What is stress grading in timber and where would you use it?

Answer 1

If I understand your query correctly, stress grading refers to how much load a particular piece of lumber can handle. The longer a board the more it will flex, thus less load it can take. Different sizes and species will vary in their abilities. Such as Douglas fir is stonger than SPF. Laminated beams (LVL) is stronger than DOuglas fir.

Stress grading of timber

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Structural grading is the process by which timber is sorted into groups (Stress Grade) with ideally, similar structural properties in each group. Structural grading can be performed in a number of ways including the following:

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Visual stress-grading

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Machine stress-grading

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Machine proof-grading

Visual stress-grading

•In softwoods, pith is the dark spot that was the upward growing twig when the tree was very, very young. The density of the wood is low, this can reduce the strength of the timber. In grading of softwoods, the visual grader has to estimate the spacing of the annual rings to determine whether or not a piece with core wood is within the specification of this grade.

•Surface checks are shallow cracks on the surface, mainly from the release of residual stresses on drying. The grader must check all cracks to see that they are not a split (that runs from one face to the other), and if a check (shallow crack) that they are not too wide, or too long.

Knots - depending on the location of the knots, different limits apply. They are estimated in these rules by the Knot Area Ratio (KAR) which requires a grader to visualise the knots going right through the cross-section. The KAR is the percentage of the cross -section that is taken up with knots. Different limits on KAR apply for knots in the margin (outer quarter of the wide face) and for the centre (central half of the wide face).

Machine stress-grading

•Machine stress-grading uses a machine to bend each piece of timber (generally about its minor axis). The machine measures the stiffness of the piece and uses a loose correlation between stiffness and strength to assign a stress grade. A sorted group with a small range of E (stiffness) can produce a larger range of strength. The E value is also used to infer all of the other structural properties, including tension, compression and shear strength.

•The machine stress grading process requires that quality checks be built into the operation of the machine so that its grading is uniform over time, and that the correlation between grading parameter and the properties of the product remains valid. These checks are on the performance of the machine, which can be found by running calibration sticks of known properties through the machine, and on the performance of the output, by periodically performing destructive tests on samples of the graded timber.

Machine proof-grading

•Proof-grading commences with the preliminary sorting of the ungraded timber being produced by the mill into sub-groups, each of which will become a stress grade classification. An accepted and well-documented grading method is used for the preliminary sort. When machine proof-grading procedures are used, after the preliminary sorting, every piece of timber in each sorted group is tested by the application of a predetermined bending load, known as the proof load.

•Each target stress grade has its own proof stress, which is used with the cross-sectional dimensions to give the proof load. As the proof load is applied, each piece of timber is loaded on edge and continuously stressed in bending to the proof stress. The proof stress is specified in and is significantly higher than the stress at serviceability loads for the particular grade being assigned. Pieces that survive the proof load without failure, excessive deformation or other signs of damage are deemed to qualify for the stress grade that corresponds to the applied proof stress

It differs from machine stress grading in the following ways:

Machine stress grading

-Timber loaded about minor axis (on flat)

-Small loads applied - timber is loaded at much less than design strength

-Intention is to find poor pieces by measuring stiffness

-High

Proof grading

Timber loaded about major axis (on edge) High load applied - timber is loaded at loads near the design strength

Intention is to find poor pieces by breaking them

Low speed process - timber may take 15 to 20 seconds to pass through the machine