Improved four-level criteria for reliability assessment of timber components subjected to decay

Safety evaluation of existing timber structures is of paramount importance for making reliable and timely decisions on repair actions. Among the inspection items, biological decay is a key factor that seriously affects the structural performance of timber components. Given the unclear limits of safety grades in the existing assessment criteria and the increased reliability requirements by the latest Chinese code GB50068-2018, improved four-level criteria are proposed to update the grading rules for timber components subjected to decay. The criteria are developed by probabilistic models and limit state functions with the depth of decay as a key variable. The revised target reliability index and load partial coefficients are taken into account. The JC method and importance sampling method are applied to obtain the relationship between the reliability index and the relative depth of decay under various load conditions. The proposed four-level criteria give explicit grading rules for the safety evaluation of decayed timber components and satisfy the revised reliability requirements. It is suggested that the ratio of the decayed area to the whole cross-section ρcr is set to be 0, 5.58%, and 8.36% for main flexural components; 0, 7.40%, and 11.92% for main tension components; 0, 5.91%, and 8.61% for main compression components as limits between au and bu, bu and cu, cu and du.