Edge-Gluing Douglas-Fir Lumber to Improve Strength Properties

Nineteen groups (samples) of Douglas-fir 2-inch lumber specimens were fabricated by edge-gluing a strip of clear wood (usually 1-in. wide) on each edge. Finished nominal sizes of specimens were 2 by 6, 2 by 8, and 2 by 10 inches built up from combinations of clear material and 2 x 4’s and 2 x 6’s of No. 3 structural grades and Ll, L2, and L3 laminating grades. Random methods of selecting pieces were used throughout the study. Six groups (175 pieces) were tested in bending; 13 groups (386 specimens) were tested in tension parallel-to-grain. In addition, 102 unglued (control) bending and 180 unglued tension specimens were tested. All edge-glued pieces with clear edges that were tested in bending improved in MOR from No 3 or L3 grade to Select Structural and dense Select Structural grades. Specimens made from L3 grade material, with clear material glued to the edges and tested in tension improved considerably but probably not enough to warrant the cost of gluing. Edge-glued pieces from L2s were strong enough to be used as critical tension laminations in laminated beams of 20F to 26F grade, while most glued specimens fabricated from Ll grade were strong enough for critical laminations in the highest grade beams (24F and 26F). Costs of edge-gluing are unknown, but the expense surely can be justified by the marked increase in strength.

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