Reliability-Based Code Calibration for Design of Wood Members Using Load and Resistance Factor Design

Abstract

The procedures underlying the load and resistance factor design (LRFD) format for engineered wood construction combine elements of reliability analysis with the experience gained from decades of successful use of allowable stress design procedures. However, the judgments applied in much of the early development or LRFD hinged directly on the load factors and underlying load distribution assumptions in the 1993 version of the ASCE 7 load standard. Since that time, load factors in ASCE 7 have changed several times and additional information is available regarding load distributions. Assumptions regarding resistance statistics also have evolved during the past decade. For example, early reliability analyses were based on resistance distributions derived from individual data sets. Unfortunately, this approach is confounded by often significant differences that occur between data sets. This approach also neglects the self-calibrating nature of engineered wood product specifications. This paper examines the range of computed reliability indices that result from updating the assumed load statistics and assumed resistance statistics. Several load cases involving dead, live, snow, and wind load are examined. The reference structural configuration is a simple flexural member. The results show that the range of computed reliability indices is most significantly influenced by regional variations in load statistics. While these results also show slight differences in computed reliability indices from one engineered wood product to another, these differences are not judged to be significant. The results of this study will not only assist the Structural Engineering Institute Design of Engineered Wood Construction Standards Committee in their upcoming revision of ASCE 16, but can also serve as a template for other material standards committees (steel, concrete) as they update their LRFD specifications to reflect evolutionary changes in ASCE 7.