In time-dependent reliability analysis, one is concerned with system reliability over a specified time period during which external loads can occur more than once. Therefore, not only the intensity or magnitude of load is important but also the number or frequency of load occurrences is an important parameter.
Over an anticipated service period, the characteristics of load to be imposed on the system could change. For example, when a watershed undergoes a progressive change, it could induce time dependence in load. More specifically, the magnitude of floods could increase as urbanization progresses, and sediment discharge from overland erosion and non-point-source pollution could decrease over time if the farming and irrigation practices in the watershed involve pollution control measures. Again, characterization of the time-varying nature of load intensity requires extensive monitoring, data collection, and engineering analysis.
The occurrence of load over an anticipated service period can be classified into two cases (Kapur and Lamberson, 1977): (1) The number and time of occurrence are known, and (2) the number and time of occurrences are random. Section 4.7.4 presents probabilistic models for describing the occurrence and intensity of load.
