SPECTRAL AMPLIFICATION OF GROUND-MOTION DUE TO TRIANGULAR HILL FEATURES

Abstract

Researchers, in the past, observed amplification due to topography from the ground motion recorded at the stations on different topography for various seismic events. This issue is not addressed in most of the design codes available today. To quantify this issue, an attempt has been made in the present study to determine the topographic amplification for triangular shaped hills. Researchers have carried out the study to obtain topographic amplification by different methods of analysis such as finite element method (FEM), finite difference method (FDM), boundary element method (BEM), digital elevation model (DEM), integral equation method (IEM), Aki-Larner method (ALM) etc. In the present study, FEM is used for the simulation of wave propagation. For preventing contamination of the results by reflection and refraction from the boundaries, proper boundary conditions are simulated which helped in reducing the size of the model as well. Modelling of damping, size of meshing, ground motion selection and proper boundary conditions are the main modelling challenges. Finite element modelling of 2D synthetic hill is carried out in ABAQUS. Parametric study is done on triangular shaped hill with different shape ratios of 0.25, 0.50, 0.75, 1.00, 1.50 and 2.00 with different base width, viz. 500 m, 1000 m, 1500 m and 2000 m. Parasitic boundary conditions are used at the sides of the model and viscous boundary conditions are used at the base. Maximum mesh size is considered as 1/15 of the wavelength corresponding to the highest frequency of interest in the ground motion. Damping is modelled as the frequency based Rayleigh damping. Far field ground motion suite from FEMA P695 is used for the parametric study. Due to the viscous boundaries applied at the base, acceleration time histories need to be converted to the velocity time history which is further converted to the uniform shear stress and applied at the base of the model. v Spectral amplification is then determined and generalized model is proposed for the calculation of spectral amplification for periods in different range. Generalized model was initially constant, than uniformly decreasing, than quadratic and followed by exponentially decaying trend in different time period ranges. Finally, this proposed model is validated using randomly selected 7 earthquake ground motions by comparing its spectral amplification with the proposed model.