Estimated wood resistance. Wood properties

When designing, developing or manufacturing wood structures, it is important to know the strength properties of the material — the design resistance of wood, which is measured as one kilogram per square centimeter. To study the indicators, samples of standard sizes are used, sawn from boards or beams of the required grade, without external defects, knots and other defects. Further, the sample is checked for resistance to compression, bending, and stretching.

Types of wood

Wood is a universal material that can be easily processed and is used in various fields of production: construction, manufacture of furniture, dishes, and other household items. The scope depends on the type of wood with different physical, chemical and mechanical properties. In construction, conifers such as spruce, cedar, pine, larch, and fir are especially popular. To a lesser extent deciduous trees - birch, poplar, aspen, oak, hazel, linden, alder, beech.

Coniferous varieties are used in the form of round timber, timber, boards for the manufacture of supporting piles, trusses, poles, bridges, houses, arches, industrial facilities, as well as other building structures. Hardwood materials account for only a quarter of total consumption. This is due to the worse physical and mechanical properties of deciduous timber, so they are trying to use them for the manufacture of structures with insignificant bearing loads. Usually they go to rough and temporary nodes of objects.

The use of timber in construction is regulated by the rules in accordance with the physical and mechanical properties of wood. These properties are dependent on moisture and defects. For load-bearing elements, the humidity should not exceed 25%, for other products such requirements do not exist, but there are standards for specific wood defects.

Chemical composition

99% of the mass of wood includes organic matter. The composition of elementary particles is the same for all rocks: nitrogen, oxygen, carbon and hydrogen. They form long chains of more complex molecules. Wood consists of:

• Cellulose is a natural polymer with a high degree of polymerization of chain molecules. A very persistent substance, it does not dissolve in water, nor in alcohol, nor in ether.
• Lignin is an aromatic polymer with a complex molecular structure. It contains a large amount of carbon. Thanks to it, lumbering of tree trunks appears.
• Hemicelluloses are an analogue of ordinary cellulose, but with a lower degree of polymerization of chain molecules.
• Extractives - resins, gums, fats and pectins.

The high resin content in coniferous trees preserves the material and allows you to maintain the original properties for a long time, helping to resist external influences. Low-grade timber with a large number of defects is mainly used in the chemical industry as a raw material for the manufacture of paper, glued wood or the extraction of chemical elements, such as tannins used in the manufacture of leather.

Appearance

Wood has the following external properties:

• Color. Visual perception of the reflected spectral composition of light. It is important when choosing a sawlog as a finishing material.
• Coloring depends on the age and species of the tree, as well as the climatic conditions where it grew.
• Shine. The ability to reflect light. The highest indicator is observed in oak, ash, acacia.
• Texture. The pattern formed by the annual rings of the trunk.
• Microstructure. It is determined by the width of the rings and the content of late wood.

The indicators are used in external assessment of the quality of logging. Visual inspection allows you to identify defects and suitability of materials for subsequent use.

Wood defects

Despite the obvious advantages over synthesized materials, wood, like any natural raw material, has its drawbacks. The presence, extent and area of ​​damage is regulated by regulatory documents. The main defects of wood include:

• defeat, rot, fungi and pests;
• slanting;
• resin pockets;
• knots;
• cracks.

Knotting reduces the strength of timber, of particular importance is their quantity, size and location. Knots are divided into types:

• Healthy. Tightly fused with the body of a tree and sit tight in pockets, do not have rot.
• Dropdowns. Peel off and fall off after sawing the material.
• Horny. They are dark in color and have a denser structure with respect to neighboring wood;
• Darkened. Knots with an initial stage of decay.
• Loose - rotten.

At the position of the knots are divided into:

• stitching;
• scapular;
• overgrown;
• stepsons.

A slanting also reduces the bending strength of wood and is characterized by the presence of cracks and spiral layers in the round timber; in the sawing material they are directed at an angle to the ribs. Products with such a defect are low-grade, they are used exclusively as temporary fortifications.

The causes of cracks depend on the external conditions and the type of wood. They are formed as a result of uneven drying, frost, mechanical stress and many other factors. They appear both on living trees and on felled trees. Depending on the position on the trunk and the shape, cracks are called:

• frost freezing;
• chamois;
• Metrics
• drained.

Cracks not only reduce the quality of wood, but also contribute to the rapid decay and destruction of fibers.

Rot is formed as a result of infection with putrefactive and other types of fungi that appear on growing and sawn trees. Mushrooms that live on live trunks are parasitic that infect annual rings and cause them to exfoliate. Other species settle already on the finished structures and cause rotting, delamination, cracking.

The reason for the appearance of harmful organisms is a favorable environment for their reproduction: humidity of more than 50% and heat. On well-dried timber, microorganisms do not develop. Insects that prefer to settle in wooden structures, making moves in them, thereby damaging the fibers and reducing their strength, belong to a special category of pests.

Wood moisture

One of the important indicators for the regulatory and design resistance of wood. It affects the percentage of water in the fibers of the trunk. Humidity - the percentage of the mass of moisture to dry material. The calculation formula looks like this: W = (m – m ₀ ) / m ₀ * 100, where m is the initial mass of the workpiece, m ₀ is the mass of the absolute dry sample. Humidity is determined in two ways: by drying and using special electronic moisture meters.

By moisture, wood is divided into several types:

• Wet With a moisture content of more than 100%, which corresponds to a long stay in water.
• Freshly sawn. With a content of 50 to 100%.
• Air dry. With a water content in the fibers in the range of 15 to 20%.
• Indoor dry. With a moisture content of 8 to 12%.
• Completely dry. With 0% water content, obtained by drying at a temperature of 102 °.

Water is in a tree in a bound and free form. Free moisture is in the cells and intercellular space, bound in the form of chemical bonds.

The effect of moisture on the properties of wood

There are several types of properties that depend on the moisture content in the wood structure:

• Shrinkage - a decrease in the volume of wood pulp fibers when removing bound water from them. The more fibers, the more moisture bound type. Removing moisture does not give such an effect.
• Warpage - a change in the shape of timber during the drying process. Occurs when the logs are not properly dried or sawn.
• Moisture absorption - the hygroscopicity of a tree or the ability to absorb moisture from the environment.
• Swelling - an increase in the volume of wood fibers when the material is in a humid environment.
• Water absorption - the ability of wood to increase its own moisture by absorbing a drop of liquid.
• Density - measured as mass per unit volume. With increasing humidity, density increases, and vice versa.
• Permeability - the ability to pass water through itself under high pressure.

After drying, the tree loses its natural elasticity and becomes harder.

Hardness

The hardness coefficient is determined using the Brinell method or the Yankee test. Their fundamental difference lies in the measurement technique. According to Brinell, a ball of hardened steel is installed on a flat, wooden surface and 100 kilogram-force is applied to it, after which the depth of the obtained hole is measured.

In the test, the Yankees use a ball with a diameter of 0.4 inches and measure how much force in pounds must be applied to push the ball into the tree half the diameter. Accordingly, the higher the result, the harder the tree and the larger the coefficient. However, within one grade, the indicators differ, which depend on the method of cutting, moisture and other factors.

Below is a table of the Brinell and Yankee hardness of wood for the most common species.

From the table of hardness of wood it is seen that:

• aspen, spruce fir, pine - very soft trees;
• birch, linden, alder and larch are soft species;
• Elm and walnut have medium hardness;
• oak, apple, cherry ash, pear and have a coefficient of normal hardness;
• beech, white acacia and yew are very hard varieties.

Solid wood is resistant to mechanical stress and is used for critical units of wooden structures.

Density

Density directly depends on the moisture content of the fibers. Therefore, to obtain uniform measurement indicators, it is dried to a level of 12%. An increase in the density of wood leads to an increase in its mass and strength. According to humidity, timber is divided into several groups:

• Breeds with the lowest density (up to 510 kg / m ³ ). These include fir, pine, spruce, poplar, cedar, willow and walnut.
• Rocks with an average density (in the range of 540-750 kg / m ³ ). These include larch, yew, elm, birch, beech, pear, oak, ash, rowan, apple tree.
• Rocks with high density (more than 750 kg / m ³ ). This category includes birch and stock.

The following is a density table for different tree species.

Coniferous species have the lowest density, and deciduous species, the highest, are the largest.

Stability

The design resistance of wood includes such a thing as stability to moisture. The degree is measured on a five-point scale when changing air humidity:

1. Instability. Significant deformation appears even with a slight change in humidity.
2. Medium stability. A noticeable degree of deformation appears with a slight change in humidity.
3. Relative stability. A slight degree of deformation appears with a slight change in humidity.
4. Stability. There is no visible deformation with a slight change in humidity.
5. Absolute stability. There is absolutely no deformation even with a large change in humidity.

The table below shows the stability of common wood species.

The indicators are calculated for wood with a moisture content of 12%.

Mechanical characteristics

The quality of wood is determined by the following indicators:

• Wear resistance - the ability of wood to resist wear during friction. With increasing hardness of the material, its wear decreases with an uneven distribution over the surface of the sample. Wear resistance is also affected by the moisture content of the wood. The lower it is, the higher the resistance.
• Deformability - the ability to restore shape after the disappearance of the acting forces. When wood is compressed, deformation of the workpiece appears, which disappears with the load. The main indicator of deformability is elasticity, which increases with wood moisture. With gradual drying, the elasticity is lost, which leads to a decrease in resistance to deformation.
• Flexibility is the natural ability of wood to bend under stress. Hardwoods have a good performance, conifers to a lesser extent. These abilities are important in the manufacture of bent products, which are first moistened, and then bent and dried.
• Impact strength is the ability to absorb impact force without chipping wood. Testing is carried out using a steel ball, which is dumped onto the workpiece from a height. Deciduous varieties show better results than conifers.

Constant loads gradually worsen the properties of wood and lead to material fatigue. Even the strongest tree is not able to withstand external influences.

Regulatory specifications

Indicators of regulatory resistance are necessary for the manufacture of various types of structures. Wood is considered suitable if the indicators are not lower than the calculated values. In the tests, only standard samples with a moisture content not exceeding 15% are used. For wood with a different humidity value, a special formula for the design resistance is used, then the indicators are converted to standard values.

When designing wooden structures, it is important to know the actual strength values ​​of the source material. In reality, they are smaller than the normative ones obtained on test samples. Reference data is obtained by loading and deforming samples of standard sizes.

Design characteristics

The design resistance of wood is the stresses in different planes of wooden samples, created by certain loads that a tree can withstand any amount of time until complete destruction. These figures are different for stretching, compression, bending, chipping and crushing.

Actual indicators are obtained by multiplying the normative data by the factors of the working condition.

A whole list of factors affects the working conditions. The above coefficients take into account such factors. Any effect of moisture on the structure leads to a decrease in the final performance.

Conclusion

When designing wooden structures, it is important to know the estimated performance of the materials used in construction. Individual nodes will experience permanent or temporary loads, which can lead to their complete destruction. The data specified in GOST and SNiP are obtained by testing standard samples. However, the real values ​​will be very different from the normative. Therefore, for the calculations, the formulas provided by the standards are used.