Load duration effects in wood members and connections: Order statistics and critical loads

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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 two 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. Other recent studies have focused on evaluating analogous load-time effects in mechanical connections in wood. While the mechanisms in connections are recognized as different from those in wood members, the treatment of time effects in design are (at least at the present time) similar. With the tendency toward engineered design of wood structures subject to natural hazards loadings, such as wind and seismic loads, load duration effects in both the primary framing members and the structural connections may be of particular importance. The evolution of the new LRFD standard for wood provides a good opportunity to re-visit 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 some recent work in (a) a simplified approach to cumulative damage analysis using order statistics, and (b) load-time effects in simple wood connections. It is shown that reliability analyses including DOL behavior can be performed more simply (efficiently) using an order statistics approach or even a simple FORM analysis. Thus, it may be possible to evaluate load duration factors for design without having to perform complex stochastic cumulative damage analyses. © 2002 Elsevier Science Ltd. All rights reserved.

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Structural Safety