Seismic behaviour of buttress dams: nonlinear modeling of a damaged buttress based on ARX/NARX models

The paper reports a full model identification study carried out on a scale model of a dam buttress subjected to seismic-like excitations generated by means of a shake table. The use of linear and non-linear models is discussed, since the buttress has an artificial crack and is subjected to high-intensity inputs. In particular, a suitable class of polynomial NARX models is considered, which captures most of the system dynamics. Several questions related to the NARX identification methodology are addressed in the paper; use of non-linear models greatly increases model accuracy and reliability, but many specific operational problems arise in practice. In particular, the validity of classical model selection approaches is questionable; satisfactory non-linear models are obtained in this case with many fewer parameters than suggested by conventional performance indices. Also, it is difficult to guarantee a satisfactory model performance in simulation and, sometimes, even stability is hard to obtain. With reference to specific identified models a further analysis step is carried out, which shows the evolution of the dynamic characteristics of the model in the various phases of the earthquake-like excitation. Also, the role of the particular non-linearities included in the model is discussed.