Volume 24, Issue 3 (May 1996)
Time-Dependent Bending Properties of Lumber
Research related to structural stressed-skin panels (SSP) and their component parts has been ongoing at Chalmers University of Technology. In recent years, attention has been focused on SSP with lumber webs and sheet metal and chipboard panels as tension and compression flanges, respectively. In this study, certain mechanical and rheological properties of structural lumber were evaluated as part of a larger project designed to predict the long-term behavior of SSP under load.
In this study, several test series were conducted to evaluate short- and long-term properties of European redwood (called Scotch pine in North America) (Pinus sylvestris). From 22 pieces of lumber measuring 25 by 125 by 4000 mm, specimens were cut for use in several test series to determine solid wood properties. Wood modulus of elasticity (MOE), accounting for the shear effects, was evaluated from initial deflections of long-term tests. Mean MOE values of about 14 500 MPa were obtained.
Four series of long-term tests were conducted under different combinations of stress level and distribution, as well as loading history. The results suggested that the magnitude of relative creep was greatest under lower stress levels. Different stress distribution (resulting from center- and third-point loading) proved to have little effect on creep behavior. Two mathematical models, i.e., power functions (Clouser function without exponential terms and power function with exponential terms)  were evaluated as predictors of the deformation-time relationship for lumber under constant loading. Differences in accuracy of predicting creep behavior between the two models studied were very modest.