Nonlinear Seismic Ground Response Analysis And Implications For Seismic Codes Implementation

Ground response analysis are used to predict ground surface motions for development of design response spectra and determine the earthquake-induced forces that can lead to instability of earth and earth-retaining structures. Evaluation of ground response is typically important and also one of the most commonly encountered problems in geotechnical earthquake engineering. The nonlinear soil characteristics have received much attention since the soil nonlinearity under strong motions was found. Construction of soil constitutive models uses basic principles of mechanics to reproduce the soil behavior under general initial stress conditions. The penalty for this increased versatility comes in the form of increased complexity, and increased computational efforts and computational instability, even increased number of model parameters, some of which are difficult to determine when incorporated into seismic ground response analyses and the computation is expensive. As a result, attempts have been made to derive a simple, straightforward and feasible new model to describe the nonlinear stress strain relationship. In addition, with dramatic development of computer technology, numerical studies with computer codes is playing increasingly significant roles in various fields of geotechnical earthquake engineering to understand the basic concepts and to model the real situations under earthquake loadings. But nonlinear seismic ground response analysis is seldom used in practice by non-expert users because parameter selection and code usage protocols are often unclear and poorly documented, the effects of parametric variability on the analysis results are unknown, and the benefits of nonlinear analyses relative to equivalent-linear analyses are often un-quantified. Thus, the development of reliable and dependable documented parameter selection for the numerical simulation is needed in practice. Moreover, it should be considered that in the Italian territory, especially in central and southern Italy, many historical centres are founded on site characterised by shear wave velocity inversion. This shear wave velocity inversion can determine a complex dynamic response of the soil column significantly affecting the in-depth and at surface ground motion. This response may be of practical interest in many geotechnical earthquake engineering problems such as the design of pile foundations and of critical buried structures.Based on the research carried out by Prof. Giuseppe Lanzo and Dr. Alessandro Pagliaroli, some further work has been accomplished with respect to nonlinear seismic ground response analysis and implications for seismic codes implementation. The main work involved in this thesis is:Firstly, Literature review, previous research on nonlinear seismic ground response analysis and the main methods to estimating ground response are summarized. Theory of wave propagation is also presented again, then the equation of motion of three dimensional wave propagation is re-derived.Secondly, the thesis focuses on the seismic ground response analysis. Relative contents are including: specification of input motion which describe how the control motion is applied as the outcrop motion or within motion correspond to the rigid base and elastic base; soil amplification, which represents the characteristics of the soil itself, and it is independent of any earthquake; time domain and frequency domain analysis in company with the nonlinear and equivalent linear method; viscous Rayleigh damping ratio which consisting of three type of formulations.Thirdly, stable numerical integration techniques and realistic constitutive models of soil are the two important and necessary components of nonlinear seismic ground response analyses. Study in this thesis is only involved in nonlinear constitutive models of soil. NHT model is new and first attempt to model nonlinear stress-strain behavior of soil based on the hyperbolic tangent function. The backbone (stress-strain) curve and unloading, reloading curves are all conducted according to the basic standard hyperbolic tangent function. It is indicated that the NHT model can exhibit the soil dynamics. Moreover, NHT model is expressed in the simple form, in which the physical meaning is clear and straightforward. The unloading and reloading can start at the point even without standing in the backbone curve, and the only thing need to do for obtaining hysteresis loop is determination of shear strain and shear stress values at the beginning of unloading and reloading.In addition, the implications for seismic codes implementation are presented. Application of five leading common used computer codes in ground seismic response is handled by creating simple homogeneous soil model. Exact solutions for body wave propagation through soil medium are carried out. The major emphasis of the research is to reduce confusions in the usage of procedures in practice and create relative clear guidelines so that the numerical analysis problem can be handled for those even non-expert users at currently available common used computer facilities. Furthermore,shear wave velocity inversion can determine a complex dynamic response of the soil column significantly affecting the in-depth and at surface ground motion. This response may be of practical interest in many geotechnical earthquake engineering problems such as the design of pile foundations and of critical buried structures. Considering the fact that in the Italian territory, especially in central and southern Italy, many historical centres are founded on large soft rock slabs overlying more deformable clay deposits, a two-layer system was established for seismic response of soil columns characterised by shear wave velocity inversion.Last but not least, the recommendations and some further efforts which can be considered in the future research are summarized.