Строительные материа ...

Unit 11. WOODWORKING.

Structure of trees. Properties of wood. Defects of wood. Preservation of wood against damage. Protection of wood against inflammation.

 

1. GENERAL INFORMATION ON WOOD

As building material, wood has a number of valuable properties: relatively high strength, small bulk density, low heat conductivity, amenability to mechanical working.

At the same time, wood has several shortcomings: its anisotropy results in different strength and heat conductivity characteristics in length and across fibres; hydroscopicity of wood causes fluctuations in properties; susceptibility to decay and inflammability. Modern wood processing techniques greatly reduce the above shortcomings.

At present, wood waste is also effectively utilized: in addition to fibrolite and xylolite items, sawdust and shavings are used with admix­ture of organic glues to make fibre-boards, fibre-slabs, etc. Besides, wood is used for the manufacture of cellulose, ethyl and butyl alcohols, paper, cardboard, organic acids, rosin and other products. Therefore, economic consumption of wood in building is a major task.

1. Structure of Trees

A tree consists of a trunk, a crown and roots. Roots are intended to implant the tree in the soil, to absorb moisture and the mineral sub­stances it contains and to supply them to the trunk. The trunk supports the crown and serves to supply water and nutrients from the roots to leaves through branches and from the leaves back to the roots. The structure of wood visible to the naked eye or at a small magnification is called macrostructure, and that apparent only at great magnifica­tions, the microstructure.

Macrostructure of wood can be studied by cutting the trunk in three directions, as shown in Fig. 1. In the cross-sectional and radial cuts, the following main parts of a tree trunk become readily apparent: bark, bast, cambium, wood and heartwood.

Bark protects wood against mechanical damage. It consists of an outside layer (the edge) and the inside layer (the bast).

Bast is a thin internal layer of bark and its purpose is to transmi nutrients from the crown downwards and to store them.

Cambium is a thin live tissue located behind bast. In the cambium wood cells grow on the side facing the core, and bast cells, on the sid facing the bast. Each cambium cell multiplies by splitting into  cells, one of which is thin-walled and laid on the outside of trunk, thi other, thick-walled and lignified one, on the side facing the core In spring, cambium consists of wide cells with thin membranes, th< so-called spring wood. In the second half of the growing season, wher the tree is loaded with developing runners and leaves, cambium cell; become flattened to perform mechanical functions and make up th< main part of the summer wood.

Fig. 1. Main cuts of a tree trunk

1 - cross-sectional; 2 - radial; 3 - tangential

Layers formed during the growing season are called annual rings. In some species, say in oak, they are distinctly visible on the cross section of the trunk. A thick layer of wood, situated behind the cambium, is composed of a number of thin concentric rings.

Wood is usually light-coloured, but in some species the heart centre is surrounded by darker wood which is known as heart wood, or dead wood. The light-coloured wood is called surrounding heartwood, sap- wood or alburnum. Species in which the central part is dark are called heartwoods (oak, cedar, pine). Species in which the central part has the properties of heartwood, but does not differ in colour from the peripheral part (spruce, fir, beech) and species with no heart- wood (birch, maple, alder) are called sapwoods. In a growing tree, sapwood consists mainly of living cells.

Heartwood, composed of thin-walled cells, runs throughout the cent­ral part of the entire trunk. The core and the first runners form the medullary sheath. This part of the trunk is the weakest, and it resists decay poorly.

In the cross-sectional direction nutrients pass from the bark to the heart through medullary rays.

The width of annual rings, even within one species, can differ greatly depending on the conditions of growth. However, the width of the annual ring affects the properties of wood to a lower degree than the percentage of late wood: the higher the latter, the stronger the wood.

Moisture in broad-leaved trees moves along vessels oriented along the trunk. Some broad-leaved species (oak, ash, elm) carry both large and fine vessels; the larger vessels are located in the earlier part of the annual ring, and the finer ones are either grouped or distributed unifor­mly over the later wood. These species are called ring-porous (Fig. 2).

 

 

Fig. 2. Anatomic structure of oak (ring-porous wood) 1 - medullary rays; 2 - vessels

Fig. 3. Anatomic structure of birch (diffuse-porous wood) 1 - vessels; 2 -. medullary rays

Some broad-leaved species (birch, aspen, lime) have no large vessels, and there are no visible differences between the earlier and the later parts of the annual rings. These species are called diffuse-porous (Fig. 3).                            ;

Conifer trees carry no vessels and they are composed of closed elon­gated cells called tracheitis (Fig.4). In most conifers, tracheids of the later part of the annual rings comprise resin ducts, or intercellular formations filled with resin. In addition to the annual rings, cross sections display narrow strips oriented along the. radii and calledmedullary rays. In the radial sections of oak, they appear as relatively broad ribbons.

Microstructure. When wood is studied under a microscope, it becomes evident that it is composed of a great number of living and dead cells of various sizes and shapes. A living cell consists of a membrane, pro­toplasm, sap and core.

Protoplasm is a granular, transparent, viscous pectose (vegetable protein) composed of carbon, hydrogen, oxygen, nitrogen and sulphur.

The core differs from the protoplasm merely by the presence of pho­sphorus, and it is generally oval.

Cell membrane consists mainly of cellular tissue and cellulose (C₆H₁₀O₅)_n. As the cell grows, the membrane changes both in structure Cross section  Fig. 4.

 

Anatomic structure of pine (conifers)

1 - resin duct; 2 - medullary rays

 

Lignification is the formation of lig- nin in the cell membrane, this increasing the strength and the hardness of the cell. When the cell membrane undergoes suberification, substan­ces lower in oxygen than lignin are formed in the membrane with the effect that the cell withstands decay and becomes impervious to water and gases. Cuticularization involves transformation of the entire mem­brane or part of it into pectose which dissolves in water. A partial cuticularization of the membrane results in holes through which the cells communicate with each other as if through vessels.

By their function, cells are subdivided into conductive, mechanical and storage. Conductive cells serve mainly to transmit nutrients from roots to branches and leaves. Mechanical cells are elongated, thick- walled and have tightly interconnected narrow interior cavities. It is precisely this type of cells that makes the wood highly strong. Storage cells are located chiefly in the medullary rays and serve to store and transmit nutrients to living cells in the horizontal direction.

Properties of Wood

Wood possesses manifold properties which become apparent when physical and mechanical characteristics of wood are considered from the engineering point of view.

Physical properties of wood. Properties of wood are greatly affected by moisture content. Wood may contain water in three forms: capillary (or free), hygroscopic and chemically bonded.

Free moisture fills all cell cavities, inter-cell spaces and vessels. Hygroscopic moisture is found in cell membranes and inter-cell spaces. Chemically bonded water is part of the wood constituents.

Capillary and hygroscopic water or hygroscopic water alone acco­unt for the major part of water in a growing tree. The state of wood which has no capillary water but contains hygroscopic water only, is called the fibre saturation point. Water content of various wood spe­cies ranges from 23 to 35%. When wood dries, water gradually evapo­rates from the surface of exterior layers, whereas remaining moisture moves outwards.

According to its moisture content, wood is divided into moist, fre­shly felled (35% and over), air-dry (15-20%) and room-dry (8-13%).

Hygroscopicity of wood is its capacity to absorb water vapours from the air. Absorption is governed by temperature and relative moisture content of the air. Moisture content of wood exposed to prolonged con­tact with air of constant relative humidity and temperature is called the equilibrium moisture content.

Because of wood hygroscopicity, the moisture content of wooden items fluctuates with the temperature and the humidity of the surro­unding air. Moist wood gives off moisture to the surrounding air, whe­reas dry wood absorbs it. Since the air humidity is not constant, wood moisture content varies accordingly. Fluctuations in wood moisture content from zero to the fibre saturation point cause corresponding volume changes in wood, or swelling and shrinkage, warping and crac­king. To reduce hygroscopicity and water absorption, wood is covered with varnishes or paints and impregnated with various substances.

Density of wood is approximately equal for all species and averages 1.54 t/m³.

Bulk density of wood depends on the volume of pores and moisture content.

For most wood species the bulk density is less than unity: for spruce, pine, lime, aspen it lies between 0.46 and 0.60; for birch, oak, larch and others, between 0.61 and 0.75; for cornel, boxtree, 0.91; for ebony and lignum-vitae, between 1.26 and 1.28 t/m³.

Bulk density of wood characterizes its physical and mechanical properties (strength, heat conductivity, water absorption). Bulk den­sity value is used to determine the quality factor which is the ratio of compressive strength to the bulk density. For pine, it equals 0.6, and for oak, 0.57.

Shrinkage of wood consists in the reduction of its linear and volu­metric dimensions in drying. Evaporation of capillary water is not accompanied by shrinkage, the latter taking place only when hygro­scopic moisture evaporates. In the process, the thickness of water memb­ranes decreases, the micellae are drawn together with the effect that the size of wood diminishes.

Because of structural non-uniformity, wood shrinks or swells irre­gularly in various directions. Linear shrinkage along the fibres lies between 0.1 and 0.3%, in radial direction, between 3 and 6%, in tan­gential direction, between 7 and 12%.

Irregularity in the variation of linear dimensions in different dire­ctions is one of the disadvantages of wood as building material. Slower drying of wood ensures a more uniform loss of moisture and produces less cracks. Irregular shrinkage causes stresses producing warps and cracks of the wood. In a round log, cracks are situated radially. Planks, cut nearer to the heart, warp less than those sawn further towards the surface of a log.

Swelling is the capacity of wood to increase both its linear and volu­metric dimensions when it absorbs water, which impregnates cell membranes. Wood swells as it absorbs water up to the fibre saturation point. Swelling, similarly to shrinkage, varies with fibre orientation. Swelling of wood along the length of fibres ranges from 0.1 to 0.8%, in the radial direction, from 3 to 5%, and in the tangential direction, from 6 to 12%.

Heat conductivity of wood is low and depends on the porosity, mo­isture content, orientation of fibres, species and bulk density of wood, and upon the surrounding temperature as well.

The coefficient of heat conductivity along the fibres is roughly 1.8 times greater than that across the fibres and averages 0.16 to 0.30 W/m '°C. As the bulk density of wood and its moisture content become greater, the amount of air entrapped inside cavities decrea­ses, the effect being a greater heat conductivity of wood.

Sound conductivity. The velocity/of sound in wood is 2 to 17 times greater than in air and thus wood may be considered high in sound transmission. Sound travels faster along the fibres, at a somewhat lo­wer rate in the radial direction and is slowest along the chord of across section.

Water permeability of wood depends on the species, initial moisture content, character of cut (cross-sectional, radial, segmental), location of wood in the trunk (core, alburnum), width of annual rings, age of wood. Water permeability is greater along the fibres than in other directions. Water permeability of wood is characterized by the quan­tity of water filtered through a unit surface area of specimen (g/cm¹).

Resistance to the action of acids, alkalis and water. Prolonged action of acids and alkalis destroys wood, and the greater their concentration, the greater the damaging effect. Weak alkali solutions fail to affect wood. In an acid medium, wood begins to decay at pH≤2, whereas concrete and steel begin to decay at pH≤4.

Conifers are more resistant to the action of sulphuric, nitric, hydro­chloric, acetic acids and sodium hydroxide than the broad-leaved species, the most resistant of the conifers being larch.

In sea water, wood is less durable than in fresh water. Wood poorly withstands bacteriologically aggressive water and, therefore, it cannot be used for sewage systems.

Mechanical properties of wood. Due to anisotropy, the resistance of wood to mechanical action differs with the fibre orientation, and this should be taken into account § when testing wood for physical and mechanical properties. Mechanical properties of wood are governed by its moisture content, its strength dropping as moisture content increases. Wood of a higher bulk densi­ty has a greater strength, the latter being also affected by the percentage of late wood, presence of defects and de­cay regions and structure.

Compressive strength of wood. To a wooden structural compo­nent stresses may be applied either parallel or perpendicular to fibres, and therefore distinction is made between compression parallel and per­pendicular to fibres. For compressive strength parallel to fibres, wood is tested in a press, the specimen being a rectangular prism, free from knots and measuring 20x20x30 mm, with the dimension parallel to fibres being not less than 30 mm.

Compressive strength parallel to fibres of wood at 15% moisture content varies greatly with the wood species, the range being from 30 to 80 MPa.

Compressive strength perpendicular to fibres of wood is much lower than that parallel to fibres and amounts to 4.1 (fir) and 25.6 MPa (white beech) in the radial section, and to 7.1 (spruce) and 15.6 MPa (white beech) in the segmental section.

Tensile strength of wood. Wood has a high tensile strength parallel to fibres. For the generally used species, this value ranges from 80 to 190 MPa. However, wooden parts restrained at their ends suffer from crushing and shearing stresses which wood resists poorly, and therefore wood cannot be extensively used in structures working under tension.

Strength of wood under static bending. Wood withstands static ben­ding well, owing to which it is widely employed for elements of buil­dings and structures (girders, slabs, rafters, trusses, etc.). Bending strength of wood is determined by testing specimen slabs measuring 20 x 20 x 300 mm. Bending strength is equal to 50-100 MPa (at 15% moisture content), depending on the species of wood.

For broad-leaved species, bending strengths in radial and tangential directions are practically the same, whereas in conifers tangential strength is somewhat greater than the radial strength.

Static bending strength is governed by the same factors as compress­ive strength.

Shearing strength along the fibres. Wood has low shearing strength along the fibres (6.5 to 14.5 MPa).

Resistance of wood to cutting across the fibres is 3 to 4 times greater than that along the fibres, but no pure shear generally takes place, since the fibres are also subjected to crushing and bending.

In building structures, shearing stresses are frequently applied to wood along the fibres, for instance, in roof trusses and structural elements.

3. Defects of Wood

Departures from the normal structure and flaws which affect its engineering properties are classified as defects of wood. Defects appear both during the growth of trees and during storage and service. Accor­ding to their causes, defects are divided into the following main groups:incorrect structure; mechanical damage; damage by fungi; damage by insects.

 

Defects due to incorrect structure of wood are as follows:

trunk eccentricity, frequent in conifers, consists in thickening of the later part of annual rings which is either one-sided or localized (Fig. 121a);

cross grain of wood appears as a twisted (helical) orientation of fibres which greatly impairs physical and mechanical properties of wood;

cross-grain wo­od presents greater shrinkage and longitudinal warping, which reduce its bending strength;

curvature is a lengthwise defor­mation of the trunk; it can be either one-sided or many-sided, the trunk being curved in one or several planes; curvature reduces the yield of stand­ard products and causes artificial cross grain;

taperingness is an excessive dec­rease in trunk diameter from butt to top; it causes cross grain and redu­ces the yield of standard products;

double heartwood, or two cores in a trunk, is found in double-crown trees (fig.121); it affects adver­sely the quality of commercial wood;

knotty wood is described by the number of knots per 1 m of length and their size and kind. Knots are found in intergrown, decayed, horny, brownrot, etc. varieties and as sound or decayed types (e.g., brownrot knots are centres of decay in sound wood);

wood cracks are formed not only as a felled tree dries, but while it is still alive, there being a variety of causes (drying of core, swinging by wind, frost, etc.). (Sacks fall into the following kinds: heart checks, cup-shake, frost cleft and plain checks.

Heart check is one or several internal cracks passing through the core in the radial and longitudinal directions but not reaching the bast. Heart checks are found in plain and cross varieties (Fig. 5). A plain heart check is one or two end cracks located along a single diameter; a cross heart check consists of two or several end cracks at an angle to one another. Heart check can either lie in one plane or it can be helical.

Cup-shake is an interior crack running along an annual ring, leng­thwise with respect to the trunk (Fig. 121c). Cup-shake may be arched or ring-like. Frost cleft is an open longitudinal crack, larger on the face of the trunk and narrowing toward the centre. Plain checks caused by drying are very frequent in all wood species (Fig. 123). They appear as the wood dries below the fibre saturation point and spread from the surface toward the centre. Cracks affect the quality of wood, reduce the quantity of useful wood and favour its decay.

 

 

 

 

Damage caused by fungi is very widespread. Abnormal colour and decay of

wood are due chiefly to fungi which are the simplest plantorganisms and feed on wood cells, but sometimes such damage is caused by physical and chemical factors. Fungi develop in the presence of oxygen, moisture and favourable temperature. Wood with a moisture content of 20% and less or wood placed in water or exposed to frost does not decay. Some fungi may develop only on growing trees, others, on felled ones, and still others, both on growing and felled trees.

Some of the fungi simply change the colour of wood and have prac­tically no effect upon its physical and mechanical properties, whereas others bring *about its decay.

Fungi that attack growing trees and continue to damage it in stru­ctures are white or brown rot, white trunk rot, etc. Fungi developing in wooden structures and buildings are called dry rot. Ones which cause the most rapid decay of wood are white dry rot and wood fungus, other types of fungi occurring much rarer. Classed with fungi that cause a slow decay of wood are moulds, blue rot and coloured stains. Decay of wood stops as soon as it dries, and all the fungi perish.

Damage by insects (burrow holes). Both growing and felled trees may be affected by burrow holes. Insects settle preferably on freshly felled trees, as well as on deadwood or weak growing trees. Most of the damage affecting growing trees is caused by bark beetle and others.

 

Damage to bark is generally in the form of shallow sinuous furrows bored by various kinds of bark beetles and their larvae. When wood damaged by bark beetles is sawn to boards, furrowed sections are sawn off and do not affect the material, but logs damaged by bark beetles are susceptible to rapid decay because beetles frequently bring spores of wood damaging fungi into the wood.

Bur row holes are deep furrows produced in wood by insects and their larvae. Wood damaged by deep burrow holes has bad mechanical pro­perties and can be used only as plain firewood. Deep burrow holes may be found in all wood species.

Sea molluscs (sea worms or bores) may cause a very rapid decay of underwater parts of sea structures.

 

4. Preservation of Wood Against Damage

Wood in structures or in storage may be attacked by fungi and insects. Various wood species have different resistances to attack by fungi and insects. Solid wood which has more summer wood and tan­ning agents is more resistant. Dry barked wood can be stored for long periods of time in dry ventilated premises. Some wood species, when placed in water, not only fail to decay, but, on the contrary, gain strength (oak, for instance).

Wood can be preserved against decay and its service life in structures increased by preventing its humidification by structural means, such as painting or coating, leaching and impregnating with antiseptics.

Painting, coating and leaching. Wood protected by paint, varnish or drying oil is more durable. Before painting, wood should be properly dried and its surface should be covered with a solid coat of paint. Dry

wood becomes very durable if tarred, because tar serves not only as paint, but also as an antiseptic, though a weak one."Leaching of wood in cold water or river logging eliminates plant sap. Wood may also be leached in hot water by boiling.

Antiseptics are substances which are poisonous for wood-attacking fungi. Antiseptics should be highly toxic with respect to fungi and durable, and neither absorb moisture nor be washed out by water. At the same time, they should be harmless to man and domestic ani­mals, to wood and metals, impregnate the wood readily and have no unpleasant odour.

Antiseptics are subdivided into water-soluble, oil and paste varie­ties. Water-soluble antiseptics are used for treating wood not exposed to moisture, the extensively employed types of antiseptics being sodium fluoride and silicofluoride, copper sulphate, sodium dinitrophenolate. Sodium fluoride (NaF) is a white powder, poorly soluble in water, odourless, harmless to wood and iron. It is used as a 3% solution for impregnating and coating wood at a temperature of 15°C. Sodium fluo­ride should not be mixed with lime, chalk or gypsum. Sodium silico­fluoride (NaaSiFe) is a powder of low solubility in water, close in anti­septic properties to sodium fluoride. It is employed as a hot liquor mixed with sodium fluoride in proportion of I : 3 and also as a com­ponent of silicate pastes or after post-alkali treatment of wood.

Sodium dinitrophenolate is manufactured from dinitrophenol and sodium carbonate. It is neither volatile nor hygroscopic, does not affect metals, but is explosive in dry powder form. It is employed in water solutions for surface treatment of wooden items not intended for service in close proximity of heated surfaces.

Because of high inflammability and sharp odour, oil antiseptics are used only for impregnating or coating wood placed in the open air, soil or water. This type of antiseptics includes coal creosote and anth­racene oils, peat creosote, coal tar and shale oil.

One of the best antiseptics is creosote oil-a black or brown liquid, weakly affected by water, neither hygroscopic nor volatile, not harm­ful to wood or metal, inflammable, of low wood-penetrating ability (1 to 2 mm), with an unpleasant odour. It forms a solid layer on the surface of wood and thus prevents drying. Creosote oil should be heated to 50-60°C prior to use. It gives the wood a dark colour, so that the latter can no longer be painted. Creosote oil should not be used inside dwelling houses or foodstuff-storage premises, in underground insta­llations and near inflammable surfaces.

Anthracene oil is a green-yellowish highly antiseptic liquid, which evaporates slowly under normal conditions, is weakly leached by water and attacks neither wood nor metals. It is manufactured from coal tar, has similar properties and finds the same applications as creosote oil.

Antiseptic pastes are subdivided according to their binders into bitumen, silicate, etc., varieties.

Bitumen antiseptic pastes are composed of 30 to 50% sodium fluo­ride, 5 to 7% peat powder, up to 30% oil bitumen of mark III or IV and up to 30% green naphthene oil. These pastes are inflammable du­ring manufacture, have a sharp odour, are water-resistant. They are intended for coating parts of structures exposed to humidification through contact with soils or atmospheric action.

Silicate antiseptic pastes contain 15 to 20% sodium silicofluoride, 65 to 80% soluble glass, 1 to 2% creosote oil and up to 20% water. These pastes are neither water-resistant nor inflammable. They find application in parts of industrial and residential buildings not subject to the action of water.

Wood preservation includes surface preservation, impregnation in hot-cold and high-temperature baths, impregnation under pressure, etc.

Surface preservation consists in smearing or spraying wood with an antiseptic solution.

Hot-cold bath technique is used to impregnate wood with water- soluble and oil antiseptics. Wood is first immersed in a hot antiseptic bath at a temperature up to 98°C, held there for 3 to 5 h, then placed in a cold antiseptic bath for 1-3 h at a temperature of 15-20°C, for water-soluble antiseptics, and 40-60°C, for oil antiseptics. This method effectively impregnates dried wood with moisture content below 30%.

Impregnation of wood in high-temperature baths (with petrolatum) is used for preserving moist wood. Wood is placed in a bath with molten petrolatum at 120-140°C, held in it for heating and elimination of mo­isture, then transferred to a cold bath with an oily antiseptic at a temperature of 65-75°C and held there for 24-48 h for impregnation. This method combines drying and preservation of wood.

Wood is impregnated under pressure with water and oil antiseptics in cylindrical steel tanks at a working pressure of 0.6 to 0.8 MPa. Wood is loaded into the tank which is then tightly sealed, a vacuum is drawn, and the tank is then filled with antiseptic, pressure is raised to 0.6 to 0.8 MPa, then brought down to normal, excess antiseptic is let out and wood is discharged.

When wood is impregnated with oil antiseptics, the latter are preheated so as not to bring the temperature down below a specified limit.

5. Protection of Wood Against Inflammation

Wood is very inflammable, this being one of its major shortcomings. Wood can be protected against fire by plastering, coating with gypsum or asbestos-cement sheets or surface treatment with fire resistant sub­stances. There are two surface treatment techniques, namely, painting and impregnation with antipyrines, or special chemicals.

By composition, fire protection paints are subdivided into silicate, casein, oil and chlorovinyl varieties.

Silicate paints are based on soluble glass and have high fire protection properties, as the paint is incombustible, resists fire for a long time and conducts heat very poorly.

Fire protection compounds, or antipyrines, are more reliable, since they either melt when the temperature rises or give off gases which in­hibit combustion. The treatment involves impregnation of dry wood with antipyrine. When treated with antipyrine, wood does not inflame even at high temperature, but merely smoulders. The best in fire protection properties are antipyrines containing salts of ammonium or boric and phosphoric acids.

Wood is impregnated with antipyrine in the same manner as with water-soluble antiseptics, the technique yielding better results as compared with coating by fire protection paints.

6. Wood Species and Their Use in Building

Both conifer and broad-leaved wood species are used in building.

Among the conifers, most extensively used are pine, spruce, terch, fir. There are several kinds of pine. Depending on growing conditions, distinction is made between two varieties of pine. Ore pine grows on elevated sandy soils and has a thin-layered solid resinous wood, a large core and a narrow bast. The other kind grows in lowlands on sandy or clayey soils and has a weaker broad-layered wood» a small core and a thick bast. Pine is used for building walls of dwelling houses, bridges, trestles, poles, window sashes, transoms, floors.

Spruce belongs to sapwood species; its colour is white with a yello­wish or rose hue. It is less resinous than pine, and, therefore, decays faster. Spruce is widely used in construction work, although in physi­cal and mechanical properties it is inferior to pine.

Larch belongs to conifer species with a red-brown heart and a white narrow bast. Larch wood has high physical and mechanical properties, its bulk density and strength being 30% higher than those of pine; it is resistant to decay and very hard, which makes it difficult to work. One of the shortcomings of larch is the big difference between its radial and tangential shrinkage which causes it to crack. Larch is used for the manufacture of poles, girders, and in general hydraulic engineering construction.

Fir belongs to sap-wood species with white wood. Fir has no resin ducts; in engineering properties, it is close to spruce and finds the same application as the latter, but is less resistant to moisture.

Broad-leaved species are very numerous and display a great variety of properties. Most used in building practice are oak, birch, alder, aspen, beech, lime, maple.

Oak belongs to ring-porous species with a yellowish bast and strong­ly pronounced medullary rays. Oak wood is solid, strong and elastic, resistant to decay, with pleasant texture and colour, but with a tenden­cy to cracking. Oak wood is a good building material, but owing to its scarcity it is used only for parquet, carpentry and finishing work in shipbuilding.

Birch is a bast species with hard and heavy wood of white or yello­wish colour. Birch is quite strong, but is susceptible of decay and warps when dried. Birch is used for veneer, turned items, furniture.

Alder is a bast species, lightweight, soft, brittle and with a high tendency to warping. In the air, alder wood decays rapidly, but if it is used for underwater structures fresh upon felling, it remains quite strong and withstands decay for a long time.

Aspen is a bast species, lightweight, soft, white in colour. In a dry environment, aspen is strong, splits readily, can easily be turned in a lathe, shows no tendency to warp or crack when dried. Aspen is used for veneer, roofs, etc.

Beech is a heavy hard wood of white colour with a reddish hue. When dried, it warps and cracks, tends to decay, especially in places with fluctuating humidity.

7. Storage and Drying of Wood

Freshly felled wood has a moisture content much greater than that admissible in service. If wood dries rapidly, it may warp and crack. Therefpre, wood is dried before it is used in building. Drying enhances its resistance to decay, increases its strength, reduces its bulk density and tendency to change shape and dimensions. The following drying methods are in current use: air (natural), chamber, electric, in hot liquids, the chief methods being air and chamber drying.

Wood can be air dried in open-air storage yards under sheds or in closed storage premises. Moisture content of wood is reduced from 60 to 20% in 15 to 60 days, depending on the season. Air drying requires no special equipment, fuel, electric power, etc. However, its main draw­back is large floorspace requirements, dependence on climatic condi­tions and seasons, poor guarantee against decay, and drying only to air-dry condition.

Chamber drying is carried out in special chambers called driers with the aid of hot and humidified air or combustion gases at a tem­perature between 40 and 105°С. In chamber drying there are special requirements for the ratio between temperature and air humidity. Departure from prescribed drying.conditions leads to cracking and war­ping, increases rejects and drying time. Artificial drying not only allows to save time, but also to dry items to below 16% moisture content and to prevent warping and cracking. Disadvantages of chamber drying are considerable consumption of fuel and electric power and large require­ments in labour, equipment and floor-space.

В. MATERIALS, ITEMS AND STRUCTURES FROM WOOD

The following kinds of wood materials and items are used in buil­ding practice: round timber (log), sawn timber and blanks, factory plank graded for door, sash, etc., floor materials, carpentry slabs, materials for roofs of temporary buildings, veneer, plaster lath and carpentry items. Wood structures include load-bearing constructions made from natural (non-glued) wood, items and parts for factory prefabricated houses and glued structures.

1.                Round Timber (Logs)

Building logs from conifer and broad-leaved species should be not less than 14 cm thick at the top and 4 to 6.5 m long. For hydraulic engineering structures and bridge elements, conifer logs should be 22 to 34 cm thick, and 6.5 to 8.5 m long. Logs for power transmission and communications lines should be made from conifers, their thickness and length ranging respectively from 20 to 32 cm and from 6.5 to 18 m. Ties for railway tracks should be sawn from conifer or broad- leaved species and should be not less than 24 cm thick and 2.7 to 5.4 m long.

Moisture content of round timber for load-bearing components of buildings and bridge spans should not exceed 25%, whereas that of constructions permanently immersed in water (piles, sheet piles) is immaterial.

Logs should be pruned flush with the surface and barked in a manner to remove bast (inner bark).

Logs for load-bearing components of permanent structures are sub­divided into three categories. The first and the second include logs in which decay and burrow holes are not admissible. Other defects (cra­cks, cross grain, number and size of knots) are more restrictive for the first category than for the second one. Defects in logs of the third cate­gory are the same as above, except for decay. The number of decayed and brownrot knots is also limited. Only surface burrow holes (damage by bark beetle) are admissible.

Round timber should be stored in stacks in accordance with species, quality categories and lengths.

2.                Sawn Timber

Sawn timber is obtained by longitudinal cutting of wood and is generally classified into the groups below:

boards up to 100 mm thick with width-to-thickness ratio greater than 2;

slabs up to 100 mm thick with width-to-thickness ratio of 2 and less;

rectangular timber (four- and two-faced) more than 100 mm thick and wide.

By finish, sawn timber falls into clean-cut variety in which both edges have been cut throughout their length and non-trimmed variety in which the edges are not cut or cut less than half their length.

Sawn timber from conifers is available in three kinds: boards, slabs and rectangular timber. Boards are sawn in thickness and width from 13 to 40 mm and 80 to 250 mm, respectively; slabs, 50 to 100 mm and 80 to 200 mm; rectangular timber, 130 to 250 mm and 130 to 250 mm.

Sawn timber from conifers is available in length up to 6.5 m insteps of 0.25 m.

Broad-leaved sawn timber is cut in lengths from 1 to 6.5 m in steps of 0.25 m, in thickness from 13 to 75 mm and in width from 80 to 200 mm.

Sawn timber for glued items and structures should have a moisture content not more than 15%, and that for bridge span structures and other load-bearing constructions should carry not more than 25% moisture.

Wood intended for sawn timber should be of high quality and free from decay, whereas sawn timber of the first and second categories must be free from burrow holes, side shoots, decayed and brownrot knots.

3. Half-Finished Materials from Conifer and Broad-Leaved Species

Half-finished materials are sawn to suit dimensions of wood items, allowances being made for shrinkage and working, as prescribed by standing specifications.

Depending on the kind of working, half-finished materials are sub­divided into sawn, glued and sized (pre-planed); according to size, materials are subclassified into thin (up to 32 mm thick), thick (over 32 mm), board (7 to 100 mm thick and more than double-thickness wide) and rectangular timber (22 to 100 mm thick and not more than double-thickness wide). Length of half-finished materials from conifer and broad-leaved wood is 0.3 to 1 m in steps of 50 mm and over 1 m in steps of 100 mm.

Moisture content of sawn half-finished materials should not exceed 18 to 22%, and that of glued and worked to size materials should cor­respond to the moisture content of finished items.

Quality of wood and conifer half-finished materials should meet the specifications of the State Standard GOST 9685-61, and that of broad- leaved species, the specifications of the State Standard GOST 7897-71.

Factory plank graded for various building applications include platbands, plinths, finished floor boards, handrails for barriers, treads, window-sill board and exterior sheathing. Planed factory plank is manufactured from conifer and broad-leaved species in lengths of 2.1 m and more in steps of 100 mm. Moisture content of finish flooring board should not exceed 12%, and that of wood for other items, 15%.

Planed factory plank may not only be single-piece, but composite as well, both in cross section and width. In all cases, the joints should be glued.

Floor materials include paper-mounted piece parquet, parquet boards, finish flooring boards, end-grain blocks and fibreboard.

Parquet is manufactured from oak, beech, birch, pine, larch, ash, maple, smooth leaved elm, elm, american elm, chestnut, common horn­beam, false acacia. Finish flooring boards are made from pine, spruce, larch, fir, cedar, birch, beech and alder, and end-grain blocks, from wood of conifers and hardwood, with the exception of fir, oak, beech and birch.

Moisture content of wood for finish flooring should not exceed 12%, that for parquet and parquet boards, 6 to 10%, and that for end- grain blocks, 28%. Floor boards and end-grain blocks should be trea­ted with antiseptics prior to laying.

Carpentry plates consist of lath panels that are faced with glued-on veneer. Depending on the jacket material, carpentry plates are subdi­vided into ones faced with planed veneer on one or both sides and unfa- ced ones; by the kind of jacket finish, into polished on one or both sides and unpolished ones; by the kind of glue for jointing the jacket to the plates, into ones glued by synthetic resins and ones glued by albumin glues.

Carpentry plates are manufactured from a single species of wood, of conifers, soft broad-leaved species or birch. Jackets for unfaced plates are made from birch, alder, beech and pine veneer and those for faced plates, from planed veneer, not below second grade, in accordance with the State Standard GOST 2977-65.

Carpentry plates are manufactured in widths of 1220, 1270 and 1525 mm, in lengths of 1800, 2120 and 2500 mm and in nine thickne­sses ranging from 16 to 50 mm. Moisture content of carpentry plates should not exceed 8%.

4. Veneer and Materials for Roofs of Temporary Buildings

Materials for roofs of temporary buildings are available in the following range: shavings, lath, wooden tiles and shingles. Materials for roofs are manufactured from pine, spruce, fir and aspen; lath may be fabricated from larch, and tiles, from cedar, f

Shavings are cut lengthwise with respect to fibres in length from 400 to 500 mm, width from 70 to 120 mm and thickness of 3 mm; lath, in length from 400 to 1000 mm, width from 90 to 130 mm and thickness from 3 to 5 mm; tiles, in length from 400 to 600 mm in steps of 50 mm in width not less than 70 mm and thickness of 13 mm; shingles, in length from 500 to 700 mm in steps of 100 mm, width from 70 to 120 mm in steps of 10 mm and thickness of the groove end of 15 mm and of the feather point of 3 mm.

Moisture content of wood for shavings and lath should not exceed 40%, that of wood for tiles and shingles, 25%.

Veneer is manufactured by glueing together thin layers (thin sheets) of wood. Three kinds of veneer-glued, bakelite-treated and decorati­ve-are generally used in building practice.

Glued veneer is subdivided into veneer of high water-resisting pro­perties, put together by phenol-formaldehyde glues (mark ФСФ); of medium water-resisting properties, glued by carbamide or albumin- casein glues (marks ФК and ФБА); of low water-resisting properties, glued together by albumin adhesives (mark ФБ). By its jacket sur­face finish, veneer is subclassified into polished and unpolished types.

Glued veneer is fabricated from birch, beech, aspen, elm, oak, lime, alder, pine, spruce, cedar and fir. Glued veneer is available in length or width from 725 to 1230 mm and thickness from 1.5 to 12 mm. Glued veneers of high water-resisting properties are used as load-bearing com­ponents (beams, arches, frames, etc.) of open-air structures that are protected against moisture by paint; for inside premises with air humi­dity less than 70% no painting is required.

Veneer of medium water-resisting properties is used for partitions and inside finishing of buildings.

Bakelite-treated veneer is manufactured from birch sheets not more than 1.5 mm thick. Veneer faces are assembled of whole sheets in width.

By the kind of resin, bakelite-treated veneer is subdivided into three grades: БФС, БФВ-1 and БФВ-2. The faces of grade БФС veneer are impregnated, the inside layers being smeared with alcohol-soluble resins; grade БФВ-1 veneer is treated respectively with alcohol- soluble and water-soluble resins, and grade БФВ-2 with water- soluble resins only.

Bakelite-treated veneer is manufactured in sizes from 770 x 2000 mm to 1580x1200 mm and 5 to 16 mm thick. Veneer moisture content should not exceed 8%. Bakelite-treated veneer finds the same appli­cation as the glued one, but no painting is required.

Decorative veneer is made of thin sheets of birch, alder, lime, and is subdivided into two kinds of facing: veneer faced with colourless glue- painted film and veneer faced with film and decorative paper. Decora­tive veneer may be faced on one or two sides and have glossy of semi- mat finish. Decorative veneer is available in the following sizes: length from 1220 to 1830 mm, width from 725 to 1220 mm and thick­ness from 1.5 to 12 mm. Moisture content of veneer should not exceed 10%.

In building practice, decorative veneer is used for finishing interior walls, partitions, panels, door panels and built-in furniture.

5. Carpentry Items

Wood is used for manufacturing the following main kinds of carpe­ntry items: parts of doors, windows, partitions and panels (sheath, paneled, dead and glazed) for residential buildings and gates for in­dustrial buildings (hinged, open, suspended and fixed, heat-insulated and non-heat-insulated). All types of carpentry items are made from conifer wood, whereas only interior doors and transoms for premises of relative air humidity of not more than 70% may be made from the broad-leaved species (beech, birch, etc.). Paneled doors, carpentry items and panels (of inside filling) are manufactured from waste of tog sawing, woodworking and veneer plants. Window sashes may also be produced from semi-finished materials with the use of water-resi­sting glues.

In recent years, wastes of insulating fibreboard are used to manufa­cture hollow paneled doors (Fig. 5a), Two waste pieces of such fibre-

 

board are glued together, then cut to strips of about 40 mm longer and 2-3 mm wider than the door frame thickness. The strips are then arran­ged as a grid. Near the cross slabs, the grids are separated by liners from the same kind of fibreboard. When the door is glued, the strips are pressed-in (since they are wider than the door frame) and thus form a solid filling that backs the facing material firmly.

Doors with more uniform surfaces are obtained by combining solid and soft fillings (Fig. 6). The solid material is wood slabs, and the softer one, insulating fibreboard strips by 3-4 mm wider than the solid

filling. Glue is applied to the solid filling only. Pressed-in soft mate rial backs the facing and ensures a smooth surface of hollow panelet doors.

Paneled doors are available either in one-colour or imitation textur» paper finishes. The latter are manufactured by glueing texture papei

 

on fibreboard to imitate a natural wood finish. To prevent fibreboard texture from showing through the imitation texture paper, the latter is backed by a sheet of appropriate colour. Fibreboard, interlayer paper-and texture paper are glued together by interlaying resin films.

 

A similar film, called finish film, covers the texture paper to i the surface of items durable and lustrous.