Kiln-Drying Douglas-Fir Pole Sections: Effects on Strength and Checking

To assess the effect of drying conditions on checking and strength properties, nine unseasoned Douglas-fir transmission poles were cut into 8-foot sections, divided into three diameter classes, and kiln-dried at three temperatures (180?, 230?, and 270?F). All sections were dried to an average moisture content of 30 percent 3 inches from the pole surface; kiln-drying time varied from 96 to 192 hours, depending on temperature and diameter. After kiln-drying and 12 months’ outside exposure, average check width increased by 0.12 inch. Depth extended from 0.6 to 1.8 inches in the small and medium diameter classes and 1.2 to 4.2 inches in the large diameter class at 180? and 230?F but did not increase at 270?F. Specimens 1.5 inches square sawn from the sections were tested in static bending and compression parallel to the grain. Strength values decreased with increasing temperature. Modulus of rupture in bending averaged a 15 percent reduction in strength; maximum crushing strength was reduced by 12 percent; and modulus of elasticity was least affected.

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