Sampling
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.