Correlation of Bending Strength and Stiffness of Southern Pine

Each of 540 pieces of southern yellow pine, 2-inch dimension stock, in 2 by 4, 2 by 6, and 2 by 10-inch sizes of No. 1 dense and No. 2 and 3 standard grades were tested in bending to determine modulus of rupture and modulus of elasticity for the purpose of correlating the two. Test pieces were 92 inches long and were tested over an 80 inch span.. All pieces were tested flat with loading at the outer fifth points. Some beams were tested with “good” face up and others with “poor” face up. Test data show a pronounced correlation between modulus of rupture and modulus of elasticity. The correlation coefficient was 0.87.5. The regression analysis established a linear relationship defined by the formula MOR=0.00619F – 1880. The 2 by 6 and 2 by 10 inch. pieces were equivalent in strength on a net section basis. Some reduction in strength was noted for the 2 by 4 inch sizes, assumed to be the effect of knots. Comparisons of testing good and poor faces in compression and tension showed no significant differences. It was assumed that unmeasurable variables were involved. Visual grading of southern pine dimension material seems to successfully separate pieces into a series of minimum value strength classes but is not efficient in defining upper limits of strength quality.

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