Regional Seismic Fragility of Bridge Network Derived by Covariance Matrix Model of Bridge Portfolios

The consequences of seismic damage to bridge portfolios are more pronounced than those to individual bridges, leading to a greater emphasis on seismic fragility analysis of bridge networks. Previous research highlighted that a systematic analysis might yield more accurate results due to incorporating correlations among components. Consequently, this research extended an analytical framework for assessing the seismic fragility of bridge networks, taking into account the partial correlation of demand across bridges. A covariance (Cov) matrix model of bridge seismic demand based on the fundamental period ratio (Tr) was developed to capture the impact of partial correlation on seismic fragility analysis. Incorporating the covariance matrix facilitates more authentic predictions, and the Cov-Tr model delineates the relationship between structural dynamic attributes and seismic demand correlation. This framework outlines a tri-level seismic fragility analysis method for bridges, road sections, and networks. Such a conventional three-tiered network structure, point, line, and surface, was examined using graph theory. By implementing this framework on a simple bridge network, a comparison was drawn between results that account for correlation and those that disregard it. The findings suggest that omitting the effects of seismic demand correlation results in fragility median estimates of approximately 50% of those considering correlation. Furthermore, the significance ranking of road sections is influenced by this correlation.