BS EN 336-2003 pdf download.Structural timber Sizes , permitted deviations.
This BS EN 336 specifies two classes of permitted deviations from target sizes for structural timber of softwood and hardwood species.
It also specifies the moisture content to be used as a reference point for the measurement of sizes, and gives average values for changes in size due to changes in moisture content.
It is applicable to sawn and prepared square-edged timber with parallel edges having sawn thickness or widths in the range 22 mm to 300 mm.
2 NormatIve references
This European Standard incorporates by dated or undated reference, provisions from other publications. These normative references are cited at the appropriate places in the text and the publications are listed hereafter. For dated references, subsequent amendments to or revisions of any of these publications apply to this European Standard only when incorporated in it by amendment or revision. For undated references the latest edition of the publication referred to applies (including amendments).
EN 1309-1, Round and sawn timber – Method of measurement of dimensions – Part 1: Sawn timber
EN 13183-2. Moisture content of a piece of sawn timber – Part 2: Estimation by electrical resistance method
3 Terms and definitions
For the purposes of this European Standard, the following terms and definitions apply.
target size
size specified (at the reference moisture content), and to which the deviations, which would ideally be zero, are to be related
3.2
deviation
difference between actual size and the corresponding target size, making allowance for the difference in size due to difference in moisture content (see clause 5)
3.3
moisture content
amount of water present in timber, expressed as a percentage of the oven-dry mass.
4.1 General
Sizes shall be measured in accordance witti EN 1309-1.
For the purposes of this standard the reference moisture content is 20 %.
4.2 Changes in size due to changes in moisture content
Unless there is evidence to the contrary, it shall be assumed that the thickness and width of a piece of timber increase by 0,25 % for every 1.0 % of moisture content higher than 20 % up to 30 %, and decrease by 0,25 % for every 1.0 % of moisture content lower than 20 %. The above values are typical. without regard to species.
4.3 PermItted cross-sectional deviations
In any cross section of each piece of timber, the actual thickness and actual width (corrected for changes due to changes in moisture content) may not deviate from the target size by more than:
Tolerance class 1
a) For thicknesses and widths ≤ 100 mm:
(-1 +3) mm
b) For thicknesses and widths >100 mm:
(-2 +4) mm
Tolerance class 2
a) For thicknesses and widths ≤ 100 mm:
(-1 +1)mm
b) For thickness and widths> 100 mm:
(-1,5 +1,5) mm
The average actual thickness and the average actual width of square-edged timber shall not be less than the target sizes, making allowance for changes in size due to changes in moisture content.
4.4 Permitted length deviations
Negative deviations are not permitted.
National annex NA (informative)
Use of target size and tolerance classes with structural timber sizes commonly available in the United Kingdom
The objective of this national annex is to overcome long-standing ambiguities in the dimensional specification and supply of structural timber and to ensure that designers. suppliers and contractors are familiar with the approach developed in European Standardization to overcome problems inherent in common practice.
Target size is the size required by whosoever requires It (I.e. buyer. specifier, designer).
Specifications of dimensions for structural timber designed to either BS 5628 and EN 1995 now has to refer to target size and tolerance class. Target size is the size used In design calculations and is therefore the size required from the supplier, with no deviations outside the specified tolerance class being permitted. Use of terms such as ‘basic size’, ‘nominal size’ or ‘regularized size’ should be discontinued.
To bring clarity and consistency to the way in which structural timber sizes are specified in the UK it is recommended that, in specifications and on drawings, tolerance class 1 and tolerance class 2 are adopted as the norm and are referred to as ‘TI’ and ‘T2’ respectively. As an example, a specification or drawing would, in its abbreviated form, read as ‘35 (T2) x 97 (T2)’, ‘47 (Ti) x 195 Cr2)’ or ‘47 (TI) x 200 (Ti)’. A note should be added: The abbreviations ‘Ti’ refers to tolerance class 1 (i.e. —1 mm +3 mm for thicknesses and widths ≤100 mm and
—2 mm +4 mm thereafter), ‘T2’ to tolerance class 2 (i.e. —1 mm +1 mm for thicknesses and widths ≤100 mm and —1.5mm +1.5 mm thereafter)
Timber structures can be designed using any size of timber, However, since producers are not normally aware of the specific end use of timber at the time of production it has become common practice to produce to a range of sizes, which have therefore become the most readily available sizes in the UK. These commonly available target sizes are listed below.
Table NA.1 lists the ranges of lengths and Table NA.2 lists the range of softwood sawn timber sizes normally available from stock in the UK for structural purposes. They will meet the requirements of tolerance class 1 in BS EN 336 (see 4.3), and the requirement (contained in 4.3) that the average size of all pieces in a batch shall not be less than the target size.
As an aid to construction, the sawn sizes in Table NA.2 are frequently machined (processed) on two opposing faces or on all faces to produce more accurate dimensions which will meet the requirements of tolerance class 2 in BS EN 336 (see 4.3). Table NA.3 and Table NA.4 list the resulting range of machined sizes, together with an indication of the tolerance classes which they can be expected to meet. The sizes in Table NA.3 are those previously described as regularized sizes.BS EN 336-2003 pdf download.