Strength calculation of plywood. Characteristics of plywood

During construction or renovation wooden house use metal ones, and even more so reinforced concrete beams ceilings are somehow out of topic. If the house is wooden, then it is logical to make the floor beams wooden. It’s just that you can’t tell by eye what kind of timber can be used for floor beams and what kind of span should be made between the beams. To answer these questions, you need to know exactly the distance between the supporting walls and at least approximately the load on the floor.

It is clear that the distances between the walls are different, and the load on the floor can also be very different; it’s one thing to calculate the floor if there is a non-residential attic on top, and a completely different thing to calculate the floor for the room in which partitions will be built in the future and stand cast iron bath, bronze toilet and much more. Therefore, take everything into account possible options and it is almost impossible to lay out everything in the form of a simple and understandable table, but to calculate the cross-section wooden beam floors and select the thickness of the boards, using the example below, I think it will not be very difficult:

EXAMPLE OF CALCULATION OF A WOODEN FLOOR BEAM

The rooms are different, most often not square. It is most rational to fasten the floor beams so that the length of the beams is minimal. For example, if the size of the room is 4x6 m, then if you use beams 4 meters long, then the required cross-section for such beams will be less than for beams 6 m long. in this case the dimensions 4 m and 6 m are arbitrary; they mean the length of the span of the beams and not the length of the beams themselves. The beams, of course, will be 30-60 cm longer.

Now let's try to determine the load. Typically, the floors of residential buildings are designed for a distributed load of 400 kg/m2. It is believed that for most calculations such a load is sufficient, and for calculations attic floor Even 200 kg/m2 is enough. Therefore, further calculations will be carried out for the above load with a distance between the walls of 4 meters.

A wooden floor beam can be considered as a beam on two hinged supports, in this case the calculation model of the beam will look like this:

1. Option.

If the distance between the beams is 1 meter, then the maximum bending moment is:

M max = (q x l²) / 8 = 400x4²/8 = 800 kg m or 80.000 kg cm

Now it is easy to determine the required moment of resistance of a wooden beam

W required = M max / R

Where R- design resistance of wood. In this case, the beam on two hinged supports bends. The design resistance value can be determined from the following table:

Calculated resistance values ​​for pine, spruce and larch at a humidity of 12%

And if the beam material is not pine, then the calculated value should be multiplied by the transition coefficient according to the following table:

Transition factors for other types of wood
according to SNiP II-25-80 (SP 64.13330.2011)

Tree species	Coefficient m n for calculated resistances
	stretching, bending, compression and crumpling along the grain R p , R i, R s, R cm	compression and crumpling across the fibers R с90, R cm90	chipping R ck
Conifers
1. Larch, except European	1,2	1,2	1,0
2. Siberian cedar, except cedar from the Krasnoyarsk region	0,9	0,9	0,9
3. Cedar of the Krasnoyarsk Territory	0,65	0,65	0,65
4. Fir	0,8	0,8	0,8
Hard Deciduous
5. Oak	1,3	2,0	1,3
6. Ash, maple, hornbeam	1,3	2,0	1,6
7. Acacia	1,5	2,2	1,8
8. Birch, beech	1,1	1,6	1,3
9. Elm, elm	1,0	1,6	1,0
Soft deciduous
10. Alder, linden, aspen, poplar	0,8	1,0	0,8
Note: the coefficients m n indicated in the table are for support structures air lines power transmission lines made from larch not impregnated with antiseptics (with humidity ≤25%) are multiplied by a factor of 0.85.

For structures in which stresses arising from permanent and temporary long-term loads exceed 80% of the total stress from all loads, the calculated resistance should be additionally multiplied by a factor m d = 0.8. (clause 5.2.in SP 64.13330.2011)

And if you plan the service life of your structure for more than 50 years, then the resulting value of the design resistance should be multiplied by one more coefficient, according to the following table:

Lifetime coefficients for wood
according to SNiP II-25-80 (SP 64.13330.2011)

Thus, the calculated resistance of the beam can be reduced by almost half and, accordingly, the cross-section of the beam will increase, but we will not use any additional coefficients for now. If grade 1 pine wood is used, then

W required = 80000 / 142.71 = 560.57 cm³

Note: Design resistance 14 MPa = 142.71 kgf/cm². However, to simplify the calculations, you can use the value 140; there will be no big error in this, but there will be a small margin of safety.

Since the cross section of the beam has a simple rectangular shape, then the moment of resistance of the beam is determined by the formula

W required = b x h² / 6

Where b- beam width, h- height of the beam. If the cross-section of the floor beam is not rectangular, but, for example, round, oval, etc., i.e. You will use round timber as beams, hewn logs or something else, then the moment of resistance for such sections can be determined using the formulas given separately.

Let's try to determine required height timber with a width of 10 cm. In this case

The height of the beam must be at least 18.34 cm. i.e. you can use a beam with a section of 10x20 cm. In this case, you will need 0.56 m³ of wood for 7 floor beams.

For example, if you plan that your structure will last for more than 100 years and more than 80% of the load will be constant + long-term, then the calculated resistance for wood of the same class will be 91.33 kgf/cm2 and then the required moment of resistance will increase to 876 cm3 and the height of the beam must be at least 22.92 cm.

Option 2.

If the distance between the beams is 75 cm, then the maximum bending moment is:

M max = (q x l²) / 8 = (400 x 0.75 x 4²) / 8 = 600 kg m or 60000 kg cm

W required = 60000 / 142.71 = 420.43 cm³

and minimally permissible height beam 15.88 cm with a beam width of 10 cm, if you use a beam with a section of 10x17.5 cm, then 9 floor beams will require 0.63 m³ of wood.

Option 3.

If the distance between the beams is 50 cm, then the maximum bending moment is:

M max = (q x l²) / 8 = (400 x 0.5 x 4²) / 8 = 400 kg m or 40000 kg cm

then the required moment of resistance of the wooden beam is

W required = 40000 / 100 = 280.3 cm³

and the minimum permissible beam height is 12.96 cm with a beam width of 10 cm; when using a beam with a section of 10x15 cm for 13 floor beams, 0.78 m3 of wood will be required.

As can be seen from the calculations, than less distance between the beams, the greater the wood consumption for the beams, but the smaller the distance between the beams, the thinner boards or sheet material can be used for flooring. And one more important point- the calculated resistance of wood depends on the type of wood and the moisture content of the wood. The higher the humidity, the lower the calculated resistance. Depending on the type of wood, the fluctuations in the calculated resistance are not very large.

Now let's check the deflection of the beam calculated according to the first option. Most reference books suggest determining the amount of deflection at distributed load and hinge support of the beam according to the following formula:

f=(5q l 4)/(384EI)

- distance between load-bearing walls;
E- elastic modulus. For wood, regardless of species, according to clause 5.3 of SP 64.13330.2011; when calculating according to limit states of the second group, this value is usually taken equal to 10000 MPa or 10x10 8 kgf/m² (10x10 4 kgf/cm²) along the fibers and E 90 = 400 MPa across the fibers. But in reality, the value of the modulus of elasticity even for pine still varies from 7x10 8 to 11x10 8 kgf/m², depending on the moisture content of the wood and the duration of the load. Under long-term load action, according to clause 5.4 of SP 64.13330.201, when calculating the limit states of the first group according to a deformed scheme, it is necessary to use the coefficient m ds = 0.75. We will not determine the deflection for the case when the temporary load on the beam is long-term, the beams are not treated with deep impregnation before installation, which prevents changes in the moisture content of the wood and relative humidity wood may exceed 20%, in this case the modulus of elasticity will be about 6x10 8 kgf/m², but we will remember this value.
I- moment of inertia, for a board of rectangular cross-section.

I = (b x h³) / 12 = 10 x 20³ / 12 = 6666.67 cm 4

f = (5 x 400 x 4 4) / (384 x 10 x 10 8 x 6666.67 x 10 -8) = 0.01999 m or 2.0 cm.

SNiP II-25-80 (SP 64.13330.2011) recommends calculating wooden structures so that for floor beams the deflection does not exceed 1/250 of the span length, i.e. permissible maximum deflection 400/250=1.6 cm. We have not met this condition. Next, you should select a beam section whose deflection suits either you or SNiP.

If you use laminated veneer lumber for floor beams LVL(Laminated Veneer Lumber), then the calculated resistance for such a beam should be determined according to the following table:

Calculated resistance values ​​for glued laminate materials
according to SNiP II-25-80 (SP 64.13330.2011)

As a rule, calculations for crushing the supporting sections of the beam are not required. But the calculation of strength under the action of tangential stresses is not difficult to do here either. The maximum shear stresses for the selected design scheme will be in the cross sections at the beam supports, where the bending moment is zero. In these sections, the value of the transverse force will be equal to the support reaction and will be:

Q = ql/2 = 400 x 4 / 2 = 800 kg

then the value of the maximum tangential stresses will be:

T= 1.5Q/F = 1.5 x 800 / 200 = 6 kg/cm²< R cк = 18 кг/см² ,

Where,
F- square cross section timber with a section of 10x20 cm;
R ck- calculated resistance to shearing along the fibers, determined from the first table.

As you can see, there is a three-roll safety margin even for timber with a maximum cross-sectional height.

Now let’s calculate which boards will withstand the design load (the calculation principle is exactly the same).

EXAMPLE OF FLOORING CALCULATION

Option 1. Floor covering made of floorboards.

With a distance between beams of 1 m, the maximum bending moment is:

M max = (q x l²) / 8 = (400 x 1²) / 8 = 50 kg m or 5000 kg cm

In this case, the design scheme for the boards, as for a single-span beam on hinged supports, is adopted very conditionally. It is more correct to consider wall-to-wall floorboards as a multi-span continuous beam. However, in this case, you will have to take into account the number of spans and the method of attaching the boards to the joists. If in some areas boards are laid between two joists, then such boards should really be considered as single-span beams and for such boards the bending moment will be maximum. It is this option that we will consider further. Required moment of resistance of boards

W required = 5000 / 130 = 38.46 cm³

since our load is distributed over the entire design area, the floor covering made of boards can be conditionally considered as one board 100 cm wide, then the minimum permissible board height is 1.52 cm; with smaller spans, the required board height will be even less. This means that the floor can be laid with standard floorboards 30-35 mm high.

But instead of expensive floorboards, you can use cheaper sheet materials, for example, plywood, chipboard, OSB.

Option 2. Plywood flooring.

The design resistance of plywood can be determined from the following table:

Design resistance values ​​for plywood
according to SNiP II-25-80 (SP 64.13330.2011)

Since plywood is made from glued layers of wood, the calculated resistance of plywood should be close to the calculated resistance of wood, but since the layers alternate - one layer along the fibers, the second across, the total calculated resistance can be taken as the arithmetic average. For example, for birch plywood brand FSF

R f = (160 + 65) / 2 = 112.5 kgf/m²

Then

W required = 5000 / 112.5 = 44.44 cm³

minimally permissible thickness plywood 1.63 cm, i.e. plywood with a thickness of 18 mm or more can be laid on the beams with a distance between the beams of 1 m.

With a distance between beams of 0.75 m, the value of the bending moment will decrease

M max = (q x l²) / 8 = (400 x 0.75²) / 8 = 28.125 kg m or 2812.5 kg cm

required moment of resistance of plywood

W required = 2812.5 / 112.5 = 25 cm³

The minimum permissible thickness of plywood is 1.22 cm, i.e. plywood with a thickness of 14 mm or more can be laid on beams with a distance between beams of 0.75 m.

With a distance between beams of 0.5 m, the bending moment will be

M max = (q x l²) / 8 = (400 x 0.5²) / 8 = 12.5 kg m or 1250 kg cm

required moment of resistance of plywood

W required = 1250 / 112.5 = 11.1 cm³

The minimum permissible thickness of plywood is 0.82 cm, i.e. plywood with a thickness of 9.5 mm or more can be laid on beams with a distance between beams of 0.5 m. However, if you calculate the deflection of the plywood (the calculation is not given in detail), then the deflection will be about 6.5 mm, which is 3 times the permissible deflection. With a plywood thickness of 14 mm, the deflection will be about 2.3 mm, which practically satisfies the requirements of SNiP.

General note: generally when calculating wooden structures a bunch of different correction factors are used, but we decided not to complicate the given calculation with coefficients; it is enough that we took the maximum possible load and, in addition, there is a good margin when selecting a cross-section.

Option 3. Flooring made of chipboard or OSB.

In general, it is undesirable to use chipboard or OSB as a floor covering (even a rough one) on floor beams, and these sheet materials are not intended for this; they have too many disadvantages. Design resistance of extruded sheet materials depends too large quantity factors, so no one will tell you what value of the calculated resistance can be used in calculations.

Nevertheless, we cannot prohibit the use of chipboard or OSB, we will only add: chipboard thickness or OSB should be 1.5-2 times larger than for plywood. Floors with failed chipboard had to be repaired several times, and a neighbor recently leveled wooden floor OSB boards, also complains about failures, so you can take my word for it.

Note: The joists can first rest on the floor beams, and then the boards will be attached to the joists. In this case, it is necessary to additionally calculate the cross-section of the lag according to the above principle.

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 more high requirements to the surface 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. Basic diagrams are compiled for standard sheets, operating as multi-span continuous beams (minimum three spans).
For standard sheet sizes, the following pitch options are obtained: cross beams.
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 moment at the time of concreting - 5.9 kNm (acceptable 7 kNm);
- maximum deflection (single-span beam) - 6.4 mm = 1/511 span.
If longitudinal beams put it 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.

Plywood is a durable, multi-layer material made of natural wood. Physical and mechanical properties and specifications plywood are determined by the process of its production itself. Namely, an odd number of sheets of thin wood veneer glued together using glue.

The veneer sheets are arranged in such a way that the direction of the wood fibers is perpendicular to each other. This makes the plywood very resistant to breaking, stretching and chipping (see table below).

Due to these parameters and affordable cost, plywood flooring is often used in construction.

Bending strength of plywood

Specification for plywood - table

Birch, coniferous, laminated and combined
(TU 5512-001-44769167-02 and TU 5512-002-44769167-98).

It should be noted that due to the fact that plywood for a long time remained almost the only material available to our compatriots; it was used everywhere. This, in turn, led to the emergence various types and types of plywood.

Types of plywood

Types of plywood are determined by the scope of its purpose:

• construction;
• furniture;
• structural;
• industrial;
• packaging

Types of plywood depend on the glue used in production:

• FC- waterproof plywood. Kabamide glue is used in its manufacture;

• FSF- plywood with increased moisture resistance. Here the veneer sheets are glued together using phenol-formaldehyde glue;

• FBA- non-waterproof plywood. In this case, albumin casein glue was used to glue the veneer. FBA plywood has little moisture resistance, but is highly valued by those who prioritize the environmental friendliness of the material;

• FB- plywood, which, thanks to the use of bakelite varnish, can be used in particularly damp conditions and in water.

And these are just the main types of plywood. There are many more stages of classification, depending on the thickness of the sheet, the number of layers, type of wood, grade, degree of finishing and type of additional processing.

1. Using plywood for flooring “+” and “-”

Advantages of plywood:

• plywood, unlike OSB and fiberboard, belongs to natural materials, and not recycled production waste. Therefore, it is more environmentally friendly;

• plywood moisture levels are in the range of 12-15%;

• plywood takes the brunt of variable loads. Thus, the screed retains its integrity, and the wood receives microcracks. However, they do not affect the quality of the floor;

• due to the fact that plywood is made from wood, it has better contact with floor coverings. As a result, the service life of the latter increases;

• plywood makes it possible to obtain a floor that will meet the specified characteristics (flatness, surface quality) with less time and resources;

• laying plywood on the floor does not require special preparation and can be performed in several stages;

• plywood plays the role of a kind of insulation, allowing to reduce heat loss through the concrete screed and floor slabs;

• If there is a significant difference in height across the floor, the use of a screed is not recommended due to its high weight and cost. But plywood, on the contrary, would be an ideal option;

• Depending on the grade and quality of polishing, plywood can be used for rough and finished flooring.

But:

• plywood is not suitable for rooms with significant temperature differences (for example, for cottages or houses not permanent residence), as well as with high level humidity (in the bathroom, bathhouse, sauna, swimming pool).

2. What kind of plywood to lay on the floor

To begin with, it is worth clarifying two important factors.

• First point - what type of floor is plywood intended for?. After all, the floor, in fact, is a two-layer structure, which consists of a rough (underlay) and a finishing (front) layers of coating.

• Second point - in which room should I lay plywood?. So, for example, in a living room, and even more so in a bedroom or children’s room, it is permissible to use only FK plywood. It contains no formaldehyde. Consequently, its use is absolutely safe, with satisfactory moisture resistance. IN production premises with good ventilation, it is permissible to use FSF brand plywood. But only 1st emission class. The class means that the formaldehyde content does not exceed 100 mg. per 1 kg. sheet of plywood.

Depending on the above points, the question of which plywood for the floor is better (which one to use for the floor) will be decided.

3. Which plywood to choose for the floor

When choosing plywood for the floor, you should pay attention to the following parameters:

• plywood grade. As already noted, for residential premises it is better to purchase FK brand plywood. Its moisture resistance indicators fully meet the operating conditions in residential premises;

• plywood class(emission class). Only class E-1 is suitable for flooring;

• grade of plywood for flooring. Plywood is divided into 4 grades. In this case, the sides of the sheet may have different variety. It is marked as 1/1, 1/2, 2/2, etc. Plywood of grades 3 and 4 is suitable for the subfloor. For finishing - 1 or 2 grade;

• moisture content of plywood. A quality sheet is one with a moisture content of 12-15%;

• number of layers of plywood. The thickness of the veneer in a sheet of plywood ranges from 1.7 to 1.9 mm. Consequently, their number determines the thickness of the sheet. The more layers a sheet has, the more durable it is. However, the thickness of plywood is selected taking into account its purpose. So for a subfloor you need plywood with a thickness of 12-18 mm, for a finishing floor 10-12 mm. When using plywood in production - at least 25 mm. Please note that if plywood is laid in two layers, then the thickness of the sheet should be divided into two;

• plywood manufacturer. European or domestic producers offer material good quality. But Chinese-made plywood causes complaints from users and often does not meet the stated characteristics.

4. Laying plywood on the floor

4.1 Plywood for subfloor

Laying a subfloor made of plywood is the fastest, most affordable and easiest way, which also has several varieties.

.
Sheet 10-12 mm thick. glued to the base. It is used when there is a level concrete screed normal quality. The main thing is not to forget about expansion joints. The gap is 3-4 mm. between the sheets, as well as between the sheet and the wall, will allow the plywood to play and adapt to its surrounding conditions.

This installation method can be used when there is a difference in height. It is enough to use special fasteners.

Adjustable floors made of plywood do not require the installation of logs, and the height difference is leveled by fasteners located under the plywood.

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Laying plywood on joists or floor beams.

Plywood, thickness over 12 mm. mounted on a prepared base. The method is labor-intensive, usually used when it is necessary to insulate the floor or raise it to a certain height.

allow you to install a sheet of plywood so that it can compensate for the height difference across the floor.

A fairly common situation is when you have partially lost appearance, but nevertheless do not cause any complaints. Then the flooring is laid on top of them.

But to fine coating has not become unusable, an intermediate floor (in this case, plywood) should be laid on the boards, which will level the surface.

Laying plywood on a wooden floor is carried out using hardware and is characterized by simplicity and high speed of work.

In order for the plywood laid under the laminate, under linoleum or to perform its functions a long period You must adhere to the following installation rules:

• securely fasten all sheets taking into account deformation gaps;

• “drown” the heads of the hardware into the sheet;

• remove irregularities using a grinder;

• fill in depressions and cracks;

• lay the substrate.

But laying plywood under a wooden floor is absolutely not required. Due to the massiveness of the floorboard, it can be laid on joists or on a flat concrete screed.

4.3 Finish plywood floor

Craftsmen can create real palace parquet from plywood. In this case, special requirements are put forward for the quality of plywood. Only the first grade may be used, surface front side the sheet must be sanded. To create a beautiful pattern, the plywood is treated with stain, and the laid plywood parquet is sanded and covered with several layers of parquet varnish.

5. Plywood for flooring - protection, operation and storage

In order for a plywood floor to serve you faithfully for a long time, you need to provide protection for the sheets at the installation stage. When working with plywood you need to consider:

• plywood needs acclimatization. Only the purchased material should not be used immediately. It needs to be given time to rest in the conditions in which it will be used.

  The exposure period depends on where, how, in what position, at what temperature conditions and the level of humidity where the plywood was stored. The acclimatization period can be:

• day. If the difference in temperature and humidity at the place of sale and installation is minimal, and the sheets were stored in a dry room, on a flat surface in a horizontal position;

• 3-5 days. If the difference exceeds 5-8°C and 10% (temperature and humidity, respectively);

• over a week. If the deviations are significant or the sheets are slightly deformed. The latter can be eliminated by pressing down the stack of sheets with weights and using more hardware per 1 square meter. leaf.

• dampness destroys plywood. Rapid fluctuations in humidity can cause serious damage to the wood from which plywood is made. At the same time, constant humidity in the room cannot be higher than 70%, and short-term - 80%. Laying plywood on a wet base is unacceptable. To check the humidity level wooden base use a special device. And concrete is covered with film for a day. The presence of condensation under the film indicates that it is worth holding off on installing the plywood;

• plywood sheets are laid at a temperature of 20-30°C. In this case, the leaf is in optimal conditions;

• additional processing improves the performance characteristics of plywood. For example, an antibacterial primer will protect the sheet from the effects of fungi and microorganisms. Impregnation with PVA-based putty will increase its moisture resistance. And the application acrylic varnish will increase the strength of the surface layer.

Conclusion

Having familiarized yourself with the types and types of plywood for flooring, as well as the nuances of its selection, storage and installation rules, you can confidently say which plywood would be better suited for floor installation.

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. It will be glued to the plywood ceramic tile, 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 relative to 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.

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