Reliability analysis of lifeline systems under seismic action is the basic problem in the lifeline systems’ layout

design

rebuilding and optimization. Finding an efficient evaluating method for large lifeline systems’ reliability is a very urgent task in practice. Two approaches-recursive decomposition algorithm （RDA） and ordered binary decision diagrams （OBDD） algorithm

which were regarded as the most effective method of evaluating systems’ terminal-pair reliability

are described and comparied in this paper. It is known that enumerating all simple paths or minimal cutset would lead to the NP-hard problem

i.e.

the number of simple paths or cuts must be non-polynomial in the size of the network. To avoid enumerating all simple paths or minimal cutset

RDA was put forward by assumption of s-independent. By the method

disjoint simple paths or cuts of a network can be obtained directly one by one

and the reliability or the failure probability of a network can be evaluated through accumulating probability of these obtained disjoint paths or cuts. When a large lifeline system is considered

RDA can get a narrow reliability bound of the lifeline system within satisfying computer run time. OBDD is a graph-based data structure for representing Boolean functions. By network traversing with edge expansion diagram

the success-path function of a given network can be constructed based on OBDD. Then the network reliability is obtained by directly evaluating on this OBDD recursively. Although a function requires

in a worst case

a graph of size exponential in the number of arguments

many of the functions encountered in typical applications have a more reasonable representation. A dramatic improvement

as demonstrated by the experimental results for a 2-by-n lattice network

is that the number of OBDD nodes is only proportional to the number of the stages.This article consists of three parts. First

the two algorithms’ basic principles and realizing process is presented in detail

and the program of evaluating terminal-pair reliability using OBDD algorithm is written with c language. Then

20 benchmark networks collected in published articles and two large practical networks-Henan power electric system and Shanghai water supply pipeline system

are analyzed using the two methods and Monte Carlo method. Finally

according to the comparison of the examples’ results using different methods

empirical conclusions are gained for practical application. Experimental results show that both RDA and OBDD algorithm have better performance than other techniques for large lifeline systems

and especially RDA is very applicable for large networks because of its’ ability of supplying approximation of lifeline systems’ reliability.