Effect of Span-To-Depth Ratio on Modulus of Elasticity of Douglas-Fir Joists

Pure modulus of elasticity and modulus of rigidity were measured on six Douglas-fir joists containing knots. Pure modulus of elasticity ranged from about 1,100,000 to 2,220,0.00 pounds per square inch. Values for modulus of rigidity ranged from 63,800 to 115,250 pounds per square inch but modulus of rigidity did not appear to be related to the pure modulus of elasticity of a joist. Six 2- by 6-inch joists in green condition were selected and tested. Size and distribution of knots varied in all boards; some members had only small knots while others had many large knots. Tests for modulus of elasticity were made on each joist over spans that started at 10 feet and were reduced by 1-foot increments to a span of 3 feet. Loads were imposed symmetrically at third points and again at fifth points. Tests were kept well within the elastic limit. Methods and speeds for: testing were essentially as described in ASTM Standard D198-27. Pure modulus of elasticity was calculated by solution of pairs of equations written for each span tested, while modulus of rigidity was determined by a curve-fitting procedure.

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