Understanding Machine Stress-Rated Lumber

Most lumber we know is graded visually by a lumber grader, who is approved by an accredited grading agency and evaluates wood based on characteristics like knots, wane, and growth rings. However, for applications that call for a greater predictability of strength performance—as in engineered structures and architectural projects—more precise methods are used to ensure the wood meets the required standards. This is where machine stress-rated, or MSR, lumber is needed.

What is Machine Stress-Rated Lumber?

Machine stress-rated lumber is subjected to machine testing on top of visual grading to reveal unseen defects. To be classified as “MSR” lumber, the wood must be a minimum of visual grade No.2, ensuring that it does not have any major defects affecting its performance. A machine then non-destructively tests the wood for a more accurate reading on strength and stiffness properties. The most commonly produced dimensions for MSR lumber include 2x4, 2x6, and 2x8.

Key Properties of Machine Stress-Rated Lumber

There are two key properties included in MSR lumber—"Fb" and "E". These two properties are indicated on MSR grade stamps, which are approved by the American Lumber Standards Committee (ALSC) or the Canadian Lumber Standards (CLS) Accreditation Board.

• “Fb” or “F” is the fiber stress in bending and indicates the predicted strength in pounds per square inch (psi). This is the most stress the lumber can withstand in bending until permanently rupturing fibers. A greater “Fb” value denotes a stronger lumber.
• “E” is the modulus of elasticity and indicates the average stiffness in millions of pounds per square inch (psi). A greater “E” value denotes a stiffer lumber that bends less.

MSR lumber may also be evaluated for additional properties, such as the allowable stress parallel-to-grain "Ft", the horizontal shear "Fv", the compression perpendicular-to-grain “Fc”, and the specific gravity "SG”.

Common MSR lumber grades include:

• 2850Fb - 2.3E
• 2400Fb - 2.0E
• 2250Fb - 1.9E
• 2100Fb - 1.8E
• 1950Fb - 1.7E
• 1800Fb - 1.6E
• 1650Fb - 1.5E
• 1450Fb - 1.3E

Quality Control (QC) Testing

The key benefit to MSR lumber is the confidence in its performance. As a result, MSR lumber is required to be quality control tested on a daily schedule to ensure up-to-date accuracy in grading. This process involves testing at least one of the strength properties and modulus of elasticity from an edgewise orientation.

Role of Species in MSR Lumber

A significant distinction between the original method of visually grading lumber and MSR lumber is that the species of the wood does not affect MSR grades. In visual grading, graders consider the species, since some are naturally stronger or stiffer than others. However, MSR lumber is based entirely on the tested properties. As a result, MSR lumber from different species can be the same MSR grade, as long as they exhibit the same strength and stiffness properties. This is an advantage for construction, as it allows for more versatility in sourcing materials for projects.

Efficiency in Usage

On a similar note, MSR lumber allows for a more efficient use of resources. For example, using a 2x4 MSR piece of lumber instead of a 2x6 visually graded piece of lumber that provides the same strength performance. All pieces of MSR lumber can be used to their maximum potential, which allows for less waste costs and a faster inventory turnover.

MSR Lumber Applications

MSR lumber is commonly used in applications that require predictable strength, such as:

• Roofing and floor trusses: These structures support the roof or floor of a building—whether it be residential or commercial—and require their components to bear hefty loads while maintaining their shape.
• Ceiling and floor joists or rafters: MSR lumber is also used in framing systems that need to brace live loads.
• MSR lumber is also used in engineered wood products, including cross-laminated timber (CLT), glue-laminated timber (glulam), and other mass timber construction. These products are designed for high strength, yet minimal weight.

Interested in learning more about MSR lumber and sourcing high-quality material for your next project? Contact us today at Olympic Industries.