Bending Creep and Load Duration of Douglas-Fir 2 By 4S Under Constant Load

Charles C. Gerhards

Abstract


Douglas-fir 2 by 4 beams of different grades were tested under various constant-load levels in a controlled environment to evaluate load duration and creep behavior. A two-parameter equation was used to model relative creep of wood beams free of partial fracture. Both parameters M and N of this equation vary considerably between specimens but can be highly correlated with each other, depending on the time base used to determine N. Stress level was partially correlated with M and N together, but with neither parameter alone.

Additional matched beams were tested at near design loads in an unheated building to determine the effect of an uncontrolled environment on load duration and creep. Based on load duration results for the controlled environment already reported, load durations do not appear to have been shortened by the uncontrolled environment, although relative creep was considerably increased.

The most important result of this study, which has implications for the safety of wood structures, is that more beams failed when loaded at near design stress than are commonly assumed would fail. This result, coupled with results from a previous study, suggests that Douglas-fir bending allowable properties should reflect greater load duration reduction factors or shorter load durations. This research is important to structural engineers and to 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;creep modeling;deflection;wood beams;lumber grade;controlled and uncontrolled environments;wood engineering;load duration;design criteria

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