Finite element analysis of an older R/C building seismically retrofitted by a base isolation system incorporating double curved surface slider devices

Reinforced concrete (R/C) structures designed in the first half of 20th century, according with the Technical Standards of the time, highlight poor performance capacities in comparison to the requirements of the latest normative generation, especially as regards their response to seismic actions. This is generally checked in terms of strength and ductility, and often of translational stiffness too, for frame building structures [1]. In addition, a lack of redundancy is noticed in special structures, among which exhibition halls [2], arcades, platform and vaulted roofs, etc, where the number of vertical members is kept to a minimum and their cross sections are optimized in size, so as to improve the effects of geometrical slenderness and architectural elegance. This imposes to carry out careful finite element-based seismic performance assessment analyses, as well as to plan proper retrofit interventions, particularly for buildings with public use [3,4]. A representative case study belonging to this class, designed by the world-famous Italian engineer Pier Luigi Nervi, is examined in this paper. The building is the swimming pool of the Naval Academy in Leghorn, Italy, whose main hall was rebuilt in 1948, after its destruction caused by air raids during the Second World War. The distinguishing structural/architectural feature of the building is represented by the barrel vault-shaped roof, constituted by prefab R/C curved beams with smoothed-V wavy section, typical of the internationally recognized Nervi’s style. At the same time, the R/C columns and beams supporting the roof and the lateral aisles — originally designed for gravitational loads only and pursuing a minimal size philosophy with respect to the calculated stress states — have very small cross sections. Furthermore, the presence of a stairwell wing situated in eccentric position in plan determines remarkable torsion contributions to the modal response of the structure. The seismic performance assessment study is carried out via time-history analysis for the four reference seismic levels established by current Italian Standards, that is, frequent design earthquake (FDE, with 81% probability of being exceeded over the reference time period VR); serviceability design earthquake (SDE, with 50%/VR probability); basic design earthquake (BDE, with 10%/VR probability); and maximum considered earthquake (MCE, with 5%/VR probability). The input accelerograms are generated in families of seven — which is the minimum number normatively fixed to offer statistical significance to the results of a dynamic analysis, and to elaborate them in mean terms — both for the horizontal and vertical components of seismic action, from the pseudo-acceleration response spectra prescribed for the municipality of Leghorn. As required by the Italian Standards, as well as by Eurocode 8 and several other international seismic regulations, in each analysis the accelerograms are applied in groups of three simultaneous components, i.e. two horizontal orthogonal ones, plus the vertical component. The time-history analyses are initially carried out by assuming an elastic behaviour of all members of the finite element model, showing general unsafe response conditions of beams and columns beginning from the BDE level of seismic action. Based on these results, a second step of the numerical assessment is developed, where the inelastic behaviour of beams and columns is investigated after incorporating plastic hinges at their end sections. Two hinge models are adopted, i.e. lumped and fiber-type, both taking into account the influence of axial force on the hysteretic moment-rotation response, which show comparable inelastic demands. Consistently with the results obtained from the elastic model, this second step of the analysis highlights a generalized plastic demand at the BDE. Moreover, the numerical collapse of the model is reached at the MCE. In order to substantially improve the assessed poor performance of the building, as well as to avoid any intrusive intervention on the exposed structural elements, the retrofit strategy proposed in the paper consists in the installation of a base isolation system incorporating double curved surface slider (DCSS) devices at the feet of all columns and below the pool tank. For the development of the time-history analyses the behaviour of the isolators is simulated by a biaxial friction-pendulum finite element with coupled friction properties for the deformations along the two reference local axes in plan, post-slip stiffness in both directions, and “gap” (no tension)-type behaviour in vertical direction. This model proved to satisfactorily reproduce the response of the DCSS devices in previous studies of the authors [5,6], and is implemented further in the paper, so as to consider the effects of possible differential settlements of the soil underlying the building. The results of the analyses in retrofitted conditions highlight the attainment of the targeted seismic performance improvement of the structure, assessed by a fully elastic response of all columns and beams up to the MCE. The checks carried out in terms of biaxial displacements and energy balance on the tentatively selected isolators, which are the smallest type of DCSS sliders in standard production, confirm that they are adequately sized to obtain the above-mentioned design objectives.