Lumber which is evaluated mechanically is called machine stress-rated (MSR) lumber. Canadian MSR lumber is manufactured in conformance with the National Lumber Grades Authority (NLGA) Special Product Standard 2 (SPS-2)
Most lumber is visually graded by lumber graders. They examine each piece, applying visual "rules" to establish its grade, which determines the structural design value. Visual grading still works well, however, the increase in demand for high-performance lumber has led to greater demand for enhanced grading techniques. Also, the emergence of managed high yield forests is producing trees whose strength is more difficult to accurately determine by visual grading alone.
In addition, building techniques have changed. The heavy roof beams and rafters of the 1930s and 1940s have been replaced by the highly engineered truss systems of today. These engineered systems have become so sophisticated that they push structural lumber elements close to the limit of their design capacity.
Machine grading makes this possible. It works by measuring a characteristic, such as stiffness or density that correlates with the structural properties of interest, such as bending strength. The process combines information about the visual characteristics of the grade with measured stiffness or density, sorts by the predicted strength, and labels each piece with a grade stamp.
As lumber is fed continuously into the mechanical evaluating equipment, stiffness is measured and recorded by a small computer, and strength is assessed by correlation methods. MSR grading can be accomplished at speeds up to 365m (1000') per minute including the affixing of an MSR grade mark. MSR lumber is also visually checked for properties other than stiffness which might affect the suitability of a given piece. The result is a more precise understanding of the strength of each piece of machine graded lumber than is possible with visually graded lumber.
The image below, shows a schematic representation of the apparatus used to rate MSR lumber. A typical Canadian MSR grade stamp looks similar to that of visually graded lumber, but in addition indicates the grade, by reference to bending strength (Fb), and stiffness (E).
MSR and MEL lumber bears a grade stamp just like visually graded lumber [see Figure 1 below]. The MSR grade stamp indicates the Fb [design bending stress] and E [modulus of elasticity or stiffness] values. Thus, the MSR grade 1650f-1.5E designates the bending stress of 1650 psi and the stiffness of 1.5 million psi in its grade name. Other design properties are as listed in the National Design Specification [NDS].
The MEL grades are designated by the letter "M" followed by a number [i.e. M-10, M-19, etc.] which is not correlated to any strength property. As with MSR, the design properties for the MEL grades are indicated on the grade stamp and are listed in the NDS.
Checks and Balances
MSR and MEL machine grading systems also include an off-line testing process for verifying the stress ratings on a daily basis. Lumber is loaded on its edge to 2.1 times its assigned design bending stress. MEL processes also require the lumber to be loaded in tension to 2.1 times its assigned tensile stress. Both MSR and MEL testing include daily monitoring of stiffness which verifies the accuracy of the machine grading process. For architects, engineers, and building designers, this testing assures a product with highly predictable strength properties. Truss designers, particularly, have taken advantage of the efficiency of MSR and MEL to design and build trusses that closely match the size and strength of lumber components to the desired span.