The Effect of Moisture Cycling On Creep of Small Glued Laminated Beams
Keywords:Douglas fir, glued laminated, creep, humidity, cycled
Creep information for sawn lumber exposed to constant and cyclic humidity environments previously published by the authors is supplemented by this study using glued laminated material of about the same size, tested on the same apparatus, and using the same general procedure.
For both sawn and glued laminated material, the relative creep was measured at about 55% of their average ultimate strength. Relative creep was measured for specimens matched with respect of modulus of elasticity (MOE) and exposed to a constant relative humidity (65 F and 65% RH) for about 1,000 hours, and a cyclic relative humidity (65 F and 90% RH for 82 hours followed by 65 F and 40% RH for 82 hours). This 164-hour cycle was repeated six times for about 1,000 hours of cyclic exposure. This is a more severe change in equilibrium moisture content of the environment than is likely to occur in building structures.
The result showed relative creep of the glued laminated material increased by 40% to 72% as a result of cycling. One very low MOE pair showed an increase of 167%. The increase for solid sawn lumber previously reported was 200% to 400% due to cycling. Results also showed that the ratio of relative creep for cycled to constant humidity exposure did not change appreciably as time under load was increased. These differences are attributed to permeability differences between glued laminated wood and sawn lumber. The gluelines are believed to retard water vapor movement and reduce the moisture change rate and extremes of wood moisture content in any given cycle.
Anderson, J. T. 1985. The effect of moisture cycling on the creep of glulam beams. M.S. thesis, Department of Civil and Environmental Engineering, Washington State University, Pullman, WA.nCouncil, K. A., and J. T. Helwig, eds. 1979. SAS/GRAPH user's guide, 1971 ed. SAS Institute Inc., Cary, NC.nHoyle, R. J., Jr., M. C. Griffith, and R. Y. Itani. 1985. Primary creep in Douglas-fir beams of commercial size and quality. Wood Fiber Sci. 17(3):300-314.nHoyle, R. J., Jr., R. Y. Itani, and J. J. Eckard. 1986. Creep of Douglas-fir beams due to cyclic humidity fluctuations. Wood Fiber Sci. 18(3):468-477.nSAS Institute. 1979. SAS user's guide. Statistical Analysis Systems, Cary, NC.nWestern Wood Products Association. 1981. Western lumber grading rules 81, Portland, OR.n
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