Specialty timbers, structural timber and the new building code
In March 2005, Standards NZ issued Amendment 4 to NZS 3603, introducing verified
With the recent release of NZS 3604:2011 all structural grades are now to be verified grades and have been renamed
This appears to follow on from a Commerce Commission cases involving Total Frame and Truss Limited and Carter Holt Harvey’s mislabelling of machine graded structural timber, where the companies knowingly sold machine graded timber that did not meet the required grades. In these cases claims made about machine graded timber products were misleading, and "those claims could not be easily verified by your average consumer" *. This is because machine grades do not have a visual basis. In contrast, visual grades can easily be verified by the consumer.
Expert structural engineers who were NZS 3604 committee members agree with industry that removal of the unverified No. 1 framing grade from NZS 3604 was unnecessary and counter-productive. There has been no reason given why SG 6 should require verification where it meets the visual No. 1 framing grading rules. The Department of Building and housing and the Minister for Building and Construction need to reconsider their stand and work with industry on this issue rather than engaging in stonewalling.
Characteristic stresses of specialty timber species
NZS 3604 and NZS 3603 apply only to radiata pine and Douglas fir. Fortunately, these Standards' provisions could be applied to other timber species. However, such use is subject to demonstration of adequate structural performance. Both NZS 3603 and NZS 3622 allow for determining characteristic stresses for grades of any timber, by testing a reference sample of timber on a species by species basis in accordance with AS/NZS 4063.1:2010.
The properties to initially evaluate for the reference sample include:
- Bending strength;
- Modulus of elasticity;
- Strength in tension; and
- Strength in compression
The initial evaluation shall:
- Identify the reference sample;
- Identify the sorting procedure;
- Identify the grades and sizes to which the procedure relates;
- Evaluate the properties of the sample according to the requirements of AS/NZS 4063 using appropriately calibrated equipment and appropriately trained staff under the supervision of an independent suitably qualified organisation;
- State or derive target bending strength and modulus of elasticity values for the grade/size combinations that the producer proposes supplying;
- State or derive machine threshold values and visual characteristics where the grading method is machine grading;
- State or define visual characteristics where the grading method is visual grading; and
- Document the evaluation in a report, copies of which shall be held by the audit organisation and the producer.
A Scion quote for this work is here. The cost per species is currently around $4200 + gst.
Where a timber design is not able to meet the requirements of NZS 3603 (e.g. using alternative timbers to radiata pine or Douglas fir) the building code allows for an "Alternative Solution". This also uses specific engineering design but must provide further evidence that the design will comply with the requirements of the building code. However because the building code now requires verified timber for structural applications, timber which cannot be verified (e.g. large beams or timber species other than radiata pine and Douglas fir) is not likely to be allowed by territorial authorities.