Multi-Hazard Assessment of Long-Span Bridges, Considering the Effects of Seismic and Wind Action

Abstract

This PhD research project focuses on the multi-hazard evaluation of long-span bridges and the development of novel solutions to improve the effectiveness of structural control schemes. In particular, the study investigates seismic response mitigation using passive devices such as tuned mass dampers and lead rubber bearings. The research aims to examine the response of both uncontrolled and passively controlled long-span bridges, specifically suspension and cable-stayed types, subjected to near-fault earthquake excitations and strong wind forces. A key objective is to quantify the probabilities of exceeding various performance limit states and to establish fragility functions for these structures under combined seismic and wind hazards. The project further explores the feasibility and efficiency of passive control systems in reducing structural fragility. These objectives are motivated by several core research questions. First, due to their considerable length and inherent flexibility, suspension and cable-stayed bridges are especially vulnerable to long-period and impulsive motions characteristic of near-fault earthquakes. The study seeks to understand the extent to which near-fault effects compromise the safety of such bridges and whether their fragility under these conditions is significantly greater than under ordinary ground motions. It also evaluates the effectiveness and practicality of using passive control devices, such as tuned mass dampers, to mitigate this fragility. In addition, the research addresses how the combined effects of stochastic wind and seismic loads can be integrated into fragility analysis and the implications for design practice. To investigate these questions, detailed finite element models were developed using SAP2000, MATLAB, and OpenSees. The models include a three-span concrete bridge, multiple five-span concrete bridges, a 300-meter span cable-stayed bridge in Iceland (Ölfusá River), and a 1500-meter span suspension bridge in China (Runyang River). Wind forces were simulated using stochastic processes based on the spectral representation method, while seismic input was derived from real ground motion records from past earthquakes. A novel simulation methodology was also implemented to generate over 55,000 pulse-type near-fault ground motions for comprehensive evaluation of their effects. The findings indicate that well-optimized passive control systems can significantly reduce structural responses under both seismic and wind loading. Fragility analyses confirm the effectiveness of tuned mass dampers in decreasing structural vulnerability and, in certain cases, enhancing pier moment capacity, particularly when vertical seismic components are considered. The study also identifies bridge bearings as critical elements influencing overall seismic performance, underscoring the importance of post-earthquake inspection and design strategies that facilitate their replacement. Given Iceland’s proximity to active fault lines, the results highlight the importance of accounting for near-fault effects in the seismic design and long-term resilience of critical infrastructure.