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Creep of lumber beams under constant bending load

University of British Columbia

Two sets of data are analyzed in the thesis. The first set was derived from the long term deformations of 2 in x 6 in x 12 ft (40 mm x 140 mm x 3600 mm) joists of Douglas-Fir loaded under constant bending stress to levels lower than or equal to 3110 psi (21.44 MPa). The second set was derived from the long term deformations of 2 in x 6 in x 12 ft joists of Hemlock loaded under constant bending stress to levels of 3000 psi (20.68 MPa) and 4500 psi (31.0 2 MPa) . The analysis shows that the creep behaviour of structural size beams depends upon the material characteristics; specifically, material with a strength lower than 5000 psi (34.33 MPa) appeared to creep 1.5 times more than, material with a strength higher than that level, over a three month period. In addition, the test results support the assumption of a linear relationship between the creep deformation of a structural-size timber beam and applied stress. A method is presented to predict the creep behaviour of a structural-size specimen at discrete times over a three month period. The method consists of expressing the creep deformation, Δ[sub c], in terms of the elastic deformation, δ[sub e], or equivalently, the fractional creep (f = δ[sub c] / δ[sub e]) in terms of δ[sub e] or in terms of the modulus of elasticity. This work is limited to the stress levels investigated and to specific temperature (10°C< θ <30°C) and moisture content (8%<MC<12%) conditions. While this method could be employed in preliminary design procedures, it has been especially designed for more complex studies of the creep behaviour of structures including floor systems, trusses, etc... The advantage of the method is that in this kind, of analysis the modulus of elasticity of the individual components can be used. This thesis also presents a set of creep curves that cover a three year span. These creep curves show that the average total deformation of beams loaded to a stress level of 3110 psi (21.44 MPa), at this time, is approximately 1.6 times the elastic deformation.

Text

2010-03-11

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http://hdl.handle.net/2429/21771

Civil Engineering

University of British Columbia

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