In order to study the longitudinal seismic capacity of shield tunnels, this work applies the structural seismic vulnerability analysis, based on incremental dynamic analysis (IDA), to a shield tunnel and explores the ground motion intensity measure suitable for the shield tunnel in different site types. The failure probability of the structure at each earthquake intensity is calculated by combining the probabilistic seismic demand model with the limits on the engineering demand parameters to establish the seismic vulnerability curve of the structure. The results indicate that the peak ground velocity (PGV) is the ground motion intensity measure suitable for the longitudinal seismic performance of the shield tunnel. The site type has the most profound influence on the extent of the longitudinal damage to the shield tunnel, and site type IV is the most dangerous under an earthquake. Further, the tunnel has stronger seismic resistance in the elastoplastic stage. The low-grade bolts between the rings damage more severely than the high-grade bolts. A tunnel with either a great depth of burial or a large cross section is more dangerous than the one with either a small depth of burial or a small cross section. The risk of the axial tension-compression failure of the shield tunnel is higher than that of the horizontal transverse shear failure.
A 1/20 scale model of a Y-shaped irregular bridge was designed, and shaking-table tests were performed to simulate its failure mechanism and performance characteristics when subjected to a multidirectional strong earthquake. The results showed that the irregular bridge structure could accelerate in the horizontal direction when subjected to vertical excitation. For both unidirectional and multidirectional excitation, the acceleration response of the pier top was more significant in the transverse direction than in the longitudinal direction. For the variable-section and branching curved beams (i.e., ramp), the response to three-dimensional excitation was equivalent to the direct superposition of the responses to bidirectional excitation and single vertical excitation. With multidirectional excitation, the girder and ramp were more prone to structural collision. However, the likelihood of structural collision was not increased with multidirectional excitation than with bidirectional excitation. The displacement of the pier and beam was very large at the junction of the variable-section main beam and branching curved beam, so bearing failure and falling beams easily occurred. Multidirectional excitation generally increased the vertical acceleration response of the two piers and pier top at the irregular bridge branch, demonstrating the need to consider shock absorption and isolation in designing these locations.
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