Influence of Loading Methods on Rolling Shear

Calculated rolling shear stresses for plywood structures were found to exceed published working stresses by a factor between 4 and 13. Thus the objective of this study was to investigate the effect of different loading methods upon the apparent shear strength of plywood specimens and, based upon the results, to recommend a new method of determining allowable rolling shear stresses in plywood structures. Unsanded Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] sheathing grade plywood, 3/8 and 3/4 inch thick, was tested in edge, center, and fully-loaded assemblies in which the specimens were bonded to steel plates with epoxy resin. The apparent rolling shear strength of specimens stressed perpendicular to their face grain increases with a decrease in the width of the plate transferring shear to a portion of the specimen, probably due to the load-sharing capacity (beam action) of the fibers adjacent to the stressed area. The strength also increases with a decrease in plywood thickness due to a relative decrease in the bending stresses in the specimen. When specimens are stressed parallel to the grain of the face ply, the apparent rolling shear resistance is equal to the sum of the longitudinal shear resistance of the face plies and the true rolling shear resistance of the perpendicular ply between the face plies. Thus the shear strength was observed to increase when the width of the plate was decreased and when the thickness of the face plies was increased. Specimens sheared over the entire surface should provide values very close to the true rolling shear stress regardless of the grain orientation of the face ply. Stress concentrations at the ends of lap-jointed specimens can be minimized by decreasing the relative width and by flaring out the ends. Consequently, long, necked-down specimens stressed over their whole surface provide the best estimate of true rolling shear stress. True rolling shear stress may be considered as the lower limits of apparent rolling shear stress in plywood structures. Therefore, it is tentatively suggested that, contrary to accepted practice, it is not necessary to apply special safety factors to currently-published working stresses of true rolling shear in structural design.

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