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Tiistilä’s wooden school of nearly 1,000 pupils promotes carbon neutrality goals of YIT and City of Espoo

One of Finland’s largest wooded schools will be completed in 2024 in Espoo’s Tiistilä. Various options were compared at the beginning of the structural engineering, and a successful outcome was ensured through cooperation between the structural engineer and the frame supplier.

When finished, Tiistilä’s wooden school in Matinkylä, Espoo, will be one of the largest timber schools in Finland. It will provide diverse facilities for 990 pupils, 110 employees and 168 children in early childhood education and care. The deep-framed wooden school of roughly 12,165 gross m2 will be leading the City of Espoo towards carbon neutrality by 2030, and this is also YIT’s goal.

‘Our aim is to halve greenhouse gas emissions from our operations and self-initiated projects, which will require practical actions, such as wood construction projects,’ says Project Manager Samuli Torkkola from YIT. ‘Due to its size, Tiistilä is a good test and reference project in wood construction for both us and the City of Espoo.’

During the project, YIT carried out carbon footprint calculator. Across its life cycle, the carbon footprint of a timber-frame school heated with geothermal heat will be 32% smaller than the carbon footprint of a school with a concrete frame and district heating. The emission reduction will be about 5,066 tonnes.

Collaboration between the structural engineer and timber frame supplier

The collaborative project management project between the City of Espoo and YIT began with a development phase. Sweco serves as the project’s main structural engineer, and it’s responsibilities have included the design of concrete elements and machine shop components. The responsibility for designing the wooden elements was shared with the timber frame contractor VVR. Torkkola praises the cooperation with the selected partners.

‘The cooperation went smoothly, and it was a pleasure to work with such solution-oriented people towards a common goal.’

According to Juliana Kalkkila, Project Manager for Timber Construction at Sweco, YIT and Sweco have worked together at several sites, such as Mansikkala´s wooden school building. ‘Projects with concrete structures include the PPP-schools in Espoo, Nummi service centre and Vuosaari General Upper Secondary School, which also won an international architecture competition.’

The main structural solution was applied in several ways

In the educational facilities, load-bearing frame walls and open-web floor trusses were used as the main structural solution. Due to fire safety and acoustics requirements, they were almost completely covered with gypsum panels. Some of the load-bearing walls are glued laminated timber frames, thanks to which the wooden frame could be left visible.

‘In addition, some of the walls are fitted with glazing, providing light to the central parts of the deep-frame building and serving as space dividers,’ Kalkkila says. On the roof, the main solution is a turned-up slab, gable walls and roof slabs.

These basic solutions had to be used in many areas. According to Kalkkila, there are some multi-ply glued veneer timber beams and pillars inside the load-bearing framework, which enabled larger openings. The long spans in the lobby and gymnasium also required structural solutions that differed from the rest of the frame. The gym consists of glued laminated ridge beams and pillars within the building.

‘The glued timber structures were left exposed, and wood is also showcased in the school’s facade cladding and central lobby.’

Large weather protection tent required temporary additional supports

Despite its complexity, the school project has remained within the budget, which indicates successful procurement, according to Torkkola. Different timber frame options were extensively tendered at an early stage. ‘It enabled us to use products that suited the supplier’s manufacturing process, which guaranteed quality and cost-effectiveness.’

The aim was also to achieve a high pre-fabrication level, which added its own requirements to the design. For example, the openings in the lightweight partition walls had to be designed differently than usual in order to maximize the benefits of prefabrication. ‘The frame elements arrive at the worksite with the facade ready, which speeds up the transition to the next steps,’ says Torkkola. Compared to building with concrete, the clean indoor work phase can begin quickly and the different phases overlap with each other. ‘While the structural installation is taking place at one end of the building, floor surfaces are already being started at the other end.’

The wooded school is being built in accordance with the Kuivaketju10 (‘Dry Chain 10’) and under a huge weather protection tent, divided into two blocks of approximately 67 x 100 metres. First, about half of the building was completed under the tent, and then the tent was moved to the next block. The second-phase tent was supported on one side to the ground and on the other onto the completed part of the roof. The worksite’s location in a windy place near the sea further increased the challenges of supporting the tent.

‘During the installation phase, the structures were subjected to higher loads than in the final state, so in addition to the actual stiffening structures, temporary additional bracings and counterweights were used during the construction,’ Kalkkila says.

Furthermore, designing the joints for the lobby and gym structures was challenging due to the accident consequence class CC3b’s requirements. Even if an individual structural part loses its load-bearing capacity, the structures may not collapse and cause a hazard to the building’s users. ‘All the structures and their joints have been approved by a third-party inspector.’

Picture: Samuli Torkkola YIT

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