Machine Graded Lumber
Machine graded lumber is identified and segregated by a characteristic
that correlates with other structural properties. Current machine
technologies op-erate on the basis of direct measurement of
either stiffness or density.
Advantages of Machine Grading
The machine identifies the lumber based on algo-rithms keyed
to the predictor property (either stiffness or density). Specific
visual overrides are applied and daily off line testing is performed
to verify the assigned properties. Because 100% of the production
goes through the machine, gets sorted on the predictor parameter
and because the process is "calibrated" daily by quality
control, production is more uniform and more predictable.
Machine Grading Technologies
There are two major types of machines available today. The first
type directly measures the stiffness of the lumber. The CLT,
or Continuous Lumber Tester, manufactured by Metriguard, Inc., is the most common type of stiffness based
grading machine in use. The second type of machine directly
measures the density of the lumber. The XLG, or X-ray Lumber
Gauge, manufactured by USNR,
is the most common type of density based grading machine in
Classes of Machine Graded Lumber
The National Design Specification for Wood Construction recognizes
two broad classes of machine graded lumber. These are Machine
Stress Rated Lumber (MSR) and Machine Evaluated Lumber (MEL).
Either the stiffness based or the density based machines can
produce these classes of machine graded lumber. There are some
engineering differences between the two classes of machine graded
lumber -primarily in the relative values of various properties
and in the level of off line testing required.
As with visual grades of lumber, machine graded lumber is identified
by the grade stamp of a lumber grading agency approved by the
American Lumber Standards Committee or the Canadian Lumber Standards
Accreditation Board. The distinguishing characteristics of the
grade stamps for machine graded lumber include supplementary design
values for the grade. For MSR, the grade stamp includes both the
allowable bending stress and bending stiffness (modulus of elasticity).
For MEL, the grade stamp adds the allowable tension stress and,
optionally, the allowable compression stress (if compression quality
control testing is performed).
Specifying Machine Graded Lumber
MSR grades are typically specified in terms of their bending
stress and stiffness -for example, one would specify 2400f -2.0E
grade. MEL grades are specified by their "M" grade
-for example, M-23.
MSR Grade Stamp:
MEL Grade Stamp:
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