NEAR-FIELD GROUND MOTIONS AND THEIR IMPLICATIONS ON SEISMIC RESPONSE OF LONG-SPAN BRIDGES

Near-field ground motions can be detrimental for long-period structures such as long-span bridges, high-rise buildings, base isolated buildings or bridges, and should be systematically considered and studied in the seismic hazard characterization of flexible structures. Until recently, however, the importance of the long-period ground motion components of the seismic response of long-span bridges was underestimated. The gradually increasing number of near-fault ground motion seismograms recorded by broadband digital strong motion instruments has recently enabled seismologists to understand and analyze the character of the near-source ground motions, and engineers to reevaluate and reconsider the design practices of long-span bridges. Despite the progress that has been accomplished, the recorded near-source strong ground motions should be complemented by analytical and numerical techniques that generate reliable synthetic ground motions appropriate for the engineering design of long-span bridges. In this direction, a modeling approach that combines (depending on the simulated frequency range) both deterministic and stochastic in nature methodologies can be employed. Alternatively, simple and reliable analytical models that adequately describe the nature of the impulsive near-fault motions both qualitatively and quantitatively may be used. Such mathematical models should be able to analytically represent empirical observations that are based on available near-field records. Furthermore, the input parameters of these models should have a clear physical meaning and be related to basic physical parameters of the fault rupture. In this paper, the authors discuss the main characteristics of the near-source ground motions, as well as their importance for the seismic response of long-span bridges. In addition, they present a simple mathematical expression for the representation of near-fault ground motions.