PLIB provides certification and inspection services for mills that manufacture Machine Stress Rated (MSR) lumber, Machine Evaluated Lumber (MEL), and glued lumber products (such as finger-jointed lumber). While not an “engineered” wood product like LVL, I-joists, trusses or glulams, MSR, MEL and some glued lumber products are often used as components in these engineered products 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 and MEL Lumber
As the names imply, machine stress rated lumber and machine evaluated lumber are lumber products that have been non-destructively evaluated by a “machine” that measures one or more physical properties of the wood, such as stiffness, density, or acoustic wave transmission. This information is then used to predict one or more mechanical properties, such as stiffness (E), bending strength, tensile strength, etc., of each piece that has been measured. In addition to being evaluated by a device, MSR and MEL are also subject to visual grade limits on 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 mechanically 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 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.
Structural Glued Lumber Products
Structural glued lumber products are structural lumber products that, as the name implies, have been manufactured by gluing two-or more pieces of lumber together to make a lumber product that can be used in a structural application, such as a wall stud, joist, or I-joist flange.
A very common structural glued lumber product is one known as finger-jointed lumber. This type of glued lumber product has been manufactured from lumber that has been machined on the ends to produce a profile of v-shaped “fingers.” These profiled pieces are then glued together end-to-end with the fingers interlocking, to produce long length pieces. Most finger-jointed lumber products are visually graded and not subject to in-line mechanical evaluation; however, there are some producers who manufacture a finger-jointed MSR product. Finger-jointed lumber is recognized by the building codes as interchangeable with non-finger-jointed lumber of the same grade.
There are two broad categories of finger-jointed lumber –- “Structural” and “Stud/Vertical Use Only.” Structural finger-jointed lumber products are manufactured with waterproof adhesives and can be used in both vertical and horizontal applications where non-finger-jointed lumber is allowed. These products can be manufactured in wider widths and are used as joists and rafters, as well as flange and chord members where bending and/or tensile forces may be significant. “Stud/Vertical Use Only” products, as the name implies, are to be used in vertical applications where the member is loaded in axial compression, where bending and tension design loads are short duration (such as wind or seismic) and where the in-service moisture content of the wood will not exceed 19%. Typical wall stud applications would fall into this category. They can be made with non-waterproof adhesives.
Other structural glued lumber products include face-glued structural lumber, for both vertical use applications and general structural applications.
One advantage of finger-jointed lumber is the ability to manufacture very long lengths that can have a wider variety of applications.
Producing MSR, MEL and Structural Glued Lumber
In addition to the necessary in-line and off-line test equipment, producers of MSR, MEL, and structural glued lumber must have their product certified by a third party inspection agency such as PLIB and conduct ongoing tests of their production. The qualification and ongoing testing requirements are established by the rules writers and agencies. There are different qualification standards across North America. In addition, different products require different test equipment, procedures and adhesives – the process of getting up and running can be complicated and even confusing. PLIB can walk interested manufacturers through the certification process and help them get their products qualified and labeled properly.
For more information on MSR, MEL, or structural glued lumber, call us at (253) 835-3344 (U.S.) or (604) 732-1782 (Canada) or contact us via email. You may also want to contact the MSR Lumber Producers Council for more general information on MSR and MEL lumber.