Effect of Joint Geometry on Strength of Finger Joints

Using Sitka spruce, Douglas-fir, and white oak, a series of finger joints was constructed with fingers perpendicular to the wide face of the test specimens. The pitch, length of fingers, tip thickness and slope were varied in making the test assembly. After gluing with resorcinol resin the varied assemblies were tested in tension parallel to the grain. Analysis of data was by multiple regression. A mathematical model for finger joint design is suggested. The major conclusions: to obtain high joint strength the fingers must be sufficiently long, the slope sufficiently low to provide an adequate glue area, and the thinner the tip, the higher the strength. Maximum stress developed in Sitka spruce joints was about 17 percent less than for the solid wood.

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