Compression Strength Adjustments for Moisture in Commercial Douglas-Fir Plywood

The Madison formula for adjusting values for compression stress in commercial plywood consistently overestimates the reduction in strength from low to high moisture contents. This is not surprising since the Madison formula was derived only for use with solid, clear material. Specifications for commercial plywood permit the use of veneer containing resinous deposits and knots, etc., which may not be affected as much by changes in moisture as is clear material. From tests of commercial material at three different moisture contents (6.6, 13.2, and 19.3 percent), it was found that a simple linear relationship described best the average compressive strength values parallel to the face grain as well as for the estimate of the lower 5 percent exclusion limits. In applying this adjustment method to data obtained from another laboratory, close agreement was obtained. Therefore, it is recommended that the linear method be employed for use in adjusting either average test values or their lower 5 percent exclusion limits from low to higher moisture contents. A further experiment involving a fourth group of specimens which were soaked in cold water for 7 weeks gave a moisture intersection point close to that published for Douglas-fir solid wood. Thus the “intersection point” thesis, established by Wilson of the Madison Forest Products Laboratory, for solid wood can be applied to plywood. This result seems to justify the application of the linear strength adjustment formula over a wide range of moisture contents from the “as fabricated” dry to soak conditions.

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