Timber is structurally graded by sorting it into groups based on its structural properties to ensure suitability for load-bearing applications.
What is Structural Grading?
According to the provided reference, structural grading is the process by which timber is sorted into groups - or stress grades - with ideally, similar structural properties in each group. This sorting is crucial because the natural characteristics of timber can vary significantly, affecting its strength, stiffness, and other properties essential for structural use.
The Structural Grading Process
The primary goal of this process is to classify timber so that engineers and builders can confidently select material with predictable performance characteristics for specific structural elements like beams, joists, and rafters.
Common methods used for structural grading include:
- Visual Grading: This method involves trained inspectors visually examining each piece of timber for characteristics that affect strength, such as:
- Knots (size, type, location)
- Slope of grain
- Splits and checks
- Wane (lack of wood on the edge)
- Fungal or insect damage
Based on these visual cues and established grading rules (which vary by region and standard), the timber is assigned to a specific stress grade.
- Mechanical Grading (Machine Stress Grading - MSG): This is a more objective method where machines measure a physical property of the timber, typically its stiffness (Modulus of Elasticity or MoE), by bending it or using vibration. Since stiffness is generally correlated with strength, the machine can then assign a stress grade based on the measured property. This method often allows for more efficient use of the timber resource by identifying the actual properties of each piece.
Understanding Stress Grades
Timber sorted through structural grading is assigned to different stress grades. These grades represent a range of assigned structural properties (like bending strength, stiffness, and tension strength) that can be used in design calculations.
As the reference notes, while the aim is to group timber with similar properties, inevitably there is a very substantial range of properties within a group and significant overlap in properties between the groups. This means that even within a single stress grade, individual pieces will have varying properties, but they are guaranteed to meet the minimum requirements assigned to that grade. Higher stress grades typically have stricter requirements or higher measured properties, indicating suitability for more demanding structural applications.
Choosing the correct structural grade for a project is vital for ensuring the safety and performance of the structure. Designers use the assigned properties of the specific stress grade to calculate member sizes and ensure they can safely carry the intended loads.
