Effects of Thickness and Moisture on the Compressive Strength of Sheathing-Grade Douglas-Fir Plywood

Testing of thin (3/8 in.) Douglas-fir sheathing-grade plywood in compression without buckling has been achieved by use of a specially designed frame. Also tested were 3/4-in. specimens formed by bonding two 3/8-in. specimens together. Tests were carried out at three moisture levels (ambient c. 5 percent m.c., approx. 15 percent m.c. and soaked). In all, five 4-ft. x 8-ft. panels from each of three manufacturers were employed; each panel provided four 7 1/2-in. x 15-in. specimens of each thickness at each moisture level. It was found that the compressive strength of the 3/4-in. material is greater, but less variable, than for the simple 3/8-in. specimens; some reasons for this are offered. Results of earlier work on the effect of moisture were confirmed in that there is evidence that the rate of reduction in strength with moisture content is a function of the thickness (or construction). Manufacturer differences, which can be related to specific gravity differences, are also revealed.

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