Timber grading Codes and Standards. Chris Holland Building Technology Group

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1 Timber grading Codes and Standards Chris Holland Building Technology Group

2 Historical situation First grading standard and permissible stress design code for the UK was CP This stayed in place with amendment unit 1978 when the permissible design code was separated from the grading standard. The permissible design code became BS5268: Part 2 Structural use of timber - Code of practice for permissible stress design materials and workmanship. The grading standard became BS 4978 (visual and machine grading). In 1979 BS 5756 was introduced to cover tropical hardwoods. In 1995 there was the first attempt at harmonisation of the European structural timber codes and the limitations were immediately obvious so it was put on hold. But machine grading then came under BS EN 519: BS EN 518 was introduced to state what was required of a European visual grading standard, BS4978 and BS5756 remain unaffected.

3 BS 4978 Visual grading of softwood Specification 2007 BS4978 is based on the Knot Area Ratio (KAR) to assess the impact of knots on strength

4 Various KAR combinations

5 Additional grading criteria Slope of gradin Wane Rate of growth Fissure Resin & bark pockets Distortion (serviceability criteria) Biological damage There are two grades GS (General structural) and SS (Special structural) Each of these criteria have a set value for each grade. The grades are attributed to a grade stress or strength class in either BS5268: Part 2 or BS EN1912.

6 BS5756 Visual strength grading of hardwoods- Specification 2007 The impact of the Knots are assessed a surface ratio rather than a KAR method.

7 Additional grading criteria Slope of gradin Wane Fissure bark pockets and included phloem Distortion (serviceability criteria) Biological damage BS5756 covers tropical and temperate hardwood HS (hardwood structural grade for Tropical) and TH 1&2 / TH A&B for temperate hardwoods. The visual grades are attributed to grade stresses / Strength classes in either Bs5268: Part 2 or BS EN1912.

8 Derivation of stresses for timbers not all ready assessed for either BS4979 or BS5756 All Building Control will requires is evidence that stresses have been derived by a reputable organisation. Usually a technical report will suffice. A sample of 40 pieces above the grade would be needed. BRE has derived stresses for species that were not covered at the time: American white oak for Portcullis House Vitex for the Chiswick boardwalk Coppiced sweet chestnut for grid shell in Sussex. Company specific stresses for Cedrela odorata and currently underway Eucalyptus grandis.

9 Historical context continued 2003 BS EN14081: Part 1 to 3 introduced replacing BS EN 518 and BS EN 519. BS EN 14081: Part 4 - machine settings, added when each timber producing country or region had to produce it own settings for the timber grown there. In 2006 there was stalled attempt to harmonise and again in When will we harmonise - sometime / never? Current default date is by If the current disagreement is cleared up soon it may be sooner. Until harmonisation takes place with CE marking 3 rd party certification on structural timber is till required in the UK.

10 Implications of harmonisation BS 5268: Part 2 will be with drawn (permissible stress design code) along with BS5268 strength classes. However, it is expected that BS5268 Part 2 will still linger for a number of years. Eurocode 5 (limit state design) will replace BS5268. Changes for UK timber graders machine setting - grading to BE EN 338 strength classes, most are already doing this. Visual grade / species combination allocated to a strength class in BS EN BS 4978 and BS 5756 will remain the same Loss of grade stresses (unless action is taken to lodge them else where).

11 Implications of the loss of grade stresses. This is important where you are dealing with known and named timber species as in Wales and other UK areas where there is a drive to use local sources of material. Strength classes aid the sale of timber in general but benefits can accrue from using grade stresses. Take the case of Douglas fir:

12 Douglas fir grade stresses v strength class Grade stress / strength class Bending strength (N/mm 2 ) Mean Stiffness (N/mm 2 ) Minimum Stiffness (N/mm 2 ) GS grade stress GS attributed to C14 in BS5268 SS grades stress SS attributed to C18 in BS 5268 SS grade stress large section SS large section attributed to C24 in BS SS large section Douglas fir to C24 BRE Digest 445

13 Things to come in near future New machine setting for Scots pine for 3 types of grading machine the MTG, X-ray grader and bending types machines. New machine setting of Douglas fir the MTG, X-Ray grader and bending types machines. Additional work on Douglas fir to investigate rate of growth criteria in BS4978 to mitigate the high rate of growth. Guidance on glulam production from Douglas fir. If sufficient budget looking at the knot plots from the strength data to see if the strength class attributions can be up lifted for visually graded material.

14 Crystal ball gazing middle to distant future Enlarged growth areas WoodWisdom project Gradewood looking at the potential to enlarge the sizes of the growth regions that constitute a growth area. Currently due to the number of growth area that machine producers have to supply to this is seen as a handicap to the spread of new technologies. Work is currently under way on testing Scots pine and European white wood from across Europe to determine if large growth areas can be achieved. Loss of the 1.12 (K V ) factor for machine graded timber. This could have significant impact on Sitka spruce yields. The characteristic value for strength of the C16 strength class is 16N/mm 2 if the material was visually graded K V factor is 1, but for machine graded timber with the application of the 1.12 K V factor the target value for strength becomes N/mm 2. The K V factor for strength over 30N/mm 2 is already 1.

15 Crystal ball gazing middle to distant future Removal of the reference depth to base line the strength classes. Currently all structural timber strength is adjusted to a reference depth of 150mm, K h factor (BS 5268 is 200mm). What does that mean for UK timber the result will be a mixed blessing depending on whether you are grading section sizes below or above the 150 mm reference depth. K h =(K/h) 0.2 -the results on the 5 th percentile for a sample with a depth of 100mm and one with s section depth 300 are as follows. K h =(150/100) 0.2 = 1.08 apply to 16 N/mm 2 (16/1.08) = 14.8N/mm2 K h =(150/300) 0.2 = 0.87 apply to 16 N/mm 2 (16/0.87) = N/mm2

16 Crystal ball gazing middle to distant future So removal of the K h factor will mean small section timber gets slightly stronger but deep section timber will get weaker. Taken with any possible reduction of the K V factor yields could be under pressure. Density profiles for the strength cases could possibly change as for higher strength classes the density requirement is higher than is actually recorded from test data.

17 Conclusions When harmonisation takes place the UK will find little immediate difference, as thanks to the UKTGC much of the industry is already grading to European standards. Many timber producers in Europe have not been kept so well informed hence the current problems. Even after harmonisation it seems likely that a period of change and uncertainty is likely to continue.

18 One last thought Strength grading of timber does not give the strength of the timber. What is does is gives a set of stresses and moduli against a set of measurable strength reducing characteristics that allows designers and engineers to design safely with the material.

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