Analysis of K-Factor for Five-Ply Plywood

K-factor has long been used as a modifier of section modulus in the calculation of bending moment for plywood beams. No comparable modification of moment of inertia is included in current recommendations for calculating plywood deflection at span-depth ratios of 30:l or greater. Logic suggests that to whatever extent “shear” strains deform beam sections from theoretical planes, they modify equally the effective moment of inertia involved in the deflection calculation and the effective section modulus used in the calculation of bending moment. Based on this logic the authors propose that the same K-factor should be involved in both calculations. Results of a limited amount of testing of 5-ply plywood at a span-depth ratio of 32:1 lend support to the basic premise of this study with average K values of 0.85 and 0.92 for Engelmann spruce and, ponderosa pine sheathing grade plywood, respectively. Other values for K-factor are anticipated at other span-depth ratios. If results of these tests are confirmed by further investigation, a method for determining an appropriate value for K based on, separate calculations of deflections due to shear and bending is suggested.

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