ABSTRACT
As previously mentioned, lifelines are highly interconnected; thus the risk of failure or deviation from normal operating conditions in one lifeline system (or part of it) can affect the risk in another. In case of an earthquake, malfunction of a system’s components can result in cascading effects within the same system and other connected systems. The nature of the identified interactions, as well as the degree of interconnectedness (type and degree of coupling) is the determinant of the interdependent systems’ seismic behavior. Equation 1 describes the seismic risk (S.R.) of interdependent lifeline systems:
{S.R.interdependent} = {S.R.independent} ∗ {Interaction function} (1)
Lets us consider three lifeline networks (systems 1, 2 and 3). Each node represents the lifeline elements that compose the system. If systems 1, 2 or 3 were independent, the failure of one system’s component would influence the operability of the connected lifeline elements inside the system and the functionality of the system as a whole lifeline. In the case of systems’ dependency, it could be considered that systems 1, 2 and 3 are dependent on the functionality of others through node N. Node N represents one lifeline system element that participates in system 1 but also influences systems 2 and 3. The functionality of node N is unsatisfactory (event EN) if system 1 fails between supply node 1 and node N (event E1) or system 2 fails somewhere between its supply node 2 and node N (event E2) or systems 3 fails between supply node 3 and node N (event E3).
