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Time and size effects for tension perpendicular to grain in wood Mau, Tak Jee

Abstract

Failure of pitched-tapered glulam beams in tension perpendicular to grain has been attributed to the inadequacy of the present method of calculating stresses. In this thesis, the factors of time and size for tension perpendicular to grain were investigated to determine how they would affect the allowable stresses. A total of 229 Douglas Fir glulam blocks of two sizes were loaded for ?0 days in uniform tension perpendicular to grain to investigate time effect. Test data were analyzed by developing estimates for the strength ratios. Results indicate that strength reductions increase at an increasing rate with the logarithm of time. The presently used time factors were shown to be conservative and could be increased for the 2-month, the 7-day and the 1-day durations. Experimental data also indicate that the present time factors may be non-conservative for load durations less than 5 minutes and more than ?0 days. In addition, short-term testing of 24-0 Southern Pine glulam blocks of three sizes was performed to investigate size effect. Results show that the relationship between specimen strength and volume is a straight line in a log.-log. plot, thus verifying the weakest-link model for Southern Pine. Comparison with Douglas Fir data taken from literature was also made. It was found that Southern Pine is definitely stronger than Douglas Fir In tension perpendicular to grain. Size effect, however, is almost identical for the two species. Significant difference was observed in the long-term test results between the two specimen volumes. This may be due to the effect of size, but it could also be caused by the different stress levels used for the two volumes.

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