Soil Dynamics and Earthquake Engineering xxx (xxxx) xxx Please cite this article as: C. Amendola, Soil Dynamics and Earthquake Engineering, https://doi.org/10.1016/j.soildyn.2020.106523 0267-7261/© 2020 Elsevier Ltd. All rights reserved. Foundation impedance functions from full-scale soil-structure interaction tests C. Amendola a , F. de Silva b, * , A. Vratsikidis a , D. Pitilakis a , A. Anastasiadis a , F. Silvestri b a Aristotle University of Thessaloniki, Thessaloniki, Greece b Universit` a di Napoli Federico II, Napoli, Italy A R T I C L E INFO Keywords: Seismic soil-structure interaction Shallow foundations On-site tests Dynamic impedance functions ABSTRACT The paper presents and discusses the dynamic foundation impedance functions calculated from full-scale feld tests on soil-structure interaction at the prototype facility of EuroProteas at Euroseistest in Greece. The experi- mental campaign included ambient noise, free- and forced-vibration tests throughout a wide frequency and amplitude range. The response of the soil-structure system was observed to be dominated by rocking. Hence, the impedances were derived under two hypotheses, i.e. considering or neglecting the foundation swaying in the dynamic equilibrium of the soil-structure system. The trend of the back-calculated impedance at high frequencies was observed to vary depending on the interpretation model and turned out to be in satisfying agreement with the available analytical solutions. Conversely, values at resonance were found almost independent of the test type and of the interpretation model. These fndings imply that i) the currently widespread non-destructive tests (for instance ambient noise tests) can be effectively used to derive impedance functions; ii) the uncertainties are minimized for the resonance frequency traditionally used to calibrate the fexible-base models for soil-structure interaction analyses. 1. Introduction The modifcation of the dynamic behavior of existing structures founded on soft soil with respect to the typical fxed-base assumption has been recognized from records of their response under white noise ([1–3]) and weak to strong ([4–7]) motions. The rising number of temporary and permanent monitored buildings is increasing the cases in which the infuence of soil-structure interaction (SSI) has been observed and, consequently, the need of smart and effective modelling approaches. In this frame, the substructure method is the most widespread and affordable procedure. It assumes that the soil compliance is modelled by equipping the structural base with a combination of linear springs and viscous dashpots associated with the translational and rotational motion of the foundation. The spring stiffness and the dashpot constant are traditionally calibrated through the complex and frequency-dependent soil-foundation impedance functions. Analytical expressions are based on simplifed assumptions on the foundation and on the soil properties. When an existing structure is analyzed, the foundation depth may be unknown or variable, its geometry very complex ([8]) or its construction material affected by ageing ([9]), as well as the soil stiffness may have not been measured below the building, so that impedance functions can result hard to be defned analytically. Establishing protocols for a straightforward measurement of the impedance from the interpretation of records of the structural motion can be more attractive and more accurate than any analytical procedure, since no simplifed assumptions are necessary. Most studies investigating the effects of SSI from data recorded on monitored buildings focussed on the variation of the fundamental period and damping of the system (e.g. Ref. [3]), or on the rocking-induced damping ([10–12]). On the other hand, very few full-scale feld tests on instrumented structures were used to evaluate foundation impedance functions. The earliest on-site investigations available in the literature provided results either relevant to a limited range of frequency ([4,13,14]) or restricted to specifc structures, such as an accelerograph station ([15]) and a scaled nuclear power plant model ([16]). More recently, forced-vibration tests were reported on a steel frame prototype structure in California ([17]) in a frequency range limited to 5–15 Hz. These studies were limited to structures founded on soil with a shear wave * Corresponding author. E-mail addresses: chiaamen@civil.auth.gr (C. Amendola), flomena.desilva@unina.it (F. de Silva), avratsik@civil.auth.gr (A. Vratsikidis), dpitilakis@civil.auth.gr (D. Pitilakis), anas@civil.auth.gr (A. Anastasiadis), francesco.silvestri@unina.it (F. Silvestri). Contents lists available at ScienceDirect Soil Dynamics and Earthquake Engineering journal homepage: http://www.elsevier.com/locate/soildyn https://doi.org/10.1016/j.soildyn.2020.106523 Received 3 August 2020; Received in revised form 23 November 2020; Accepted 25 November 2020