A new proposal for the evaluation of the amplification coefficient due to valley effects in the simplified local seismic response analyses

Summary Many evidences from in situ registrations of strong and weak motions, both on rock and soils, and from several comparisons of one-dimensional (1D) and two-dimensional (2D) numerical simulations of local seismic response along shallow and deep basins filled by soils, demonstrated that the concave shape of basins and the transformation of body waves into surface waves across the valley produce surface amplification as high as 2 or 3 times the 1D analysis predictions. Moreover, such amplifications involve a large range of periods and are associated with longer durations of seismic events; thus, these effects can strongly affect structures more than the topographic ones. Unfortunately, such phenomenon, called “valley effect”, can be estimated only case by case by means of specific 2D numerical simulations, and is quite difficult to prescribe simple rules in the body of Technical Codes or Guidelines. In the present paper, a simplified approach to predict “valley effects” has been carried out by means of 2D simple sketches of 30m depth basins filled by soils, which have been characterized by VS30 values calculated according to the dynamic soil categories proposed by the Italian building code. Results from such analyses have been compared with previous studies carried out by the authors in the Tuscany Region territory and by many researchers worldwide over the last thirty years. The main objective is to take into account the “valley effect” even in a simplified fashion by means of a “geometric coefficient” akin to the “topographic coefficient” reported by the Italian building code. Keywords: Valley effect, local seismic response, geometric coefficient