Bending Creep and Load Duration of Douglas-fir 2 by 4s Under Constant Load for up to 12-Plus Years

Authors

  • Charles C. Gerhards

Keywords:

Bending creep, relative creep, deflection, wood beams, lumber grade, controlled and uncontrolled environments, wood engineering, load duration, design criteria

Abstract

This paper finalizes research on graded Douglas-fir 2 by 4 beams subjected to constant bending loads of various levels and durations. Compared to results for testing in a controlled environment, results confirm that load duration did not appear to be shortened by tests in an uncontrolled environment, at least extending out to 12-plus years. By the same comparison, relative creep was considerably increased, however.

The extended data also confirm that no evidence was found for a threshold below which stress levels for lumber can be maintained indefinitely.

Based on the finalized prediction equations of this study and those of two previous studies, a factor of 2.0 for a 10-year load duration is more appropriate for Douglas-fir bending allowable properties than the 1.62 factor currently recommended. Also, bending deflections due to creep doubled sooner than commonly accepted. This research is important to structural engineers and code groups responsible for the safe design of wood structures when establishing new design criteria for load duration and deflection limits.

References

American Forest & Paper Association (AFPA). 1997. National design specifications for wood construction. ANSI/AF&PA NDS-1997. Appendix B (non-mandatory) load duration. Washington, DC.nAmerican Society for Testing and Materials (ASTM). 1981. Standard methods for establishing structural grades and related allowable properties for visually graded lumber. ASTM D 245-81. American Society for Testing and Materials, Philadelphia, PA.nEllingwood, B., and D. V. Rosowsky. 1991. Duration of load effects in LRFD for wood construction. J. Struct. Eng. ASCE 117(2):584-599.nFridley, K. J., R. C. Tang, L. A. Soltis, and C. H. Yoo. 1992a. Hygrothermal effects on load-duration behavior of lumber. J. Struct. Eng. ASCE 118(4):1023-1038.nFridley, K. J., R. C. Tang, L. A. Soltis, and C. H. Yoo. 1992b. Creep behavior model for structural lumber. J. Struct. Eng. ASCE 118(8): 2261-2277.nFridley, K. J., M. O. Hunt, and J. F. Senft. 1998. Effect of 85 years of service on mechanical properties of timber roof members. Wood Fiber Sci. 30(1):72-80.nGerhards, C. C. 1985. Time-dependent bending deflections of Douglas-fir 2 by 4s. Forest Prod. J. 35(4):18-26.nGerhards, C. C. 1988a. Effect of grade on load duration of Douglas-fir lumber in bending. Wood Fiber Sci. 20(1):146-161.nGerhards, C. C. 1988b. Effect of high-temperature drying on bending strength and load duration of Douglas-fir 2 by 4's. Forest Prod. J. 38(4):66-72.nGerhards, C. C. 1991. Bending creep and load duration of Douglas-fir 2 by 4s under constant load. Wood Fiber Sci. 23(3):384-409.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.nLebatteux, M., P. J. Galimard, and P. Morlier. 1996. Duration of load of structural lumber. Proc. International Wood Engineering Conf. 4:106-112. Oct 28-31, New Orleans, LA.nRosowsky, D. V., and K. J. Fridley. 1992. Stochastic damage accumulation and reliability of wood members. Wood Fiber Sci. 24(4):401-412.nRouger, F., C. Le Govic, P. Crubilé, R. Soubret, and J. Paquet. 1990. Creep behavior of French woods. International Timber Engineering Conf. October, Tokyo, Japan. Pp. 330-336.nShen, Y., and R. Gupta. 1994. Creep of structural lumber in natural environment-initial observations. Proc. IUFRO S5.02 Timber Engineering Conf. July, Sydney, Australia. Pp. 61-77.nShen, Y., and R. Gupta. 1997. Evaluation of creep behavior of structural lumber in a natural environment. Forest Prod. J. 47(1):89-96.nToratti, T. 1994. Creep experiments in variable humidity-three years of loading. Proc. IUFRO S5.02 Timber Engineering Conf. July, Sydney, Australia. Pp. 93-111.nToratti, T., and P. Morlier. 1994. Long-term bending of structural timber. Pacific Timber Engineering Conf. 2: 173-177. July 11-15, Gold Coast, Australia.n

Downloads

Published

2007-06-19

Issue

Section

Research Contributions