An Analysis of Creep-Inducing Stress in Sitka Spruce
Small specimens of Sitka spruce (Picea sitchensis) were placed under a parallel-to-the-grain compressive loading situation in a water-saturated state to examine the adequacy of three-element and four-element spring and dashpot model analogs in describing creep response curves for wood. Stress levels of 10, 20, 40, and 60% of ultimate compressive strength were studied for load durations up to 20 days. Deformation-time measurements were analyzed statistically by means of a computerized, nonlinear, least squares regression analysis and with the aid of graphic analysis.
Creep deformation was found to occur at stress levels as low as 10% and at very short time periods. In general, three- or four-element model analogs were adequate to describe time-dependent deformation phenomena for applied purposes; further refinements were deemed superfluous. For time periods of 24 hr or longer, the use of a four-element model is advised, particularly at higher stress levels.
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