Fracture At Butt Joints in Laminated Pine

On the basis of test results it is suggested that the theory of linear fracture (“brittle”) mechanics be considered in the design of structural tension members with butt-jointed laminations. The fracture postulate was tested with tension members consisting of three laminations of radiata pine. A butt joint was placed at midlength of each center lamination. The proportions of all the specimens were geometrically similar except for lamination width, which was 1.875 inch for every lamination. The average moisture content was 9 percent, the density was 32 lb. per cu.ft., and the slope of the face grain was 1 in 20 in the vicinity of the butt joint. The average modulus of rupture was 13,000 psi with a standard deviation of 3,300 psi. The initial fracture load as indicated by arch gages usually coincided with the appearance of a visible crack in the wood at the butt joint. No dominant type of crack was apparant. The increase of fracture stress with decrease in lamination thickness suggests the possibility that by the use of sufficiently thin laminations, the tension strength of laminated structural timber can be made to approach that of defect-free wood.

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