Bilotta, E. et al. (2015). Ge ´otechnique 65, No. 5, 391–400 [http://dx.doi.org/10.1680/geot.SIP.15.P.016] 391 Importance of seismic site response and soil–structure interaction in dynamic behaviour of a tall building E. BILOTTA  , L. DE SANCTIS†, R. DI LAORA†, A. D’ONOFRIO  and F. SILVESTRI  A tall public building in Naples (Italy) has recently undergone a seismic vulnerability assessment, following the new Italian code requirements. The building is about 100 m high and is founded on a piled raft floating in a thick layer of soft pyroclastic and alluvial soils. On the basis of a conventional subsoil classification, the inertial seismic actions on the building would lead to expensive measures for seismic retrofitting. By contrast, if site effects and soil–structure interaction are adequately addressed the picture is completely different. First, free-field seismic response analyses highlighted the beneficial effects of a peat layer, acting as a natural damper on the propagation of shear waves. Finite- element analyses of pile–soil kinematic interaction were then carried out to define the foundation input motion, which was found not to be significantly affected. The effects of inertial interaction were evaluated accounting for soil–foundation compliance; they resulted in an increase of the structural period of vibration, while the overall damping did not change compared to that of the fixed-base structure. The increased structural period led to further reduction of spectral acceleration. The results could lead to significant impacts on the seismic assessment of slender buildings founded on piles embedded in deformable soils. KEYWORDS: dynamics; footings/foundations; piles; soil/structure interaction INTRODUCTION Soil–structure interaction (SSI) may be an important issue in the assessment of the seismic vulnerability of a building. Depending on the relative stiffness between the structure and the soil–foundation system, it is generally expected that the dynamic SSI induces a significant increase of the fundamen- tal period of the structure and an increase of damping, thus reducing the seismic demand on the structure (e.g. Veletsos & Meek, 1974). Recent studies (e.g. Han, 2002) have shown that the seismic response of a tall building supported on a pile foundation may be difficult to predict correctly, if the com- plex dynamic interaction problem is not handled with care. Neglecting such interaction, for instance by modelling the tall building as having a fixed base, cannot represent the actual seismic response, since the overall stiffness of the system is overestimated and the damping is underestimated. Equally, simplifying the problem by modelling a real pile foundation as a fictitious equivalent footing leads to no better prediction. Particularly in the case of large-diameter piles, the important contribution of the foundation system to the rocking stiffness of the building would be neglected. As a consequence, too low natural frequencies and too large displacements would be calculated. In such a case, the assessment of the seismic vulnerability of the building would be inaccurate, hence expensive and likely useless retrofitting could be undertaken to meet the seismic safety requirements. An adequate procedure to consider the soil–foundation– building interaction is based on the substructures method (Gazetas, 1984; Makris et al., 1996; Mylonakis et al., 1997) and it is implemented by subdividing the analysis into three different stages (a) assessment of the seismic input on the foundation accounting for kinematic interaction (FIM ¼ foundation input motion) (b) calculation of the dynamic impedance functions asso- ciated with vertical and horizontal translation, as well as with torsional rotation and rocking (c) analysis of the inertial interaction of the building subjected to the FIM and supported by visco-elastic springs, characterised by the impedance functions determined above. In this paper stages (a) and (b) are described in detail, with reference to a case study of a tall building on a pile founda- tion floating in a deformable subsoil (section entitled ‘Case study’). The seismic actions on the building are calculated in terms of response spectra by free-field seismic response (section entitled ‘Seismic site response’) and kinematic inter- action (section entitled ‘Foundation input motion’) analyses. The numerical calculation procedure of the (six-components) impedence matrix and the relevant modifications of the spectral ordinates due to the changes of the natural frequen- cies and the overall damping are finally assessed (section entitled ‘Influence of pile–foundation compliance’). CASE STUDY The analysed building, located in the eastern area of Naples (Italy), is a 29-storey reinforced concrete tower, with a height of 107 . 4 m, built in the early 1980s. The tower, with a stiffening core, is rigidly connected to a pile founda- tion by a reinforced concrete box structure, made up by a lower raft of thickness up to 1 m and an upper 40 cm slab, joined by vertical reinforced concrete walls 6 m high. The 82 piles are unevenly distributed on a large area of 3300 m 2 (Fig. 1); they were drilled in alluvial and volcanic soils with a length of 42 m and a diameter varying between 1800 mm and 2200 mm (Viggiani & Vinale, 1983; Mancuso et al., 1999). The reconstruction of the subsoil layering was based on the results of boreholes and cone penetration tests (CPTs) Manuscript received 1 April 2014; revised manuscript accepted 11 March 2015. Discussion on this paper closes on 1 October 2015, for further details see p. ii.  University of Napoli Federico II, Naples, Italy. † University of Napoli Parthenope, Naples, Italy. Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution