Plywood or OSB (OSB): what to choose? Characteristics, properties and applications of plywood Comparison of deflection of wood and plywood.
When constructing plywood, the following rules are observed:
- the plywood sheet should be symmetrical relative to the middle layer
- The number of veneer layers in plywood is usually odd.
The thickness of the veneer used for the outer layers of plywood does not exceed 3.5 mm, and inner layers- 4 mm.
The special properties of plywood are imparted through the use of various resins and varnishes.
Based on water resistance, there are three types of plywood:
- FC– plywood is glued with urea resin. Used indoors.
- FSF- plywood is glued together with phenolic resin. Used both indoors and outdoors.
- FB– Bakelized plywood – impregnated with bakelite varnish and then glued together. Used in tropical climates, aggressive environments and sea water.
Based on the degree of mechanical surface treatment, plywood is divided into:
- NS- unpolished;
- Ш1- polished on one side;
- Ш2- polished on both sides.
Plywood is also divided according to the type of wood from which it is made: birch, coniferous and combined plywood. Plywood is considered to be made from the species from which its outer layers are made.
High physical and mechanical properties Birch trees combined with a multi-layer structure provide plywood with unusual strength. Such properties as warm shades and beautiful wood structure are also important. |
|
This type of plywood is made mainly from pine, the properties of which provide not only an attractive and harmonious appearance, but also excellent strength and low weight, which is successfully used in house construction. |
|
Attractive appearance along with an attractive price (due to alternating layers of coniferous and birch veneer) make it advisable to use plywood in furniture production, interior decoration and gyms, design of design solutions. |
|
The laminated surface of the board creates high resistance to various natural and chemical conditions, which makes laminated plywood indispensable in production (reusable forms concrete formwork, caravan upholstery and floors, etc.) |
For all types of plywood, it is mandatory to indicate the emission class of free formaldehyde E1 and E2 (up to 10 or from 10 to 30 mg/100g of dry product, respectively).
The quality of plywood is also assessed by the strength of chipping, static bending, tensile strength of samples, moisture content, the presence, structure, color of knots, and the presence of defects.
The thickness of plywood sheets (slabs) is produced from 4 to 40 mm.
The grade of plywood is determined by the number of knots per 1 square meter of the surface of the outer sheet and is indicated by Roman numerals from I to IV or Latin letters “A”, “B”, “C” and their combinations.
| Grade I- practically without defects, only a few healthy fused knots with a diameter of up to 8 mm and minor brown veins are allowed. | |
| Grade II- repair of the sheet surface is allowed. Knots and open defects are sealed with veneer inserts. Covered with various finishing materials and paints. | |
| Grade III- this grade includes plywood sheets rejected from grade II(BB). Intended for the manufacture of structures hidden from external view, various special containers and packaging. | |
| Grade IV- All manufacturing defects are allowed. Knots are allowed in unlimited quantities, only good gluing is guaranteed. Used for the manufacture of durable containers and packaging. |
Physical and mechanical indicators
Standard plywood size: 1525x1525 mm
Dimensions, mm (inches): 1525x1525 (60x60), 1525x1270 (60x50), 1270x1525 (50x60), 1270x1270 (50x50), 1525x1475 (60x58), 1475x1525 (58x60), 1475x1475 (5 8x58), 1830x1525 (72x66), 1830x1475 (72x58 ), 1830x1270 (72x50).
Brand: FC, FSF
Thickness, mm: 3; 4; 5; 6; 8; 9; 10; 12; 15; 18; 21; 24; 27; thirty.
Standard size: 1250(1220)x2500(2440), 1525x3050 mm
Dimensions, mm: 1250x2500, 1220x2440, 2500x1250, 2440x1220, 1525x3050.
Brand: FSF Thickness, mm: 4.0; 6.5; 9; 10; 12; 15; 18; 21; 24; 27; 28; thirty; 35; 40.
The element of the floor formwork that takes the pressure of concrete and all other loads is plywood. The above mentioned types of plywood have, depending on the direction of work different meanings for both elastic modulus and flexural strength:
- in floors with low surface requirements f - in floors with higher surface requirements f The deflection of plywood (0 depends on the load (thickness of the floor), the characteristics of the plywood itself (modulus of elasticity, sheet thickness) and support conditions.
Appendix 1 (Fig. 2.65) shows diagrams for the main types of plywood supplied by PERI - birch plywood (Fin-Ply and PERI Birch) and coniferous plywood (PERI-Spruce). The diagrams are based on a sheet thickness of 21 mm. In this case, the dotted line marks the areas where the deflection exceeds 1/500 of the span. All lines end when the plywood's tensile strength is reached. The basic diagrams are based on standard sheets operating as multi-span continuous beams (minimum three spans).
For the running dimensions of the sheets, the following options for the pitch of the transverse beams are obtained.
Table 2.7
When assessing deflections during addition: for birch plywood, the same values are taken for the modulus of elasticity and tensile strength as for the main sheets, since it is not always known in which direction the additional sheets are laid. For coniferous plywood,
in which, when the sheet is turned, these characteristics change sharply.
Using the diagram (Fig. 2.65) for birch plywood with 3 or more spans, we use the X axis to find our value for the floor thickness (20 cm) and determine the values for deflections:
For our sheet length, two options are acceptable - either 50 cm or 62.5 cm. Let’s focus on the second option, since it saves on the number of transverse beams. The maximum deflection is 1.18 mm. Let's look at the diagram for a single-span system. With this scheme, the line for a span of 60 cm ends exactly at the value of the floor thickness of 20 cm (the tensile strength of plywood). The deflection is 1.92 mm.
It follows from this that in order to avoid excessive deformations of the extension, one should either limit the span of this extension to 50 cm, or place an additional transverse beam under this extension (the design diagram of a uniformly loaded 2-span beam has the smallest values for deflections, but it has an increased ratio reference moment to multi-span schemes).
Determination of the span of transverse beams (step of longitudinal beams b)
According to the step of the transverse beams selected in the previous paragraph, we check the table corresponding to our type of beams. 2.11 the maximum permissible span of these beams. As mentioned above, these tables are compiled taking into account all design cases, for transverse beams, primarily moment and deflection.
When choosing the pitch of the longitudinal beams, it is necessary to take into account that the outermost longitudinal beam is located at a distance of 15-30 cm from the wall. Increasing this size can lead to the following unpleasant results:
- increase and uneven deflections on the consoles of the transverse beams;
- the possibility of overturning transverse beams during reinforcement work.
The reduction makes it more difficult to control the struts and creates the risk of the transverse beams slipping off the longitudinal beams.
For the same reason, and also taking into account normal operation at the end of the beam (especially when using truss beams), a minimum beam overlap of 15 cm is assigned on each side. In no case should the actual pitch of the longitudinal beams exceed the permissible value according to the table. 2.11 and 2.12. Remember that the span in the formula for determining the moment is present in the square, and in the deflection formula even to the fourth power (formulas 2.1 and 2.2, respectively).
Example
For simplicity, choose a rectangular room internal dimensions 6.60x9.00 m. Floor thickness 20 cm, PERI Birch plywood 21 mm thick and sheet dimensions 2500x1250 mm.
The permissible value for the span of transverse beams with their pitch of 62.5 cm can be found from the table. 2.11 for GT 24 truss beams. In the first column of the table, find the thickness of 20 cm and move to the right to the corresponding pitch of the transverse beams (62.5 cm). We find the utmost permissible value span 3.27 m.
We present the calculated values of the moment and deflection for this span:
- maximum torque at the time of concreting - 5.9 kNm (acceptable 7 kNm);
- maximum deflection (single-span beam) - 6.4 mm = 1/511 span.
If we place the longitudinal beams parallel to the length side of the room, we get:
6.6 m - 2 (0.15 m) = 6.3 m; 6.3:2 = 3.15 m 3.27 m; 8.7:3 = 2.9 m We get three spans with a beam length of 3.30 m (minimum 2.9 + 0.15 + 0.15 = 3.2 m). Cross beams are less loaded - most often this is a sign of excess material consumption.
In some cases, for example, when it is necessary to install formwork around pre-installed large equipment, beams have to be calculated. The following prerequisites should be taken into account. As a design scheme in “MULTIFLEX” type systems, only a single-span hinged beam without consoles is always considered, since when installing formwork and during concreting we always have intermediate stages where the beams work exactly according to this scheme. For large spans of beams without additional support, loss of stability is possible even at small loads. Any floor formwork after concreting must be pulled out from under the finished floor, sometimes from an enclosed space, so it is advisable to limit the length of the beams (a problem of weight and maneuverability).
If there are no values in the table, you can still use it. For example, to increase the span, you want to reduce the pitch of the beams - as a result, you must check the permissibility of the span. For example, they decided to install the beams in increments of 30 cm, the thickness of the floor is 22 cm. The calculated load according to the table is 7.6 N/m2. We multiply this load by the pitch of the beams: 7.6-0.3 = 2.28 kN/m. We divide this value by one step of the transverse beams, which are present in the table: 2.28:0.4 = 5.7 ~ 6.1 (load on floors 16 cm thick); 2.28:0.5 = 4.56 - 5.0 (load on floors 12 cm thick).
In the first case, for a floor thickness of 16 cm and a beam pitch of 40 cm, we find a span of 4.07 m, in the second case, a thickness of 12 cm and a beam pitch of 50 cm - 4.12 m.
We can take the smaller of the two values minus the difference of these values (taking into account the change in live load, which is present only in the calculation for the moment), without wasting time on lengthy calculations. IN specific example obtained by accurate calculation
4.6 m, but accepted 4.02 m.
The scope of plywood application depends on the characteristics of each type. One of the main parameters is the strength of plywood or resistance to destruction.
Peculiarities
This is a layered material where veneer from various types of wood alternates with adhesive composition based on resins. By combining the layers into a single whole by pressing, the result is fabrics with different properties, including resistance to loads. This is due to some differences in technology, characteristics of wood and glue. A special production technology makes it possible to obtain such products that if you compare the strength of plywood and boards, the former will be more resistant to loads, and this quality is used not only in interior design, but also in construction and mechanical engineering.
Parameters that determine the strength of plywood:
- Thickness;
- Type of wood;
- Grade;
- Adhesive for production;
- Lamination.
Thickness
The standard thickness that industrially produced products can have is usually in the range from 3 to 30 mm, although sheets with a thickness of 40 mm can be produced under an agreement with the enterprise. Naturally, high-strength plywood will have a sheet thickness of about 20 mm or more.
Type of wood from which veneer is made
Almost any wood is used for production - coniferous and hardwood veneer, which give it different qualities. In the first case, pine, larch or cedar are used, and deciduous trees are represented mainly by birch, alder or poplar. If we evaluate the influence of wood species on resistance to destruction, then hardwood plywood has an advantage; its tensile strength is higher due to the fact that the wood used for its production is more dense.
Note! Due to differences in density, even similar-looking canvases have different weight. For example, with the same thickness of 21 mm coniferous standard size 1.52 m by 1.52 m weighs about 32 kg, and the same sheet made from birch veneer will weigh 34.5 kg.
Grade
The grade is determined by the number of defects on one square meter. Plywood, the tensile strength of which is sufficiently high, should not have defects that reduce its resistance to destruction. There are five grades in total, which determine the number of defects and their size. Products of elite varieties are considered the best, without any damage to the surface, and are able to withstand significant loads. Products of the first and second grade can be considered quite durable, because a small number of defects allows them to be laminated or used as a basis for finishing materials, including as a basis for flooring.
Note! The lower the quality of the canvas, the smaller the margin of safety it has, so it is used either where there will not be large loads or to level a surface reinforced with another material, for example, if it is necessary to level wooden floor before finishing coating, you can also use sheets of the fourth grade.
Adhesive for production
Depending on what resins are used for the production of glue, products of the FK or FSF brand are obtained, which have almost the same strength and the difference between them is manifested in how these types react to humidity. Plywood, for the production of which bakelite glue is used, has the highest strength. Such products are designated as FB, FBS or BS and can be used for almost any operating conditions.
Sheathing a boat from bakelite sheets
Lamination
Lamination, when veneer is covered with a thermosetting film before gluing, allows you to create sheets that have very high strength and resistance to damage. At the same time, the cost of the material is quite affordable, and its appearance allows it to be used almost anywhere, because furniture made from such sheets is indistinguishable from real wood, but unlike it, it is not afraid of high humidity.
Note! Laminated fabrics can be not only dark shades. Bright, saturated shades are also popular and are used to create original interiors.
Material flexibility
Is in particular demand flexible plywood With unique properties, which distinguish it as a special species. It is indispensable for creating decorative elements in the interior and furniture with curved lines that cannot be created from other materials.
The high bending strength of plywood is ensured by the wood of trees exotic for our latitudes - ceibe and kuruing, which, in addition to high flexibility, have good resistance to impacts and are not afraid of moisture. Such canvases are made in a special way, placing all layers of veneer so that the fibers are in the same direction.
What are the advantages of flexible plywood:
- High flexibility allows you to create forms where the canvas can be bent 180° without damage, which allows you to create elements of any shape;
- Possibility of processing by any means, which does not require the purchase of special equipment to work with the material;
- It has a smooth surface with high decorative characteristics, which allows it to be used for making furniture, including kitchen furniture, taking into account its resistance to high humidity;
- The low density makes the material quite light and allows the manufacture suspended structures that do not require reinforced fastening;
- The absence of odor and resins that emit compounds harmful to health, which allows it to be used even for decorating children's rooms.
IN Lately Such a material has appeared from birch veneer, and such plywood has a bending strength higher than usual due to a specially developed technology that almost halves the density of the material.
So there is a cell with clear dimensions of 50x50 cm, which is planned to be covered with plywood with a thickness of h = 1 cm (actually, according to GOST 3916.1-96, the plywood thickness can be 0.9 cm, but to simplify further calculations we will assume that we have plywood with a thickness of 1 cm), a flat load of 300 kg/m2 (0.03 kg/cm2) will act on the plywood sheet. Ceramic tiles will be glued to the plywood, and therefore it is very desirable to know the deflection of the plywood sheet (calculation of plywood strength is not discussed in this article).
Ratio h/l = 1/50, i.e. such a plate is thin. Since we technically cannot provide such fastening on supports so that the logs perceive the horizontal component of the support reaction that occurs in the membranes, then it makes no sense to consider a plywood sheet as a membrane, even if its deflection is quite large.
As already noted, to determine the deflection of the plate, you can use the corresponding design coefficients. So for a square slab with hinged support along the contour, the calculated coefficient k 1 = 0.0443, and the formula for determining the deflection will have the following form
f = k 1 ql 4 /(Eh 3)
The formula does not seem to be complicated and we have almost all the data for the calculation, the only thing missing is the value of the elastic modulus of wood. But wood is an anisotropic material and the value of the elastic modulus for wood depends on the direction of action of normal stresses.
Yes, if you believe regulatory documents, in particular SP 64.13330.2011, then the modulus of elasticity of wood along the fibers E = 100,000 kgf/cm 2, and across the fibers E 90 = 4000 kg/cm 2, i.e. 25 times less. However, for plywood, the values of the elastic modulus are taken not simply as for wood, but taking into account the direction of the fibers of the outer layers according to the following table:
Table 475.1. Moduli of elasticity, shear and Poisson's ratios for plywood in the plane of the sheet
It can be assumed that for further calculations it is enough to determine a certain average value of the elastic modulus of wood, especially since the plywood layers have a perpendicular orientation. However, such an assumption will not be correct.
It is more correct to consider the ratio of elastic moduli as an aspect ratio, for example for birch plywood b/l = 90000/60000 = 1.5, then the calculated coefficient will be equal to k 1 = 0.0843, and the deflection will be:
f = k 1 ql 4 /(Eh 3) = 0.0843 0.03 50 4 /(0.9 10 5 1 3) = 0.176 cm
If we did not take into account the presence of support along the contour, but calculated the sheet as a simple beam with a width b = 50 cm, a length l = 50 cm and a height h = 1 cm under the action of a uniformly distributed load, then the deflection of such a beam would be (according to the calculated diagram 2.1 table 1):
f = 5ql 4 /(384EI) = 5 0.03 50 50 4 /(384 0.9 10 5 4.167) = 0.326 cm
where the moment of inertia I = bh 3 /12 = 50 1 3 /12 = 4.167 cm 4, 0.03 50 is the reduction of a plane load to a linear load acting across the entire width of the beam.
Thus, supporting along the contour allows you to reduce the deflection by almost 2 times.
For plates that have one or more rigid supports along the contour, the influence of additional supports creating the contour will be less.
For example, if a sheet of plywood is laid on 2 adjacent cells, and we consider it as a two-span beam with equal spans and three hinged supports, not taking into account the support along the contour, then the maximum deflection of such a beam will be (according to design diagram 2.1 of Table 2):
f = ql 4 /(185EI) = 0.03 50 50 4 /(185 0.9 10 5 4.167) = 0.135 cm
Thus, laying plywood sheets over at least 2 spans allows you to reduce the maximum deflection by almost 2 times, even without increasing the thickness of the plywood and without taking into account the support along the contour.
If we take into account the support along the contour, then we have, as it were, a plate with rigid pinching on one side and hinged support on the other three. In this case, the aspect ratio is l/b = 0.667 and then the calculated coefficient will be equal to k 1 = 0.046, and the maximum deflection will be:
f = k 1 ql 4 /(Eh 3) = 0.046 0.03 50 4 /(0.9 10 5 1 3) = 0.096 cm
As you can see, the difference is not as significant as with hinged support along the contour, but in any case, an almost twofold reduction in deflection in the presence of rigid pinching on one of the sides can be very useful.
Well, now I would like to say a few words about why the elastic moduli for plywood differ depending on the direction of the fibers, because plywood is such a tricky material in which the directions of the fibers in adjacent layers are perpendicular.
Determination of the modulus of elasticity of a plywood sheet. Theoretical background
If we assume that the modulus of elasticity of each individual layer of plywood depends only on the direction of the fibers and corresponds to the modulus of elasticity of wood, i.e. impregnation, pressing during manufacturing and the presence of glue do not affect the value of the elastic modulus, then you must first determine the moments of inertia for each of the sections under consideration.
Plywood with a thickness of 10 mm usually has 7 layers of veneer. Accordingly, each layer of veneer will have a thickness of approximately t = 1.43 mm. In general, the given sections are relatively perpendicular axes will look something like this:
Figure 475.1. The given sections are for a plywood sheet with a thickness of 10 mm.
Then, taking the width b = 1 and b" = 1/24, we get the following results:
I z = t(2(3t) 2 + t(2t 2) + 4 t 3 /12 + 2t(2t 2)/24 + 3t 3 /(24 12) = t 3 (18 + 2 + 1/ 3 + 1/3 + 1/96) = 1985t 3 /96 = 20.67t 3
I x = t(2(3t) 2 /24 + t(2t 2)/24 + 4 t 3 /(12 24) + 2t(2t 2) + 3t 3 /12 = t 3 (18/24 + 2/24 + 1/72 + 8 + 6/24) = 655t 3 /72 = 9.1t 3
If the elastic moduli were the same in all directions, then the moment of inertia about any of the axes would be:
I" x = t(2(3t) 2 + t(2t 2) + 4 t 3 /12 + 2t(2t 2) + 3t 3 /12 = t 3 (18 + 2 + 1/3 + 8 + 1 /4 =43 3 /12 = 28.58t 3
Thus, if we do not take into account the presence of glue and other factors listed above, the ratio of elastic moduli would be 20.67/9.1 = 2.27, and when considering a plywood sheet as a beam, the elastic modulus along the fibers of the outer layers would be (20.67/28.58)10 5 = 72300 kgf /cm 2. As you can see, the technologies used in the manufacture of plywood make it possible to increase the calculated value of the elastic modulus, especially when the sheet bends across the fibers.
Meanwhile, the ratio of the calculated resistances when bending along and across the fibers of the outer layers (which can also be considered as the ratio of moments of inertia) is much closer to what we determined and is approximately 2.3-2.4.