Breitinger Consulting Testing Services

What types of structural tests should be performed to evaluate my structural product/s?

There are 10 design properties that are assigned to a timber stress grade: Bending Strength, f’_b; Bending Stiffness, MOE; Tension Strength, f’_t; Compression Strength, f’_c; Beam Shear, f’_s; Modulus of Rigidity, G; Bearing Strength perpendicular to the Grain, f’_p; Bearing Strength parallel to the Grain, f’_l; Joint Shear Strength, f’_sj; Tension perpendicular to the Grain, f’_tp.

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What do I need to consider when collecting production samples for a structural compliance testing program?

The primary goal of sampling structural timber boards is to gain a representative sample of the general production, and correctly reflects the variability of timber quality for a specific grade/size combination.

• Production Matrix
  List all structural grade/size combinations produced. Next to each grade/size combination note the percentage of the total structural product output for grade/size combination. This is done to make sure that the higher volume grade/size combinations are given preferential selection as test batches.
• Number of Test Batches
  It is important to cover the extremes of structural sections produced, because structural product performance can be interpolated between sizes tested. Therefore select a commonly produced small section size, a commonly produced large section size, and either 1 or 2 section sizes in-between those two extremes.
• Test Replications
  Strength properties are based on the lower 5% strength values. Accurate and confident estimations of lower percentile strength values can only be achieved by larger test populations. The recommended minimum number of test replications to determine strength properties is 30 boards, although 100 test replications significantly reduces the sampling error factors applied when determining characteristic strength properties.
• Sample Stick Length
  The length of the sampled structural boards should be the longest marketed lengths sold for that grade/size combination. For four-point bending tests the minimum structural test length is 20 times the wide face of the board. Any extra board length above 20 times the board’s wide face allows the bending test specimen to be randomly positioned within the board length, as per AS/NZS4063 structural test methodology.
• Sampling Period/Sampling Rate
  The sample of any one grade/size combination should come from a reasonable number of production runs, enough to truly reflect the general wood quality processed by your mill for that grade/size combination. Sampling periods could be shortened if logs from different resource allotments are mixed prior to being processed. If mixed species are processed allow sufficient time to ensure that the all species in the mix are proportionately sampled.
  A reasonable sampling period could be between 2 to 3 months. The sampling rate per production shift is determined by first establishing the sampling period, and then the number of production shifts of the grade/size combination occurring over the sampling period.
• On-going Sampling Period/Sampling Rate
  a/ Mills should have already completed an initial evaluation of its structural production before proceeding to the on-going monitoring of structural properties, refer to the points above.
  b/ Mills already having completed an initial evaluation of their structural products can proceed directly to the next step of their compliance testing program, annual on-going monitoring of structural properties. This consists of sampling and testing the most common grade/size combination, and possibly one other grade size combination. For more frequent feedback the boards sampled for the annual monitoring of structural properties could be externally tested in 6 monthly intervals.

The simple answer to this question is YES. The onus is now on you, the timber Processor/Manufacturer/Importer, to ensure that the structural timber products you have branded/marked with a stress-grade is in compliance with the structural timber properties applicable to that designated stress-grade, and also ensure that such compliance is capable of being verified. The only manner in which this can be achieved is to have a structural performance test database generated from a suitable compliance testing program.

Mechanically Stress-Graded Timber
Clause 5 of AS/NZS1748 “Timber-Mechanically stress-graded for structural purposes” clearly states that timber branded/marked as mechanically stress-graded timber must be initially evaluated, and also have its properties continuously monitored.
Appendices A1 and A3 (both informative) of that Australian Standard refer to AS/NZS4490 and AS/NZS4063, and provide guidance on issues related to;

• Definition of resource through a parent population
• Checking structural properties (in-grade testing)
• Linkage between reference population and its structural properties
• Sampling requirements for on-going monitoring
• On-going monitoring of structural properties

Visually Stress-Graded Timber
The Softwood (AS2858) and Hardwood (AS2082) Visual Grading Standards have now placed the linkages between visual-grades and stress-grades into the informative Appendices. The “Identification of Grade” Clause in both of these visual-grading Standards now make it the Manufacturer’s responsibility to ensure that timber branded/marked with a stress-grade is in compliance with the structural properties applicable to the designated stress-grade, and also ensure that such compliance is capable of being verified. AS2082 Appendix A (Informative) even states;

“Validation of the link between structural grade defined by this Standard and stress grade, using in-grade testing in accordance with AS/NZS 4063, is recommended’”
“Any stress grades evaluated through in-grade testing of full sized structural material in accordance with AS/NZS4063 should be taken to supersede all of the information presented in this Appendix.”

The compliance of the New Zealand softwood processing industry’s structural products have been put into doubt, and as a result the industry has been regulated to sampling their production of structural timber products, at the rate of 1 in a 1000 structural boards, to ensure grade compliance. The ACCC in Australia has directed the timber industry itself to find its own solutions, and to self-regulate their quality assurance programs.

One response from the Australian Timber Industry was to draft an Industry Best Practice document, “Guidelines for Verification of Structural Timber Properties”. This project was managed by Australian Plantation Products & Paper (A3P), and the main author being Geoff Boughton, TimberEd Services, who invited the timber processors to provide feedback at each stage of the drafting process. This Guidelines document has been written in Standard-eze, and is due for release in July 2008. The impact of this document will be assessed by most timber processors/importers and other stakeholders over approximately a 12 month period, then it will most likely be quickly adopted and republished as an Australian Standard.

Contact_me to discuss the benefits of setting up an appropriate sampling and testing program for your company’s structural timber products, and the costs involved for this service.