Bending and Shear Properties of Grooved, Hardwood Plywood Paneling

Lauan 3-ply plywood panels of nominal thicknesses from 3.6 mm to 1/4 in. were tested in static bending and in 2-rail shear. Most of the panels had grooves through the faces to give the appearance of planking. Some of the panels were machined with a groove 1 in. wide in the back ply opposite a groove in the face ply to simulate the effect of a split. The average values of the modulus of rupture and modulus of elasticity of the individual plys as estimated from the bending tests were approximately 13,500 psi and 1.8 x 10 psi respectively, only slightly below the mean values published for lauan species corrected to the same moisture content as the test specimens. All 21 of the 3.6 mm thick panels tested in shear buckled laterally without fracture. Estimates of the buckling loads obtained from the theory for orthotropic plates with simply supported edges in shear, and from Southwell plots of the lateral deflection vs. load relationship, were in good agreement with the test buckling loads. The results indicate that the Southwell procedure may be used with the 2-rail shear test to estimate the buckling loads of thin panels. The thicker panels failed by fracturing in shear. The nominal shear stress at failure, based on the net section of the grooved specimens and without allowance for stress concentrations at the groove or variation along the length of the 2-rail shear panel, ranged between 640 psi and 1230 psi. There was no significant difference between the shear stresses at failure for specimens with grooves in the face ply with and without a simulated split in the back ply. Comparison of the shear stresses at failure with values calculated from empirical formulae published by the United States and United Kingdom Forest Products Laboratories appeared to confirm that an interior glue does not increase the properties of the plywood. Omission of the term in the formulae for the reinforcing effect of the glue-line led to underestimates of the shear stress at failure by the United States formula and generally small overestimates by the United Kingdom formula. The bending and shear properties were sufficiently high to show that grooved, thin plywood panels may be used structurally. Standard procedures for estimating the strength of the panels seem to be satisfactory. However, due allowance must be made for the effect of the grooves and splits, if present, on the section properties, and for the possibility of buckling.

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