Narrow Results

Theoretical analyses show that the velocity pulse characteristics of ground motions adversely affect structural responses of mega-sub isolation systems. This study presents an extensive shaking table test conducted to investigate the seismic responses of a mega-sub isolation system under near-fault ground motions with velocity pulses. Two steel frames were used as test specimens, representing aseismic and seismic isolation models. Two representative groups of actual ground motion records with velocity pulse characteristics were selected as inputs, along with their corresponding synthetic counterparts with the same acceleration spectrum, but without velocity pulses. Test results showed that near-fault ground motions with velocity pulses had an adverse effect on the seismic responses of the mega-sub structure system, especially on the displacement of the isolation layer in the isolation structure. Compared with the mega-sub isolation system, more nonlinear behaviors were observed in the aseismic system. Finite element analysis of the mega-sub aseismic and isolation systems was conducted by using SAP2000. Satisfactory agreement was observed between the simulation and test results, and the differences between them were discussed in detail. The obtained conclusions can provide a scientific basis and valuable reference for the seismic design and safety evaluations of mega-sub isolation systems under near-fault ground motions with velocity pulses.

The construction of high-speed railways continues to extend in seismic prone areas, but there is limited research on the seismic-induced track irregularities occurred in positions where high-speed railway bridges are involved. Taking a five-span high-speed railway simply supported beam bridge as the engineering example, the corresponding nonlinear system model, with CRTS II slab ballastless track, is established by using ANSYS finite element analysis software. The distribution mode of post-earthquake track residual irregularity is studied, the statistical characteristics of its power spectral density are analyzed, and a representative seismic-induced track geometric irregularity spectrum is constructed. The effects of the site categories and the epicenter distance on seismic-induced track spectrum are also discussed. The results show that after the earthquake, the alignment irregularity is significant, while the vertical, cross-level and gauge irregularities can be ignored. The amplitude of track geometric irregularity, caused by earthquake, increases significantly with the increase of ground motion intensity. The seismic-induced track spectrum obeys lognormal distribution at all frequencies. The track spectra almost coincide for all five site categories, thus, the impact of site categories on the seismic-induced track spectrum of high-speed railway bridges is considered negligible. Due to the pulse effect, the seismic-induced track spectrum of near-field earthquakes is significantly larger than that of far-field earthquakes.

Instability is one of the major failure modes of long span arch bridges, and its possibility of occurrence will be increased as triggered by earthquake excitations. However, the randomness of each ground motion causes the difficulty in achieving a reliable assessment of the safety of the bridges in regard to its stability issue based on certain time history analysis. Therefore, a failure probability-based instability evaluation method and corresponding instability damage index are proposed in this study to solve this problem, converting the deterministic analysis of a ground motion into a probability analysis of a group of random ground motions. The results find that the input direction, the velocity pulse and the pulse period of the ground motion have a significant impact on the stability of the bridge, while seismic moment and PGV/PGA ratio do not. The fragility curves show that the bridge has more than 60% probability of slight instability when input PGA reaches 0.2 g, 50% probability of moderate instability when input PGA reaches 0.6 g, and 20% probability of collapse when input PGA reaches 1.0 g. Moreover, when the PGA approaches 1.0 g, it is discovered that the velocity pulse and the pulse period can increase the chance of the occurrence of bridge instability by 20%–30%.