Evaluation of a reduced section modulus model for determining effects of incising on bending strength and stiffness of structural lumber

Abstract

A quantitative model is presented for evaluating the effects of incising on the bending strength and stiffness of structural dimension lumber. This model is based on the premise that bending strength and stiffness are reduced when lumber is incised, and the extent of this reduction is related to the reduction in moment of inertia of the bending members. Measurements of voids and damage caused by incisor teeth showed that actual incision depth was not always equal to nominal depth. For lumber with 660 incisions/ft2, the actual depth of the incisions was approximately equal to the targeted nominal depth. However, for lumber with 800 incisions/ft2, the actual depth of the incisions was approximately 65 percent of the targeted nominal depth. The amount of damaged wood below the incision was found to extend 1.38 to 2.18 times the depth of the void left by the incising process. Using these data as input, the reduced moment of inertia model predicted lumber bending strength loss resulting from incising to within 7 percent accuracy, on average. To predict lumber bending stiffness loss resulting from incising, a complementary virtual work method was used, which resulted in predictions that were accurate to within 1 percent, on average. It was then showed that bending strength and stiffness losses can be calculated for lumber of various sizes. Currently, the National Design Specification (NDS) publishes a single reduction factor for bending strength, which is applied to all sizes of structural lumber and to all incising densities, up to the maximum allowed. This factor is overly conservative for large members and does not account for various types of incisor tooth dimensions and incision densities. The reduced section modulus model presented here would provide the much needed adjustment to current NDS incising factors.

Citation

Hernandez, Roland; Winandy, Jerrold E. 2005. Evaluation of a reduced section modulus model for determining effects of incising on bending strength and stiffness of structural lumber. Forest products journal. Vol. 55, no. 9 (2005): p. 77-83.