This paper presents an efficient simulation-based technique to estimate the
system reliability of structures that experience variation in loads and re
sistance over time. To arrive at a realistic reliability estimate of such s
tructures, random process description of load variation and consideration o
f resistance degradation and periodic repair must be included. An adaptive
importance sampling methodology for estimating the time-dependent system re
liability of brittle structures is presented in this paper. In the proposed
method, a simple enumeration scheme is used to identify one complete failu
re sequence of the structural system. This defines the initial failure doma
in to start the simulation. As the simulation progresses, the initial failu
re domain is refined adaptively to include other significant failure sequen
ces and to accurately estimate the failure probability of the system. The p
roposed method removes the idealized simplifying assumptions of earlier ana
lytical methods, facilitates easy addition of the reliability analysis modu
le to structural analysis, and provides a fast and accurate estimate of thr
failure probability. Numerical examples presented to illustrate the applic
ation of the proposed method demonstrate its efficiency and promise.