Another Look at Load Duration Effects in Wood

摘要

Load duration behavior, arising from creep rupture, is one of the most significant effects distinguishing wood materials from other structural materials. The phenomenon of creep rupture has been widely studied over the past 2 decades. Recent experimental programs have focused on duration-of-load (DOL) effects in full-size lumber and a number of different cumulative damage models have been proposed. These models have been used in reliability analyses that take into account the stochastic nature of the loading process to evaluate appropriate load duration adjustment factors for use in design. More recently, the stochastic damage accumulation process itself has been investigated. This has resulted in proposals for simplified cumulative damage analyses and the re-emergence of the ''killer pulse'' concept for load duration effects in wood. The evolution of the new LRFD standard for wood provides a good opportunity to revisit this important issue. This paper will briefly review DOL research, with particular emphasis on the work used as the basis for the time effects factors in the LRFD standard, and describe a simplified approach to cumulative damage analysis using order statistics. It is shown that reliability analyses including DOL behavior can be performed more simply (efficiently) using an order statistics approach or even a simple first order reliability method analysis. Thus, it may be possible to evaluate load duration factors for design without having to perform complex stochastic cumulative damage analyses.