NUMERICAL STUDY OF 1D AND 2D SEISMIC RESPONSE OF SEDIMENTARY BASINS: CASE STUDY OF PERSIAN GULF BRIDGE

The experience obtained from recent earthquakes has indicated the influence of local geologic conditions on the significant characteristics of strong ground motion including amplitude and frequency content. In this regard, the role of material properties of alluvial layers and the topography of the site can be significant. This paper studies the effect of mentioned parameters on the Basin of Persian Gulf Bridge, considering the complex seismic characteristics by 1D (equivalent column) and 2D analysis, using FLAC2D software based on Visco-elastic soil behavior. In addition, in order to verify the numerical modeling, the 1D analysis is conducted by FLAC2D and DEEPSOIL programs for each pier of bridge due to 18 input motions. The obtained results indicated that the local variations of topography and sediment deposit might affect the propagation of seismic waves, resulting in a significant difference in the spatial variation of ground motions. Furthermore, the results represent the significant influence of mentioned parameters on the seismic amplification patterns, predominant period and response spectra of site. As a result, in comparison of 1D and 2D results, the maximum amplification is observed in periodic bound of 0.5 to 1s at surface of sediment canyon. Moreover, in periods greater than 1s the amplification values of 1D and 2D analysis almost coincide. The obtained PGA values of both analyses more than the input motion is observed. Besides, the predominant period of 2D model has been greater than 1D model especially in the corners of basin. In comparison with the obtained response spectra of 1D and 2D models and the design spectrum of AASHTO code, it is observed that the results of AASHTO in the periods greater than 1s can be conservative. Finally, it can be concluded that the 1D results can lead to unrealistic results in design of Persian Gulf Bridge. Hence, a careful consideration of 2D model can provide more accurate results.