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 Specialty timbers, structural timber and the new building code

 Specialty timbers, structural timber and the new building code

May, 2011

Background

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 as a means of reliably establishing properties of sawn timber. This compromise was accepted by the wood processing and construction industries, and span tables for its use existed in NZS 3604NZS 3604 provides methods and details for the design and construction of timber-framed structures not requiring specific engineering design. at the time.

With the recent release of NZS 3604:2011 all structural grades are now to be verified 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

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.

The situation

Although NZS 3604:2011 states that properties for timber grades are specified in NZS 3603, it also overrides NZS 3603 by removing unverified No. 1 framing grade. Industry have now realised the consequences
  • Efficiency of scale: Smaller producers of timber will not be able
    to afford the test rig and audit costs for their output
  • Larger industry and corporate sawmills are unlikely to make their
    verification facilities available to smaller operations seeking to verify
    their timber
  • Larger beams cannot be verified
  • Specialty timbers will effectively be excluded from the
    structural market
of this over-simplification and are currently lobbying the Department of Building and Housing and the Minister for Building and Construction to allow an unverified visually graded SG 6. Unfortunately both the minister and his ministry appear resolute on verification for all timber. 

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

Durability requirementsNZS 3602 Timber and Wood-based Products for Use in Building. This standard allows untreated Cypress species for structural members protected from the weather would also need to be met.

The initial evaluation shall:

  1. Identify the reference sample;
  2. Identify the sorting procedure;
  3. Identify the grades and sizes to which the procedure relates;
  4. 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;
  5. State or derive target bending strength and modulus of elasticity values for the grade/size combinations that the producer proposes supplying;
  6. State or derive machine threshold values and visual characteristics where the grading method is machine grading;
  7. State or define visual characteristics where the grading method is visual grading; and
  8. 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.  

The "alternative"


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.


Disclaimer: While every effort is made to ensure the accuracy of the information provided on this site, Farm Forestry Timbers Society do not accept liability for any consequences arising from reliance on the information published. If readers have any doubts about acting on any articles they should seek confirming, professional advice.

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