The Effect of Geometry on the Performance of Structural Finger-Joints

Summary

The author reviewed and compared the results of several studies in which the geometry of a finger-joint was related to its performance.

From this review it was concluded that there is no one best design for a structural finger-joint. There are, in fact, many possible variations, all of which will perform equally well. This flexibility is felt to be good, because it allows one to make allowances for practical manufacturing considerations and still develop a joint that will give the required performance.

In designing a finger-joint it must be kept in mind that the net section of the pieces being joined must be kept as large as possible. It is this net section that has to resist the stresses that will be applied. Secondly, the strength of a finger-joint depends on the strength in shear parallel to the grain of the pieces being joined. Since the shear strength of the wood parallel to the grain is normally one-tenth or less of the tensile strength, it is apparent that the area to be stressed in shear, or the effective glue joint area, must be 8 to 10 times at least greater than the net section in order for the joint to develop a significant proportion of the tensile strength of the wood.

Maintained at Madison, Wis., in cooperation with the University of Wisconsin.