Effect of high-temperature drying on bending and shear strengths of western hemlock lumber

Western hemlock (Tsuga heterophylla) 38- by 140-mm (2 by 6) dimension lumber visually graded as No.2 & Better was dried using conventional and accelerated kiln schedules to determine if the accelerated kiln schedule could be used to reduce the time required to dry this size and grade of structural lumber. After being dried, the lumber was tested in bending and shear to assess the potential effect of high temperature on the allowable bending strength, bending stiffness, and shear properties. The bending test was conducted on full-size lumber, and the shear test was done using small, clear specimens. The time required to dry the western hemlock lumber by the conventional schedule was 48 hours; the accelerated schedule resulted in a drying time of approximately 24 hours, a 50 percent reduction. Drying conditions had no significant effect on the mean bending strength or stiffness; however, the mean shear strength was reduced by 6.4 percent. The variation associated with the bending and shear properties was not affected by the drying schedule. The accelerated drying schedule did reduce base design bending stress by 8.1 percent, mainly because of lack of equivalence and the calculation procedure, and base design shear stress by 14.0 percent.

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