System Reliability and Load‐Sharing Effects in Light‐Frame Wood Construction

Abstract

The wood industry is developing a load and resistance factor design (LRFD) specification for wood construction. Specification provisions are intended to apply to light‐frame as well as heavy, engineered timber construction. The current national design specification contains a repetitive use factor, which permits allowable stresses in light‐frame construction to be increased by 15% under certain conditions (no less than three members, spaced no more than 24 in. on center, joined by a floor, roof, or other load‐distributing element). This factor accounts for load sharing and partially composite action in a simple way. A factor that serves a similar purpose in LRFD can be determined on the basis of a system reliability analysis of a simple model representing a light‐frame floor system. This analysis accounts for duration‐of‐load (DOL) effects, which are known to be significant in wood construction. The analysis shows that the load sharing and DOL effects are interrelated, and that failure to include the DOL effect in system reliability analysis leads to an overly optimistic appraisal of the system effect.