Time-Dependent Bending Deflections of Douglas-Fir 2 By 4’s

The time-dependent deformation known as creep is an important material characteristic of wood that is not well understood. This study evaluates the time-dependent deflections of Douglas-fir 2 by 4 beams tested at three different constant load levels for up to 220 days. Few of the beams exhibited pure creep during the full time on test because most had partial fractures sometime during the experiment. Also, most of the beams failed completely during the experiment because the level of constant load was relatively high for their static strength. The partial failures resulted in jumps in the creep curves limiting the extent to which creep could be modeled. A power law was used to model creep, at least out to the first jump in the creep curve. Based on the power law model, the rate of creep relative to initial deflection appears to be lower for lumber than for small clear wood. Because of the jumps and subsequent additional creep deflection that followed, other means were used to quantify the time-dependent deflection characteristics of the beams. The results should prove useful to engineers when designing new wood structures or evaluating existing wood structures for continued use.

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