Frequency-domain analysis of long-span bridges subjected to nonuniform seismic motions

Nonuniform seismic motion affects the seismic behavior of long-span bridges. Three main reasons for this nonuniformity have been identified. They are local soil conditions, wave passage, and incoherency effects. Other effects, such as extended source and attenuation, are relatively small. The importance of non-uniform seismic motions, especially for sensitive and important structures, has led to the development of several methods of analysis. These methods can be subdivided into two general categories, deterministic and stochastic. Because of the inherent uncertainty of the nonuniform seismic motions, deterministic methods, mainly time integration methods, can be computationally inefficient. Stochastic methods have been based mainly on modal analysis methods. The input/output are described in terms of either power spectral density or response spectra. This paper presents a direct frequency-domain method that is based on formulating the whole soil-structure problem. The use of this direct frequency-domain method for solving nonuniform seismic support motions is shown. The application of the proposed method to a simple 2D bridge and a long-span suspension bridge is presented. Several observations were made. It is observed that nonuniform support motions may result in a large shifting of resonant frequencies of the structure. Also, large redistribution of bridge responses and internal forces were observed when using nonuniform seismic motions when compared with the uniform seismic motions case.