The tensor polynomial (Tsai-Wu) strength theory with the interaction stress coeffi cient, F12, determined from the Hankinson formula, has been applied in this study to predict the strength of wood under plane-stress conditions. The biaxial failure envelope for Sitka spruce, based on this choice of F12 and the mechanical properties from the Wood Handbook, is presented. F12 is very sensitive in biaxial loading tests and can be extremely difficult to obtain in such tests on wood. Since the Hankinson formula has been verified repeatedly and has been popular in the wood industry, it is recommended that F 12 be based on the Hankinson formula.
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