Types Of Structural Walls

Structural framework consists of studs mounted between a top plate and a bottom plate, with diagonal bracing added.There have been many variations on this theme through time. Early versions are known as timber framing and use a few large wooden members in the range of 15 to 30 cm thick joined using mortice and tenon or more

13.39

Timber frame construction. Source: Rolf Jacobsen, Gaia Tj0me.

complex joints that are usually fastened using only wood pegs (Figure13.39). Later the tendency has been toward smallerdimensions of timber components and more industrialized production (Figure 13.40).This has reduced the quantities of wood needed, but also the quality and strength of the structure to a certain extent. In modern framework the distance between the studs can vary somewhat, from 300 mm to 1.2 m. Until recently bracing was done with diagonal lengths of timber or metal strips, but nowadays it is more usually braced with sheets of fibreboard, plasterboard or chipboard. The spaces between the studs are then filled with different types of insulation. In earlier

13.40

Conventional timber framework. (a) platform construction; (b) balloon construction; (c) balloon construction without sills to allow easy refilling of loose fill insulation from the attic.

13.40

Conventional timber framework. (a) platform construction; (b) balloon construction; (c) balloon construction without sills to allow easy refilling of loose fill insulation from the attic.

'The little grey house'. Swedish prefabricated house in log construction. Architects: Sandell and Landstrom. Source: Bertil Harstom and George van der Weyden.

13.41

'The little grey house'. Swedish prefabricated house in log construction. Architects: Sandell and Landstrom. Source: Bertil Harstom and George van der Weyden.

times they were filled with clay (in wattle walls), firewood, or bricks (known as half-timbered brickconstruction).

Modern methods of structural framework usetimber very economically, but it is seldom easy to recycle. The many and very strong fixings used make the demolished structure suitable only for recycling as pulped raw material for production of wood based boards or burningforenergy recovery.'Traditional timber framing is more flexible, and easy to dismount and move, or modify.

The timber used in frame construction has to have high strength. It should not be too elastic or deform too much when exposed to moisture. The timbers best suited for this

13.42

Floor elements of massive wood. (a) With horizontal wood dowels; (b) Prestressed with drilled-in rods; (c) Glued; (d) With vertical wood dowels. (e) Glued box elements that can be filled with insulation.

13.43

The 'climate cube' building system consists of massive wooden blocks, see Figure 13.44, piled up to complete walls. The cubes are connected to each other with wooden plugs. No glues are used and the blocks can be easily disassembled for re-use. Gaia Lista, 2005.

13.43

The 'climate cube' building system consists of massive wooden blocks, see Figure 13.44, piled up to complete walls. The cubes are connected to each other with wooden plugs. No glues are used and the blocks can be easily disassembled for re-use. Gaia Lista, 2005.

13.44

A climate cube consists of recycled timber panelling and residues from sawmills, rippled and fixed together with wooden dowels. All sides measures 37,5 cm and the thermal insulation value is substantial because of all the closed air gaps formed between the layers. Gaia Lista, 2005.

13.44

A climate cube consists of recycled timber panelling and residues from sawmills, rippled and fixed together with wooden dowels. All sides measures 37,5 cm and the thermal insulation value is substantial because of all the closed air gaps formed between the layers. Gaia Lista, 2005.

13.45

Common log joints.

are fir, spruce, larch and oak. For smaller structures, birch, aspen, ash and lime can be used.

Timber frame construction is the dominant structural system in the timber building industry today.

Log construction. In this method, logs are stacked directly over each other and notched together in the corners.These buildings are usually rectangular, but can have upto10sides.

A solid timber wall has good acoustic properties and fire resistance.The thermal insulation isalso good. For 700 to 800years it has been considered the warmest alternative. However, a log wall needs extra thermal insulation by today's low energy standards. This should be placed on the outside, so as to keep the thermal mass and humidity regulating qualities of the logs in the interior.

Pine has been the timber most used in log construction. It has often been left open and exposed to the weather, so it has been well tested for hardiness. In log construction with external panelling, spruce can be used. For outhouses birch, aspen and lime can be used. In particularly damp areas, exceptionally durable timber such as oak must be used for the bottom log or sill plate.

There are many ways of shaping the logs and their joints, depending upon which timber is used (see Figure 13.45). Some of these joints are complex and require a high level of skill. Pine should have its surface worked by profiling, while spruce needs only the removal ofthe bark to keep its strength. Accessible technology and rationality have played a crucial role in the development of techniques. The type shown on the right in Figure 13.45 belongs to the nineteenth-century style of building and was well-suited to the newmachinery of the period-sawmills.The disadvantage wasthat it was difficult to make airtight, and the knots were less strong.

The massive wood technique is afairlyrecentdevelopment.The basic idea isto use fairly worthless low quality planks.These planks are stacked and assembled together into elements.They can be fixed together by glue, nails or with wooden dowels. Large

(A), (B), (C), (D). 4 phases in the erection of 13.47

a kindergarten in massive wood ele- Apartment block in massive wood at Svartlamoen, Trondheim (Norway). Architects: Brendeland and ments. Rauli, Flekkefjord (Norway). Gaia Kristoffersen, 2005. Lista, 2005.

(A), (B), (C), (D). 4 phases in the erection of 13.47

a kindergarten in massive wood ele- Apartment block in massive wood at Svartlamoen, Trondheim (Norway). Architects: Brendeland and ments. Rauli, Flekkefjord (Norway). Gaia Kristoffersen, 2005. Lista, 2005.

elements can be prefabricated, forming entire sections of walls, floors and roofs. Advanced prefabrication at the factories includes openings for windows, conduits for electric cables and often also the cladding. These elements are then delivered to the site and the whole structure of a building can be mounted with a small crane within just afewdays.

Within the massive timber technique quite a number of variations are developing (Figure 13.42).The elements may be from 8 cm to 40 cm thick. Laid as loadbearing floors they can easily span more than 10 metres. Rough or fine surface quality can be ordered. Interior surfaces can therefore be left exposed. An acoustic profile can be given to the underside of a ceiling. In the thickest varieties, plugged together without glue, there are small air spaces between the planks that increase the insulation

13.48

Reggio Emilia kindergarten established in a former motor car repair shop in Trondheim (Norway). Massive woods elements are used as partition walls. Architects: Brendeland and Kristoffersen, 2007.

13.48

Reggio Emilia kindergarten established in a former motor car repair shop in Trondheim (Norway). Massive woods elements are used as partition walls. Architects: Brendeland and Kristoffersen, 2007.

13.49

Stovewood construction in Latvia. Source: Rolf Jacobsen.

13.49

Stovewood construction in Latvia. Source: Rolf Jacobsen.

value so that the wooden element itself needs no further thermal insulation even in cold climates.

Massive timber construction requires fairly low quality wood. Even recycled wood can be used in some of the products (see Figure 13.44). The glued types use synthetic glues, mostly based on melamin, phenol and polyurethane. The amounts used are of the order of 2 to 4% of the total volume of the element. In some types made up with smaller wood flakes, this may be nearer to 6%. Many of these glues have negative environmental effects, as well as possible health effects. Since the surfaces are often left exposed, emissions to the indoor air are a risk (see Chapter 17). Continuous layers of glue may also reduce the humidity regulating performance of the elements.

Glued products will also pose a problem after the building's lifetime and some of these products may have to be treated as special waste. Elements without glue can easily be re-used, especially if they are made in standardized dimensions.

Massive timber elements can be produced in quite small local sawmills using local timber. Ifthetrendmovestowards large centralized units, however, thetransport consequences will cancel out some of the environmental advantages.

Massive timber construction is, together with log construction, the best method available for increasing the use oftimber in construction in order for it to becomean important carbon sink for mitigating greenhouse gas emissions.

Stovewood constructions originate in the nineteenth century and represent a building tradition of recycling, where bits of plank and spill from the sawmills are built up into walls using a mortar of pure clay mixed with water and sawdust or chaff (Figure 13.49). A similar tradition is based on cordwood, sometimes set in lime mortar.

The thermal insulating properties of these walls is moderate because of the many cold bridges due to the mortar joints and because wood insulates less in the direction

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13.50

Roof trusses constructed in solid timber, some with additional steel wire.

13.50

Roof trusses constructed in solid timber, some with additional steel wire.

13.51

Possible combinations of double curved shells.

Source: Schjodt, 1959.

13.51

Possible combinations of double curved shells.

Source: Schjodt, 1959.

of the fibres. Stovewood constructions need a couple of years to settle before panelling or wallpapering is done.

Floor structures

Floor structures in timber usually consist of solid timber joists, composite beams, laminated timber beams or a combination of these. As mentioned above, recent techniques with massive timber elements can provide large spans. For floors between apartments, extra sound and fire protection layers normally have to be added, although the massive timber itself has quite good fire characteristics.

Roof structures

The use of wooden components in roof structures is almost the same as for floors, with beams and massive wood elements as the main options. Many structural alternatives are available through combining them in different ways, including the use of tensile elements such as steel cables (Figure 13.50).

Roofs fall into three main categories: single raftered, purlin and forms made of trusses, with a smaller group known as shell structures.

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