Effect of shoulders on bending moment capacity of round mortise and tenon joints

Tests were conducted to determine the effect of close-fitting shoulders on the bending moment capacity of round mortise and tenon joints. Results indicate that close-fitting shoulders can substantially increase the strength of the joints and that useful estimates of the contribution of shoulders to the bending moment capacity of round mortise and tenon joints may be obtained by means of the following expression: F-s = 0.934 x D-1.66/2w x F-ns where F-s and F-ns = bending moment capacities of joints with and without shoulders, respectively (in-lb); w = distance from the longitudinal axis of the tenon to the lower edge of the stretcher (in); and D = diameter of the tenon (in). Simple working relationships between shifts in neutral axis, which result from the use of shoulders, and bending moment capacity were not found. Close fits can be obtained through the use of somewhat complex relationships, but these were not judged useful for practical design purposes.