Effect of Grade On Load Duration of Douglas-Fir Lumber In Bending
Keywords:Static strength, grade, rate of loading, duration of load, constant load, bending, lumber, time
AbstractSelect Structural, No. 2, and No. 3 Douglas-fir 2 by 4 specimens were tested in bending at several rates of loading and several levels of constant load to determine the effect of grade on load duration. The constant load results suggest that lower grades of lumber have shorter times to failure; however, differences in the load duration effect between lumber grades may not be statistically significant. These results also suggest that allowable bending properties of lumber may be nonconservative for any design load that really exists for the design duration. Recommended load duration design factors based on traditional methods of derivation are included along with discussion of stress level threshold and absolute stress effect. This report should be useful to engineers responsible for wood structural design and to grading agencies for evaluating the safety of recommended allowable lumber properties, from the standpoint of both real loads and their durations as well as code loads.
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