When designing, developing or manufacturing wooden 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 timber of the required grade, without external defects, knots and other defects. Next, the sample is tested for resistance to compression, bending, stretching.
Types of wood
Wood is a versatile material that can be easily processed and is used in various areas of production: construction, furniture, utensils and other household items. The area of application depends on the type of wood with different physical, chemical and mechanical properties. In construction, such conifers as spruce, cedar, pine, larch, 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, and other building structures. Hardwood materials account for only a quarter of total consumption. This is due to the worse physical and mechanical properties of hardwood timber, so they are trying to be used for the manufacture of structures with low bearing loads. Usually they go to draft and temporary object nodes.
The use of timber in construction is regulated by the rules in accordance with the physical and mechanical properties of wood. These properties depend on humidity and the presence of defects. For load-bearing elements, the humidity should not exceed 25%, for other products there are no such requirements, but there are standards for specific wood defects.
Chemical composition
In 99% of the mass of wood are organic substances. The composition of elementary particles for all rocks is the same: 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. Very stable substance, does not dissolve in water, alcohol or ether.
- Lignin is an aromatic polymer with a complex molecular structure. Contains a large amount of carbon. Thanks to him, lignification of tree trunks appears.
- Hemicellulose is an analogue of ordinary cellulose, but with a lower degree of polymerization of chain molecules.
- Extractivesubstances - resins, gums, fats and pectins.
The high content of resins in coniferous trees preserves the material and allows it to retain its original properties for a long time, helping to resist external influences. Low-grade wood products with a high number of defects are mainly used in the wood 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. Important when choosing sawlogs as a finishing material.
- The coloration depends on the age and type of tree, as well as the climatic conditions where it grew.
- Shine. The ability to reflect light. The highest rate is noted in oak, ash, acacia.
- Texture. The pattern formed by the annual rings of the trunk.
- Microstructure. Determined by ring width and latewood content.
Indicators are used in the external assessment of the quality of logging. Visual inspection reveals defects and the 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, degree and area of the lesion is regulatednormative documents. The main wood defects include:
- defeat, rot, fungus and pests;
- oblique;
- resin pockets;
- knots;
- cracks.
Knotness reduces the strength of timber, of particular importance is their number, size and location. Knots are divided into types:
- He althy. Tightly grow together with the body of the tree and sit firmly in pockets, do not have rot.
- Drop-down. Peel and fall off after sawing the material.
- Horny. Dark in color and have a denser structure in relation to neighboring wood;
- Darkened. Knots with the initial stage of decay.
- Loose - rotten.
According to the location, knots are divided into:
- stitched;
- clawed;
- overgrown;
- stepsons.
Slant also reduces the bending strength of the wood and is characterized by the presence of cracks and spiral layers in the round timber, in the sawn material they are directed at an angle to the ribs. Products with such a defect are low-grade, used exclusively as temporary fortifications.
The causes of cracks depend on external conditions and wood species. They are formed as a result of uneven drying, frost, mechanical stress and many other factors. They appear both on living trees and on cut ones. Depending on the position on the trunk and shape, cracks are called:
- frosty;
- sernitsa;
- metics;
- shrink.
Cracks not only reduce the quality of the wood, but also contribute to the rapid decay and destruction of the fibers.
Rot is formed as a result of infection with putrefactive and other types of fungi that appear on growing and felled trees. Fungi that live on live trunks are parasitic, which infect the annual rings and cause them to peel off. Other species settle already on finished structures and cause decay, delamination, cracking.
The reason for the appearance of harmful organisms is a favorable environment for their reproduction: humidity over 50% and heat. On well-dried timber, microorganisms do not develop. A special category of pests should include insects that prefer to settle in wooden structures, making moves in them, thereby damaging the fibers and reducing their strength.
Wood moisture
One of the important indicators for the normative and design resistance of wood. It affects the percentage of water in the fibers of the trunk. Moisture - percentage of the mass of moisture to dry material. The calculation formula looks like this: W=(m–m0)/m0 100, where m is the initial mass of the workpiece, m 0 - weight of absolute dry sample. Moisture is determined in two ways: by drying and using special electronic moisture meters.
Wood is divided into several types according to moisture content:
- Wet. Withmoisture content of more than 100%, which corresponds to a long stay in the water.
- Freshly cut. With a content of 50 to 100%.
- Air dry. With fiber water content ranging from 15 to 20%.
- Room-dry. With a moisture content of 8 to 12%.
- Totally dry. With 0% water content, obtained by drying at 102°.
Water is in the tree in bound and free form. Free moisture is in the cells and intercellular space, bound - in the form of chemical bonds.
Influence of moisture on wood properties
There are several types of properties depending on the moisture content in the wood structure:
- Shrinkage is a decrease in the volume of wood pulp fibers when bound water is removed from them. The more fibers, the more moisture of the bound type. Removing moisture does not give such an effect.
- Warping - a change in the shape of timber in the process of drying. Occurs when logs are not properly dried or sawn.
- Moisture absorption - the hygroscopicity of wood 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 dripping liquid.
- Density - measured as mass per unit volume. As humidity increases, density increases, and vice versa.
- Permeability - the ability to pass water through itself under high pressure.
After dryingwood 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 hardened steel ball is placed on a flat, wooden surface and 100 kilogram-force is applied to it, after which the depth of the resulting hole is measured.
The Yankee test uses a 0.4 inch ball and measures how much force, in pounds, it takes to push the ball half the diameter into the tree. Accordingly, the higher the result, the harder the tree and the greater the coefficient. However, within the same variety, the indicators differ, which depend on the method of cutting, humidity and other factors.
Below is a table of Brinell and Yankee wood hardness for the most common species.
| Name | Brinell hardness, kg/mm2 | Yankee hardness, pounds |
| Acacia | 7, 1 | |
| Birch | 3 | 1260 |
| Karelian birch | 3, 5 | 1800 |
| Elm | 3 | 1350 |
| Pear | 4, 2 | |
| Oak | 3, 7-3, 9 | 1360 |
| Spruce | 660 | |
| Linden | 400 | |
| Larch | 2, 5 | 1200 |
| Alder | 3 | 590 |
| European walnut | 5 | |
| Spanish Walnut | 3, 5 | |
| Aspen | 420 | |
| Fir | 350-500 | |
| Rowan | 830 | |
| Pine | 2, 5 | 380-1240 |
| Cherry | 3, 5 | |
| Apple tree | 1730 | |
| Ash | 4-4, 1 | 1320 |
From the table of wood hardness it can be seen that:
- aspen, spruce fir, pine - very soft trees;
- birch, linden, alder and larch are soft woods;
- elm and walnut are medium hard;
- oak, apple, cherry ash, pear and have a coefficient of normal hardness;
- beech, locust and yew are very hard varieties.
Hardwood is durableto mechanical stress and is used for critical components of wooden structures.
Density
Density is directly related to the moisture content of the fibers. Therefore, in order to obtain homogeneous 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 moisture, timber is divided into several groups:
- The lowest density rocks (up to 510 kg/m3). These include fir, pine, spruce, poplar, cedar, willow and walnut.
- Reds with medium density (in the range of 540-750 kg/m3). These include larch, yew, elm, birch, beech, pear, oak, ash, rowan, apple.
- Rocks with high density (more than 750 kg/m3). This category includes birch and stock.
Below is a density table for different tree species.
| Breed name | Rock density, kg/m3 |
| Acacia | 830 |
| Birch | 540-700 |
| Karelian birch | 640-800 |
| Beech | 650-700 |
| Cherry | 490-670 |
| Elm | 670-710 |
| Pear | 690-800 |
| Oak | 600-930 |
| Spruce | 400-500 |
| Willow | 460 |
| Cedar | 580-770 |
| European maple | 530-650 |
| Canadian maple | 530-720 |
| field maple | 670 |
| Larch | 950-1020 |
| Alder | 380-640 |
| Walnut | 500-650 |
| Aspen | 360-560 |
| Fir | 350-450 |
| Rowan | 700-810 |
| Lilac | 800 |
| Plum | 800 |
| Pine | 400-500 |
| Poplar | 400-500 |
| Thuya | 340-390 |
| Bird cherry | 580-740 |
| Cherry | 630 |
| Apple tree | 690-720 |
Coniferous species have the lowest density, while deciduous species have the highest density.
Stability
The calculated resistance of wood includes such a thing as stability toexposure to moisture. The degree is measured on a five-point scale when the air humidity changes:
- Instability. Significant deformation appears even with a slight change in humidity.
- Average stability. A noticeable degree of deformation appears with a slight change in humidity.
- Relative stability. A slight degree of deformation appears with a slight change in humidity.
- Stability. No visible deformation with slight change in humidity.
- Absolute stability. There is absolutely no deformation even with a large change in humidity.
Below is a stability chart of common wood species.
| Breed name | Degree of stability |
| Acacia | 2 |
| Birch | 3 |
| Karelian birch | 3 |
| Beech | 1 |
| Cherry | 4 |
| Elm | 2 |
| Pear | 2 |
| Oak | 4 |
| Spruce | 2 |
| Cedar | 4 |
| European Maple | 2 |
| Canadian Maple | 2 |
| Field maple | 1 |
| Larch | 2-3 |
| Alder | 1 |
| American Walnut | 4 |
| Brazil Nut | 2 |
| Walnut | 4 |
| European walnut | 4 |
| Spanish Walnut | 3 |
| Aspen | 1 |
| Fir | 2 |
| Poplar | 1 |
| Bird cherry | 1 |
| Cherry | 2 |
| Apple tree | 2 |
The figures 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 an increase in the hardness of the material, its wear decreases with an uneven distribution over the surface of the sample. The moisture content of the wood also affects wear resistance. 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, which disappears with the load. The main indicator of deformability is elasticity, which increases with the moisture content of wood. With gradual drying, elasticity is lost, which leads to a decrease in resistance to deformation.
- Flexibility - the natural ability of wood to bend under loads. Deciduous species have 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 - the ability to absorb impact force without chipping wood. Testing is carried out using a steel ball, which is dropped onto the workpiece from a height. Deciduous varieties show better results than conifers.
Constant loads gradually deteriorate the properties of wood and lead to fatigue of the material. Even the most durable tree is not able to withstand external influences.
Regulatory Specifications
Indicators of normative 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 tests, only standard samples with a moisture content not higher than 15% are used. For wood with a different moisture value, a special design resistance formula is used, then the indicators are converted to standard values.
When designing timber structures, it is important to know the actual strength values of the source material. In reality, they are less than the normative ones obtained on test samples. Reference dataobtained by loading and deformation of 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 it is completely destroyed. These figures differ for stretch, compression, bending, shearing and crushing.
Actual figures are obtained by multiplying the normative data by the coefficients of the working conditions.
| Name | Design wood resistance factor | ||
| Stress along fibers | Tension across the fibers | Chipping | |
| Larch | 1, 2 | 1, 2 | 1 |
| Siberian cedar | 0, 9 | 0, 9 | 0, 9 |
| Pine | 0, 65 | 0, 65 | 0, 65 |
| Fir | 0, 8 | 0, 8 | 0, 8 |
| Oak | 1, 3 | 2 | 1, 3 |
| Maple, Ash | 1, 3 | 2 | 1, 6 |
| Acacia | 1, 5 | 2, 2 | 1, 8 |
| Beech, birch | 1, 1 | 1, 6 | 1, 3 |
| Elm | 1 | 1, 6 | 1 |
| Poplar, alder, aspen, linden | 0, 8 | 1 | 0, 8 |
Working conditions are influenced by a whole list of factors. The above coefficients take into account such factors. Any exposure to moisture on structures results in a reduction in final performance.
Conclusion
When designing wooden structures, it is important to know the calculated indicators of the materials used in construction. Individual nodes will experience permanent or temporary loads that can lead to their complete destruction. The data specified in GOST and SNiP were obtained by testing standard samples. However, the actual values will differ greatly from the normative ones. Therefore, the formulas provided by the standards are used for calculations.
