BS EN 14272-2011 pdf download.Plywood – Calculation method for some mechanical properties.
In this BS EN 14272, where a value for a given property of the veneers is derived with a specified test method (including exploitation of the results), the models in this standard will provide a panel value for that property as if derived with the specified test method.
EXAMPLE For instance, ii, in a panel composition, a specified percentile of a bending property of veneers is determined with EN 789 and EN 1058, the calculated value of the bending property of the panel will be its specified percentile as If determined by using EN 789 and EN 1058.
4 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
characteristic strength
population fifth percentile value relating to a temperature of 20 °C and a relative humidity of 65 %
4.2
characteristic stiffness
either the population fifth percentile or the mean value relating to a temperature of 20 °C and a relative humidity of 65 %
4.3
characteristic density
population fifth percentile value with mass and volume corresponding to equilibrium at a temperature of 20 °C and a relative humidity of 65 % either of the wood species or of the panels, single species or mixed species
NOTE The density values found from calculation refer to the minimum acceptable density of veneers used in the lay- up of plywood. In the case of single species plywood these values are taken as the characteristic values for the plywood. In the case of mixed species plywood the characteristic value of density of the panel is calculated from the characteristic densities of the individual veneers according to the proportion of each species
4.4
veneer value or basic value
characteristic value of a property to be used for each layer in the equations of the calculation method
NOTE Characteristic values of the wood species, along and across the grain, are fifth percentile values for strength but either mean values or fifth percentile values for stiffness (modulus of elasticity).
4.5
reference panel value
value of a given mechanical property of a panel composition
6 Calculation method
6.1 General
The mechanical properties of plywood, in bending, tension, compression panel shear and planar shear can be derived by calculation.
The calculation method described in this standard can be applied to plywood panels of any composition. symmetrical or not.
For the classification according to appearance classes of plywood, see EN 635-2 and EN 635-3.
6.2 Properties relevant to the calculation methods
For calculation of characteristic values of mechanical properties for different plywood compositions, the values of the properties of the veneers compounding the layers shall be derived in accordance with Annex A.
The relevant properties are as listed in Table 1.
6.3 Wood species
For calculating the characteristic values of panel mechanical properties, test values shall be used where available for each wood species in the plywood panel composition: where not, imposed values from EN 338 shall be used.
NOTE EN 338 values, related to the density of the wood species, are quite conservative and therefore should be used as a last resort option
6.4 Factors for plywood composition
The characterization of plywood composition is shown in Table 2.
For the purposes of calculation, if two or more plies are glued together in the same grain direction, they can be considered as one single layer provided that they belong to the same wood species: otherwise they shall be considered as independent layers.
The calculations are based on the following equations:
7.4.3 Resistance
7.4.3.1 General
For strength properties, the veneer value of a specified layer depends on:
the corresponding property of its wood species;
its stress level.
7.4.3.2 Common procedure to bending, tension and compression
Firstly, the 5”’ percentile value of the strength of the wood species of each layer is picked (in accordance with Annex A).
Secondly, the stress level in the layer shall be derived prior to enter the stress value: though similar in principle, the procedure for tension-compression and bending differ slightly.
Thirdly, In case of mixed species panels or of mixed grade single species panels, as the weaker layer(s) may fail before reaching the maximum load, the procedure of calculation shall be resumed.
Once the first calculation Is over, the weaker layer(s) and the corresponding panel strength is recorded in the relevant direction of grain, another calculation shall be carried out with the input, for them, of close to 0 values for strength and modulus of elasticity (but the input shall be such that the value of the ratio fnJEmi shall not be the lower in the composition).
NOTE 1 For instance, an equal value of 1/1000 N/mm2 can be input for and fm. Indeed, for the weaker layer(s), zero values cannot be input in the equations (or the spreadsheets of Annex B) because the calculation of the ratio fmdEm of the layer(s) cannot be determined.
This result of the calculation is then recorded. This procedure shall be repeated as long as the result of strength is greater than the one obtained previously. As soon as the strength value is less than the previous one, the higher value in the set of results provided by the repetition of the procedure shall be accepted as the panel strength.
NOTE 2 The repetihon of the procedure is justified by the fact that EN 789 aims at determining the maximum force as the failure force.BS EN 14272-2011 pdf download.