Plywood or OSB (OSP): What to choose? Calculation of permissible plywood spans (transverse beams a) and durable plywood having thickness

Separation formwork element, perceiving concrete pressure and all other loads, it is plywood. The above mentioned plywood species are dependent on the direction of work. different values For both the modulus of elasticity and the bend strength:
- In the floors with low requirements for the surface F - in overlaps with higher requirements for the surface F, the deflection of plywood (0 depends on the load (the thickness of the overlap), the characteristics of the plywood itself (the module of elasticity, the thickness of the sheet) and the content of the support.
Appendix 1 (Fig. 2.65) shows the charts on the main types of plywood supplied by Peri - Birch Plywood (Fin-Pry and Peri Birch) and conifer (Peri-Sprice). The diagrams are composed for a sheet thickness of 21 mm. At the same time, the dotted line is allocated areas where the deflection exceeds 1/500 span. All lines end when the strength limit is reached. The main charts are made up for standard sheets working as multiplet continuous beams (minimum three spans).
For the running size of sheets, the following variants of the step of transverse beams are obtained.
Table 2.7.

When evaluating the deflection at a good: for birch plywood, they take the same values \u200b\u200bfor the modulus of elasticity and strength limit, as for the main sheets, as it is not always known, in which direction there are a goodwear. For coniferous plywood
in which, when turning the sheet, these characteristics change dramatically.
According to the diagram (Fig. 2.65) for birch plywood with 3 or more spans, along the X axis, we find our overlap thickness value (20 cm) and determine the values \u200b\u200bfor the deflection:


For our sheet length, two options are acceptable - either 50 cm, or 62.5 cm. Let us dwell on the second version, as it gives savings by the number of transverse beams. The maximum deflection is 1.18 mm. We look into the diagram for a single-spare system. With such a scheme, a line for a span of 60 cm just on the value of the overlap thickness in 20 cm ends (plywood strength). The deflection is 1.92 mm.
From this it follows that in order to avoid inflated deformations of the problem, you should either limit the span of this kind to 50 cm, or to put the additional transverse beam (the calculated scheme of a uniformly loaded 2-touch beam has the smallest values \u200b\u200bfor the defaults, but it has an increase in relation to to multiplet schemes supporting moment).
Determination of the span of transverse beams (pitch of longitudinal beams b)
According to the transverse beams selected in the previous paragraph, check the tables in the corresponding bar. 2.11 The maximum allowable span of these beams. As already mentioned above, these tables are compiled, taking into account all the calculated cases, for transverse beams, first of all, a moment and deflection.
When choosing a step of longitudinal beams, it is necessary to consider that the extreme longitudinal beam is at a distance of 15-30 cm from the wall. An increase in this size may result in the following unpleasant results:
- increasing and unevenness of deflection on cross-beam consoles;
- Opportunities for overturning transverse beams during reinforcement work.
Reducing complicates rack management and creates a risk of scaling transverse beams from longitudinal.
For the same reason, as well as taking into account the normal operation of the end of the beam (especially when using Baok Farm), a minimum beam beam is assigned to 15 cm on each side. The actual step of longitudinal beams in no case should exceed the permissible value in Table. 2.11 and 2.12. Recall that the span in the formula for determining the moment is present in the square, and in the deflection formula even in the fourth degree (according to formula 2.1 and 2.2).
Example
For simplicity, choose a rectangular room internal dimensions 6,60x9.00 m. The thickness of the overlap 20 cm, Peri Birch plywood with a thickness of 21 mm and the size of the sheet 2500x1250 mm.
The permissible value for the span of the transverse beams during their step in 62.5 cm will be found along Table. 2.11 for GT 24 beam beams. In the first column of the table we will find a thickness of 20 cm and move to the right to the appropriate step of cross-beams (62.5 cm). We find the maximum permissible value of the span 3.27 m.
We present the calculated values \u200b\u200bof the moment and deflection for this span:
- the maximum moment at the time of concreting is 5.9 kNm (permissible 7 kNm);
- Maximum deflection (single-break beam) - 6.4 mm \u003d 1/511 span.
If a longitudinal beams We put in parallel the length of the room, we get:
6.6 m - 2 (0.15 m) \u003d 6.3 m; 6.3: 2 \u003d 3.15 m 3.27 m; 8.7: 3 \u003d 2.9 Mf Three spans with a length of beams of 3.30 m (minimum 2.9 + 0.15 + 0.15 \u003d 3.2 m). Transverse beams are less loaded - most often it is already a sign of material overpower.
In some cases, for example, if necessary, the installation of the formwork around the predetermined large-sized equipment has to be calculated by the beams. The following prerequisites should be taken into account. As the calculated scheme in the MultiFlex type systems, only a single-break hinged leaf beam without consoles is considered, since when installing the formwork and during concreting, we always have intermediate stages, where the beams work precisely according to such a scheme. For large beam spans without additional support, the loss of stability is possible already with small loads. Any formwork of overlapping after concreting should be pulled out from under the finished overlap, sometimes from a closed room, so it is desirable to limit the length of the beams (the problem of weight and maneuverability).
In the absence of values \u200b\u200bin the table, it can still be used. For example, to increase the span, you want to reduce the beam step - as a result, you must check the permissibility of the span. For example, the beams decided to put in a step of 30 cm, the overlap thickness is 22 cm. Calculated load It is according to the 7.6 N / m2 table. We multiply this load on the beam step: 7.6-0.3 \u003d 2.28 kN / m. We divide this value for one step of transverse beams, which are present in the table: 2.28: 0.4 \u003d 5.7 ~ 6.1 (load on overlapping thickness 16 cm); 2.28: 0.5 \u003d 4.56 - 5.0 (load on overlapping with a thickness of 12 cm).
In the first case, we find for the thickness of the overlap 16 cm and the beam steps 40 cm. The span is 4.07 m, in the second case - the thickness is 12 cm and step 50 cm - 4.12 m.
We can take a smaller of two values \u200b\u200bminus the difference of these values \u200b\u200b(Accounting for changing the temporary load, which is present only in the calculation at the time), without losing time for long-term calculations. IN specific example It turns out exactly the calculation
4.6 m, and accepted 4.02 m.

According to SP 64.13330.2011:

3.3. Estimated resistance construction plywood Led in Table. 10.

Table 10.

Type of plywood	Estimated resistance, MPa (kgf / sq. CM)
	Stretching in the sheet plane R. F.R.	compression in the sheet plane R. F.S.	bending from sheet plane R. F.I.	rocking in the sheet plane R. F.Sc.	cut perpendicular to the sheet plane R. F.Sh.
1. Plywood glued birch grades FSF varieties in / bb, in / s, bb / s
a) seven layer 8 mm thick and more:
along the fibers	14(140)	12(120)	16(160)	0,8(8)	6(60)
	9(90)	8,5(85)	6,5(65)	0,8(8)	6(60)
at an angle of 45 ° to the fibers	4,5(45)	7(70)	_	0,8(8)	9(90)
b) five-layer 5-7 mm thick:
along the fibers of the outer layers	14(140)	13(130)	18(180)	0,8(8)	5(50)
across the fibers of the outer layers	6(60)	7(70)	3(30)	0,8(8)	6(60)
at an angle of 45 ° to the fibers	4(40)	6(60)	_	0,8(8)	9(90)
2. Plywood glued from the larch wood of the FSF grade varieties in / explosive and BB / with a semi-layer thickness of 8 mm and more:
along the fibers of the outer layers	9(90)	17(170)	18(180)	0,6(6)	5(50)
across the fibers of the outer layers	7,5(75)	13(130)	11(110)	0,5(5)	5(50)
at an angle of 45 ° to the fibers	3(30)	5(50)	_	0,7(7)	7,5(75)
3. Plywood Bakelized FSB grads with a thickness of 7 mm and more:
along the fibers of the outer layers	32(320)	28(280)	33(330)	1,8(18)	11(110)
across the fibers of the outer layers	24(240)	23(230)	25(250)	1,8(18)	12(120)
at an angle of 45 ° to the fibers	16,5(165)	21(210)	_	1,8(18)	16(160)

Note. Estimated resistance to crumpled and compression perpendicular plane sheet for birch plywood FSF brand R. F.S.90 \u003d. R. F.Sm.90 \u003d 4 MPa (40 kgf / cm 2) and FBS brands R. F.S.90 \u003d. R. F.Sm.90 \u003d 8 MPa (80 kgf / cm 2).

In the necessary cases, the values \u200b\u200bof the calculated resistance of the construction plywood should be multiplied by the coefficients m. in, m. t, m. d, m. N I. m. And given in PP. 3.2, and; 3.2, b; 3.2, in; 3.2, g; 3.2, to these standards.

Plywood is considered popular building material. It has aesthetic characteristics, and after treatment it becomes durable, elastic and resistant to moisture. This makes it possible to significantly expand the scope of its use. When we are talking The ability of this material to resist deformations, then in this case the quality of the goods determines the two main criteria - the strength of plywood on the gap, as well as the fuel of the bending strength.

Of course, the definition of the strength characteristics of the plywood sheets is a whole process in which it is worth considering the set of nuances. The tree breed is taken into account, the condition of raw materials, moisture content, processing technology and other criteria:

• impact viscosity - the ability to absorb work when hit without any destruction;
• wear resistance - the degree of destruction of the material at regular exposure to its surface. Experience has shown that wet wood is wears much faster than dry;
• the ability to hold metal fasteners - Important property. The fact is that the installation of the fastener is capable of running the deformation processes. So, if the material is not durable enough, then when clogging the nail or screwing the self-pressing, the risk arises that the plywood sheet will give a crack;
• deformativeness - the appearance of deformations inevitably when exposed to loads.

In general, plywood is a unique building material. Its secret lies in the technology of laying veneer. The latter is a thin layer of wood cut from the tree trunk. This is not the most durable raw materials. To eliminate this disadvantage, it is placed so that the fibers are in mutually perpendicular directions. Usually the minimum number of such layers - 3, but the maximum amount in theory may be unlimited, although more than 30 is rare in practice.

The strength of plywood of various brands and thicknesses

However, the correctness of laying the fibers is not the main secret of the strength of this material. After all, plywood only partly consists of a tree, and everything else is represented adhesive compositionused to bond each layer. For this, various substances are used:

• urea-formaldehyde - a mixture of carbamide resins with a small amount of formaldehyde. Usually, this composition is used during the production of goods of the FC brand - an environmentally friendly and safe product. It has uncommon challenges in terms of strength, but copes well with internal finishing works;
• phenol formaldehyde - Here the main danger carries a substance called phenol, which is toxic to humans. But he repels moisture well, therefore it is used for the production of FSF - sufficiently strong and reliable building materials;
• melaminormaldehyde is a safe substance used by milking FKM brand. The only drawback of the product is a high cost;
• bakelite resins - make it possible to create high-strength products with which no wood can be compared. But if the level of flexibility has a value for you, it is actually completely lost by means of such treatment.

If you are interested in the strength of the material, then when studying technical characteristics, pay attention to the density indicator. On average, this value varies in the range of 550-750 kg / m³. For comparison, the density of the bakelite plywood is 1200 kg / m³.

The thickness of the building material also matters. Of course, the strength of plywood 10 mm will be lower than that of a sheet with a thickness of 12 mm. These features also need to be considered.

How to calculate the strength of plywood yourself?

Push into account the strength of plywood is necessary when the roof is arranged, the construction of the supporting structure, during the manufacture of furniture (rack, cabinet, etc.) or laying outdoor coating. This will help determine what load it will be able to withstand and choose suitable materials.

Special online calculators will help you to make the necessary calculators, you can also ask for help from a specialist or calculate the strength of plywood yourself to make sure your choice is correct.

To do this, use the definition formula plywood sheetwhich looks like this:

f \u003d k1ql4 / (EH3), where:

• k1 - estimated coefficient;
• E - wood elastic module;
• h - the thickness of the plywood sheet;
• l - length;
• q - Flat load value.

At first glance, the formula seems simple, but we advise you to be attentive in the calculations and recover the result received several times. Data for calculations you can find on the Internet.