costs for buying the area, developing the concept (R&D), engineering costs and construction costs, as well as all internal costs in BOB (project management etc)

Average price for an apartment: 6,500€ per m2

Envelope performance

Treet is built on top of a concrete garage (level 0). The construction process was divided into three stages in ascending order: 1st-5th, 6th-10th and 11th-14th storeys.

Structural elements

-Modules: The wooden frame modules are the size of an apartment and were directly assembled on the building site. The modules were improved from the original design concept, even though the insulation and air tightness qualities were already according to standard Norwegian regulations and/or low energy buildings. This was done in order to achieve Passive House standards and was attained by incrementing the insulation of the exterior walls and roof as well as focussing on air tightness and improved U-values on windows.

The modules were 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 of the building was significantly shorter with the developers able to erect 4 storeys in only 3 days.

- Glulam (550 m3): The main load bearing is handled by glulam truss work alone. Large glulam sections are block glued. A typical column is 405 x 650 and 495 x 495 mm and a typical diagonal is 405 x 405 mm. The levels with only stacked wooden modules (up to four levels high) are connected one by one at bottom slabs.

- Cross-Laminated Timber (CLT) (385 m3): CLT is used in the staircases, elevator shift (15 stops), some inner walls and balconies, but is not structurally connected to the glulam.

-Concrete deck platforms: 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. These elements are concrete deck platforms located at the top of levels 0, 5th, 10th, 13th and 14th to improve dynamic building behaviour with the extra weight.

-Connecters : All wooden elements use plated truss connectors (slotted-in steel plates) and dowels.

-Permanent weather protection: To protect the glulam structure from strong winds, the building has glassed-in balconies installed on north and south facades. This gives the building a unique appearance, with the glulam structural elements being visible through the glass facade. The east and south facades have metal cladding on the walls which are lined and insulated.

Envelope elements

-Exterior walls: U-value average is 0.12 W/m2.K. Insulation is approximately 350 mm

-Roof: U-value is not detected, but insulation is approximately 500 mm

-Windows and doors: U-value average is 0.8 W/m2.K

Air leakage factor: 0,5 (at 50 Pa pressure difference)

Energy consumption

Primary energy consumption: 84 kWh/m2.year

Final energy consumption: 71 kWh/m2.year*

Energy label: A

Calculation method is according to NS3700/3701

(*) Final consumption breakdown:

Space heating: 3 kWh/m2.year

Ventilation heating: 4.5 kWh/m2.year

Domestic hot water: 29.8 kWh/m2.year

Fan administration: 4.8 kWh/m2.year

Pump administration: 0.1 kWh/m2.year

Lighting: 11.4 kWh /m2.year

Technical equipment: 17.5 kWh/m2.year

Energy systems

Ventilation system with heat recovery

Each flat has its own balanced ventilation system with heat recovery having efficiency above 80%. Ducting and choice of fans are designed to ensure low fan operating energy consumption.

District heating system for heating and DHW

There is an internal heating system connected to the local district heating system via heat exchanger. This supplies room heating (when needed) and hot potable water via a heating distribution cabinet in each flat. Energy consumption is measured in each flat.

Carbon footprint and lifecycle of the building

Timber is flexible, sturdy, and lighter than materials like concrete or steel and has a high strength-to-weight ratio.

There are significant environmental benefits: Buildings made primarily of wood have a significantly lower carbon emissions and use less energy than those made from traditional materials.

"Wood in the construction binds CO2 throughout the building’s lifetime… Wood products used, store approximately 1,000 metric tons of CO2. 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." (as cited in Malo, K.A., Abrahamsen, R.B. & Bjertnæs, M.A. Eur. J. Wood Prod. (2016) 74: 407. doi:10.1007/s00107-016-1022-5).

Additional building code information

Fire safety

Timber is not a pre-accepted material for high-rises in Norway. However, Norwegian regulations open up for alternate materials as long as required documentation is produced. The fire safety requirements are done according to the Eurocode.

Timber can burn and, in this case, the glulam is thick enough that the estimated burning time is 90 minutes without failing and without extra gypsum used. Also, all steel connections are hidden inside the timber in order to maintain the fire resistance time. In addition there are sprinklers, pressurized escape stairs and painted surfaces to improve fire safety.

Building movement

According to curves given in ISO 10137:2007, residents on top floors might, in rare cases, feel vibrations but it is very unlikely that they will become uncomfortable. The chosen structural solution for "Treet" using glulam truss works and stacked prefabricated building modules gives a robust design and is likely to have insignificant effects from vibrations caused by wind exposure. The calculated value is a 71 mm of maximum horizontal deflection (level 14).

Awards and recognition

Winner of the prize “Årets Trebyggeri 2015” (wooden construction of the year 2015) celebrated during Building Week - 8 March, 2016 (link).

Overall winner of the contest “Prefab house of the year 2016” held in Estonia (link).

Additional information

Official project website

Related news in Buildup

Technical information source in Norwegian language

Video in English language of project design and construction

Timelapse video of construction stages

Video in Norwegian language with explanations during construction stages