Farm Forestry New ZealandNZS 3622 Verification of Timber Properties
Introduction
With the release of NZS 3604:2011 Timber Framed Buildings all structural framing timber grades are now to be verifiedRandomly sampled timber is independently tested on a properly calibrated testing machine for stiffness and bending strength in accordance with NZS 3622 Verification of Timber Properties. grades and have been renamedThe renaming simplifies structural grading for the market because there will be no distinction between framing timber which is machine graded (MSG) and verified, or visually graded (VSG) and verified. SG 6, SG 8 and SG 10. This was to "clarify and simplify requirements for structural timber framing and to improve quality assurance processes for better assurance that the product meets assigned properties and is fit for purpose" *Formal agreement between Wood Processors Association, NZ Timber Industry Federation and Department of Building and Housing, signed on 12 July 2010.
However NZS 3604 also states that properties for timber grades are as specified in NZS 3603.
NZS 3603 Timber Design Standard contains design stresses and methods for specific engineering design of timber structures to meet the performance requirements of the New Zealand Building Code.
In March 2005, Standards NZ issued Amendment 4 to NZS 3603, introducing verifiedIn 2005 a new standard, NZS 3622 was introduced setting out how timber properties are to be verified i.e. samples are independently tested for strength and stiffness. structural grades. NZS 3603 makes a distinction between verified product and unverified product and requires designers to discount the engineering values for unverified No1 Framing gradeTimber, which has, as a minimum, been visually graded as No. 1 framing to the requirements of NZS 3631 by 25%. This recognises deficiencies in sole reliance on visual grading without verification as a means of reliably establishing properties of sawn timber.
Verification
Both VSGVSG (Visual Stress Grades) are sorted on the basis of visual characteristics. Visual characteristics are good for determining strength but not so good for predicting stiffness. In the case of visual stress grades, the verification process is more pertinent for checking the stiffness characteristics. and MSGA machine stress grader is only capable of testing the stiffness of timber but not strength. There is correlation between stiffness and strength but visual characteristics such as knots are the main determinant of strength. Therefore visual grading is also undertaken after the wood has been machine graded for stiffness. In order to determine whether the strength properties for machine stress graded product are being achieved, samples of the production must be independently tested on a static testing machine in accordance with the Verification Standard NZS 3622:2004. This tests for both stiffness and also bending strength. grading systems assign timber into Structural Grades (SG) according to their stiffness and strength.
The key aspect to ensure both grading systems are compliant is the verification process. Both grading systems are subject to the same rules and grade acceptance criteria as set out in the Verification Standard NZS 3622:2004.
The verification process described in NZS 3622:2004 is an output control system, which means that timber is continuously taken from production and tested for stiffness and bending strength by the producer. Independent auditing also takes place. The Verification Standard is based on random sampling and statistical analysis to ensure the population of timber within a stress grade is meeting the requirements of the grade. Essentially, each grade has an average and minimum stiffness rating (Modulus of Elasticity).
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