Shear Strength of Douglas-Fir Laminated-Veneer Lumber

A sample of sixty 2 x 8-inch Douglas-fir laminated-veneer lumber (LVL) specimens was obtained from a commercial manufacturer. The material was divided into matched specimens and from each sample block-shear specimens, joist, and plank full-size components were obtained. The material which was tested indicated an allowable stress when oriented as a joist about 5 percent greater than the published value of 95 psi, which is applicable for all grades of Douglas-fir dimension lumber. Butt joints appear to have no influence on joist shear strength. It is evident, however, that this material was made from veneers which had rough surfaces, were deeply checked, and possibly overdried. On the other hand, in LVL material, which was made in the laboratory from veneers peeled and dried under controlled conditions, stresses were well above published lumber values. Plank strength, on the basis of tests of structural-sized material, indicated an allowable shear strength which was 20 percent greater than the published value for lumber (95 psi), but 31 percent less than the published value (165 psi) for glued-laminated beams (CSA-086-1970). Since the small block-shear plank specimens indicated allowable stresses less than the published allowable lumber stress, LVL as represented by the quality of the commercial experimental sample should not be employed as a plank. However, when manufactured using a supervised quality-control program which employs shear parallel to grain and glueline strength criteria, in addition to percent wood failure, it is expected that LVL could be employed as a lumber plank in bending as well as for joists. However, in the highest shear-stress areas of glued-laminated beam assemblies and other built-up shear-critical beam sections, LVL is not under any circumstances recommended for use when oriented as a plank. Due to the effects of veneer peeling and drying variables on shear strength of LVL, it is recommended that any manufacturer of LVL establish quality-control supervision over the veneer peeling and drying as well as the gluing process.

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