Structural reliability evaluation considering capacity degradation over time

Due to the ageing of the existing infrastructure on a worldwide scale and to the lack of codes for the continued safety management of structures during their lifetime, it is necessary to develop mathematical tools to evaluate their structural reliability over a time interval taking into consideration the reduction in their structural capacities. In this paper, an approach to evaluating structural reliability that takes into account capacity degradation over time by means of closed mathematical expressions is proposed. Two alternative indicators of structural reliability are considered: (1) The expected number of failures over a time interval, taking into account the uncertainty due to both aleatory and epistemic uncertainties; (2) The confidence factor, λconf(t), and the confidence level, Kx(t)Kx(t), as functions of time (t)(t) are presented within a Demand-and-Capacity-Factor Design format (as used in FEMA 351). Both indicators are extended (based on the approach proposed by Cornell and collaborators) to estimate the reliability of a structure over a time interval. This reliability analysis takes into account events of different intensities (all possible events at the site) by means of environmental hazard curves. It is assumed that the structural capacity decreases linearly with time. The proposed expressions can be applied to different types of structures and can consider different conditions and levels of cumulative damage. An illustrative example is presented using a simplified model of a fixed steel jacket platform located at Campeche Bay in the Gulf of Mexico. The damage condition considered is the fatigue crack sizes at the critical joints caused by service and storm waves. The environmental hazard is represented in this example by means of exceedance rate curves of the maximum wave heights. The results obtained from the proposed expression are analyzed and compared with those obtained from numerical integration.