History

The first mention of buildings constructed completely from timber in European history isTacitus' writing about German houses in his Histories in AD 98, characterizing them as something 'not pleasing to the eye'.These houses had either palisade walls with columns fixed directly into the earth or clay-clad wattle walls. And they had thatched straw roofs. Excavations from a Stone Age village in Schwaben, Germany showed that houses like these have been built over a period of at least 4000 years.

Remains of log timber buildings from about 1200-800 BC have been found in the village of Buch outside Berlin. Even in China and Japan there are traces of this technique from an early period, but most likely from a completely separate tradition to that of Europe.

In areas where there is a milder climate, such as the British Isles, the coasts of the continent and Scandinavia, an alternative structural technique developed alongside log construction: the stave technique.This technique is best exemplified in all its magnificence by the stave churches, and creates tall airy timber structures from specially grown timber, held together by wooden plugs.

The rendered wattle wall really started to develop when masonry walls were enforced by law. After a series of town ires during the seventeenth century, rendered wattle walls were almost the only alternative to brick and stone.

13.6.1 Structural elements in timber

Materials in solid timber occur in different dimensions, either as round logs or rectangular sections. There is an obvious limitation depending upon the size of the tree that is used, and this varies between different types of tree. Generally, the smaller the size of the component, the more effective the use of the timber available. To resolve the problem of the limitations of some components, timber jointing can be used.

It is necessary to differentiate between timber jointing for increasing the length and for increasing the cross-section. Jointing for increasing the length can be achieved with timber plugs, bolts, nails or glue. It is normal to use spliced joints for sills, logs, columns or similar

13.28

Mechanical timber joints for increasing the length.

components where compressive strength is more important than the tensile strength (Figure 13.28). Certain spliced joints, such as the glued finger joint, have a good tensile strength.

Increasing the cross-section can be achieved by using solid connections or I-beams. Solid connections consist quite simply of the addition

13.29

A roof joint bolted together and stiffened by dowels. No glue is used. Gaia Lista, 1987.

13.29

A roof joint bolted together and stiffened by dowels. No glue is used. Gaia Lista, 1987.

j___X 1 ___ of smaller sized timbers to each other. The fixing elements are bolts,

- . - .. - - ■ -- —=t-i nails or glue. Bolted joints are often complemented by steel or timber 13.30 dowels to stop any lateral movement between the pieces of timber, as

Toothed beam joint put together of three in Figures 13.29 and 13.30. Dowels and toothing were used until the pieces. 1920s. Solid laminated timber joints have been in use since the turn of the century, and nowadays usually consist of 15-45 mm-wide spruce planks. Normally there is about 2% glue; common glues used are based on melamin-formaldehyde (MUF and MUPF) or phenol-formaldehyde (PF), but polyurethane adhesives (PU) are also used.

Laminated veneer lumber uses multiple layers of thin wood assembled with adhesives, (see pages 339-340 in Chapter 15). The amounts of glue are somewhat higher here and phenol-formaldehyde adhesive (PF) is commonly used. Oriented strand lumber is manufactured from flakes of wood (see pages 339-340 in Chapter 15). In this case there is 6 to 8% glue, often combinations of glues such as melamin-formalde-hyde in the centre and the rest with polyurethane glues.

I-beams consist of an upper and lower flange with a web in between (Figure 13.31). The web can be formed of solid timber, steel, veneer, chipboard or fibreboard. The first two are usually fixed by plugging, bolting, nailing with nails or nail plates, while the others are glued. I-beams are a very economical use of material in relation to their strength, and can be used in roof, floor and wall construction.

The energy consumed in the production of glued products is considerably higher than for plain timber products. However, glued products often need less timber since the glues compensate for weaknesses in the timber to some extent.

Structural elements that are bolted together can be more easily dismantled and recycled. Larger nailed and glued products offer a more difficult recycling. In structures where dismantling and re-assembly are anticipated, high quality timber should be used. One should also consider slightly over-designing the components to prevent long-term warping or deformation.

The synthetic glues most often used in industrial timber products are usually derived from fossil oils, causing pollution during the production phase, and in many cases carry definite health risks (see Chapter 17). Hazardous chemicals are also often used to provide protection against insects and fungus, see Chapter 19.

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