Comparative assessment of dynamic soil-structure interaction models for fragility characterisation

Dynamic soil–structure interaction (SSI), involving the coupling of structure, foundation and soil, is a crucial and challenging problem, especially when soil nonlinearity plays an important role. This paper shows the impact of adopting different SSI models on the assessment of seismic fragility functions. The substructure approach is initially adopted by implementing two models, the first of which includes only a translational elastic spring and a dashpot. The second and more refined model is a Lumped-Parameter Model (LPM) accounting for frequency dependence of the impedance. An additional approach including soil nonlinearities is also employed. A nonlinear macro-element is introduced to model the near-field behaviour by condensing the entire soil-foundation system into a single nonlinear element at the base of the superstructure. Energy dissipation through radiation damping is also accounted for. The comparison between the adopted approaches is evaluated in terms of their effects on the characterisation of fragility functions for unreinforced masonry buildings.