Beam Strength as Affected By Placement of Laminae

Thick veneers, sawn from 124 bolts of slash pine (Pinus elliottii Engelm.), were kiln-dried and conditioned to an EMC of approximately 8.9 percent, planed to 1/3-inch thickness S2S, straight line jointed, and ripped to 3-inch width. Specific gravity of each veneer in its entirety was determined. The stiffness of each veneer was then measured by use of a deflection apparatus. Following stiffness measurements, 600 veneers were numbered and randomly divided into five major groups, each containing 120 veneers. The 120 veneers in each of the five major groups were then randomly assigned to five groups of 24 veneers, and following rejection of scant or broken pieces, five beams of 21 laminations each were laminated for each major group. The 25 beams so produced were then tested in bending to destruction along with five solid sawn beams obtained from bolts of the same description and history as those used to obtain the veneers. Beams in each of the five major groups differed in construction according to the following criteria, based on placement of the laminae within each beam: A) laminae arranged by specific gravity (highest S.G. placed outermost), B) laminae arranged by stiffness (stiffest placed outermost), C) laminae arranged by appearance (most knot-free placed outermost), D) laminae arranged randomly, and E) laminae arranged by stiffness as in Group B, followed by gluing of 0.05-inch-thick aluminum strips 3 inches wide to the tension and compression faces. The beams with veneers arranged by stiffness appeared to be stronger and stiffer than those made by any other arrangement. Beams with randomly arranged laminae (Group D) were significantly (0.05) weaker and less stiff than the rest. Those with laminae arranged by specific gravity (Group A) had a significantly lower MOR than did the wood cores of the Group E beams. The pertinent values of the beam groups arranged by decreasing strength, are: Group B, with laminations arranged by elastic modulus, had an average MOR of 10,200 psi and an average MOE of 1,940,100 psi. Group C, with laminations arranged by appearance, had an average MOR of 9,590 psi and an average MOE of 1,760,000 psi. Group F, a solid sawn arrangement’ had an average MOR of 9,540 psi and an average MOE of 1,830,000 psi. Group A, with laminations arranged by specific gravity, had an average MOR of 8,390 psi and an average MOE of 1,740,000 psi. Group D, randomly arranged, had an average MOR of 6,630 psi and an average MOE of 1,480,000.

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