Wood in the construction process
Car park with glulam and CLT structural frame, Ekorren, Skellefteå. Photo Patrick Degerman.
Modern wood construction
Building with wood has a long tradition in Sweden, with the nation’s oldest preserved wooden buildings dating from the 13th century. Wooden buildings are therefore strongly associated with Sweden’s cultural heritage.
Today, over half of our housing stock comprises wooden buildings, a figure that has grown since the turn of this century. To mark the difference between tradition, the present and the future, “modern wooden construction” is often used to describe construction since the 1994 transition from material-based to function-based building regulations across Europe. Modern wood construction thus has no tradition to fall back on, but at the same time also has no limits. Architecture, materials, construction techniques and construction processes are now in a dynamic phase of development that is made even more exciting by the fact that it is pan-European.
Modern wood construction is characterised by more use of prefabrication, shorter construction times, a more meticulous planning process and quality assurance of the production system. Taken together, these new circumstances entail a different process from concept to completion than is the case in the traditionally organised construction industry. It is much closer to the model that other industrial production systems apply, with clearer specification of the product and its performance right from the computerised drawing board. Similarly, the manufacture and final assembly involves major automation and follows very strict procedures that ensure consistent and correct completion of every project.
There are several ways of building with wood, with the main difference relating to the degree of prefabrication. It is important to understand the link between prefabrication, construction time, moisture content of the material and the need for climate protection, as related to the choice of wood material and production method. These links are described briefly in the section Building with wood, page 79 and Moisture in the construction process, page 81. More and more developers and building contractors will be encountering modern wood construction in the future and they need to deepen their knowledge of wooden structures. This is why this edition of Choosing wood now also has a section on the end product, the building. Wooden buildings are built up using the wood components described earlier in Choosing wood. This section addresses the construction process when wood or wood components are the load-bearing material for the structural frame.
Building with wood
High-rise buildings with eight or more floors, sports halls, car parks and road bridges are structures that are not instantly associated with wood construction. However, wood has become an increasingly common structural material for these since the mid-1990s.
In this context, a wooden structure is defined as a structure where the carcass is made of wood. A wood façade is not part of the load-bearing system, and many of today’s wooden buildings do not actually display the system material, wood, on the façade. And conversely, a building may have a wooden façade and a different carcass material, and such a building is not defined as a wooden building.
Technical advances in wood-based structural elements and the new system for building regulations are the main reasons why wood construction has grown in scope. Modern international research on fire development and fire risks has strongly helped to change the view of fire in buildings. With today’s fire safety equipment, the risk of city fires is now greatly reduced, especially with the predictable way that wood retains its load-bearing function during a fire. This is considered a major plus.
Wood as a construction material has also recently come to drive the advancement of the construction process. It is the material’s strength in relation to its weight that increases the opportunity to build larger and more composite components. It has therefore been possible to substantially increase the degree of prefabrication in wooden structures, and to change the construction site process from manufacture to assembly. This allows for logistical planning at a minute level, as in other industrial processes. Wooden bridges and high-rise buildings built from box units are examples of how industrial construction techniques are changing the construction market – industrial wood construction is simply so much faster.
There are several different techniques for constructing wooden buildings. Stick-built structures may be detached houses or larger buildings. The construction technique is very common in the USA for high-rise buildings of up to 7 floors. This method is suitable for unique buildings and when building without advanced lifting equipment. Most of the work is carried out by tradesmen on the construction site, often outdoors. The basic material is either ready-cut timber or timber from a builders’ merchant or timber merchant that is cut on site. Construction timber is usually supplied with a target moisture content of 16% on delivery and is encased with a surface moisture content of max. 18%.
Site-built walls are usually assembled horizontally on the foundation slab or on an upstairs floor. The bottom plate and top plate are laid out and the vertical studs placed between them. The nailed frame is then raised and secured in place. Beams and roof trusses are fixed to the top plate and a floor or ceiling is fitted.
The whole carcass is normally built as an open structural frame with no sheet material or insulation. A protective roof is then built. With taller buildings, it may be necessary to fit some boards for wind stabilisation while the frame is being built. Only once the roof is weathertight can the installation of moisture-sensitive materials such as insulation and plasterboard begin.
A stick-built house with a wooden façade and a structural frame in wood and concrete, Villa Nilsson, Höganäs, nominated for the Swedish Timber Prize 2012. Photo Åke E:son Lindman.
The construction time on site can be reduced by having parts of the structural frame or the building prefabricated as modules. These may be surface units for walls, floors and ceilings or box units with walls, floor structure and ceiling forming one or more rooms. The degree of prefabrication varies; in the most complete units, box units, the surface layer and installations for electricity, telephony, data and plumbing can be pre-installed, so that it all simply needs to be connected on the construction site.
Assembling a box unit, Vinsta, Stockholm. Photo Johan Ardefors.
Walls as surface units often have the same structure as traditional stud walls. They are usually pre-insulated and ready for internal surface treatment. Exterior wall units may also have the final façade cladding fitted and primed. Window frames are generally also pre-installed.
Surface units are also available as a solid wood structure, with the wall made from a solid sheet of cross laminated timber (CLT). The wall unit may then be built up around the CLT frame in various ways. The edges are profiled to fit with the floor structure and create a complete, load-bearing system.
Walls separating apartments in a high-rise block are manufactured in two halves for sound insulation reasons. They are then assembled next to each other, but without material contact.
Surface units for floors in low-rise buildings often have the same structure as stick-built wooden floor structures, with load-bearing joists and a sheet material on top. The spaces in the floor structure are filled with insulation to reduce sound transmission. Floors in high-rise blocks require more advanced structures to achieve fire barriers and impact sound insulation between different apartments. These often comprise two layers, one structural and one as a fire and sound barrier. The load-bearing structure may comprise traditional beams, a CLT sheet or a cassette structure.
Building with surface units is a very common method that is used for all types of wooden building, low-rise housing, high-rise apartment blocks and office blocks. This construction method requires some form of lifting equipment that can handle weights of up to 3 tonnes. A truck mounted crane can usually lift as high as 3–4 storeys.
Wall and floor units are generally delivered fully finished, with insulation and a surface layer, and so must be protected from rain during the construction period. Low-rise buildings are usually assembled in a day and so can have a weathertight roof on that same day. High-rise buildings that cannot be assembled in a day require some form of protective covering system. Many such systems are available on the market.
The use of surface units can normally allow around 20% of the on-site construction work to be moved to an industrial workshop. Building with box units is a wood construction technique that moves up to 80% of the work indoors to tailored manufacturing premises. The high degree of prefabrication significantly shortens the time needed on the construction site.
Box units may contain one or more rooms or parts of the building, such as bathrooms with all their installations. The units comprise walls plus floors and ceilings. They form their own self-supporting unit, which is linked up with other units.
The degree of prefabrication varies, but it is common for installations such as electricity, telephony, data and plumbing to be pre-installed, so all that remains is for these to be connected up once the units have been secured in place on site. The size of the box units is restricted by the transport limitations. 4150 mm is the normal width for roads, and slightly less for the railway.
Installation takes place during concentrated periods at a rate of 20–30 units per day. Parts of a high-rise building can thus be installed to full height in one day and capped with a prefabricated roof. Box units are sensitive to precipitation before they are covered by a weatherproof roof. It is important to have a properly functioning temporary roof cover, if it is not possible to achieve full height in one working day.
Concrete and wood floor structure, other structural parts in glulam, office block, Växjö. Photo: Ole Jais.
Glulam structures often apply a post and beam system, where prefabricated units are joined together on site. Here too, there is an urgent drive to quickly get a weatherproof roof fitted, but often some form of protective cover is required for the components of the structural frame. This protection is also required during transport. There are several other frame systems that are used for halls and bridges, including trusses, frames and arches.
Glulam arches support the structure in Göranssons Arena, Sandviken. Photo Åke E:son Lindman.
Moisture in the construction process
Wood used in constructing buildings is dried down to a moisture content that matches the climate of the site. This also means that the moisture content is adapted to the production method.
Industrial production methods with shorter production times are thus based on less moisture being built into the structural frame and so the wood used has a lower moisture content than what is sold via builders’ merchants and timber merchants. At the same time, there has been a growing requirement to ensure that moisture issues are keep fully at bay during the open assembly time, i.e. the time during construction when the structure is unprotected from the elements. This requirement relates most explicitly to the fully finished prefabrications, i.e. box units. On the other hand, the open assembly time is shortest when using these units and there are plenty of good ways to protect them from the elements.
Surface units for low-rise buildings, covered with transport protection, can tolerate a certain amount of dampness, but the place where the units connect to the foundation slab and the fixed locating sills is a sensitive point. Nowadays, water and diffusion proof underlays are placed under the sills to prevent moisture from being sucked up into the sill. The top of the units should have similar protection unless they are clad in some way.
When building taller structures, carefully considered protection measures are required during the construction period. A number of different designs for weatherproofing have been developed, the most advanced of which combine weather protection with a gantry so that it also serves as an assembly crane.
Installing surface units under a roof cover, Skellefteå. Roof covers are used on high-rise buildings that cannot be assembled in one day. Photo: Patrick Degerman.
With stick building, the open assembly time is longer, even for low-rise buildings. Good weather conditions can be sufficient for single houses, but for continuous professional operations, some type of roof cover is required so that moisture is not built into the structure. There is no actual danger of microbial growth if the wood is exposed to moisture for a short time, as long as it then has the opportunity to quickly dry out. Surface moisture will not have time to penetrate very far into the wood – particularly if it is spruce – and the wood will dry out just as quickly as it became wet if the wood is not encased within the structure. The critical issue is whether moisture can penetrate contact surfaces between different pieces of wood, end-grain wood surfaces and under sills (see above on surface elements units), places where it is difficult to achieve rapid drying. The moisture content on such surfaces must be checked before encasing. Insulation, a vapour barrier, cladding or sheet material must not be fitted until the surface moisture content is below 18%. Note that it is the surface moisture content that determines the risk of microbial growth.
To prevent absorption of water in studs, impregnation and waxing of sills and end-grain wood on the studs has been trialled, along with the use of composite wood materials that do not absorb water. A production process that does not allow moisture an opportunity to penetrate these surfaces is always the best solution.