Simplified seismic soil classification: the Vfz matrix

Site effect assessment studies aim at predicting the effect of seismic shaking on structures by modeling the subsoil as an oscillator coupled to another oscillator representing the construction. The resulting amplification functions and response spectra depend on so many strong assumptions and parameters that, in the standard engineering practice, simplified seismic classifications appear preferable to complex modeling procedures which can only offer an illusory better accuracy. Since stratigraphic seismic amplification is not properly related to the absolute rigidity of subsoil but to impedance contrasts, the standard simplified approaches based on the ‘average’ rigidity of subsoil in the first few meters (e.g. Vs30) can hardly be effective. Here it is proposed a simplified soil classification approach that takes into account the basic Physics of seismic amplification and its parameters, i.e. the average shear wave velocity of the cover layer, the resonance frequency and the impedance contrast between the cover and the bedrock, which we summarize as VfZ. A possible classification approach is illustrated through a set of examples.