Development of Working Stresses for Stress-Grade Lumber

This paper is a review of former lumber stress-grading, which contributed to the development of 1949 regulations concerning stress-graded lumber and structural design. Before World War II there were four basic grades designated as Sl, 52, S3, and S4, which correlated to the respective factors of 87-1/2, 75, 66-2/3, and 50 Percent. These were applied to the lumber of limit knots and other characteristics to assure that timber had 7/8, 3/4, 2/3, and 1/2 of the strength of the basic clear wood stress. During the war a 20 percent increase in working stresses was put into effect to conserve material. After the war there was a re-examination of working timber stresses by the forest products laboratories of Canada and the United States. Buildings built during the war using the increased stress code were also examined for durability. These studies resulted in a 10 percent increase in the basic stresses for bending and horizontal shear, and increases in all properties except modulus of elasticity. It was also established that wood could withstand greater stresses for a short time than it could over an extended period.

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