Life-cycle Cost Analysis for the Seismic Isolation of Bridges in a Region of Low to Moderate Seismicity

A nonlinear approach for the life-cycle cost (LCC) analysis is proposed. Computational procedure for the analysis of LCC for seismically isolated bridges in a region of low to moderate seismicity is established. To compute the failure probability of critical structural components, most probable failure modes for the structure-isolator system are defined as unseating failure of a superstructure, local shear failure of an isolator, and damage of a pier. Multi-level damage state for a pier is introduced according to the level and type of visual damages and the level of corresponding damage result. Since the relationship between the damage states and the damage index of a pier structure virtually does not exist, or has not yet been established, especially for a region of low to moderate seismicity, a correlation between the damage index and the damage state of a pier structure is established by performing quasi-static cyclic loading test. The probability that a certain level of damage state occurs at a pier is calculated from nonlinear analyses for thousands of artificially generated earthquake records. Damage probability matrix is then constructed for a specific combination of seismic conditions such as acceleration and site condition specified in design codes. The proposed procedure has been applied to the optimal design of seismically isolated bridge based on the minimum LCC. It has also been used for investigating the effect of using different return period of design earthquakes, as is suggested in the NEHRP Provisions (1998). Detailed examples will be presented in the paper.