Strength of Structural Lumber Under Combined Bending and Tension Loading

Full scale southern pine structural lumber was subjected to failing loads under combined tensile and bending stress. A pre-load tensile stress was applied and held constant while bending stresses were increased to failure. Tensile loads, combined with deflections due to bending, create significant counter-moments that must be recognized in interpreting the results. The test results show that bending strength, particularly at low tensile levels, does not follow a relationship to applied tensile stress as implied by the long accepted interaction equation. The low tensile stress levels, as defined here, include common working ranges for wood roof trusses of sizes and slopes common in residential and similar light construction. Consequently, it would seem that for this particular use of structural lumber the current practice of using a very conservative value of allowable tensile stress coupled with further reduction in bending stress through the interaction equation should be reviewed, and perhaps revised, if further research warrants design modifications. The potential savings to the light frame construction industry and the potential gain in competitive advantage for wood are obvious.

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