Research Needed on Tensile Strength of Wood

The need for additional information on the testing of the tensile strength of wood is discussed. Early studies were made on small clear specimens and standard procedures evolved. Tensile strength was found to be two to three times compressive strength and one and one-half times bending strength. Working stresses allowing tension equal to bending were considered satisfactory until subsequent development of improved fastenings, glued joints, and glue laminated construction has resulted in the design of members with higher tensile stresses than were previously developed. Tensile strength is reduced greatly by slight deviations from parallel loading, a 10 percent reduction resulting from deviation of 1:30. Local cross grain is also important. Research supports the validity of available theories but fundamental behavior is not fully understood. The effect of moisture content on tensile strength has been calculated as being exponential in form but needs experimental verification. Theoretical relationships suggest that a 1:8 slope of grain gives a 50 percent reduction in tensile strength. Shear strength of one-half the average results in 75 percent reduction in tensile value and is more important than a proportionate weakness in tension parallel or perpendicular. Slope of grain also affects MOE but to a lesser extent than it affects tension. The effect of defect location on internal stresses in structural members and the need to develop standard techniques which will simulate conditions encountered in use are discussed. Preliminary results indicate type, shape, and length of member; grip design; and control of introduced bending moments are important. Local cross grain effects need to be studied in relation to the other characteristics of the member. Basic theoretical analysis and experimental verification is needed of stress and strain in lower chords and cambered, pitched and notched beams, effects of checks and lamination thickness, and of various methods of prestressing and reinforcement of wood beams.

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