ABSTRACT
ABSTRACT: Japan has been exposed to many natural hazards such as typhoons, tsunamis and earthquakes. However, road networks have not been designed to protect against all such natural hazards. Moreover, even the most modern design specifications can not guarantee the absolute safety due to economic constraints. Therefore, it is necessary to develop a comprehensive disaster prevention program based on the recognition that road networks may be unavoidably damaged when very strong earthquakes occur. After earthquake disaster, road networks play important roles in rescue, evacuation activities, extinguishing fires, and disaster-relief activities. The damage of the road network is associated with severe effects on the daily life and economic activities of people. In this study, the seismic performance of road network is evaluated based on the following three factors: (a) the characteristics of road network such as existence or non-existence of detour, (b) importance of bridge such as existence or non-existence of a medical center near a particular bridge of the network, and (c) road traffic condition. The purpose of this research is to provide a framework for optimal allocation of strengthening cost for increasing the seismic performance of road networks. In this paper, an attempt is made to discuss the relationship between strengthening cost of an entire road network and safety level of this network by using Multi-Objective Genetic Algorithm (MOGA). Namely, the following two objective functions are considered: (a) strengthening cost of road network is minimized; (b) safety level of road network is maximized. There are trade-off relations among these two objective functions. For example, safety level decreases when strengthening cost decreases. Multi-objective optimization can provide a set of Pareto solutions that can not improve an objective function without making other objective functions worse. The objective functions are defined as follows:
where SC is the strengthening cost of the road network, SCi is the strengthening cost of the i-th bridge of the network, SL is safety level of the road network, Bi is the importance of the i-th bridge and Si is the safety level of the i-th bridge.
