Edgewise Shear Properties of 3-Ply, 4-Ply, and 5-Ply 1/2-Inch Southern Pine Plywood

The ultimate edgewise shear strengths of 1/2-inch southern pine plywood of three constructions were determined by the FPL rail shear test, while the edgewise moduli of rigidity were obtained by the ASTM plate shear test. Moduli of rigidity of each construction was also estimated by a formula reported by Ueda (1968). At 12 percent moisture content, the edgewise shear strength of southern pine plywood was practically the same whether face grain orientation was parallel or perpendicular to the direction of applied shear loads. At 12 percent moisture, the ultimate edgewise shear strength of the 5-ply, 1/2-inch southern pine plywood was approximately 10 percent and 4 percent greater than that of the 4-ply and 3-ply constructions, respectively. In addition, the edgewise modulus of rigidity of the 5-ply plywood was 12 percent larger than those of the 3-ply and 4-ply plywood. These differences in shear strength and moduli of rigidity exhibited by the 5-ply construction is attributed partially to its higher specific gravity and partially to its veneer thickness. It is therefore concluded that edgewise shear properties of 3-ply, 4-ply, and 5-ply, 1/2-inch southern pine plywood for practical application should be considered equal. The average edgewise shear strength of 1/2-inch southern pine plywood at 12 percent MC is 980 psi, while the average value of edgewise modulus of rigidity is 98,000 psi. At 24 percent moisture content, values of modulus of rigidity of 1/2-inch flat plywood panels decreased approximately 30 percent from values corresponding to 12 percent moisture. However, when panels did not remain flat the reduction appeared to be smaller. The modulus of rigidity of a southern pine plywood panel can be estimated with good accuracy from certain elastic constants of strips taken from the panel or from a unidirectionally laminated veneer panel constructed with veneer of the same species and quality.

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