ABSTRACT
Seismic performance assessment is one of the most important steps for the design and maintenance of a structural system subjected to hazards of strong ground motions. In seismic performance assessment, nonlinear time history analyses are widely employed using ground motions that are either selected from recorded ground motions or artificially generated for the site of interests. For building structures, uniform base excitation is often used for seismic analysis because their horizontal dimensions are typically much smaller than the wavelength of traveling ground motion waves. By contrast, cable-stayed bridges have a relatively large distances between their supports such that the input ground motions at multiple supports are inevitably different during earthquake excitation. Such variations stem from the loss of coherency, wave-passage effect, and site amplification effects, which could impact the seismic response of the structural system significantly. To investigate the influence of the asynchronous ground motions on the seismic response of long-span cable-stayed bridges, this study first proposes a ground motion simulation algorithm that generates two orthogonal horizontal seismic waves that are multi-variate and spectrum-compatible. The algorithm first calculates the power spectral density matrix producing ground motions that are compatible with the target response spectrum. Then, the orthogonal horizontal ground motions are generated using their empirical coherency function. Second, the Incheon Grand Bridge is selected and modeled for numerical investigation, which has the main span length of 800 m and the total length of 1,480 m. The estimated modal properties are verified against those available in the literature. The influence of spatially varying ground motions on the seismic response of the bridge is investigated by comparing the seismic responses of the bridge under three ground motion sets: (1) uniform support excitation (i.e. synchronous), (2) ground motions with wave passage effect (i.e. asynchronous), and (3) ground motions with wave passage and incoherency effects (i.e. asynchronous). The results confirm that the spatially varying earthquake ground motions may affect the response of the cable-stayed bridge system significantly and thus should be considered for a proper assessment of seismic responses.
