Seismic Fragility Assessment of Seismically Isolated Multi Span Continuous Box Girder Bridge in Nepal Considering Local Site Effects

The seismic vulnerability of highway bridges in Nepal has received limited attention despite the country’s high seismic risk. Bridges along the Kathmandu Terai Fast Track (KTFT), a strategic expressway corridor, traverse complex geological formations, including Siwalik and Quaternary deposits, and are exposed to near-fault effects as they closely traverse and directly intersect the Main Frontal Thrust (MFT) and Main Boundary Thrust (MBT). Limited research has been conducted on how site-specific near-fault effects and complex geological conditions influence conventional PSHA-based bridge fragility assessments in Nepal. This study evaluates the impact of PSHA-based, near-fault, and site-response-modified ground motions (GMs) on the fragility of a seismically isolated RC box girder bridge using nonlinear finite element modeling. System-level and component-level fragility curves for bridge piers and lead rubber bearings (LRBs) are developed through Incremental Dynamic Analysis (IDA). Results indicate that the bridge is most vulnerable under near-fault GMs, where fragility estimates exceed those from PSHA-based and site-response-modified GMs. LRBs exhibit significantly higher fragility than bridge piers, reaching extensive and collapse damage states at lower intensities, particularly under near-fault excitations where displacement demands surpass design expectations. System-level fragility confirms that bridge failure is primarily governed by LRB fragility, highlighting that seismic isolation effectiveness depends strongly on ground motion characteristics. The study findings demonstrate that site-response analysis improves fragility estimations over empirical PSHA-based approaches, but near-fault effects introduce additional challenges. Seismic isolation design & maintenance must explicitly account for near-fault scenario, and fragility assessments should integrate detailed site-response analysis for improved accuracy in Nepal’s seismic bridge design provisions. These findings emphasize that seismic isolation design and maintenance must explicitly account for near-fault scenarios, particularly considering LRB displacement sensitivity under strong ground motions. Additionally, fragility assessments should integrate detailed site-response analysis and ground motion scaling sensitivity studies to enhance the accuracy of seismic bridge design provisions in Nepal.