The Study Centre at Darwin College, Cambridge (Fig.
4.2), which occupies a narrow site overlooking the River Cam, is designed to accommodate both books and computers. It is a load-bearing masonry and tim- ber building which features the extensive use of English oak, including massive paired columns to the first- floor reading room which is partly cantilevered over the river. The columns in green oak have characteristic shakes and splits giving an impression of great age, and these contrast with the refined oak and oak veneer of the floors, windows frames and furniture. Joints in the green oak are held by stainless steel fixings, which can be tightened as the timber dries and shrinks. The use of oak throughout gives unity to the building, which sits comfortably within its highly sensitive location.
METABOLISM OF THE TREE
The tree, a complex living organism, can be consid- ered in three main sections: the branches with their leaves, the trunk (or bole) and the roots (Fig. 4.3).
The roots anchor the tree to the ground and absorb water with dissolved minerals from the soil. The leaves absorb carbon dioxide from the air and in the pres- ence of sunlight, together with chlorophyll as a catalyst, combine carbon dioxide with water to produce sugars.
Oxygen, a by-product of the process, diffuses out of the leaves. The sugars in aqueous solution are transported down the branches and trunk to be subsequently con- verted, where required for growth, into the cellulose of the tree. The trunk gives structural strength to the tree
and acts as a store for minerals and food such as starch and also as a two-way transport medium.
The tree is protected from extremes of temperature and mechanical damage by the bark, inside which is the bast layer which transports the sugars synthesised in the leaves downwards. Radial rays then move the food into the sapwood cells for storage. Inside the bast is the thin and delicate cambium, which is the growing layer for the bark and sapwood. Growth only takes place when the cambium layer is active, which in temperate climates is during the spring and summer seasons.
A transverse section through the bole shows the growth rings. These are sometimes referred to as annual rings, but unusual growth patterns can lead to multiple rings within one year, and in tropical cli- mates, where seasonal changes are less pronounced, growth rings may be indistinct and not annual. The growth rings are apparent because theearly woodpro- duced at the start of the growing season tends to be made from larger cells of thinner walls and is thus softer and more porous than thelate woodproduced towards the end of the growing season. Each year as the tree matures with the production of an additional growth ring, the cells of an inner ring are strengthened by a process ofsecondary thickening. This is followed by lignification in which the cell dies. These cells are no longer able to act as food stores, but now give increased structural strength to the tree. The physi- cal changes are often associated with a darkening of the timber due to the incorporation into the cell walls of so-called extractives, such as resins in softwoods or tannins in oak. These are natural wood preser- vatives which make heartwood more durable than sapwood.
T I M B E R A N D T I M B E R P R O D U C T S 1 0 7
Fig. 4.2 Green oak construction—Darwin College Study Centre, Cambridge.Architect: Dixon Jones.Photographs: Courtesy of Dennis Gilbert
CONSTITUENTS OF TIMBER
The main constituents of timber are cellulose, hemi- cellulose and lignin, which are natural polymers.
Cellulose, the main constituent of the cell walls, is a polymer made from glucose, a direct product of
photosynthesis within the leaves of the tree. Glucose molecules join together to form cellulose chains con- taining typically 10,000 sugar units (Fig. 4.4). Alternate cellulose chains, running in opposite directions to each other, form a predominantly well-ordered crystalline material. It is this crystalline chain structure which
Fig. 4.3 Metabolism of the tree (after Everett, A. 1994:Mitchell’s Materials. 5th edition. Longman Scientific & Technical)
Fig. 4.4 Structure of cellulose
T I M B E R A N D T I M B E R P R O D U C T S 1 0 9 gives cellulose its fibrous properties, and accounts for
approximately 45% of the dry weight of the wood.
Hemicelluloses, which account for approximately 25% of the weight of wood, have more complex partially crystalline structures composed of a vari- ety of other sugars. The molecular chains are shorter than those in cellulose producing a more gelatinous material. Lignin (approximately 25% by weight of the timber) is an insoluble non-crystalline polymeric material. Its main constituents are derivatives of ben- zene combined to form a complex branched-chain structure.
The three major components are combined to form microfibrilswhich are in turn the building blocks for the cell walls. Crystalline cellulose chains are sur- rounded by semi-crystalline hemicellulose, and then a layer of non-crystalline cellulose and are finally cemented together with lignin (Fig. 4.5). Millions of these microfibrils are built up in layers to form the indi- vidual cell walls. It is this composite structure which gives timber its physical strength, with the cellulose contributing mainly to the tensile properties and the hemicellulose and lignin to the compressive strength and elasticity.
Fig. 4.5 Cell structure of timber (after Desch, H.E. 1981:Timber:
its structure properties and utilisation, 6th edition. Macmillan Education—Crown Copyright)
In addition to the three major constituents and significant quantities of water, timbers contain many minor constituents; some, such as resins, gums and tannins, are associated with the conversion of sapwood to heartwood. Starch present in sapwood is attractive to fungi, and inorganic granules such as silica make working certain tropical hardwoods, such as teak, dif- ficult. The various colours present in different timbers arise from these minor constituents, as the various celluloses and lignin are virtually colourless. Some colours are fixed to the polymeric chains, but others are light-sensitive natural dyes which fade on prolonged exposure to sunlight, unless the timber is coated with an ultraviolet-absorbing finish.
HARDWOODS AND SOFTWOODS
Commercial timbers are defined as hardwoods or softwoods according to their botanical classifica- tion rather than their physical strength. Hardwoods (angiosperms) are from broad-leafed trees, which in temperate climates are deciduous, losing their leaves in autumn, although in tropical climates, when there is little seasonal variation, old leaves are constantly being replaced by new. Softwoods (gymnosperms) are from conifers, characteristically with needle-shaped leaves, and growing predominantly in the northern temper- ate zone. Mostly they are evergreen, with the notable exception of the European larch (Larix decidua) and they include the Californian redwood (Sequoia sem- pervirens), the world’s largest tree with a height of over 100 metres.
Although the terms hardwood and softwood arose from the physical strength of the timbers, paradoxi- cally, balsa (Ochroma lagopus) used for model making is botanically a hardwood, whilst yew (Taxus baccata), a strong and durable material, is defined botanically as a softwood. Under microscopic investigation, soft- woods show only one type of cell which varies in size between the rapid growth of spring and early summer (early wood) and the slow growth of the late summer and autumn (late wood). These cells, or tracheids, per- form the food- and water-conducting functions and give strength to the tree. Hardwoods, however, have a more complex cell structure with large cells or vessels for the conducting functions and smaller cells or wood fibres providing the mechanical support. According to the size and distribution of the vessels, hardwoods are divided into two distinct groups. Diffuse-porous hardwoods, which include beech (Fagus sylvatica), birch (Betula pendula) and most tropical hardwoods,
Fig. 4.6 Cell structures of hardwoods and softwoods
Fig. 4.7 Limed oak—Jerwood Library, Trinity Hall, Cambridge.Archi- tects: Freeland Rees Roberts.Photograph: Arthur Lyons
have vessels of a similar diameter distributed approx- imately evenly throughout the timber. Ring-porous hardwoods, however, including oak (Quercus robur), ash (Fraxinus excelsior) and elm (Ulmus procera), have large vessels concentrated in the earlywood, with only small vessels in the latewood (Fig. 4.6). The Jerwood library of Trinity Hall, Cambridge (Fig. 4.7) illustrates the visual quality of limed oak as an architectural feature within the context of a sensitive built environ- ment.
TIMBER SPECIES
Any specific timber can be defined through the correct use of its classification into family, genus and species.
Thus oak, and beech are members of the Fagaceae family; beech is one genus (Fagus) and oak (Quer- cus) another. The oak genus is subdivided into several species, including the most common, the peduncu- late oak (Quercus robur) and the similar but less common sessile oak (Quercus petraea). Such exact tim- ber nomenclature is, however, considerably confused by the use of lax terminology within the building
T I M B E R A N D T I M B E R P R O D U C T S 1 1 1 industry; for example, both Malaysian meranti and
Philippine lauan are frequently referred to as Philip- pine mahogany, and yet they are from a quite different family and genus to the true mahogany (Swietenia) from the West Indies or Central America. This impre- cision can cause the erroneous specification or supply of timber, with serious consequences. Where there is the risk of confusion, users should specify the correct genus and species.
The standard BS EN 13556: 2003 lists both hard- woods (dicotyledons) and softwoods (gymnosperms) used within Europe, with a four-letter code. The first two letters are a distinctive combination referring to the genus (e.g. oak – Quercus – QC). The third and fourth letters refer to the particular species; thus, Euro- pean oak –Quercus petraeais QCand American red oak –Quercus rubrais QCXR. Typical softwoods are west- ern red cedar –Thuja plicata– THPL and Scots pine – Pinus sylvestris– PNSY.
Softwood accounts for approximately 80% of the timber used in the UK construction industry. Pine (European redwood) and spruce (European white- wood) are imported from Northern and Central Europe, whilst western hemlock, spruce, pine, and fir are imported in quantity from North America. For- est management in these areas ensures that supplies will continue to be available. Smaller quantities of western red cedar, as a durable lightweight cladding material, are imported from North America, together with American redwood from California, pitch pine from Central America and parana pine from Brazil.
Increasingly, New Zealand, South Africa and Chile are becoming significant exporters of renewable timber.
The UK production of pine and spruce provides only about 10% of the national requirements while Ireland plans to be self-sufficient early in the next century.
Over 100 different hardwoods are used in the UK, although together beech, oak, sweet chestnut, meranti, lauan, elm, American mahogany and ramin account for over half of the requirements. Approximately half of the hardwoods used in the UK come from temperate forests in North America and Europe including Britain, but the remainder, including the durable timbers such as iroko, mahogany, sapele and teak, are imported from the tropical rain forests. The Great Oak Hall at Weston- birt Arboretum, Gloucestershire (Fig. 4.8), illustrates the use of ‘medieval’ construction systems within a modern building by using ‘green’ oak fixed with dowels and wedges.
Since 1965, 6.5% of the Amazon forest has been lost, but much of this deforestation has been for agri-
cultural purposes, with more than three quarters of the timber felled used as a local fuel rather than exported as timber. With the growing understanding of the environmental effects of widespread deforesta- tion, some producer governments are now applying stricter controls to prevent clear felling and encour- age sustainable harvesting through controlled logging.
Other imported naturally durable hardwoods, avail- able in long lengths, include ekki, greenheart and opepe, whilst UK-produced sweet chestnut is durable and an appropriate structural timber. Some timbers not previously used within the UK, such as jatoba (Hymenaea courbaril), are now being imported from South America.
CONVERSION
Conversion is the process of cutting boles or logs into sections prior to seasoning. Subsequent further cut- ting into usable sizes is called manufacture. Finishing operations involving planing and sanding produce a visually smooth surface but reduce the absorption of penetrating wood stains. Timber for solid sections is sawn, whereas thin layers for plywood are peeled and veneers are usually sliced across the face of the log to maximise the visual effect of colour and figure, which is the pattern effect seen on the longitudinal surface of cut wood.
Types of cut
The two main types of cut, plain sawn and quarter sawn, refer to the angle between the timber face and the growth rings. This is best observed from the end of the timber, as inFig. 4.9. If the cut is such that the growth rings meet the surface at less than 45◦then the timber is plain sawn. Timber with this type of cut tends to have a more decorative appearance but a greater tendency to distort bycupping. Timber cut with the growth rings meeting the surface at not less than 45◦is quarter sawn. Such timber is harder wearing, weather- resistant and less likely to flake. If a log is cutthrough and through, which is most economical, then a mixture of plain and quarter-sawn timber is produced. Quarter sawing is more expensive as the log requires resetting for each cut and more waste is produced; however, the larger sections will be more dimensionally stable. The centre of the tree, the pith, is frequently soft and may be weakened by splits or shakes. In this case, the centre is removed as aboxed heart.
Fig. 4.8 Traditional oak construction—Great Oak Hall, Westonbirt Arboretum, Gloucestershire.Architects: Roderick James Architects.Photograph:
Arthur Lyons
Fig. 4.9 Conversion of timber
Sizes
BS EN 1313-1: 1997 defines the standard sizes of sawn softwood timbers at 20% moisture content (Table 4.1).
Widths over 225 mm and lengths over 5 m are scarce and expensive, but finger jointing (BS EN 385: 2001), which can be as strong as the continuous timber, does allow longer lengths to be specified. Regular- ising, which ensures uniformity of width of a sawn timber, reduces the nominal section by 3 mm (5 mm over 150 mm) and planing on all faces or ‘processed all round’ (PAR) reduces, for example, a 47×100 mm section to 44×97 mm (Table 4.2 ). Hardwood sizes are more variable due to the diversity of hardwood species, but preferred sizes to BS EN 1313-2: 1999 are specified inTable 4.1. Hardwoods are usually imported in random widths and lengths; certain structural hard- woods such as iroko (Chlorophora excelsa) are available in long lengths (6–8 m) and large sections. Tolerances for acceptable deviations from target sizes for softwood are given in BS EN 1313-1: 1997 and BS EN 336: 2003 (Table 4.3). The latter defines two tolerance levels for
T I M B E R A N D T I M B E R P R O D U C T S 1 1 3
Table 4.1 Standard sizes of softwoods and hardwoods
Standard sizes of sawn softwood (20% moisture content) to BS EN 1313-1: 1997.
Thickness (mm) Width (mm)
75 100 115 125 138 150 175 200 225 250 275 300
16 V V V V
19 V V V V
22 V V V V V V V
25 V V V V V V V V V V
32 V V V V V V V V V V V
38 x √
x √
x √
x x x x x x
47 x x x x x x x x x
50 x √ √ √ √ √ √
x x
63 √ √ √ √
x x
75 x x √ √ √ √
x x x
100 x x √
x x x x
150 x x x
250 x
300 x
Sizes marked with a tick indicate preferred EU sizes.
Sizes marked with a cross are the complementary UK preferred sizes.
Sizes marked with a V are the additional UK customary sizes.
Customary lengths of timber to BS EN1313-1: 1997
Length m
1.80 2.10 3.00 4.20 5.10 6.00 7.20
2.40 3.30 4.50 5.40 6.30
2.70 3.60 4.80 5.70 6.60
3.90 6.90
Lengths over 5.70 m may not be readily available without finger jointing.
Standard sizes of sawn hardwood (20% moisture content) to BS EN1313-2: 1999 Preferred thicknesses
EU 20 27 32 40 50 60 65 70 80 100 mm
Complementary thicknesses
UK 19 26 38 52 63 75 mm
Preferred widths
EU 10 mm intervals for widths between 50 mm and 90 mm, 20 mm intervals for widths of 100 mm or more.
Preferred lengths
EU 100 mm intervals for lengths between 2.0 m and 6.0 m, 50 mm intervals for lengths less than 1.0 m.
sawn surface dimensions (tolerance class 1, T1 and tolerance class 2, T2) with T2 specifying the smaller tolerance limits, also appropriate to planed timber.
Customary lengths for structural softwood timber and hardwood are given inTable 4.1.
MOISTURE CONTENT AND SEASONING
As a tree is a living organism, the weight of water within it is frequently greater than the dry weight of wood itself. The water content of a tree is equal in winter and in summer, but one advantage of
Table 4.2 Maximum permitted reduction from target sawn sizes of softwoods and hardwoods by planing two opposed faces Maximum reductions from sawn softwood sizes by planing two opposed faces (BS EN1313-1: 1997)
Typical application Reduction from basic size (mm)
15–35 36–100 101–150 over 150
Constructional timber 3 3 5 5
Matching and interlocking boards (not flooring) 4 4 6 6
Wood trim 5 7 7 9
Joinery and cabinet work 7 9 11 13
Maximum reductions from sawn hardwood sizes by planing two opposed faces (BS EN1313-2: 1999)
Typical application Reduction from basic size (mm)
15–25 26–50 51–100 101–150 151–300
Flooring, matchings, interlocked boarding and planed all round
5 6 7 7 7
Trim 6 7 8 9 10
Joinery and cabinet work 7 9 10 12 14
winter felling is that there is a reduced level of insect and fungal activity. After felling, the wood will lose the water held within the cell cavities without shrinkage, until the fibre saturation point is reached when the cells are empty. Subsequently, water will be removed from the cell walls, and it is during this pro- cess that the timber becomes harder and shrinkage occurs. As cellulose is a hygroscopic material, the tim- ber will eventually equilibrate at a moisture content dependent on the atmospheric conditions. Subsequent reversible changes in dimension are called movement.
The controlled loss of moisture from green timber to the appropriate moisture content for use is called seasoning.
Moisture content=
weight of wet specimen−dry weight of specimen dry weight of specimen ×100%
Table 4.3 Permitted deviations on structural timber sizes to BS EN 336: 2003
Maximum deviations from target sizes
Tolerance Class T1 Tolerance Class T2
Thicknesses and widths≤100 mm
−1 to + 3 mm −1 to + 1 mm Thicknesses and
widths >100 mm
−2 to + 4 mm −1.5 to + 1.5 mm
The primary aim of seasoning is to stabilise the tim- ber to a moisture content that is compatible with the equilibrium conditions under which it is to be used, so that subsequent movement will be negligible. At the same time, the reduction in water content to below 20% will arrest any incipient fungal decay, which can only commence above this critical level. Drying occurs with evaporation of water from the surface, followed by movement of moisture from the centre of the tim- ber outwards due to the creation of a vapour–pressure gradient. The art of successful seasoning is to control the moisture loss to an appropriate rate. If the mois- ture loss is too rapid then the outer layers shrink while the centre is still wet and the surface sets in a distended state (case hardening) or opens up in a series of cracks or checks. In extreme cases as the centre subsequently dries out and shrinks it may honeycomb.
Air seasoning
Timber, protected both from the ground and from rain, is stacked in layers separated by strips of wood called stickers which, depending on their thickness, control the passage of air (Fig. 4.10). The air, warmed by the sun and circulated by the wind, removes mois- ture from the surface of the timbers. The timber ends are protected by waterproof coatings (bituminous paint) to prevent rapid moisture loss, which would cause splitting. Within the UK a moisture content of between 17% and 23% may be achieved within a few months for softwoods, or over a period of years for hardwoods.