Effect of length on the tensile strength of lumber

An experimental study was conducted to evaluate the effect lumber length has on the parallel-to-grain tensile strength of lumber. Four hundred pieces of visually graded Select Structural and 370 pieces of No. 2 kiln-dried nominal 2 by 4 inch (38 by 89 mm) Spruce-Pine-Fir lumber were tested at gauge lengths of 104, 145, and 192 inches (2642, 3683, 4877 mm). The specimens were sorted into matched groups according to their average flatwise modulus of elasticity values. The effect of gauge length was isolated from the effect of grade by positioning the grade controlling defect of No. 2 grade specimens within the test span for all gauge lengths. The length effect of the mean and the fifth percentile tensile strength were estimated. For doubling the gauge length from 100 to 200 inches (2540 to 5080 mm), the estimated length effect factors were in the range of 0.877 to 0.925 for the fifth percentile tensile strengths and in the range of 0.934 to 0.936 for the mean tensile strengths. Based on the summary statistics from test results, it as shown that variability of an exponential parameter quantifying length effect was higher in No. 2 compared to Select Structural lumber.

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