Time-dependent reliability of aging structures in the presence of non-stationary loads and degradation

Civil infrastructure performance and reliability may be affected by deterioration in strength or stiffness caused by service or environmental conditions or by systemic changes in load beyond the baseline conditions assumed for design. These changes should be considered when assessing a structure for its continued future reliability in service. This paper presents an improved method for evaluating time-dependent reliability of structures taking these factors into account. The method enables the impact on safety and serviceability of non-stationarity in the load and resistance deterioration processes to be assessed quantitatively. Parametric analyses show that the reliability is sensitive to the load intensity at the end of the service period, moderately sensitive to the initial and final mean occurrence rates of load events and the nature of these increases in time, and relatively insensitive to the nature of the increase in mean load intensity. A realistic time-dependent model of structural resistance is proposed and the role played by the auto-covariance in the resistance degradation process is investigated. The auto-covariance in stochastic resistance plays a significant role in time-dependent reliability assessment. Assuming that the time-dependent resistance is 'fully correlated' generally gives a reasonable estimation of structural reliability, while assuming that the resistances are statistically independent at two points in time may cause the failure probability to be overestimated. (C) 2014 Elsevier Ltd. All rights reserved.