As the names imply, machine stress rated (MSR) lumber and machine evaluated lumber (MEL) are lumber products that have one or more physical properties, such as stiffness or density, non-destructively evaluated. This information is then used to predict one or more mechanical properties, such as stiffness (E), bending strength, tensile strength, etc., of the piece that has been measured. In addition to being evaluated by a device, MSR and MEL are also subject to visual grade rules that limit allowable characteristics such as knots, wane, etc. Finally, MSR and MEL production is subject to off-line quality control tests which measure the actual stiffness and/or strength of samples pulled from daily production.
A variety of methods can be used to non-destructively measure the wood and the manufacturer has many options when choosing MSR or MEL machines. In all cases, MSR and MEL machines must be approved by the American Lumber Standards Committee (ALSC) or the Canadian Lumber Standards Accreditation Board (CLSAB).
MSR and MEL are often used as components in engineered products like LVL, I-joists, trusses, and glulam as well as being used as single members in traditional stick frame construction. MSR and MEL products may include E-rated structural laminations and tension laminations.
MSR lumber grades are designated by a Fb-E classification system, where Fb is the assigned bending strength and E is the assigned modulus of elasticity for the grade. MEL grades are designated by a grade code that references a specific set of characteristic property values. A full listing of the property values assigned to each MSR or MEL grade can be found in the rule books of the various rules writing agencies across North America or by contacting PLIB.
A full description of the MSR grade rule can be found in section 8, paragraph 206-a of Standard no. 17 Grading Rules for West Coast Lumber. A listing of the property values for MSR lumber can be found in Table 13 and Table 14 of paragraph 206-a. Other MSR and MEL grades can be found in the rule books of the various rules writing agencies across North America or by contacting PLIB.