Moisture Effects On Load-Duration Behavior of Lumber. Part II. Effect of Cyclic Relative Humidity

Authors

  • Kenneth J. Fridley
  • R. C. Tang
  • Lawrence A. Soltis

Keywords:

Load-duration (creep-rupture), strength, failure, relative humidity, moisture content, lumber, damage models

Abstract

The effect of cyclic moisture conditions on the load-duration behavior of structural lumber is presented. Select Structural and No. 2. Douglas-fir nominal 2 by 4 specimens were tested in bending in two cyclic relative humidity (RH) environments: 35% to 95% RH on 24- and 96-hour cycles. A constant temperature of 73 F was maintained in both tests. Constant bending loads based on the 15th percentile of the static strength distributions for each grade at 73 F and 50% RH were used to load the beams. The load-duration behavior in the two cyclic RH environments is compared to previously reported results observed from three constant RH environments (35%, 50%, and 95% RH at 73 F). Analysis of test results indicated a trend toward shorter times-to-failure in cyclic RH conditions as compared to constant RH conditions. The effect, however, was no more evident in the No. 2 specimens than in the Select Structural specimens. To predict the load-duration behavior, an existing damage accumulation model was modified to account for the effect of changing moisture contents on the long-term strength of structural lumber. The developed model was found to predict the observed behavior quite well.

References

Barrett, J. D., and R. O. Foschi. 1978a. Duration of load and probability of failure in wood. Part I. Modelling creep rupture. Can. J. Civil Eng. 5(4):505-514.nBarrett, J. D., and R. O. Foschi. 1978b. Duration of load and probability of failure in wood. Part II. Constant, ramp, and cyclic loadings. Can. J. Civil Eng. 5(4):515-532.nFoschi, R. O., and J. D. Barrett. 1982. Load-duration effects in western hemlock lumber. ASCE J. Struct. Div. 108(ST7):1494-1510.nFridley, K. J., R. C. Tang, and L. A. Soltis. 1988. Effect of temperature on duration of load of structural lumber. Pages 390-394, vol. 1, in R. Y. Itani, ed. Proceedings of the 1988 International Conference on Timber Engineering. Forest Products Research Society, Madison, WI.nFridley, K. J., R. C. Tang, and L. A. Soltis. 1989a. Effect of cyclic temperature on duration of load of solid lumber in bending. Pages 179-184, AMD vol. 99 in R. W. Perkins, Jr., ed. Mechanics of cellulosic and polymeric materials. ASME, New York, NY.nFridley, K. J., R. C. Tang, and L. A. Soltis. 1989b. Thermal effects on load-duration behavior of lumber. Part I. Effect of constant temperature. Wood Fiber Sci. 21(4):420-431.nFridley, K. J., R. C. Tang, and L. A. Soltis. 1990. Thermal effects on load-duration behavior of lumber. Part II. Effect of cyclic temperature. Wood Fiber Sci. 22(2):204-216.nFridley, K. J., R. C. Tang, and L. A. Soltis. 1991. Moisture effects on load-duration behavior of lumber. Part I. Effect of constant relative humidity. Wood Fiber Sci. 23(1): 114-127.nGerhards, C. C. 1979. Time-related effects on wood strength. A linear cumulative damage theory. Wood Sci. 11(3):139-144.nGerhards, C. C. 1982. Characteristics of Douglas-fir 2 x 4 duration of load lumber sample. Unpublished report.nGerhards, C. C. 1988. Effect of grade on load duration of Douglas-fir lumber in bending. Wood Fiber Sci. 20(1): 146-161.nGerhards, C. C., and C. L. Link. 1987. A cumulative damage model to predict load duration characteristics of lumber. Wood Fiber Sci. 19(2): 147-164.nHoyle, R. J., 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.nHwang, W., and K. S. Han. 1986. Cumulative damage models and multi-stress fatigue life prediction. J. Composite Mater. 20(4):125-153.nMiner, M. A. 1945. Cumulative damage in fatigue. J. Applied Mech. ASME 12(3):A159-A164.nMurphy J. F. 1983. Discussion of "Load-duration effects in western hemlock lumber," by R. O. Foschi and J. D. Barrett. J. Struct. Eng. 109(12):2943-2946.nNational Forest Products Association (NFPA). 1986. National design specifications for wood construction. NFPA. Washington, DC.nStamm, A. J. 1964. Wood and cellulose science. The Ronald Press, New York, NY.n

Downloads

Published

2007-06-28

Issue

Number 1 / January 1992

Section

Research Contributions

License

The copyright of an article published in Wood and Fiber Science is transferred to the Society of Wood Science and Technology (for U. S. Government employees: to the extent transferable), effective if and when the article is accepted for publication. This transfer grants the Society of Wood Science and Technology permission to republish all or any part of the article in any form, e.g., reprints for sale, microfiche, proceedings, etc. However, the authors reserve the following as set forth in the Copyright Law:

1. All proprietary rights other than copyright, such as patent rights.

2. The right to grant or refuse permission to third parties to republish all or part of the article or translations thereof. In the case of whole articles, such third parties must obtain Society of Wood Science and Technology written permission as well. However, the Society may grant rights with respect to Journal issues as a whole.

3. The right to use all or part of this article in future works of their own, such as lectures, press releases, reviews, text books, or reprint books.