Evaluation of creep behavior of structural lumber in a natural environment

Creep behavior of structural lumber is being observed over time in a natural environment at the Forest Research Laboratory of Oregon State University. In a bending test, 20 Douglas-fir beams were subjected to a constant load. Deflections of the beams and daily fluctuations in temperature and relative humidity are being measured every day. This study reports the results of the first 14 months from April 1994 to June 1995. The objective of the study is to evaluate the long-term creep behavior of full-size structural lumber. Stiffness of the beams appears to have a strong influence on the magnitude of creep strain, and creep strain appears to follow the fluctuations in air temperature closely. The mechano-sorptive creep strain in this experiment was shrinking and swelling of the beam surfaces and was not tied to beam moisture content, which changed little over the experimental period. An existing five-element creep model did not describe creep behavior of the structural lumber in the natural environment. A four-element model and an empirical model were developed to include stiffness of the beams and air-temperature effects. The four-element model fits the experimental data well.

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