Visually Graded and Machine Graded Lumber and Design Values

The below companies below determine lumber grades and design values.

Visually Graded Lumber

Trained graders, following the requirements of ASTM Standard D1990-16 in the mill, visually sort each piece of lumber by key characteristics of the wood, such as knot size, slope of grain, and amount of wane. Visually graded lumber is suitable in the most common applications.

An example of a Visually Graded Lumber Stamp is shown below. See "Understanding Grade Stamps" at the bottom of this article for description of information on grade stamps.

Visually Graded Lumber stamp, courtesy of SPIB. — Courtesy of SPIB

Machine Graded Lumber

Machine Stress Rated Lumber (MSR) Lumber
Machine Evaluated Lumber (MEL) Lumber

For structural applications and more demanding engineered uses, such as commercial/industrial roof and floor systems, glued laminated beams, can often be used instead of steel and concrete.

Machine Stress Rated Lumber (MSR) and Machine Evaluated Lumber (MEL) are accepted by regulatory agencies and all major building codes.

For more detailed information on grading lumber, visit Southern Pine Inspection Bureau (SPIB) and the companies listed above.

Machine Stress Rated (MSR) Lumber

SPIB states, "For MSR products, the phrase "MSR" or "machine rated" must appear, and the grade name consists of two numbers that designate the fiber stress in bending, Fb and the E (e.g., 2400f-2.0E). MSR producers may opt to apply a No.1 grade wane limitation on their MSR, which is designated by the "1W" on the grademark."

An example of an MSR grade stamp is shown below.

Machine Evaluated Lumber (MEL)

SPIB states, "A grade name for MEL is the letter M followed by a number code, e.g., M-9. The grade mark must also show the design values for which quality control is performed, namely bending strength, E, and tension strength."

An example of an MEL grade stamp is shown below.

Lumber Design Values

The ASTM Standard D1990-16 establishes the allowable properties for visually graded dimensional lumber from “in-grade tests” to determine design strengths of full size visually graded solid sawn dimension lumber.

All timber produced by machine stress-grading is required to be in-grade tested.

The basic principle of in-grading testing is to:

Take a large sample of full sized commercial timber and test it fairly to the true strength distribution of a grade.
Use strength values representing the low end of the strength distribution as the basis for design.

The National Design Specification, "Design Values for Wood Construction (NDS)" are embedded in the software used to create nearly all roof and floor trusses in North America.

The six design values are:

fiber stress in bending (Fb),
tension parallel-to-grain (Ft),
horizontal shear (Fv),
compression parallel-to-grain (Fc),
compression perpendicular-to-grain (Fc | )
modulus of elasticity (E).

Each piece of lumber is sorted into various strength classifications in a MEL production-approved machine after meeting certain visual requirements.

MSR requires daily quality control for bending strength and stiffness.

Specifically required for MEL are daily quality control tests for tension strength before a grade stamp can be applied.

When ordering grade-marked MEL lumber, specify the grade name corresponding to the required design values. Alternately, you can contact individual suppliers for the grades which they produce. Specify based on values of:

Bending Strength or Modulus of Rupture (MOR) measured in F_b or fiber stress in bending
Modulus of Elasticity (MOE) measured in million psi or E

Click here to view the free option of the 2024 National Design Specification, "Design Values for Wood Construction (NDS)" from the American Wood Council.

Example of Design Values for Machine Stress Rated Lumber (MSR)

Example of Design Values for Machine Evaluated Lumber (MEL)

Adjustment Factors

Adjustments to values are taken for duration of load, repetitive member situations, beam and column stability, and other factors as summarized in Table 2.3.1 in the NDS supplement. Machine graded lumber enjoys specific advantages in beam stability, column stability, and buckling stiffness factors as a result of the consistency of E compared to visually graded lumber.