Apartment block PUUMERA

When completed, the PuuMERA apartment block at Kivistö in Vanta will be the biggest individual woodbuilt apartment block in Europe, in terms of area. The project, developed by the construction firm Reponen, was on show at the Vanta Housing Fair.

The preliminary sketch designs for the Kivistö wood-built apartment block area were begun in 2012. The design was launched on the basis of a preliminary master plan and massing studies of slab blocks, transverse to the site, forming closed city blocks with internal courtyards. In the master plan, the site is specified for central area use and construction has to be tied to the street lines on each side of the city block.

When the building owners joined the project, the site was divided into two lengthways, in the middle. At this stage, work also started on a clearer division into flat types, as desired by the clients. The building mass was resolved as a closed block. The roof-level saunas were located and it was possible to study the scheme as a whole, as specified in the master plan, and create street areas and outdoor spaces that were intensive and of high architectural quality.

Originally, the idea was to build the scheme in the form of timber construction from the ground up, but as the project progressed, word came that the area was to be built with as many residents’ parking spaces as possible. That being the case, it was natural to construct the ground floor in concrete to form a deck and build the residential floors in wood on top of it. As well as parking spaces, the ground floor houses auxiliary spaces for the apartments, plant rooms and the shops called for by the master plan. At this point, flexible, easygoing cooperation with the city planners was of key importance, as it made it possible to build the concrete ground floor in addition to the number of storeys permitted in the master plan.

The wood-built residential storeys on top of the pre-stressed concrete deck were designed using the same hybrid construction system as the PuuERA building, completed in Vierumäki in 2011, in which the system became a commercial product and was extensively tested. In terms of size, the Kivistö block is around six times larger than the PuuERA building, and construction took place, sheltered from the weather, in the three-section Gibson Tower – the first time it was used in Finland.

The roof shape is a monopitch sloping in the same direction as the long side of the whole block, with the roof-level saunas and terraces being housed in the higher part. Altogether, there are five residential floors. Double storey-height openings in the building mass proved to be extremely challenging architecturally and from the construction angle. The support for the upper floors was studied in close cooperation with the open-minded structural engineer and, in the end, solid timber V-columns were used, aligned with the loadbearing structure.

Laminated timber balconies were studied thoroughly with the end result that three different types were used. A more refined version of the balcony developed for the PuuERA building at Vierumäki was produced, in which laminated timber columns are located outside the balcony glazing where they are easier to maintain. In addition, the loadbearing beam system has been simplified. We also set out to develop two entirely new balcony types, as building the whole development with only one balcony type would not have been sustainable in terms of townscape. We thus decided on a balcony with a central column, the ‘serpent’ balcony, in which the solid parts skirting the central column vary on each floor, and the ‘container’ balcony, which is completely suspended from the facade.

The colouring of the facades was also studied from different starting points. The original idea was to have a light-coloured building mass with the backdrops to the balconies forming a colourful fabric. The spirit of the master plan, however, was better fulfilled as a negative version of this, with darker shades used for the timber boarding and the balconies repeating as white. However, the courtyard side is kept as light as possible as a counterweight to the colourful external envelope, as the space is narrow and intensive.

The apartment balconies act as alternative means of escape, so the surfaces have to be of non-combustible materials, as in the cladding to the ground-floor and the courtyard deck. The building is sprinklered throughout with a high-pressure water-mist system right down to the staircases, balconies and car parking. As an exception to the surface material classification category, we were allowed to build the stair-flights out of solid wood, because of compensatory fire-safety measures. In this, we took the lessons learned from Vierumäki a stage further and the stairs were constructed with a centre string to which laminated timber treads, worked using CNC, were attached, with a wear-resistant granite plank on top. We were also allowed to leave the untreated solid timber construction visible, contrary to the regulations.

Construction

The storeys constructed of wood and of concrete differ from each other due to the 400 mm in situ concrete vault, where the cold car park has a post-tensioned slab, and on top of the warmer spaces is a reinforced in situ slab. The timber construction above is located partially on a different line from the concrete structure beneath. The in situ vault construction was challenging, because uneven thermal movement takes place between the cold car park with the warmer internal spaces above it, and the cold internal courtyard. The lowest floor of the building is a ventilated hollow slab with foam-glass insulation beneath it, which acts as a base for groundworks.

The wood frame of the building is of stud construction with large elements and composite intermediate slabs and beams. The partition elements are cross-framed, as are the lower and upper beams. In the intermediate floor junctions there are different element beams to support the intermediate floors. Wall thicknesses are defined according to the intermediate floor junction. This is being developed further for coming projects and in future, the walls will be thinner. Stiffening stresses are controlled by the partitions, which have boarding and anchors which are checked on the basis of stiffening calculations.

The external walls are of laminated timber stud construction (270 mm, 600 c/c), fitted with facade panels and boarding at the factory. Windows and balcony doors are also installed at the factory and all metal flashings are complete. Only one horizontal panel with its corner battens was installed on site, in the joints between the elements. The intermediate floors were finished wooden elements that were screeded on site. The frame of the intermediate floor elements is made of split laminated timber with connector plates in the deck to take care of the adhesion of the screed. The stairs are in fire-proofed laminated timber, of surface class B, which is approved by both the fire authorities and the building control authorities. On the basis of measurements taken, the sound insulation in the project is excellent

The roof construction is essentially in spatial elements built of trusses, with the 600 mm parts in between being constructed on site. Wall panels, ventilation pipes, fire-stops and soundbreaks, eaves flashings, bottom layer of felt, etc were all installed at the factory. The wide joints turned out to slow the work down too much, so that in forthcoming projects these, too, will be finished at the factory.

VTT has defined the building as passive in terms of energy. It has an air-leak factor of less than 0.6. The building uses solar panels for electricity generation. The U-value of the walls is 0.12 and of the roof 0.08. Floor to floor height is 3200 mm.