9872342 Mahadevan The objective of this study is to develop a practical method to estimate the time-variant reliability of mechanical and structural systems. Currently used analytical methods have several simplifying assumptions regarding the loads and the resistance to deformation which lower their applicability to practical structures. The project will develop a simulation- based methodology which has advantages of robustness, ease of implementation and visualization, and the capability to model realistic situations. The methodology will be sufficiently general to include ductile and brittle structural behavior. An adaptive importance sampling simulation technique will be developed to estimate the system reliability at different instances of time. A significant failure sequence of the structure is the starting point for the adaptive simulation. This project will extend the idea of importance sampling to structures with multiple failure sequences and time-varying loads and resistance to deformation. ***