Effects of the soil-structure interaction and seismic vertical component on the response of a concrete surge tank

The seismic safety of hydroelectric power plants is of high concern in earthquake-prone regions. Of specific interest to this paper is the seismic performance assessment of a critical item of a typical plant, which is represented by the surge tank, via dynamic analyses both using rigorous and complex models, and both recurring to different assumptions and simplifications. Dynamic time-history analysis, considering non-linearities in the soil but constraining the structure to remain in the elastic uncracked state, is recognized as a suitable analysis method for the problem at hand. In this context, the contribution focuses on a specific Italian case study with the aim of assessing the influence of two main modeling assumptions that engineers could include: (i) considering vs neglecting the soil-structure interaction phenomena; (ii) considering vs ignoring the vertical component of the ground motion. To this aim, 7 seismic inputs have been defined starting from compatibility criteria with a spectrum provided by a site-specific seismic-tectonic study. First, they have been used in a non-linear propagation analysis, to assess the waveform propagation from the bedrock up to the foundation basis. Then, 3D Finite Element (FE) analyses of an empty surge tank have been conducted to assess the effect of the dynamic soil-structure interaction on its seismic response. The effects of the vertical component of the ground motion have been investigated, thus providing some guidance on the modeling choices that should be adopted for a safe assessment of the seismic performance of such kind of structures.