Resistance Factor for Wood in Bending or Tension

Abstract

The reliability of dimension lumber in tension or bending is examined using advanced first‐order second‐moment reliability analyses. The material strength data are obtained from the literature. The reliability index for existing designs ranges from 1.1 to 3.7 for both bending and tension. Load combinations from the American National Standards Institute (ANSI) A58.1 are considered. The load‐duration effect is included by applying the appropriate Madison curve value to the resistance, considering that load in the load combination that was at its lifetime maximum value. A load‐ and resistance‐factor design (LRFD) procedure for dimension lumber in bending or tension is then calibrated to existing design practice. Due to the wide range of reliability indices, the selection of a target reliability index is not possible at this time. Rather, a direct calibration approach is used to obtain a single resistance factor for bending or tension. The use of this resistance factor in combination with the load factors and load combinations from ANSI A58.1 is examined. For a given species, grade, and size, the reliability levels over a load combination are made more consistent. However, this direct calibration approach retains the inconsistent levels of safety, which now exist in the NDS between the various species, grades, and sizes. The technique might be useful for a first (lateral) calibration from the current National Design Specification for Wood Construction (NDS) to an LRFD‐based NDS, but future work toward more consistent safety levels would still be in order. The technique might also be useful for calibration of species where little in‐grade test data are available.