The use of timber in building

Timber is a many-faceted structural material and can be used in foundations, walls and roof structures.

Foundations

The most important timber construction methods for foundations are raft and pile foundations. Their main areas of use are as bases for foundation walls and to stabilize weaker ground conditions.

Timbers have varying properties in relation to damp. Some timbers, such as maple and ash, decompose very quickly in both earth and water; spruce is similar. Many types of timber can survive longer in damp and low-oxygen environments rather than in the open air. Pine, alder, elm and oak can last over 500 years in this sort of environment;

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A lattice I-beam in a school building. All joints are fixed by bolting, no glue is used. Gaia Lista, 1998.

larch can survive for 1500 years. As soon as the relative moisture content in timber drops below 30-35%, rot sets in, and durability falls drastically. Certain types of timber are better than others even in these conditions. Oak can survive between 15 and 20 years, whilst larch and pine rich in resin can probably last between 7 and 10 years.

An optimal condition for a permanent timber foundation is an even, rich dampness. The timber should be completely embedded in clay and lie below the groundwater table.

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Demountable timber joints with metal components and plugs. Source: Janebo.

TIMBER-BASED METHODS OF FOUNDATION WORK

Bulwarking has been used since the Middle Ages, especially when building along the edge of beaches and cutwaters. It is basically a structure of logs laid to form a rectangle, jointed at the corners, measuring 2 to 3 m on each side and filled with stones to stabilize it (Figure 13.37). A bulwark has an elasticity in its construction that allows it to move, and it can therefore cope with waves better than stone or concrete. If robust timber is used, a bulwarkcan keep its functional properties for hundreds of years.

Raft and pile foundations. Many large coastal towns are built on raft or pile foundations. If the foundations are continually damp then durability is high. Excavations have uncovered pile foundations of alder and aspen from the Middle Ages that are still in perfect condition, with even the bark of the tree preserved (Liden, 1974; Minke, 2006; Rybczynskietal.,1974).Through the increase oftunnelling and drainagesystems in many towns, the level of the ground water has been lowered, and because of this, fungus attackon the foundations occur, causing a settling of the buildings.

The simplest form of raft foundation is a layerof logs laid directly onto the ground tied to logs laid across them (Figure 13.38). Masonry columns or perimeter walls are built

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A structure designed for re-use. The structure is produced of prefabricated and standardized monomaterial components in wood and concrete, which can easily be dismantled and re-used. Gaia Lista, 1995.

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A structure designed for re-use. The structure is produced of prefabricated and standardized monomaterial components in wood and concrete, which can easily be dismantled and re-used. Gaia Lista, 1995.

on thisfoundation, and around the edges layers of clay are packed in. Raft foundations were in common use around the seventeenth century and quite normal up to about 1910, when they were slowly replaced by reinforced concrete foundations.

In pilefoundations the raft is replaced by vertical logs, which are rammed down into the ground. It is usual to lay three or four horizontal logs onto the piles to distribute the weight evenly, before building the walls. The weight of the building and the bearing

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Playroom at Westborough Primary School, Southend (UK), constructed largely from cardboard made from recycled paper. The cardboard can be recycled again once the products have served their useful life. Architects: Cottrell & Vermeulen Architecture 2001.

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Playroom at Westborough Primary School, Southend (UK), constructed largely from cardboard made from recycled paper. The cardboard can be recycled again once the products have served their useful life. Architects: Cottrell & Vermeulen Architecture 2001.

Bulwark method of foundation work. Source: Drange et al., 1980.

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Bulwark method of foundation work. Source: Drange et al., 1980.

Raft foundation. Source: Bugge 1918.

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Raft foundation. Source: Bugge 1918.

capacity of the earth decide howclosethe piles need to be to each other. Foundations for smaller buildings usually have thinner piles, from the thickness of an arm down to the thickness of a finger. To distribute the load, a filled bed of round stones may be used. In sandy earth without humusthe piles can extend up intothe open air.This can provide a simplified solution in certain cases, but even with good impregnation and high-quality timber it is doubtful that such foundations will last longer than perhaps 75 years.

Structural walls

By timber buildings we usually mean buildings with loadbearing timber walls. It is necessary to differentiate between light and heavy structures.

The most important lightweight construction is the framework, which is economical in the use of materials and takes advantage of both the tensile and compressive strengths of timber.

The log building technique is the traditional technique of heavy structures. This method uses a lot of timber and is statically based on the compressive strength of timber. The recently invented massive wood technique exploits both the tensile and the compressive strength. It is therefore also used for loadbearing floors and roof structures.

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