Tensile Strength of AITC 302-24 Grade Tension Laminations

Distributional characteristics of tensile strength for laminating grades of lumber in the United States are necessary for the reliability based design of glued laminated timbers (glulam). The most important of the laminating grades is the American Institute of Timber Construction visually graded tension lamination grade called 302-24. This study was conducted to provide tensile strength distributional characteristics of 302-24 tension laminations, which can be used as input for reliability based design models and as baseline criteria for alternated grades and products that might serve as substitutes for the 302-24 grade. A total of 691 on-grade 2 by 6’s, 12 feet long, were tested in tension parallel to grain. Included were 362 Douglas-fir/larch, 219 southern pine, and 110 hem-fir specimens. No end joints were used. Testing followed standard procedures with the exception that the specimens were tested to failure in an average time of 2 minutes. Three-parameter Weibull distributional estimates are reported because they fit quite well the tensile strength and modulus of elasticity for all the species. There were no significant differences between on-grade and as-graded samples. The resulting tensile strength of the Douglas-fir/larch specimens was in the range expected, but the tensile strengths of the southern pine and hem-fir specimens were higher than expected; at those high lumber strengths, the strength of end joints could become the controlling factor. The current near minimum level of 4,000 pounds per square inch used as a criterion for the acceptance of proposed alternate tension lamination grades or products was substantiated.

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