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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Keywords: pile foundation; pile-supported bridge pier; foundation input motion (FIM); pile-soil kinematic interaction; soil-structure interaction

1. Introduction The assessment of the response of pile-supported bridge piers in seismic conditions is an important stage in the design and verification of these relevant structures. The common way to handle this task is to model the bridge pier as a Single Degree of Freedom (SDOF) system under fixed base assumption and to apply the horizontal component of the free-field motion, which is usually represented by the response spectrum provided in national seismic codes or by the average response spectrum derived via seismic response analyses. This procedure is simple but neglects both inertial and kinematic interaction mechanisms, while assuming a compliance base would allow, at least, to account for the inertial interaction. Nevertheless, also in the latter case the kinematic interaction is neglected, and the seismic action is still represented by the free-field motion and not by the seismic motion at the foundation level (Foundation Input Motion, FIM) as modified by the presence of the pile foundation. A more rational way to study this problem is to apply the sub-structure method (Kausel and Roesset, 1974, Kausel et al., 1978). This method, summarized in Figure 1, consists of three steps: the assessment of the FIM; the computation of the impedance functions of the soil-foundation system; the analysis of the superstructure response resting on a compliant-base at which is applied the FIM. The main limitation of the sub-structure method is that is rigorous under linear viscoelastic assumption and the analyses are commonly performed in the frequency domain.

Fig. 1. Sub-structure method

The only way to overcome this limitation is to study the entire superstructure-foundation-soil model with advanced numerical tools based on the Finite Difference Method, Finite Element Method, or Boundary Element Method. These allow to account for the effect of Soil-Structure Interaction (SSI), which, especially in the case of pile-supported structures on soft soil conditions, would be beneficial. In this contribution a specific software developed by the authors (Stacul and Squeglia, 2018, 2020, 2023), and capable to compute the FIM considering the filtering effect exerted by the piles, will be used to show the relevance of its proper assessment while studying the seismic response of pile-supported bridge piers.