The effect of creep on the performance and reliability of wood structures

Current structural wood design specifications include limited provisions to account for the time-dependent strength and deformation of wood members and systems in bending. However, these guidelines have been based, to a large extent, on past performance and engineering judgement rather than on rigorous mechanics-based probabilistic analyses. In the research described herein, the effect of creep on the reliability of wood members and systems of repetitively-used wood members is examined for serviceability and strength limit states. A method for including creep behavior in an analysis of structural reliability is presented. Renewal pulse process models, which can simulate intensity, duration, and frequency of occurrence, are used to represent static structural loads. The time-dependent material response of wood is represented by a mechanical analog mathematical model with reproduces the observed time-dependent characteristics of a viscoelastic material. Models which approximate the behavior of systems are employed. The time-dependent load and resistance models are coupled in the analysis of single beams and systems. Resistance factors and system factors for a Load and Resistance Factor Design format are recommended. The effect of creep on the ultimate strength of woods members is also examined. In the past, ultimate failure has been assumed to occur as a result of damage accumulated in response to a load history. An alternative failure criterion based on the notion of a critical energy density for the material is presented and developed herein. Using models based on the alternative failure criterion, reliability indices for single wood members are computed, and the results are compared with reliability indices computed using the damage accumulation models