SERVICEABILITY DESIGN IN LRFD FOR WOOD

The phenomenon of creep causes the deflection of a wood beam under sus tained loading to increase throughout time. Current design codes inclu de provisions to account for the time-dependent deflections of wood me mbers in bending, but these guidelines have been based, to a large ext ent, on past performance and engineering judgment only. Previous relia bility studies for wood have not explicitly considered creep effects, and consequently, questions as to the true limit-state exceedence prob abilities associated with serviceability design procedures remain unan swered. In this paper, the effect of creep deformations on the reliabi lity of single dimension-lumber wood beams is examined. A stochastic v iscoelastic stress-strain relationship is used to model the time-depen dent 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 det ermined, and resistance factors that include creep effects are suggest ed for use in a checking equation for load and resistance factor desig n.