Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct Discrete model for circular and square rigid tanks with concentric openings – Seismic analysis of a historic water tower Angeliki A. Zanni, Michail S. Spyridis, Dimitris L. Karabalis ⁎ Department of Civil Engineering, University of Patras, 26504 Rio, Patras, Greece ARTICLE INFO Keywords: Water tower Hollow tanks Seismic analysis Fluid–structure-soil interaction ABSTRACT The dynamic characteristics of rigid, cylindrical or square tanks with concentric openings (manholes) are studied using detailed FEM and approximate discrete models. Parametric studies are presented, in the form of practical design charts, for various geometrical configurations. Further on, the dynamic analysis of a historic water tower is attempted, where the hollow cylindrical tank and the supporting structure are modelled in detail. Comparison studies to other approximate analyses are discussed. Seismic analyses of the water tower are performed con- sidering fixed base conditions, the usual design assumption, and complete fluid–structure-soil interaction effects. The dynamic characteristics of the soil are accounted via a simplified frequency-independent mass-spring- damping model for rigid ring foundations. 1. Introduction The aim of this article is dual: (a) a rigorous parametric study of sloshing in cylindrical or square water tanks with concentric manholes and rigid walls, and, based on the above, (b) the seismic analysis of a historic water tower taking into consideration the complete fluid–- structure-soil interaction effects. To the author’s best knowledge dy- namic analyses of tanks with manholes are not available in the acces- sible literature. Elevated water towers are among the most seismically vulnerable structures because of their tall and slender load-bearing structural system and the positioning of the largest portion of their vibrating mass at a high elevation. The water tower under investigation, supporting an approximately 100tn tank, was constructed in 1930 in Florence, at Santa Maria Novella Station, and is still in use. It was designed by the famous Italian engineer Pier Luigi Nervi, introducing several innova- tions (Sorace, Terenzi and Mori [1]), in comparison to similar structures of the same era. These innovations are associated, mainly, with the geometrical and structural characteristics of the load bearing system, where reduced member sections along with high-strength concrete and increased quantities of reinforcing steel are used. In contrast to similar structures of the same era, this structure lacks inclined columns at the bottom portion of the staging as well as bracings and intermediate slabs within the structural framework. Fig. 1 shows a view of the structure and representative plans of the studied water tower reproduced mostly from Ref. [1], while several details of the actual geometry are clarified and completed with the kind assistance of Sorace [2]. The motivation for this study comes from the work of Sorace, Terenzi and Mori [1] and the particular way they model the circular water tank with the concentric manhole at the Santa Maria Novella Station. Their model is based on the discrete impulsive-convective ap- proach initiated by Housner [3] and Haroun and Housner [4] for cir- cular tanks, without manholes. In order to account for the manhole, Sorace, Terenzi and Mori [1] add some arbitrary modifications to the original model, i.e. subtracting the volume of the manhole from the total water volume and, thus, calculating an equivalent water mass corresponding to an idealized circular tank of reduced water content. Thus, the first objective of this study is to investigate the effect of the approximation introduced in Ref. [1] with regards to the dynamic characteristics of water tanks with manholes of various proportions. To this end, a parametric FEM model is developed for water tanks with manholes and rigid walls, and rigorous parametric studies are per- formed. As a byproduct of these analyses, an accurate impulsive-con- vective-type model is established in the form of a series of charts, useful for design purposes. Subsequently, the above models are used for a more accurate seismic analysis of the water tower at Santa Maria No- vella Station. Moreover, in addition to the fixed base conditions, which is the usual design hypothesis, the soil-structure interaction effect is considered along with a detailed FEM model of the filled water tank, including its unconventional geometry, and the supporting structure. Wherever possible, comparisons to the results shown in Ref. [1] are attempted and, finally, some useful conclusions are drawn. https://doi.org/10.1016/j.engstruct.2020.110433 Received 7 May 2019; Received in revised form 8 February 2020; Accepted 24 February 2020 ⁎ Corresponding author. E-mail address: karabali@upatras.gr (D.L. Karabalis). Engineering Structures 211 (2020) 110433 0141-0296/ © 2020 Elsevier Ltd. All rights reserved. T