Tensile Strength of Redwood Dimension Lumber I. Relation to Grade and Working Stress

The ultimate tensile strength parallel to the grain of redwood dimension lumber was determined for 519 pieces. The lumber represented both structural and nonstructural material from Clear, Select, and Construction grades in 2 by 6 and 2 by 10 sizes, obtained from seven mills. Matched small clear specimens were also tested. The small clear specimen properties were slightly lower than in previously published results for redwood probably due to differences in sampling schemes. The average specific gravity was 0.368 (green volume and ovendry weight base), while the average green bending strength and stiffness was 6930 psi and 1,108,000 psi, and the average dry tensile strength and stiffness (13.2 percent moisture content) was 11,700 psi and 1,401,000 psi respectively. There was only a minimal increase in clear tensile strength of dry specimens over green specimens. Clear strength properties were related to grade, the most pronounced relationship being a higher density and small clear specimen strength for structural grades than for nonstructural grades. A regression analysis revealed that modulus of rupture is highly correlated with density (r = 0.81), while bending stiffness and tensile strength were poorly correlated to density (r = 0.55). The correlation between green and dry tensile strength was high (r = 0.81), as was the correlation between tensile and bending stiffness (r = 0.77). The correlations between all other small clear specimen properties tested were low. The average tensile strength of the dimension-size pieces free of defects was 5512 psi. There was a considerable range of tensile strength values in each grade, and average tensile strength differed for each grade. The clear 2 by 10 structural grade had the highest average, 5350 psi, while the construction 2 by 10 nonstructural grade had the lowest average, 2260 psi. The 2 by 6 specimens were 16 percent stronger in tension than the 2 by 10 specimens, all else being equal. This difference was attributed to the presence of a size effect. Structural grades were stronger than nonstructural grades, and the yard-grade mix was generally intermediate in tensile strength. Flatwise static modulus of elasticity for the full size specimens was less sensitive than tensile strength to differences in size and grades. For. example, the stiffness values for 2 by 6 pieces exceeded those for 2 by 10 pieces, but a “t” test indicated this difference was not significant.

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