Time-dependent deflection of Douglas fir beams in bending and shear modes Public Deposited

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  • Creep in wood has been observed since 1833 but only recently has it been studied quantitatively. The aim of this paper is to study the time-dependent deflections of Douglas fir, Coast Region beams in bending and shear modes. The general method of approach used is that of large scale observation. Three straight-grained clear wood specimens were used for testing. The beams were simply supported and loaded at the quarter span points, thus, each beam had a section of pure bending moment and sections of constant shear with linearly varying bending moment. The beams had a 40 inch span with a width of 1-1/2 inches and a depth of 3-1/2 inches. The maximum stresses induced were f = 1370 psi and v = 120 psi. The moisture content of the wood was brought down to approximately seven percent and the beams were placed in a testing environment of approximately uniform relative humidity and temperature. Deflection data were collected for the quarter span and mid-span points. These measurements were made for 70 days at which time the creep rate had decreased to a very small value. The time-dependent deflections were plotted versus log time in days. These curves showed, as expected, that creep varies linearly with the log of time. The final values of creep were 14 percent of the initial elastic deflections. The concept of effective moduli was used to compare the bending and shear contributions to the creep behavior. The elastic shear deflection of beam #1 appears to have been partially recovered with time. Creep due to shear in beam #2 remained in the approximate ratio of its contribution to the elastic deflection. This lack of correlation made definite conclusions unattainable. The results suggest that the shear and bending modes of creep behavior are dependent upon different variables of the internal structure. It is obvious to the author that more sophisticated and complex methods must be used to thoroughly investigate the time-dependent deflections of wood.
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