Timber is a highly variable material. Its properties depend on the species of timber, but also vary greatly in response to differences in density of the fibre material, and to the presence of knots and other defects. In order to obtain reliable performance, structural timber is sorted into strength classes
(sometimes called grades) with more closely defined properties. Grading may be done visually, or by machine. The European strength classes for timber are based on a unified system set out in EN 338 ‘Structural timber – Strength classes’. Strength classes are annotated by an alphabetical letter followed by a numerical number (ie C16, C24, D30, etc.). The letter ‘C’ represents softwood (coniferous) species and poplar, whilst ‘D’ represents hardwood (deciduous) species. In addition, the numerical number represents the characteristic lower 5th percentile value of the bending strength of 150 mm deep timber in N/mm2. EN 1995-1-1 requires that timber members comply with EN 14081-1. This is currently available as EN 14081-1: 2000 ‘Timber structures – Strength graded structural timber with rectangular cross section. Part 1: General requirements’. The EN 14081 suite of standards has been rejected by the CEN committee TC
124 ‘Timber structures’, but it is expected that they will be passed at a future vote without major changes. Round timber must comply with EN 14544. This is currently available as EN 14544: 2002 ‘Timber structures – Strength graded structural timber with round cross-section – Requirements’. Timber to EN 14544 must be graded in accordance with an appropriate standard, but currently no such standards are published.
EN 14081-1 requires timber to be strength graded either to an approved visual grading standard, or by machine using settings determined in accordance with EN 14081-2. This is currently available as EN 14081-2: 2000 ‘Timber structures – Strength graded structural timber with rectangular cross section. Part 2:
Machine grading – Additional requirements for initial type testing’. EN 14081-3 deals with ‘Timber structures – Strength graded structural timber with
rectangular cross section. Part 3: Machine grading – Additional requirements for factory production control’ whilst EN 14081-4 deals with ‘Timber structures – Strength graded structural timber with rectangular cross section. Part 4: Machine grading – Grading machine settings for machine controlled systems’. The strength classes of EN 338, the grades into which timber would normally be graded, are based on measured values of bending strength, mean modulus of elasticity in bending and density. Strength and modulus are measured to EN 408 and density to ISO 3131: 1975 ‘Wood – Determination of density for
physical and mechanical tests’, and these are evaluated to EN 384. For allocation to a strength class, the requirements for characteristic bending strength, mean modulus, and both characteristic and mean density must be met.
EN 408 and EN 384 are currently available as 1995 editions. However, draft revisions, EN 408: 2000 ‘Timber structures – Structural timber and glued
laminated timber – Determination of some physical and mechanical properties’ and EN 384: 2000 ‘Structural timber – Determination of characteristic values of mechanical properties’, have been prepared. The principal effect of adopting these revisions would be to change the method of measuring modulus of elasticity in bending to demonstrate compliance with EN 338. Currently the local modulus of elasticity (MOE) measurement is a true shear-free
determination of the modulus. The revised standards would require
measurement of the global modulus of elasticity ( ie including shear deflection in addition to bending deflection), with results then adjusted to subtract the estimated shear deflection. Direct measurement of the true modulus requires measurement along the neutral axis over a relatively short span. A note included with EN 384 points out that it has been shown that these test results tend to be inconsistent both within and between laboratories. Global MOE is measured from the tension face over a larger span. Measurements have been shown to be far more consistent, and can be made more conveniently than the local
modulus, particularly in conjunction with determining bending strength. The conversion formula to adjust values to determine the pure bending modulus was developed from comparative testing work undertaken by several countries (including work by BRE), and so there should be no systematic bias that might cause a substantial change in the structural performance of timber allocated to
For visual strength grading, CEN accepts certain national grading schemes including those used in the UK. EN 14081-1 gives requirements for visual grading schemes in an annex, and refers to EN 1912 ‘Structural timber –
Strength classes – Assignment of visual grades and species’ which assigns grades and species to strength classes. Once a particular source and species grade has been accepted as meeting a strength class, timber is graded according to visual rules, and any requirement for testing to demonstrate that the timber continues to meet the EN 338 strength class requirements will depend on the grading scheme used.
The principles of machine strength grading of timber are set out in BRE Digest 476(o). There are two alternative approaches to machine strength grading. The output-controlled system is relatively straightforward and very efficient as samples from the graded timber are proof tested in accordance with EN 408 at regular intervals, and the results may be used to optimise the machine settings determining the grades. However, this process requires long runs and high throughputs of timber of a consistent type, and is rarely feasible in Europe. The machine-controlled system requires detailed calibration of the machine’s output with any input source and species of timber to be graded. These settings are published, and accurate grading relies on always using the appropriate settings for the input timber and output grades on a correctly calibrated machine. As with visual grading, once appropriate machine settings have been accepted for grading a particular source of timber to specific strength classes, there is no requirement for further testing to EN 408 to demonstrate that the timber continues to meet the EN 338 strength class.
Machine strength grading is currently dominated by a few brands of grading machine that work by bending the timber across its smallest dimension to measure its stiffness as it passes between rollers. Methods of grading using alternative scanning systems have been investigated, and may be introduced on a significant scale. Systems based on X-rays or stress wave technology have the operational advantage of being able to grade virtually the full length of a piece timber with a single pass and reduced or no mechanical contact with the timber. Bending-type machines are unable to grade about 0.5 m at each end of a piece, and these ends must be assessed visually. Hence alternative systems offer faster throughput with lower labour costs. However, alternative systems have not yet achieved as accurate a prediction of the strength and other grades properties as bending-type machines. Thus, to meet the grade requirements, such grading machines must be set to allocate less timber to the higher grades and more to the lower or reject grades. Neural networks are also being evaluated, to sort timber into grades by evaluating a number of indicating parameters instead of using a single regression parameter. Such assessment has the potential to grade more efficiently than would be possible from just one of the parameters, but complex systems are harder to calibrate accurately and confidently than single- parameter algorithms. EN 14081-2 specifies a method for evaluating the
relationship between indicating property and timber grade for machine-
controlled grading, describing size and global cost matrices that shall be used to determine the appropriate machine settings.
Historically, strength grading schemes in the UK require third-party certification. It is currently unclear whether this will continue with the introduction of EN 14081-1 but, in any case, it will be possible to CE mark timber to EN 14081-1 and
import it into the UK for structural use without third-party certification. Such timber may also have been visually strength graded to a system that is currently unfamiliar in the UK, and may thus produce a slightly different distribution of grade properties.
Whilst bending strength, modulus of elasticity and density must be measured to establish that a grade of timber meets a particular EN 338 strength class, the other properties tabulated in EN 338 are derived from the bending strength values for the measured properties. However, methods of measurement for these properties are currently given in EN 408 and EN 1193, whilst all of these tests are brought together in EN 408: 2000. In principle, direct measurement might demonstrate that a particular type of timber had superior properties to the derived values. However, neither BS 5268-2 nor EN 1995-1-1 encourage this by stating explicitly that such values are permissible for design, whilst the
extensive test programme that would be required makes it highly unlikely that such an approach would be adopted for structural timber due to the cost involved in testing.