Serviceability Design in LRFD for Wood

Abstract

The phenomenon of creep causes the deflection of a wood beam under sustained loading to increase throughout time. Current design codes include provisions to account for the time‐dependent deflections of wood members in bending, but these guidelines have been based, to a large extent, on past performance and engineering judgment only. Previous reliability studies for wood have not explicitly considered creep effects, and consequently, questions as to the true limit‐state exceedence probabilities associated with serviceability design procedures remain unanswered. In this paper, the effect of creep deformations on the reliability of single dimension‐lumber wood beams is examined. A stochastic viscoelastic stress‐strain relationship is used to model the time‐dependent behavior of wood, and stochastic pulse process models are used to account for the temporal variability of the loads. Reliability levels associated with current serviceability design procedures are then determined, and resistance factors that include creep effects are suggested for use in a checking equation for load and resistance factor design.