Duration of Load Tests for Dry Lumber in Bending

Testing of 450 pieces of 2 by 6 Hem-Fir #2 Grade with stepwise ramploadings showed that strength was highly dependent upon stress level. The compression failures of clear material were different from the tension failures observed in material containing knots; the investigation showed that the time dependent behavior was different in the two cases. The tests confirmed the Madison curve for clear material, but this did not apply to commercial material containing knots; no decrease in strength was observed for material weaker than 2500 psi and the tests indicated that for low strength lumber the presently used reduction factor for time needs further study. It was also observed that the test results were not “gaussian” distributed and that assumption of normality may lead to erroneous results.

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