Creep of Douglas Fir Beams Due to Cyclic Humidity Fluctuations

Authors

  • Robert J. Hoyle, Jr.
  • Rafik Y. Itani
  • James J. Eckard

Keywords:

Beams, deflection, relative creep, creep, humidity

Abstract

This report describes tests of the hypothesis that creep in Douglas fir beams of commercial size is influenced by cyclic humidity fluctuations.

Twenty 3.5- x 3.5-inch (89- x 89-mm) beams were creep-tested for a period of approximately 1,200 hours (7 weeks). Deflection data were recorded for loaded beams while the relative humidity of their environment was cycled at 70 F. The environmental EMC was varied from 7 to 20% for periods of 24 and 168 hours, respectively. Three regression models were fitted to the relative creep data. The effect of stress levels and humidity cycles were compared.

The study showed that creep for cycled specimens greatly exceeded creep for uncycled specimens.

References

Armstrong, L. D. 1962. The effect of moisture content changes on the deformation of wood under stress. Australian J. Appl. Sci. 13(4).nArmstrong, L. D., and G. N. Christensen. 1961. Influence of moisture changes on deformation of wood under stress. Nature 191(4791).nArmstrong, L. D., and R. S. T. Kingston, 1960. Effect of moisture content changes on creep in wood. Nature 185(4716).nHall, H., J. Haygreen, and B. Neisse. 1977. Creep of particleboard and plywood floor deck under concentrated loading. For. Prod. J. 27(5):23-32.nHoyle, Robert 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):3000-313.nKingston, R. S. T., and L. D. Armstrong. 1951. Creep in initially green wooden beams. Division of Forest Products, CSIRO, Melbourne, Aust. J. Appl. Sci. 2(1):306-325.nSAS Institute. 1979. SAS user's guide. Statistical Analysis System Institute, Cary, NC.nSchniewind, Arno P. 1967. Creep rupture life of Douglas-fir under cyclic environmental conditions. Wood Sci. Technol. 1(4):278-288.nArno P. Schniewind, and Duane E. Lyon. 1973. Further experiments on creep rupture life under cyclic environmental conditions. Wood Fiber 4(4):334-341.nSenft, John F., and S. K. Suddarth. 1971. An analysis of creep inducing stress in Sitka spruce. Wood Fiber 2(4):321-327.nUniform Building Code. 1985. Int. Conf. Bldg. Officials, Whittier, CA. Sec. 2307:128.n

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Published

2007-06-28

Issue

Number 3 / July 1986

Section

Research Contributions

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