All tree species are divided into coniferous and deciduous. Coniferous species differ from deciduous ones in greater straightness of fibers and the presence in their composition more resinous substances. It is resinous substances that increase the resistance of wood to decay. Therefore, wooden building structures are made mainly from coniferous wood. Proceeding from this, we will dwell in more detail on the structure and properties of coniferous wood.

STRUCTURE OF CONIFEROUS WOOD

Wood has a tubular layered fibrous structure. In cross section, a tree trunk consists of bark, a thin layer of cambium, sapwood, heartwood and pith. Cambium is the living part of the trunk, located under the bark. Feeding on the ascending juices, the cambium is directly involved in the growth of the tree, organizes the growth of the main wood and bark. The core is the central inner part of the trunk with a diameter of only 3-5 mm. It refers rather to defects in natural growth than to a useful part of wood, as it consists of loose low-strength cells. Therefore, lumber of small assortments (boards) with a core belongs to the second and third grades and is not recommended for use in tensioned elements of load-bearing structures.

The entire main part of the tree trunk, located between a thin layer of cambium and the core and containing strong and dense cells, consists of two parts: sapwood and core. Sapwood is a young, undead part of the wood, which is closer to the outer contour of the trunk and carries out an upward movement of juices from the roots to the crown of the tree. The core is the oldest, most durable and dense part of the wood, which does not take part in the movement of juices. It is in the sound part that the largest amount of resins lies, which give the material strength and release volatile substances. With the age of the tree, the size of the core increases due to the transition of part of the sapwood into the heartwood, and the width of the sapwood gradually decreases. The most durable building material is obtained from heartwood. To distinguish the sapwood from the heartwood, you need to pay attention to the color: the sapwood is usually lighter, the heartwood is darker. The exception is spruce wood, in which it is more difficult to distinguish the heartwood from sapwood. From the point of view of microstructure, the bulk of wood (up to 95%) is made up of wood fibers located along the trunk of a growing tree and consisting of elongated hollow shells of dead cells called tracheids.

Tracheids in cross section have an almost rectangular hollow shape. Their porous walls are a multilayer plexus of thin fibers - fibrils formed from filamentous cellulose molecules. Cellulose is part of the fibers, forming their frame and providing them with strength. The gaps between the cells-fibers are filled with an intercellular substance of an amorphous structure - lignin, which sticks the fibers together. Thus, cellulose and lignin are the main components of the wood substance. A simplified, but visual representation of the structure of coniferous wood gives a comparison with a bundle of straw, in which individual straw-fibers are glued together in the transverse direction with amorphous glue.

The growth of the tree occurs due to the division of cambial cells only in the spring-summer-autumn period. The tree does not grow in winter. Every year the tree adds one layer of wood. At the same time, early and late wood is present in each annual layer. Early wood has tracheids with large cross-sectional dimensions and a thin wall. In late wood - tracheids with smaller cross-sectional dimensions, but with much thicker walls. Thus, late wood in its structure has fewer voids and more woody substance. Therefore, it is denser, darker in color and stronger than early wood. In coniferous trees, 70-90% of the annual layer is early wood and only the Annual layer is a layer of wood, which in a growing tree is formed annually from the outer part of the trunk under the bark. On a transverse section of a coniferous tree, annual layers are presented as alternating light and dark stripes, the number of pairs of which corresponds to the age of the tree in years. Early wood - part of the annual layer, which was formed in the spring with an excess of moisture, when growth is intensive. Late wood is part of the annual layer, which was formed in the summer-autumn period, when there is less moisture, growth is slowed down, but there are still enough nutrients. Knots - radially directed wood fibers (bases of branches); cause curvature of the hairs of the main trunk. The wood of the knots differs from the main mass of the trunk by increased hardness, a darker color and has an independent system of growth rings. Knots reduce the strength of wood, make it difficult to process, create internal stress in wooden elements.

Svil (pilosity) - a tortuous or tangled arrangement of fibers that forms a curl. Curl increases the density of wood at its location. Just like knots, it makes it difficult to process wood and creates internal stresses.

10-30% - late wood. The more late wood in the annual layers, the stronger the “clean” (that is, without knots, twist, slant layer and other defects) wood. In wooden structures, timber should be used that contains at least 20% late wood in its structure.

In the structure of wood, core rays are also distinguished, which in coniferous species occupy about 7% of the total volume of wood, and in hardwood - 18%. Their cells have a radial direction, therefore they help the wood work on shearing in the tangential direction (along the fibers) and increase the crushing strength in the radial direction (across the fibers). It is they who form the branches (and hence the knots).

Hardwood has a slightly different structure than softwood, in which the walls of wood fiber cells are formed by three layers of microfiber. Each of the layers of microfiber is directed in a spiral with different angle inclination to the longitudinal axis of the cell. The spiral direction of the cell walls of hardwood, in particular, the most common birch in Russia, leads to warping and cracking of lumber during drying and deterioration of nailability. The presence of these shortcomings and low resistance to decay limit the use of hardwood for wooden structures. At the same time, the high strength properties of hard hardwood (including birch) make it possible to use them for the manufacture of small connecting elements (pins, dowels, overlays), as well as critical supporting parts. Such details from oak wood can not be antiseptic, but from birch they must be antiseptic.

In addition to hollow fibers, intercellular substance, resin and core rays, wood contains a large amount of moisture (water solutions of salts). All moisture contained in wood can be divided into three types: free, hygroscopic and chemically bound moisture. Free and hygroscopic moisture can be removed from wood by drying. Chemically bound moisture is released from wood only during its chemical processing, as well as during decay or combustion. By the way, when rotting 1 cu. m of wood, about eight liters of water are released from it.

The amount of water in wood is estimated by such an indicator as moisture content. Freshly cut wood has a moisture content of up to 80-100%, and the moisture content of driftwood can reach 180-200%. For building parts, wood with a moisture content ranging from 8 to 20% should be used. This indicator is achieved in the process of properly organized drying.

Moisture reduction up to 30% is achieved by air drying in stacks. The most difficult and responsible in the general process of drying wood is the process of drying from 30 to 8-20% moisture. It is generally accepted that the maximum amount of hygroscopic moisture that wood can pick up at a temperature of 20 ° C is approximately 30% (this is the so-called saturation point of the fibers). The saturation point of the fibers is the boundary for changing the strength of wood depending on its moisture content. This is explained by the fact that when the moisture content decreases from 200 to 30%, only free moisture is removed from the wood, and the removal of free moisture does not cause shrinkage, and hence deformations. (The approximate duration of drying lumber from freshly cut wood to a moisture content of 30% is indicated in the table). Further return of moisture (already hygroscopic) occurs much more slowly. The movement and return of moisture during drying occurs both across and along the fibers, however, with greater intensity, moisture moves along the fibers. The movement of moisture across the fibers during drying leads to a state when the outer layers of the wood are already dry, while the inner ones remain damp. This creates undesirable internal stresses in the section of the wooden element, which cause it to crack or warp.

To avoid this undesirable effect, it is important that the outer and inner layers dry evenly. Such conditions are created by a soft drying mode, in which all processes occur more slowly and at a lower temperature than in a hard or normal mode.

On the contrary, with an increase in humidity from 0 to 30%, the cell membranes are saturated with water, the wood swells, and building parts increase in volume. Free moisture is moisture that partially or completely fills the internal cavity of the wood cells and the intercellular space. Hygroscopic moisture - moisture that has been absorbed by the porous walls of cells; its quantity is limited by the ability of cells to absorb, that is, hygroscopicity. Chemically bound moisture - water included in chemical composition wood substance. Wood moisture - the ratio of the mass of water contained in wood to the mass of absolutely dry (that is, not containing free and hygroscopic moisture) wood, expressed as a percentage. Shrinkage is a decrease in the linear dimensions and volume of wood when hygroscopic moisture is removed from it. Removal of free moisture does not cause shrinkage. The more cell walls per unit volume of wood, the more hygroscopic moisture in it and the higher the shrinkage.

Changing the shape of a tree during drying

Warping - a change in the shape of lumber and blanks during drying, as well as sawing and improper storage. Most often, warping occurs due to the difference in the amount of shrinkage in different structural directions (that is, in the radial and tangential directions). At the same time, in cramped conditions (for example, in the wall of a house), significant internal stresses can occur in the wooden elements, which will also lead to to deformations (buckling) of wooden elements and structures. It is also important to know that the denser the wood, the more sizes shrinkage and swelling, other things being equal. In accordance with this, the size of shrinkage in the radial and tangential directions in late (more dense) wood is much larger than in early (more porous).

The standard wood moisture content is 12%. It is at this humidity that all the properties of wood are compared.

Advantages of coniferous wood

Along with such relevant characteristics as environmental friendliness, natural beauty, the ability to “breathe” and create a favorable indoor climate, coniferous wood has a number of positive properties that make a wooden house strong, warm, reliable, durable and economical.

Light weight. Coniferous wood used in construction, with an average density of 500 kg / m3, is 15.7 times lighter than steel and 4.8 times lighter than concrete, which makes it possible to significantly reduce material costs for transportation, foundations, do without heavy lifting mechanisms in the construction of buildings and structures. High specific strength. One of the indicators of the effectiveness of the use of structures made of various materials is the so-called specific strength of the material. If we keep in mind that the design resistance (that is, the tensile strength) of wood is on average 14 MPa (megapascals), steel is 230 MPa, and concrete of class B25 is 30 MPa, then for wood the ratio of design resistance to density is 28, for steel - 29.3, and for concrete - 1 2.5 units. Thus, the specific strength of wood is only 4.4% less than steel, and 122% higher than concrete. This indicator confirms the feasibility of using wooden and, in particular, glued wooden structures on a par with metal structures in large-span buildings, where the own weight of structures is of decisive importance.

elasticity and viscosity. Of all the traditional building materials, only wood, having high elasticity, allows the building to respond to uneven settlement of foundation bases without the appearance and development of cracks in wooden details, and also makes it possible to manage the foundations of shallow deepening. The viscous nature of the destruction of wood structures allows the redistribution of forces in the structural elements, which eliminates the possibility of their instantaneous collapse.

Slight thermal expansion. The thermal expansion of wood during heating or cooling is much less than that of other building materials. For example, the coefficient of thermal expansion of wood along the fibers is only 3.6x10"6, steel - 11.5x10"6, aluminum - 23.8-27x10"6, concrete - 12.6x10"6 degrees". This suggests that under conditions of strong heating, wooden elements will have elongations 2.5 times less than steel, 2.8 times less than concrete, and 5.7 times less than aluminum. That is why there is no need to divide wooden buildings into blocks of limited length by means of a device expansion joints.

The development of modern engineering and technology depends on the production and use of various structural materials: wood, metal, plastics, glass, etc. The use of wood has become widespread. Products from it are used in almost all spheres of our life. Paper, cardboard, rayon, plastic, furniture, building elements, musical instruments and souvenirs, and many other necessary things are made from this material.

All tree species are divided into two groups: coniferous and deciduous (Fig. 13).

Rice. 13. Tree species: a - coniferous; b - deciduous

Conifers have needle-shaped leaves. These include: spruce, pine, cedar, larch, fir, etc. Hardwoods are alder, linden, oak, beech, hornbeam and others (Fig. 14).

Rice. 14. Wood of various tree species: a - oak; b - linden; c - birch; g - alder; d - spruce; e - pine

Trees are used to make structural wood materials. Wood materials can be easily processed with various cutting tools: saws, knives, chisels, drills, files and others. Elements of structures made of wood materials are securely and firmly connected with nails, screws, as well as gluing. Trees are the tallest of all plants, although there are also dwarfs among them, up to several centimeters tall (Fig. 15).

Rice. 15. Tall (a) and dwarf (b) trees

Rice. 16. Tree structure

Each tree consists of three parts: root, trunk and crown (Fig. 16).

Root absorbs moisture and nutrients dissolved in it from the soil and conducts them to the trunk.

Trunk is the main part of the tree. It conducts water with nutrients dissolved in it from the root to the branches and leaves.

Crown- the upper part of the tree, consisting of branches and leaves. The leaves of trees absorb carbon dioxide and release oxygen, which is why forests are called the "lungs of the planet." They improve the condition environment, purifying the air and water, contribute to the development of flora and fauna - all life on Earth.

Protection of Nature is an important responsibility of every person. In Ukraine, the protection of natural resources has become one of the most important tasks, and such rare trees as Polish larch, Siberian pine, Cretaceous pine, Austrian oak, Dnieper birch and others, listed in the Red Book of Ukraine, are protected by law and prohibited for industrial use.

In our country, there are forestries - specialized forestry enterprises that grow trees for industrial processing and production of wood materials. They grow different types of trees in vast areas. After a certain time, when the tree reaches industrial age, that is, it will have a certain height and diameter of the trunk, it is harvested. At the same time, forestry enterprises also take care of the renewal of forest plantations - new young trees are planted in places of cut trees.

In forestry, trees are first cut down (Fig. 17, a). Then the trunks cleared of branches, which are called whips, are moved to the place of shipment. This process is called skidding. For skidding, special skidders are used (Fig. 17, b). Then the wood is loaded and transported to a special overpass, where the whips are sawn into pieces - decks. This process is called bucking (Fig. 18).

Rice. 17. Wood harvesting: a - sawing; b - hauling

Rice. 18. Bucking wood

The decks are called commercial wood, and the top of the whip (where there are many knots) is called wood (Fig. 19).

Rice. 19. Business (a) and firewood (b) wood

Rice. 20. Sawmill

To obtain wood materials, commercial wood is cut along the trunk on special machines - sawmills (Fig. 20). Enterprises that process wood are called woodworking. They also process wood waste: sawdust, bark, branches, root. Various materials are made from them: glue, rayon, paper, cardboard, wood boards, etc.

As a result of sawing commercial wood, a variety of sawn timber is formed (Fig. 21). A variety of products are made from lumber. However, in order for the product to be reliable in use, have an attractive appearance and a number of other quality features, it is necessary to take into account the peculiarities of the structure of wood in its manufacture. It is studied in three sections of the trunk: transverse (end), radial and tangential (Fig. 22).

Rice. 21. Types of lumber

Rice. 22. Main sections of a tree trunk: 1 - tangential; 2 - radial; 3 - transverse (end)

Rice. 23. Annual rings on the cross section of the trunk

Rice. Fig. 24. Texture of some types of wood: a - oak; b - birch; c - walnut; g - hornbeam

By the cross section of the trunk and the number of rings that are visible on it, you can determine how old the tree is, how fast it grew, how the weather changed during its growth, etc. (Fig. 23). On the transverse section, an alternation of light and dark rings is observed.

The cut of wood along the trunk through the core is called radial. It shows longitudinal stripes formed as a result of the growth of the tree. By cutting the trunk at some distance from the core, a tangential cut is obtained. On it you can see a pattern of a certain color characteristic of each tree, which is called texture (Fig. 24). It depends on the structural features of each type of wood and the direction of the cut of the trunk.

You will learn about other properties of wood materials from the following paragraphs of the textbook.

Laboratory and practical work No. 3. Acquaintance with the texture of wood materials

Equipment and materials: a carpenter's workbench, samples of different types of wood, a magnifying glass, a set of colored pencils, a ruler, chalk.

Work sequence

1. Consider samples of different types of wood.
2. Mark each sample with chalk.
3. Compare the texture of each wood sample with the texture of different types of wood shown in figure 24 of the textbook.
4. Explain the similarities and differences between the samples (placement and width of annual rings, wood color, smell, other signs).
5. According to the above properties and the pattern of the corresponding texture shown in the textbook, determine the type of wood.
6. Complete the table as follows:

New terms

hardwood, softwood, root, trunk, crown, industrial wood, firewood, industrial age, whip, deck, bucking, texture.

Basic concepts

• A beam is a sawn-off tetrahedral deck.
• The Red Book of Ukraine is a book that records plants and animals that are protected by the state and prohibited for industrial use.
• Sawmill - a device with an electric motor, designed for sawing logs into lumber.
• Nutrients are substances dissolved in water that nourish the plant.
• A tree species is a set of certain features, properties that characterize a tree.
• Natural resources are the reserves of something in nature that can be used when needed.
• Property, sign - a feature characteristic of something (for example, smell, color, sound conductivity, etc.).

Fixing the material

1. What types of wood are coniferous? To leafy?
2. What kind wood materials made in woodworking factories?
3. What is the texture of wood?
4. What is the structure of a tree?
5. What types of lumber do you know?
6. Describe the role of the forest in human life.
7. How do green spaces improve the natural environment?
8. What trees of your region are listed in the Red Book of Ukraine?

Test tasks

1. Belong to conifers

A birch
B pine
In alder
G oak
D el
E hornbeam

2. To lumber belong

A whip
B timber
The deck
G board
D all of the above
E none of the above

3. What is made from decks?

And the tables
Used lumber
The chairs

4. Hardwoods belong

A maple
B spruce
The aspen
G pine

5. What is the name of the natural pattern on the treated wood surface?

And the structure
B longitudinal stripes
The texture
G sapwood

The first main advantage of wood over other construction materials is the constant renewal of its supply. This is typical for our Motherland, a significant part of which is covered with forests. Soviet Union possesses a boundless green factory, on the territory of which, daily, hourly, the beneficial forces of nature create wonderful material needed in various sectors of the national economy. When creating other structural materials (steel, concrete, plastic, etc.), a large amount of raw materials is consumed, the reserves of which are not renewed, but are constantly running out. In addition, the creation of most structural materials requires large amounts of energy, the shortage of which is already felt in many countries. In the process of creating wood, the energy of the sun is used, the reserves of which are enormous.

The second advantage of wood is its low density and relatively high specific strength and rigidity. The corresponding table describes these indicators for wood and basic structural materials.

This table gives the maximum (numerator) and minimum (denominator) tensile strengths and elastic moduli of pine (coniferous), ash (hardwood annulus), and birch (hardwood diffuse vascular) at 12% moisture. From the above data it can be seen that the maximum specific strength of wood of all species is approximately equal to the specific strength the best varieties steel and 4 times greater than the specific strength of steel. The maximum specific stiffness of wood of all species is approximately equal to the specific stiffness
steel and significantly exceeds the specific rigidity of duralumin and fiberglass.

The third advantage of wood over other construction materials is its easier workability.

A decisive role in the choice of wood for the manufacture of many products and structures is also played by its low thermal and electrical conductivity, high sound insulation, biological compatibility, high acoustic properties, aesthetics, chemical resistance, etc.

Long-term observations indicate that wooden houses, equipped with items made of natural wood, a person feels much better than in stone and reinforced concrete with interiors made of plastic. The replacement of reinforced concrete and stone buildings with wooden ones in agriculture helps to increase the productivity of animal husbandry. Studies of the acoustic properties of materials have shown that wood is the best and so far indispensable for the manufacture of soundboards of musical instruments. The presence of aggressive environments in chemical production shops dictates the need to replace metal and reinforced concrete structures with wooden ones, as they are more resistant to chemical influences.

However, defects that significantly reduce the quality of wood products, low strength and stiffness in directions perpendicular to the fibers, a significant decrease in mechanical characteristics with increasing humidity, creep even at normal temperatures in some cases give rise to distrust of wood as a structural material. This mistrust is for the most part a consequence of relatively little knowledge of the strength and rigidity of wood products. Thorough theoretical and experimental studies these issues are necessary to develop recommendations for the rational use of wood and products and determine their reliability and durability.

Particular attention deserves the use of wood in combination with other structural materials. In this case, you can use the positive properties of wood and compensate for its shortcomings. The use of various materials (wood, metal, plastics, reinforced concrete) in combination ensures the most efficient use of the properties inherent in each of them. Thus, the role of wood as a structural material must constantly increase.

| Service life |

Wood as a natural material

Wood- a traditional material for the manufacture of flooring, which includes parquet, parquet board and solid wood board. Wood is understood as the body of woody and shrubby plants, surrounded by cambium and bark.

The texture and surface pattern of wood products for different sawing options depend on the width and visibility of the growth rings of a tree. It is believed that from an aesthetic point of view, the value of wood is the higher, the more uniform the structure of the annual layers and the smaller the difference in the width of individual layers.

From the point of view of sawing wood, three main types are considered: transverse (or end); radial; tangential.

The use of natural wood for the production of the described building materials (parquet, solid wood and parquet boards) determines the transfer of the advantages and disadvantages of this natural material on the properties of floor coverings. The greatest difficulty is created by the dependence of the geometric dimensions of products on the temperature and humidity conditions of storage, transportation, installation and operation. In this regard, in addition to proper packaging and compliance with storage and transportation conditions, there are certain restrictions on the temperature and humidity of the premises (including enclosing structures - base and walls) when laying and operating parquet.

The same considerations determine mainly the choice of size ranges of products, including the ratio between length and width and thickness, sheet pile parameters and tolerances for machining accuracy during the manufacturing process. The quality of natural wood flooring depends on the type of wood, the conditions of its growth, processing and operation.

wood color(Fig. 4) is due to the tannins, dyes, resinous substances and their oxides contained in it and depends on the type of tree, its age, soil composition and climatic conditions of the area where it grew. Over time, the color of the wood changes, it seems to be patinated, which, on the one hand, creates an aura of antiquity, and on the other, makes it difficult to repair the floor associated with the replacement of individual planks.

wood texture- This is a natural pattern formed by the fibers and layers of wood and due to the peculiarities of its structure. Depends on the location of wood fibers, the visibility of annual layers, the color scheme of wood, the number and size of core rays. The type of wood is determined by color and texture.

wood hardness primarily depends on the type of wood, and also to a large extent on the conditions of tree growth, humidity, etc. Within the same species, the spread of values ​​\u200b\u200bcan be very significant. Usually, the average relative hardness values ​​according to Brinell are indicated as a percentage in relation to oak, the relative hardness of oak is taken as 100%.
Brinell hardness is determined by pressing a hardened steel ball with a diameter of 10 mm into the test specimen with a certain force. Then the formed hole is measured and the Brinell hardness value is calculated (the smaller the hole, the harder the wood). The harder the wood, the higher the number on this scale.

Wood is a hygroscopic material, which has the property of absorbing moisture from the environment and giving it away. Its humidity changes with changes in the climatic characteristics of the surrounding air. For example, at a relative humidity of 50% and a temperature of +20 °C, the equilibrium moisture content of wood will be 9%, at an air humidity of 30% and a temperature of +25 °C, this figure is 5%. The rate of change in wood moisture content depends on the species.

When the moisture content of the wood changes, the linear dimensions of the planks also change, characterized by linear expansion coefficient. This indicator is expressed as a percentage of the bar width.

The diagram (Fig. 3) presents data on the change in the width of the plank depending on the type of wood when the moisture content of the wood changes by 1%.

Using this coefficient, it is possible to determine the theoretical deformation of the parquet laying by calculation (the actual deformation, as a rule, turns out to be less than the calculated one).

The deformation of wood, which is an anisotropic material, occurs unequally in different directions and depends on the type of cut and on the presence of residual stresses after drying.

It should also be noted that at normal indoor humidity (40-65% humidity is considered the norm), there are significant linear changes in quality dried parquet will not occur, i.e. the quality of drying depends on how the parquet floor will behave during its operation, how durable it will be. Good results, from the point of view of minimizing residual stresses, are obtained by vacuum or vacuum-convective drying.

Moisture content of plank wood according to GOST 862.1-85 when shipped to the consumer should be 9±3%. Such humidity is optimal from the point of view of maintaining parquet of its geometric dimensions. Under normal operating conditions, 19% wood moisture corresponds to 55% relative air humidity at 20°C.

A freshly cut tree may have a relative wood moisture content of 50-70%. There are various ways of drying wood, incl. hot air, microwave and vacuum chambers. During technological process it is important not only to bring the moisture content of the wood to the required value (9 ± 3%), but also not to create residual stresses, which can later lead to warping of the parquet or its cracking.

It must be understood that even well-dried parquet will react to changes in humidity in the room. But at the same time, the changes occurring in it will not be critical if the relative humidity and temperature in the room correspond to normal conditions.

Based on the assessment criteria common for different types of wood, it is possible to determine characteristics that are transformed into consumer properties of wood products, and compile the corresponding table ("Properties of wood of various species used in parquet production", see the link on the CD-ROM below). The following are used as criteria for assessing the properties of wood:

• hardness and resistance to stress, affecting wear resistance - the service life of a parquet floor;
• stability and degree of shrinkage, which characterize the reaction of wood to changes in temperature and humidity and determine, among other things, the compatibility of different species in the structures of artistic parquet;
• the degree of oxidation, which determines the stability of the color of wood during operation;
• the expressiveness of the texture characterizing the aesthetic properties of the wood surface.

Wood protection implies a relatively wide range of measures and means designed to prevent the impact on it of influences that destroy it or change its characteristics in an undesirable direction. This is, first of all, protection against moisture, which involves the application of varnishes, wax mastics or oils to the surface of the tree (with impregnation to a certain depth). Protection against humidity during storage and transportation involves the use of appropriate packaging that protects against both moisture and mechanical stress during transportation.

For certain operating conditions, impregnation of wood with pyrophobic and antiseptic agents is provided.

In order to increase the hardness of wood in the manufacture of certain types of floor coverings, it is subjected to special pressing, which increases the density of the surface layers. For such types of floor coverings as parquet boards and pronto-parquet, a multilayer structure is used in the subbase of the material with mutually perpendicular fastening of the layers, which helps to increase the stability of the geometric dimensions of the flooring elements.

And, finally, the task of protecting natural wood is to maintain normal operating conditions for floor coverings made of it. For despite all the protective coatings and measures for waterproofing the floors, we value natural wooden floors, among other things, for their ability to "breathe", i.e. provide moisture exchange with the surrounding air. Excessive humidity or, on the contrary, overdried air is harmful to us as well as to the natural wood products we use. However, it should be borne in mind that neither the packaging used, nor any type of protective coating used for floors, provide complete moisture resistance.

Basic properties of wood as a structural material. Advantages and disadvantages.

Physical Properties

Density.

temperature expansion. α

Thermal conductivity λ ≈ 0.14W/m∙ºС.

.

Heat capacity C \u003d 1.6KJ / kg ∙ºС.

Mechanical properties of wood

strength - the ability to resist destruction from mechanical influences; rigidity - the ability to resist changes in size and shape; hardness - the ability to resist the penetration of another solid body; toughness - ability to absorb work on impact.

Wood like others Construction Materials, has its advantages and disadvantages.

Advantages:

Availability of a wide, constantly renewable raw material base;

Relatively low density;

High specific strength - the ratio of tensile strength along the fibers to density: 100/500 = 0.2 (approximately equal to steel);

Resistance to salt aggression, to the effects of other chemically aggressive environments;

Biological compatibility with humans and animals - wood buildings have the best microclimate;

High aesthetic and acoustic properties - the best concert halls in the country are lined with wood;

Low coefficient of thermal conductivity across the fibers - a wall made of timber 200 mm wide is equivalent in thermal conductivity to a brick wall 640 mm wide;

Low coefficient of linear expansion along the fibers - in wooden buildings there is no need to arrange expansion joints and movable supports;

Less labor intensive mechanical processing, the possibility of creating curved glued structures.

Disadvantages:

Anisotropy of wood structure;

Susceptibility to rotting and damage by carpenter beetles;

Combustibility in fire conditions;

Changes in physical and mechanical characteristics under the influence of various factors (moisture, temperature);

Shrinkage, swelling, warping and cracking under the influence of atmospheric influences;

The presence of defects (knots, oblique and others), significantly reducing the quality of products and structures;

Limited assortment of timber.

Types of engineering plastics Their physical and mechanical characteristics. Advantages and disadvantages. Application area.

Depending on the type of resins under the influence of temperature, plastics are divided into two types: a) thermoplastic plastics (or thermoplastics) based on thermoplastic resins; b) thermosetting (reaplasts) based on thermosetting resins.

Thermoplastics usually named after the binder, based on the name of the monomer with the addition of the prefix "poly-" (polyvinyl chloride, polyethylene, polystyrene, etc.)

thermoset- by type of filler (fiberglass, wood plastics, etc.)

Depending on the structure, plastics can be divided into two main groups:

1) plastics without filler (not filled);

2) filled plastics (filled).

The plastics that find and will find the greatest use in building structures in the future include fiberglass, plexiglass, vinyl plastic, polyethylene, heat and sound insulating materials, and wood plastics.

Fiberglass.

Fiberglass is a material consisting of a glass fiber filler and a binder.

As a binder, thermosetting resins (polyester, epoxy, phenol-formaldehyde) are usually used. Glass fiber is a reinforcing element, the strength of which reaches 1000-2000 MPa. The basis of glass fibers are elementary fibers.

Elementary fibers (primary filaments) are obtained from molten glass mass, pulling it through small holes - spinnerets; elementary fibers (about 200) with a diameter of 6-20 microns are combined into threads, and several dozen threads into bundles (twisted threads).

In fiberglass used in construction, the following fiberglass fillers are used:

a) rectilinear continuous fibers introduced in the form of bundles, threads or elementary fibers.

b) chopped glass fiber in the form of randomly arranged segments approximately 50 mm long.

The mechanical properties of fiberglass depend on the type of fiberglass filler. Glass-reinforced plastics reinforced with continuous rectilinear fiberglass have the highest mechanical properties. In the direction of the fibers, their tensile strength reaches 1000 MPa, and the elastic modulus is up to 40,000 MPa, however, in the transverse direction, the strength of glass-reinforced plastics is not high (about 10 times less).

All fiberglass reinforced in one or two mutually perpendicular directions are anisotropic materials.

Glass-reinforced plastics reinforced with chopped glass fiber are isotropic materials.

There are the following types of fiberglass:

1) Press - materials like SVAM(Fiberglass Anisotropic Press Material) is one of the first high-strength glass-reinforced plastics obtained by pressing glass veneers (unidirectional fiberglass veneers).

It is obtained in this way: after winding a certain number of layers of impregnated thread, the unidirectional material is cut off. In the development, it is a square sheet measuring 3x3 m 2. Then the sheet is rotated 90 degrees and the layer of threads is wound again. Thus, a glass veneer with a mutually perpendicular arrangement of fibers is obtained. The ultimate strength of SVAM in tension and compression is 400-500 MPa, and in bending, approximately 700 MPa.

2) Press materials AG-4S and AG-4V.

AG-4S is a unidirectional tape obtained on the basis of twisted glass threads and amino-finol-formaldehyde resin. AG-4S is designed to produce high-strength products by direct pressing or winding.

The ultimate strength in compression and bending is lower than that of SVAM - 200-250 MPa, and slightly higher in tension.

Press - material type AG-4V is a fiberglass based on sections of the primary thread. A specially prepared glass fiber filler is mixed with phenol-formaldehyde resin, then dried.

Fiberglass type SVAM, AG-4S and AG-4V are used for the manufacture of connecting parts (bolts, gussets) and for profile products operated in chemically aggressive environments, where the metal quickly corrodes. All of the above glass-reinforced plastics are opaque. However, translucent fiberglass is most often used in construction. In our country, translucent polyester sheet fiberglass is produced in large volumes.

3) Polyester fiberglass are made on the basis of chopped fiberglass and transparent polyester resins, due to which polyester fiberglass is translucent. It is produced in products in the form of wavy or flat sheets, often with different colors. Strength characteristics are significantly lower than those of previous materials, and amount to 60-90 MPa in tension and compression.

Polyester glass-reinforced plastics are widely used in enclosing structures (wall and roof panels), stair railings and balcony railings, canopies, etc. structures. Fiberglass plastics for combined spatial structures are very promising.

Wood plastics.

Materials obtained from the processing of natural wood, combined with synthetic resins, are called wood plastics.

Wood laminates(chipboard) are made from thin sheets of birch (sometimes alder, lime or beech) veneer impregnated with resin and pressed with high pressure 150-180 kg\cm 2 and temperature t=145-155ºC.

Depending on the relative position of the veneer layers in the package, there are 4 main grades of chipboard:

DSP-A- all layers are parallel to each other, DSP-B- every 10-12 parallel layers one transverse, DSP-V- cross arrangement, and the outer layers are located along the plate, DSP-G- star-shaped, each layer is shifted in relation to the previous one by 25-30º.

In all cases, the strength of chipboard exceeds the strength of solid wood, and for some grades, under the action of forces along the veneer fibers, it is not inferior to the strength of steel.

Currently, due to the high cost of chipboard, it is mainly used for the manufacture of means for connecting structural elements.

fibreboard(Fibreboard) is made from randomly arranged wood fibers (sawdust) glued together with a rosin emulsion. The raw material for fiberboard is sawmill and woodworking waste. For the manufacture of hard and superhard boards, phenol-formaldehyde resin is added to the wood fiber mass. With prolonged exposure to a humid environment, the fibreboard is very hygroscopic, swells in thickness and loses strength, so it is not recommended to use fiberboard in wet conditions. The tensile strength of superhard fiberboards with a density of at least 950 kg / m 3 is about 25 MPa.

Particle boards(PS and PT) are obtained by hot pressing wood chips mixed, or rather pollinated with phenol-formaldehyde resins.

Chipboards, depending on the density, are divided into:

Light γ \u003d 350-500 kg / m 3

Average PS γ \u003d 500-650 kg / m 3

Heavy PT γ \u003d 650-800 kg / m 3

Tensile strength of PT and PS plates is 3.6-2.9 MPa and 2.9-2.1 MPa, respectively. PS and PT are cheap and available material, it is widely used in construction as partitions, suspended ceilings. The moisture absorption of the plates varies widely, while they swell in thickness by 30-40%.

Airtight fabrics - a new, unusual construction material, consisting of textiles and elastic coatings.

Technical textiles are the strength basis of airtight fabrics. It is made from high strength synthetic fibers. Polyamide fibers of the "kapron" type are used most widely. They have high strength, significant elongation and low resistance to aging. Polyester fibers of the "lavsan" type are less extensible and more resistant to aging.

virtues this material:

limitations

The use of plastics as a material for building structures explained next virtues this material:

High strength, which for most plastics (except for foam plastics) is 50-100 NPa, and for some fiberglass, the strength reaches 1000 NPa;

Low strength (bulk weight) ranging from 20 (for foam plastics) to 2000 kg / m 3 (for fiberglass);

Resistance to chemically aggressive environments;

Biostability (non-susceptibility to decay);

Ease of shaping and easy machinability;

High electrical insulating properties and some other positive properties.

However, plastics have limitations , such as, for example, deformability, creep and strength drop under long-term loads, aging (deterioration of operational properties over time), combustibility, use of scarce oil products as raw materials.

The impact of plastic deficiencies can be reduced in a variety of ways. Thus, a decrease in deformability is achieved by using rational forms of the cross-section of structures (three-layer, tubular).

Combustibility and aging can be reduced by introducing special additives.

Physical Properties

Density. Wood belongs to the class of light structural materials. Its density depends on the relative volume of the pores and their moisture content. The standard density of wood should be determined at a moisture content of 12%. Freshly chopped wood has a density of 850 kg/m 3 . The calculated density of coniferous wood as part of structures in rooms with a standard air humidity of 12% is taken equal to 500 kg / m 3., in a room with an air humidity of more than 75% and in the open air - 600 kg / m 3.

temperature expansion. Linear expansion during heating, characterized by the coefficient of linear expansion, in wood is different along and at angles to the fibers. Linear expansion coefficient α along the fibers is (3 ÷ 5) ∙ 10 -6, which allows you to build wooden buildings without expansion joints. Across the wood fibers, this coefficient is 7-10 times less.

Thermal conductivity wood due to its tubular structure is very small, especially across the fibers. Thermal conductivity coefficient of dry wood across the fibers λ ≈ 0.14W/m∙ºС. A beam 15 cm thick is equivalent in thermal conductivity to a brick wall 2.5 bricks thick (51 cm) will, as well as when sawing logs as a result of their escape.

fins, sawmills. .- butts.nivaniyu than needles.

Heat capacity wood is significant, the coefficient of heat capacity of dry wood is C \u003d 1.6KJ / kg ∙ºС.

Another valuable property of wood is its resistance to many chemical and biological aggressive environments. It is chemically more resistant material than metal and reinforced concrete. At ordinary temperatures, hydrofluoric, phosphoric and hydrochloric (low concentration) acids do not destroy wood. Most organic acids do not weaken wood at ordinary temperatures, so it is often used for structures in chemically aggressive environments.

The mechanical properties of wood are characterized by: strength- the ability to resist destruction from mechanical influences; rigidity- the ability to resist changes in size and shape; hardness- the ability to resist the penetration of another solid body; toughness- the ability to absorb work on impact.

For the manufacture of wooden load-bearing structures, usually coniferous forest materials are used: pine, spruce, larch, cedar and fir. Among the forest plantations of Russia, coniferous forests are the most common. Coniferous wood is superior in strength to most common hardwoods and is less prone to decay. The trunks of coniferous trees have a more regular shape, which allows fuller use of their volume. Pine is the most commonly used.

Pine, according to the place of growth, is divided into myand pine and ore pine. Myandovaya prefers low-lying soils, its wood is loose, loose, less layered than that of the ore pine and therefore prone to decay in a humid environment. It is very well processed, perfectly impregnated and little subject to warping. The ore pine, unlike the myandova, grows on hills, various elevations and prefers stony loamy or sandy loamy soil. Its wood is resinous and fine-layered, has a fairly high density. It is these qualities that provided the ore pine with a worthy place in the field of house-building technologies (floors, roof structures, walls, internal partitions).

Spruce is inferior to pine in a number of characteristics. It is less processed, less dense and less durable than pine. Significantly impairs the consumer properties of spruce, its branchiness and increased hardness. Spruce wood's tendency to rot limits its use in areas subject to moisture. In housing construction, spruce is used in the manufacture of door blocks, floors, internal partitions, and furniture.

Larch is characterized by high density, resistance to decay, hardness. The latter significantly complicates the processing of larch, which to some extent limits its use in construction. But other qualities, plus having a high resistance to warping, provide larch with a reputation as a valuable building material.

Larch, like no other material, requires a very moderate drying regime with all precautions. The fact is that with intensive drying, cracks appear in the larch. In housing construction, larch is used primarily where high resistance to decay is required. In addition, larch has established itself as a good material for the manufacture of parquet planks.

Siberian cedar in its physical and mechanical properties is intermediate between spruce and fir. The wood of the cedar is soft, light, well processed. With special processing, it acquires increased resistance to decay. In housing construction, it is mainly used in the same place as pine. But this is a good material for assemblies and structures that experience fluctuations in humidity and temperature conditions.

Siberian fir is similar in its qualities to spruce wood, but inferior to it in strength and density. And in what is not inferior to spruce only Caucasian fir. The use of fir is quite common (especially Caucasian fir). These are door and window blocks, floors, plinths, layouts, friezes and many other products. Fir is not used in external wooden structures due to its low resistance to decay.

The use of hard hardwood (oak, beech, ash, hornbeam, maple) is allowed only in those areas where these species are local building material.

Pedunculate oak (summer) has great strength and resistance to decay and is used mainly for small critical parts of wooden structures in the form of dowels, dowels, liners, etc. The only thing that should not be forgotten is that oak wood is prone to splitting when nails are driven into it or screws are screwed in without first drilling the hole channel with a smaller diameter drill.

Bukpo basic qualities (strength and hardness) are not much inferior to oak, but its wood has a high hygroscopicity and therefore is more prone to decay. At the same time, beech wood is high-tech: it is well processed by any tool, it bends well under steam. In housing construction, it is not used as widely as oak (due to hygroscopicity), but it is very much in demand in finishing work.

For the manufacture of open layered rafters and lathing in the coverings of permanent buildings with an attic, as well as for the construction of temporary buildings (warehouses, sheds, sheds, etc.) and auxiliary structures (flyovers, towers, etc.), soft hardwood should be widely used - aspen, birch, beech, linden, poplar and alder, but with mandatory enhanced protection against decay.

Round timber. Timber used in industrial and civil construction is divided into round timber and sawn timber. For each of these types of materials, their classification, grade, assortment, type of processing, quality requirements, permissible deviations from normal sizes and acceptance conditions are established by the relevant standards.

A building log can be used in a round form or as a raw material for lumber production. Saw logs have the following standard sizes.

Table 1.1.

The length of the logs is from 3 to 6.5 m with a gradation of 0.5 m. An increase in the thickness of the log along the length is called a run. On average, the run is 0.8 cm per 1 m of length. The more massive part of the log is called the butt, and the opposite part is called the upper pipe. The diameter of the log is measured in the upper cut. Logs with a length of more than 6.5 m are prepared by special order for power transmission and communication poles.

Sawn timber. Sawn timber products include:

two-edged bars, in which only two sides are sawn off (Fig. 1.2.a);

four-edged bars, in which all four sides are sawn off (Fig. 1.2.b and c);

Bars, sawn from four sides, not more than 10 cm thick and not more than twice the width (Fig. 1.2.d);

boards with a thickness of not more than 10 cm and a width of more than double thickness: boards are divided into thin, up to 3.2 cm thick (Fig. 1.2.e) and thick - more than 3.2 cm (Fig. 1.2.e).

Rice. 1.2. Sawn timber: a - two-edged timber,

b - wane four-edged timber, c - cleanly cut

four-edged timber, g - bar, d - thin board,

Assortment of wood

Timber obtained by construction is divided into round and sawn.

Round timber, also called logs, are parts of tree trunks with smoothly sawn ends - ends. They have a standard length of 3 - 6.5 m with a gradation every 0.5 m. The logs have a natural truncated-conical shape. A decrease in their thickness along the length is called a run. On average, the run is 0.8 cm per 1 m of length (for larch 1 cm per 1 m of length) of a log. Medium logs have a thickness of 14 to 24 cm; large ones - up to 26 cm. Logs with a thickness of 13 cm (undercarriage) and less are used for temporary construction structures. Round timber depending on quality are subdivided into 1,2 and 3 grades.

lumber obtained as a result of longitudinal sawing of logs on sawmills or circular saws. Lumber is divided according to the nature of processing: edged (sawn from 4 sides along the entire length); wane (part of the surface is not sawn along the entire length due to the run-off of the log); unedged (two edges are not sawn off).

Rectangular lumber is divided into boards, bars and beams. The wider sides of the lumber are called layers, and the narrow ones are called edges. Timber has a standard length of 1–6.5 m with gradation every 0.25 m. The width of lumber ranges from 75 to 275 mm, thickness - from 16 to 250 mm. According to the quality of wood and processing, boards and bars are divided into five grades (selective, 1, 2, 3, 4th), and bars into four (1, 2, 3, 4th).

Density of wood.

The density of wood is the ratio of the mass of wood to its volume. Density is determined by the amount of wood substance per unit volume. The density is expressed in kg / m3 (kilogram per cubic meter) or g / cm3.

There are voids in wood (cell cavities, intercellular spaces). If it were possible to press the wood so that all the voids would disappear, then a solid woody substance would be obtained. The density of wood due to its porous structure is less than the density of the woody substance, the same rule can be applied to wood products, for example the density of birch or spruce is lower than the density of birch or softwood plywood.

There is a close relationship between the density and strength of wood. Heavier wood is generally more durable.

Wood density values ​​vary over a very wide range. Boxwood wood has the highest density - 960 kg / m3, iron birch - 970 kg / m3 and saxaul - 1040 kg / m3; Siberian fir wood has the lowest density - 375 kg/m3 and white willow - 415 kg/m3. As the moisture content increases, the density of the wood increases. For example, the density of beech wood at a moisture content of 12% is 670 kg/m3, and at a moisture content of 25% it is 710 kg/m3. Within the annual layer, the density of wood is different: the density of late wood is 2-3 times greater than that of early wood, therefore, the better developed the late wood, the higher its density.

By density at a moisture content of 12%, wood can be divided into three groups:

Rocks of high density - 750 kg / m3 and above - white acacia, iron birch, hornbeam, boxwood, saxaul, pistachio, dogwood.

Breeds of medium density - 550 - 740 kg / m3 - larch, yew, birch, beech, elm, pear, oak. Elm, elm, maple, plane tree, mountain ash, apple tree, ash.

Breeds of low density - 510 kg / m3 or less - pine, spruce, fir, cedar, poplar, alder, linden, willow, chestnut, Manchurian walnut, velvet tree.

Coniferous wood has a low density, and scattered vascular hardwood has a high density, so it is cleanly processed, well varnished and polished.

Rice. 12.11. Segmented metal-wooden truss with a glued linear top chord

1 - steel shoe of the support unit; 2 - the same, the lower belt; 3 - metal insert

Rice. 12.13. Determination of the calculated bending moment in the upper chords of segmented metal-wooden trusses.

Diagrams of bending moments in a truss with a split (a) and continuous (b) upper chord and diagrams of the operation of a curved element - a constant load throughout the span and a temporary (snow) load on half the span.

Snow load is taken according to scheme 2 app. 3 SNiP (1) for vaulted roofs, while the most unfavorable combination of loads is usually obtained when taking into account one-sided snow load distributed according to the triangle law.

The geometric dimensions of the truss elements are determined by replacing the curvilinear upper chord with a straight one, i.e. connecting the nodes of the upper belt with straight lines - chords.

The constructive calculation of trusses consists in the selection of the section of chords, braces, the design and calculation of nodes. The upper chord, due to curvilinearity and load application between the nodes, is calculated as a compressive-bent element.

The calculated bending moment in the panels of the upper chord is determined as the sum of the moments from the transverse load and the moment from the longitudinal force arising due to the bending of the panel (Fig. 12.13).

With a split upper chord, the moment is determined by the formula

(12.3)

where M 0 is the bending moment determined by the beam scheme,

D 1 - horizontal projection of the panel between the centers of the nodes;

q is the calculated conditionally uniformly distributed load (within the panel);

N is the calculated compressive force in the panel of the upper chord;

f 0 - lifting arrow (curvature) of the panel;

d is the length of the panel along the chord;

R is the radius of curvature of the upper chord,

l - truss span;

f is the height of the truss in the middle of the span between the axes of the chords.

With a continuous upper chord, the design bending moments in the span and on the supports are determined as for a continuous multi-span beam with equal spans according to the approximate formulas:

for supporting (extreme) panels

(12.4)

(12.5)

for middle panels

(12.6)

(12.7)

The moments from longitudinal forces are determined based on the assumption that each panel is a single-span beam, with the outer panels being considered hinged at one end and with the other end rigidly fixed, and the middle panels with both rigidly fixed ends. When determining flexibility, the estimated length of the outer panels is taken equal to 0.8 chord length, and the middle panels - 0.65d.

The section of the lower chord is selected according to the formula for centrally tensioned steel elements in terms of the net area, that is, taking into account the weakening from the holes for the nodal bolts. When the nodal bolt is located with eccentricity relative to the axis of the lower chord, the lower chord is checked for eccentric tension, taking into account the load from its own weight.

Compressed braces are calculated for buckling with a calculated length equal to the length of the brace between the centers of the truss nodes. Stretched braces are calculated for tension, taking into account the existing weakening. For the purpose of unification, all braces are assumed to have the same section.

Then the number of capercaillie (dowels) necessary for attaching the plates to the braces is determined, considering the most loaded element. Steel plates are checked for tension along the weakened section and for stability from the plane, taking the calculated length of the bar equal to the distance from the nodal bolt to the brace bolt closest to it. To reduce the estimated length of the slats, an additional tie bolt is placed outside the brace.

The supporting node of the farm is designed and calculated:

The end of the upper belt is checked for crushing;

The dimensions of the base plate are assigned from the condition of support and fastening with anchor bolts;

The required length of welds is determined for attaching the corners of the lower belt to the gussets of the support assembly.

If necessary, a steel insert is calculated in the nodes of the split upper chord and the nodal bolt. The nodal bolt, on which the bracing plates are put on, is calculated for bending from the resultant forces R b arising in the adjacent braces under one-sided loading. Moment in knotted bolt

where a is the shoulder of application of the force R b,

a \u003d δ + 0.5 δ 1 (δ - thickness of the plate - tip, δ 1 - thickness of the extreme edge of the nodal insert).

The construction lift of the trusses is assigned equal to 1/200 of the span. The truss is checked for mounting loads.

See p18

Figure 8 - Geometrical and design scheme of the arch

In lancet arches, the angle of inclination α and the length l of the chord, the central angle φ and the length S / 2 of the semi-arch, the coordinates of the center a and b, the angle of inclination of the reference radius φ 0 and the equation of the arc of the left semi-arch are determined. Then half of the span of the arch is divided into an even number, but not less than six equal parts, and in these sections the coordinates x and y, the angles of inclination of the tangents α and their trigonometric functions are determined.

Static calculation

The support reactions of a three-hinged arch consist of vertical and horizontal components. Vertical reactions R a and R b are determined as in a single-span freely supported beam from the condition that the moments in the support hinges are equal to zero. Horizontal reactions (thrust) H a and H b are determined from the condition that the moments in the ridge hinge are equal to zero.

It is convenient to determine reactions and forces in sections of only one left semi-arch in the following order:
- first, the forces from a single load on the right and left, then from left-handed, right-handed snow, wind from the left, wind from the right and the weight of the equipment.

Bending moments should be determined in all sections and illustrated with diagrams.

Longitudinal and transverse forces can only be determined in sections at the hinges, where they reach their maximum values ​​and are necessary for calculating nodes. It is also necessary to determine the longitudinal force at the site of the maximum bending moment for the same combination of loads.

Forces from bilateral snow and dead weight are determined by summing up the forces from unilateral loads.