Duration of Load Effects in LRFD for Wood Construction

Abstract

The wood industry is developing a load and resistance factor design (LRFD) specification for wood construction. This effort provides an opportunity to reevaluate and restructure the current bases for design with wood, and to develop a specification using concepts of probability‐based limit‐states design. Wood design must take into account the fact that wood is a natural material with large variations in mechanical properties, sensitive to the rate of application and duration of structural loads. These complications are addressed by modeling structural loads as random processes, rather than as random variables, and by postulating limit states that reflect the possibility of failure by either progressive damage accumulation or overload. Current treatment of duration of load effects in the national design specification is found to be inconsistent from a uniform reliability viewpoint. Practical LRFD criteria that are consistent with a desired reliability measure can be developed to reflect the time‐dependent nature of wood behavior.