| Summary: | Mill visits were conducted to evaluate the effect of Visual Quality Level (VQL) restrictions
of edge knots on the yields of Machine Stress Rated (MSR) lumber. This initial study indicated
that edge knots contribute significantly to the amount of MSR lumber that is being downgraded
because of visual override requirements, representing in excess of $15,000 per month in lost
revenue to MSR producers. Therefore, an experimental study was initiated to study the effect of
large VQL (edge knots) on the strength properties of Spruce-Pine-Fir (SPF) MSR lumber.
Two groups of material were studied that had been graded by a Continuous Lumber Tester
(CLT) to receive a 1650E-1.5Fb MSR grade designation. One group called on-grade complied
with the visual override requirements and the other group, VQL, should contained at least one
edge knot with a VQL greater than 25 % of the cross sectional area. All specimens were nominal
38 mm x 89 mm SPF lumber collected from the same MSR production run.
Manual measurements of the VQLs was conducted on all 750 specimens and they were
then tested nondestructively to determine flat-wise and edge-wise modulus of elasticity. All the
specimens were randomly divided into either tension or third point bending testing since these are
the most commonly used destructive tests.
There were weak relationships noted between the measured VQL size and destructive
strength values. Strength results were as expected in that tension strength proved to be more
important and restrictive. Even when some boundaries were placed on the VQL size the tension
specimens failed to meet the grade requirements while the bending specimens exceeded the
requirements in all cases. This results from the fact that MSR production of SPF lumber is governed by the MOE value determined by the CLT. It is not controlled by the bending strength
as is the case for Douglas-fir; whose strength values were used in the VQL size determination.
Two procedures, simulation and proportionality, were used to determine the amount of
large VQL material that can be included with the current on-grade production and meet grade
requirements. These procedures were conducted with the various VQL groups for both
destructive tests and confirmed that tension strength is the governing factor. The results highlight
the importance of the amount of VQL material to include in a grade; therefore, proper and
thorough testing by organizations is required for large VQL inclusion.
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