Effect of drilled holes on the bending strength of large dimension Douglas-fir lumber

Falk, R.; Devisser, D. P. ume, G.; Fridley, K.

In this study, experimental bending tests were performed on nominal 4- by 8-inch (actual 89-mm by 191-mm) lumber members to determine how a notch and holes drilled in the wide face affect edgewise bending strength. Holes were drilled at the midspan in three locations relative to the edge. The results appear to justify an allowable hole one-half the allowable knot size that is currently permitted for cedar in No. 1 Beams & Stringer grade. Furthermore, the data indicate that hole location may be as important as hole size. A 1-inch (25.4-mm) hole had about the same effect on strength as a 1-3/4-inch (44-mm) hole when the holes were 1 inch (25.4 mm) from the tension edge.