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Publication: Forest Products Journal
Authors: Hoyle R J Jr
Reference ID: 11(6):251-254
Publish Year: 1961
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Title: A Nondestructive Test for Stiffness of Structural Lumber
Description:
The inefficiency of visual grading methods results in much stress-graded lumber being rated below its true capabilities. A case is made for the need for development of a continuous, nondestructive test method for every-piece testing of structural lumber which would permit assignment of more realistic design stresses and provide for better utilization. Such a machine has been developed by Potlatch Forests and its design and operation are described. The machine measures the stiffness (EI) of each piece in a bending test when calibrated to the size of the piece tested. Although lumber may be tested either on edge or flatwise, the flat orientation is preferred because it allows larger deflections at low fiber stresses, which are easier to measure. Studies have shown moisture variations are not important because of the balancing effects of moment of inertia (I) and modulus of elasticity (E). The reduction in on drying is approximately offset by an increase in E. Temperature effects are minor and it is suggested that a production machine be calibrated seasonally. Speed of testing does not appear to affect stiffness, which will permit rapid through-put in production. Although stiffness often is the limiting factor for certain lumber applications, bending strength is also a necessary design factor. The relationship between stiffness and bending strength for four species was studied and described as having significant correlation. A method to predict a working stress from stiffness data is suggested and shown to be conservative and provides improved values over those for conventionally graded structural lumber. Although additional development is necessary, the potential for rapidly measuring stiffness and correlating with strength for grading lumber has been demonstrated.
The inefficiency of visual grading methods results in much stress-graded lumber being rated below its true capabilities. A case is made for the need for development of a continuous, nondestructive test method for every-piece testing of structural lumber which would permit assignment of more realistic design stresses and provide for better utilization. Such a machine has been developed by Potlatch Forests and its design and operation are described. The machine measures the stiffness (EI) of each piece in a bending test when calibrated to the size of the piece tested. Although lumber may be tested either on edge or flatwise, the flat orientation is preferred because it allows larger deflections at low fiber stresses, which are easier to measure. Studies have shown moisture variations are not important because of the balancing effects of moment of inertia (I) and modulus of elasticity (E). The reduction in on drying is approximately offset by an increase in E. Temperature effects are minor and it is suggested that a production machine be calibrated seasonally. Speed of testing does not appear to affect stiffness, which will permit rapid through-put in production. Although stiffness often is the limiting factor for certain lumber applications, bending strength is also a necessary design factor. The relationship between stiffness and bending strength for four species was studied and described as having significant correlation. A method to predict a working stress from stiffness data is suggested and shown to be conservative and provides improved values over those for conventionally graded structural lumber. Although additional development is necessary, the potential for rapidly measuring stiffness and correlating with strength for grading lumber has been demonstrated.
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