Out-Of-Plane Strength and Stiffness of Dowel Joints

The design of furniture as an engineered structure requires methods for systematically designing the joints. Of particular concern are dowel joints loaded in flatwise bending, since neither their strength nor stiffness characteristics have been investigated and yet they are often heavily loaded in bending. Sixty dowel joint specimens of black walnut were tested in flatwise bending. The joints were assembled with 3/8-inch-diameter plain sugar maple dowels and a gap-filling urea-formaldehyde resin. Ten joints each were constructed with rails of the following thicknesses: 3/4, 1, 1-1/4, and 1-1/2 inches. Similarly, 10 joints with rails 3/4 inch thick were assembled with gaps between rail and post of 1/16 and 1/8 inch. A set of edgewise bending specimens was included in the study to provide a comparison. The strength of the joints could be predicted by the formula F4 = (D3 / 16)Su F2 (W D)/2 where F4 refers to the ultimate bending strength of the joint, pound-inches; D is the diameter of the dowel, inches; F2 is the ultimate withdrawal strength of the dowels, pounds; Su is the MOR of the material of which the dowels were constructed, and W is the thickness of the rail, inches. Values predicted by this expression agreed within 2 percent of observed test values over the range of rail widths evaluated. Joints were quite flexible in flatwise bending as compared to edgewise bending; specifically, joints with 3/4-inch-thick rails were 14 times more flexible. Joint stiffness was closely related to tightness of fit of the joint; i.e., the tighter the fit between rail and joint, the stiffer the joint.

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