Bending Creep and Load Duration of Douglas-fir 2 by 4s Under Constant Load for up to 12-Plus Years

Charles C. Gerhards

Abstract


This paper finalizes research on graded Douglas-fir 2 by 4 beams subjected to constant bending loads of various levels and durations. Compared to results for testing in a controlled environment, results confirm that load duration did not appear to be shortened by tests in an uncontrolled environment, at least extending out to 12-plus years. By the same comparison, relative creep was considerably increased, however.

The extended data also confirm that no evidence was found for a threshold below which stress levels for lumber can be maintained indefinitely.

Based on the finalized prediction equations of this study and those of two previous studies, a factor of 2.0 for a 10-year load duration is more appropriate for Douglas-fir bending allowable properties than the 1.62 factor currently recommended. Also, bending deflections due to creep doubled sooner than commonly accepted. This research is important to structural engineers and code groups responsible for the safe design of wood structures when establishing new design criteria for load duration and deflection limits.


Keywords


Bending creep;relative creep;deflection;wood beams;lumber grade;controlled and uncontrolled environments;wood engineering;load duration;design criteria

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