Comparative Study on Frequency and Time Domain Analyses for Seismic Site Response Ciro Visone, Filippo Santucci de Magistris University of Molise – Structural and Geotechnical Dynamic Laboratory STReGa – via Duca degli Abruzzi – Termoli (CB) - Italy Emilio Bilotta University of Napoli – Department of Hydraulic, Geotechnical and Environmental Engineering – via Claudio, 21 – Napoli - Italy ABSTRACT In this paper, a comparative study on frequency and time domain analyses for the evaluation of the seismic response of subsoil to the earthquake shaking is presented. After some remarks on the solutions given by the linear elasticity theory for this type of problem, the use of some widespread numerical codes is illustrated and the results are compared with the available theoretical predictions. Bedrock elasticity, viscous and hysteretic damping, stress-dependency of the stiffness and nonlinear behaviour of the soil are taken into account. A series of comparisons between the results obtained by the different computer programs is shown. KEYWORDS: dynamic analyses, local site response, soil damping modelling, finite elements I NTRODUCTI ON Dynamic interaction problems involve the determination of the response of a structure placed in a seismic environment created by an earthquake or some other source such as vibrating machine foundation. Such an environment is defined in terms of free field motion prior to placement of the structure. The spatial and the temporal variation of the free field motion used as input must be such that they satisfy the equations of motions for the free field. They may be obtained from a site response analysis. Thus, a free field solution must be available before a true interaction problem can be solved (Lysmer, 1978). Numerical methods is often adopted to predict the behaviour of the geotechnical systems (e.g. retaining walls, pile foundations, embankments, dams) under seismic loadings, both for scientific and practical applications. Dynamic finite element analyses can be considered one of the most complete available tools in geotechnical earthquake engineering for their capabilities to provide indications on the soil stress distribution and deformation/displacements and on the forces acting on the structural elements that interact with the ground (PIANC, 2001). However, they require at least a proper soil constitutive model, an adequate soil characterization by means of in situ and laboratory tests and a proper definition of the seismic input. The response of a finite element model is also conditioned by the setting of several parameters influencing the sources of energy dissipation in time-domain analyses. The amount of