Multi-support seismic input generation: an harmonization with code prescriptions

The seismic analysis of Submerged Floating Tunnels moored by cables is addressed with particular attention to spatial variability of the excitation. A uniformly modulated random process, whose spatial variability is governed by a single coherency function, is deemed adequate to model the multi-support seismic input for the given structure of large dimension, and moderately long periods, undergoing limited plastic deformation. A novel method to obtain response spectrum compatible accelerograms is proposed, based on the explicit expression of the median pseudo-acceleration response spectrum induced by the adopted power density function (PSD). This expression is used to identify the parameters of the PSD function that minimize the difference with the elastic response spectrum prescribed by EN 1998. The minimization process is discussed and parameters for the PSD spectra are obtained for the soil types considered by Eurocode 8. With reference to soil type C, samples of the free-field motion are then generated using a proved and theoretically sound approach. They show a satisfactory agreement with the prescribed response spectra. To model the cables, a 3 node isoparametric cable element is enriched by including hydrodynamic loading, within a numerical procedure for the dynamic time domain step-by-step analysis of non-linear discretized systems. An example of application is shown that makes reference to the bed profile of Qiandao Lake (People’s Republic of China), where a plan exists to build the first SFT prototype.