In-grade testing helps to establish the true strength distribution of graded timber for some of the more commonly used structural species. It involves the sampling of commercially available timber stocks and full-sized lengths of commercially available material with data reviewed to ensure a minimum expected performance across each specified grade.
In-grade testing involves the following:
- Sampling the stocks of commercially available timber in a particular species and grade. In some cases, this may require shipping timber from different parts of the country to ensure that the sample is as representative as possible.
- Testing the full sized lengths of commercially available timber in a manner similar to the loading expected in service for the product. For example, bending strength is derived using a third point loading over a span of 18 times the depth of the timber. The bending moment diagram is similar in shape and scale to that produced by a uniformly distributed load, and the span-to-depth ratio is characteristic of bending members.
- Modifying the data from the tests to allow for the sampling so that it can be used to represent the strength of the entire population of timber in that species and grade. The design properties can be chosen so that they are less than the characteristic values indicated by the tests. In many cases, different sized cross-sections will have been tested enabling a size factor to be derived.
The procedures for performing in-grade tests and appropriately presenting the data are detailed in AS/NZS 4063 Timber - Stress-graded - In-grade strength and stiffness evaluation.
In-grade testing has been completed for the group of species known as Australian pine and Victorian ash. With Australian pine, the data has been used to create a number of grades, where the properties have been derived from the in-grade test data. These classes are known as MGP10, MGP12, MGP15 with the '10' referring to 10,000 MPa, a rounded characteristic modulus of elasticity. The other MGP grades follow the same convention. All timber produced by machine stress-grading is required to be in-grade tested.
In-grade testing involves the careful sampling of commercial timber in the sizes and grades produced, and the testing of hundreds of pieces of timber to get as close as possible to the true strength distribution of a grade. The true strength distribution of timber in the marketplace is used as a starting point to assign or confirm a set of design values. These values can be tied to an existing grade or can be product specific.
In-grade testing can also be used to periodically test timber to ensure that the product is delivering the properties implicit in the grade stamp applied. This gives a higher level of reliability to the product properties.
In-grade testing to determine design strength
The following in-grade testing process is used to determine the design strengths of timber:
- Large numbers of pieces of commercial sized structural timber of a single species and grade are tested. The resulting distribution tends to follow a log-normal distribution rather than the normal distribution as for the small clear specimens. The fifth percentile strength from this distribution is used for all subsequent calculations.
- The fifth percentile of the strength distribution is modified to account for the sampling process. The result of the modification is an estimate of the fifth percentile strength of the entire population of the single size and grade that was sampled and tested. The modification algorithm allows for both sample size and the variance of the properties from the tests, and gives 75 per cent confidence that the data reflects the fifth percentile of the population.
- This figure is the characteristic strength of the timber, which is used as a basis for setting the design value.
Some structural timber products have most or all of their major characteristic properties (eg bending strength, stiffness, compression and tension) based on in-grade testing, whilst some secondary properties, such as compression perpendicular to grain, are based on strength grouping.
In-grade testing to determine design strength
A similar process can be used to monitor properties of continuing production of timber products. The process can be used to monitor strength or stiffness. It is illustrated here for strength:
- Sampling over a long period of time gives material that represents the long-term production of that product. Testing of this sample of material uses the same test method outlined in AS/NZS 4063.
- The fifth percentile of the test results is calculated.
- This value is compared with an acceptance criterion given in AS/NZS 4490. Where the fifth percentile is above the acceptance criterion, the production is said to match the design value within expected sampling error. Where the fifth percentile is below the acceptance criterion, then there is doubt about the properties of the material and corrective action may be required.
In-grade testing is a versatile concept that has acceptance around the world. Different test protocols are used in different parts of the world, and analysis methods differ slightly too. However, the basic principle remains the same:
• Take a large sample of full sized commercial timber, and test it fairly.
• Use strength values representing the low end of the strength distribution as the basis for design.