Effect of High-Temperature Drying on Tensile Strength of Douglas-Fir 2 By 4’S

High-temperature kiln-drying can reduce strength of Douglas- fir. Results of high-temperature drying research, reported in this paper, suggest that drying process variables may need consideration in establishing structural design values for Douglas-fir lumber. To help determine the effect of high-temperature drying with differing kiln schedules, 8-foot-long 2 by 4’s cut from 12 Douglas-fir logs were dried by three different kiln schedules: 1) conventional, not exceeding 185?F; 2) progressive low-high temperature ending at 2300F; and 3) constant high temperature of 230?F. Top-of-load restraint was used during drying by each type of schedule, but one conventional and one progressive schedule were also run without restraint. Drying restraint did not significantly affect either tensile strength or MOE. Tensile strength was significantly affected by type of kiln schedule but MOE was not. Average tensile strength was 18 percent lower for the high temperature schedule and 10 percent lower for the progressive low-high temperature schedule than for the conventional schedule. These results support strength loss data reported by others for similar test on Douglas-fir. The average kiln schedule effects were applicable as well to the lower quality Douglas-fir lumber specimens.

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