Effect of high-temperature drying on bending strength and load duration of Douglas-fir 2 by 4’s

Matched samples of Douglas-fir 2 by 4’s kiln-dried either conventionally or by high temperature were tested for static bending strength or duration to failure under constant bending load. The high-temperature schedule consisted of 19 hours at 230?F, 4 hours at 210?F, and 3 hours of equalizing at 195?F dry bulb and 180?F wet bulb. Results show that Douglas-fir subjected to high-temperature drying (HTD) had load durations less than one-tenth that of Douglas-fir subjected to conventional drying (CD) when both types carried equal constant loads for periods up to 12 weeks. HTD Douglas-fir had load durations equivalent to CD Douglas-fir when HTD Douglas-fir was subjected to constant loads that were 90 percent of those for CD Douglas-fir. The 10 percent reduction in constant load for HTD Douglas-fir was the approximate HTD effect on static strength observed in this study. Engineers, grading agencies, lumbermill operators, and building code officials should be concerned with the HTD effect on Douglas-fir and other species in which strength loss has been reported. Care in the selection of HTD kiln schedules to minimize structural lumber damage is imperative.

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