PSI - Issue 62

Stefano Stacul et al. / Procedia Structural Integrity 62 (2024) 617–624 Stefano Stacul and Nunziante Squeglia / Structural Integrity Procedia 00 (2019) 000 – 000

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For the OC and NC clays the shear modulus degradation ( G/G 0 ) and the damping ( D ) increase with increasing shear strain were modelled with the curves proposed by Vucetic and Dobry (1991) (considering a plasticity index of 40%) while for the Sand the upper limit of the Seed and Idriss (1970) curves was adopted. A set of seven spectrum compatible motions (target spectrum = Ground Type A spectrum for the municipality of Vicchio, Italy according to the Italian seismic code (NTC, 2018), see Figure 6a) have been selected to study the seismic response of the pile supported bridge pier via the boundary element method (BEM) based code KIN SP, developed by the authors (Stacul and Squeglia, 2018, 2020). This code can compute the FIM when studying solely the soil foundation system, or the superstructure (modelled as a SDOF) response when studying the entire simplified system (SDOF-foundation-soil). A preliminary seismic response analysis (SRA) is needed to perform the analysis with KIN SP, this means that the results of SRA in terms of nodal displacements and velocities in free-field conditions are applied as input as shown in Figure 6b. SRA in free-field was carried out in the time domain assuming a nonlinear soil response (Ramberg-Osgood constitutive law). SRA results (in terms of free-field motion at surface) obtained with the seven spectrum compatible motions have been also applied at the base of both a fixed-base SDOF (Figure 6c) and a compliant-base SDOF (Figure 6d).

Fig. 6. a) spectrum compatible motions used in the analyses, b), c), d) analysis types considered

In Figure 7a and 7b are shown the comparison between KIN SP results (in terms of maximum acceleration of the SDOF-superstructure) and those obtained according to the approaches in Figure 6c (fixed-base case) and in Figure 6d (compliant-base case). In Figure 7 the large squares in the graphs represent the average of the results obtained with the 7 spectrum compatible motions. It is possible to see that the compliant-base SDOF system can reproduce well the results obtained with a more advanced code (KIN SP), while the fixed-base SDOF system leads, in general, to an overestimation of the maximum acceleration of the superstructure compared to the other two approaches, especially in the case of NC clay soil (soft soil condition). Despite everything, also the results with the fixed-base model are quite satisfactory, nevertheless it should be remembered that this reasonable agreement is mainly justified by the fact that in all the considered approaches (KIN SP, fixed-base and compliant-base SDOF) the results of an advanced SRA were used as input.