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Don’t Forget! You Saw it in the
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By Linda Brown and Frank Woeste
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June 2021 #13263 Page #80
Machine Graded Lumber: Technologies, Quality, and Benefits
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or more than 70 years, people have been using machines to assist in the grading of lumber. This article summarizes the machine grading methodologies for producing Machine Stress Rated Lumber (MSR) and Machine Evaluated Lumber (MEL) and highlights the similarities and differences between MSR and MEL. In addition, some structural design benefits from selecting MSR and MEL are presented.
Grading Technologies When mechanical devices were introduced to North America in the 1960s, they evaluated the stiffness of lumber by observing the load required to cause the deflection of each piece of lumber by a preset amount (see Figure 1). By the 1990s, x-ray technology was developed, whereby variations in lumber density were correlated with strength properties. More recently, acoustic technology has been adapted for use in machine grading of lumber; a sound wave is induced by striking each piece with a hammer and detecting the transmission speed with a microphone, so that the speed of sound transmission can be related to the stiffness of the lumber.1 Importantly though, none of these technologies can be used to grade lumber independent from a visual override to ensure overall appearance.
Figure 1. Mechanical stress-rating process. Diagram courtesy of the Western Wood Products Association (WWPA), Technical Guide 4: Machine Stress Rated Lumber. Additional information on the evolution of machine-graded lumber is available in the USDA Forest Products Laboratory’s GTR 238 Nondestructive Evaluation of Wood, Second Edition. Continued next page
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