Relative Reliability of Dimension Lumber in Bending

Abstract

The advanced first‐order, second‐moment method was used to examine the relative reliability of dimension lumber in bending for designs made in accordance with the National Design Specification for Wood Construction. System behavior was not included, and the load duration effect was approximated using the existing time‐strength relationship, the “Madison curve.” The species of lumber had an effect on the reliability. For a given grade and size, the reliability indices between certain species differed by more than one unit. The grade of lumber also affected the reliability. Generally, higher grade material exhibited higher reliability indices. One notable exception is No. 1 grade material, which often exhibits relatively low reliability. Increased size (depth) of dimension lumber produced a decrease in the reliability index. Considering lognormal and Weibull 2‐parameter distributions for ultimate bending stress, the reliability is greater for the lognormal distribution assumption. The difference in the reliability index can be as much as one unit.