Grading Timber
Find out more about how timber strength is graded.
Grading Timber: Ensuring Structural Integrity and Quality
Ensuring the strength and safety of structural timber begins with proper grading. At Verified Timber Ltd, we use two primary grading systems to assess the quality of timber: Machine Stress Grading and Visual Stress Grading. Each method has its unique approach and focus, both aimed at maintaining the highest standards for timber used in construction.
Why Grade Timber
Grading timber is a crucial process that determines its suitability for various construction applications based on its structural properties.
The grading process assesses both the strength and stiffness of the timber, ensuring it meets the required standards set out in the New Zealand Standard NZS3622 for Verification of Timber Properties.
To ensure the highest quality and structural integrity of timber used in construction, we employ two primary grading methods: Machine Stress Grading and Visual Stress Grading.
Machine Stress Grading
Machine Stress Grading utilises advanced machinery to test the stiffness of timber. This method is highly effective in measuring how much a piece of timber will bend or flex under a given load.
The machines used in machine stress grading evaluate the modulus of elasticity (MoE), which directly correlates with the timber's stiffness. While machine stress grading provides a consistent, objective assessment of stiffness, it does not measure strength directly. To compensate, all machine stress graded timber undergoes additional visual inspections to identify any large defects or anomalies, such as knots or splits, that might impact its structural integrity.
How Machine Stress Grading Works
Timber pieces are fed through a machine that applies a controlled load, measuring the deflection to calculate stiffness. This grading method is precise and repeatable, making it a reliable choice for assessing timber's structural capability.
Importance of Machine Stress Grading
By providing an accurate measure of stiffness, machine stress grading ensures that the timber used in construction can handle the loads and forces it will face, contributing to the overall safety and stability of the structure.
Visual Stress Grading
Visual Stress Grading relies on a thorough visual inspection process to evaluate timber based on visible characteristics. This method focuses on identifying features such as knots, grain deviations, splits, and other natural defects, which are strong indicators of the timber's strength.
While these visual features are effective for assessing potential weak points that could affect the timber's ability to bear loads, they are less reliable for predicting stiffness. As a result, visual stress grading is primarily concerned with ensuring that any visually identified defects do not compromise the timber's structural performance in use.
How Visual Stress Grading Works
Graders visually inspect each piece of timber, sorting it according to the presence and size of knots, grain patterns, and other imperfections that might reduce its strength. Timber that passes visual stress grading is considered suitable for applications where strength is the primary concern.
Importance of Visual Stress Grading
By identifying and categorising timber based on visual characteristics, visual stress grading ensures that only strong, defect-free timber is used in construction, particularly in applications where load-bearing capacity is critical.
Minimum Target Values for Machine Stress Graded and Visually Graded Pinus radiata and Douglas fir Timber
All timber that has been graded either by machine or visually is given a Structural Grade (SG) depending on their strength. The table below provides the bending, compression and tension strength of different grades of timber, alongside the levels of elasticity you should expect from them.
| Moisture condition | Grading Technique | Grade | Bending strength fb (MPa) | Compression strength fc (MPa) | Tension strength ft (MPa) | Modulus of elasticity E (GPa) | Fifth percentile modulus of elasticity (GPa) |
|---|---|---|---|---|---|---|---|
| Dry (m/c = 16%) | Machine | SGS15 | 41 | 35 | 23 | 15.2 | 11.5 |
| Machine | SG12 | 28 | 25 | 14 | 12 | 9 | |
| Machine / Visual | SG10 | 20 | 20 | 8 | 10 | 7.5 | |
| Machine / Visual | SG8 | 14 | 18 | 6 | 8 | 5.4 | |
| Machine / Visual | SG6 | 10 | 15 | 4 | 6 | 4 | |
| Green (m/c = 25%) | Machine / Visual | SG8 (Wet) | 11.7 | 12 | 4 | 6.5 | 4.4 |
Note: A producer can offer other species or grades with different stresses and moduli subject to verification in accordance with the standard NZS3622.
SG8 (wet) is a grade which has been verified in the green condition for when the timber will be used in a service situation where the moisture condition may be 25% or over.
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