In this paper

some problems concerned in the determination of seismic fortification criteria

such as multi-level earthquake-resistant design criteria

seismic reliabilities

optimal design earthquake levels

are discussed for earthquake-resistant structures. It is suggested that the multi-level seismic design criteria should be improved so that performance-based requirements of the structure may be better satisfied. The target reliabilities are proposed to be the seismic design parameters of structures

and the oplimal degree of safety in structural design may be determined on the basis of minimum expected life-cycle cost consisting of initial cost and failure cost of the structure. The relationship between multi-level seismic design criteria and seismic reliability of the structure is discussed

and multi-level target reliabilities are adopted in order that the multi-level earthquake-resistant design criteria can be accomplished. By this operation

it is unnecessary to determine minor

moderate and major earthquakes in seismic design of the structure

so that difficulties for determining these earthquakes may be eliminated. The seismic reliability of the structure can be assessed basd on the analysis of seismic hazard at the structural site and uncertainties in structural system. Finally the cost benefit analysis method is recommended for deciding the optimal seismic fortification criteria of structures.