infrastructures Article Assessment and Fragility of Byzantine Unreinforced Masonry Towers Emmanouil-Georgios Kouris 1 , Leonidas-Alexandros S. Kouris 1, * , Avraam A. Konstantinidis 1 , Chris G. Karayannis 2 and Elias C. Aifantis 1   Citation: Kouris, E.-G.; Kouris, L.-A.S.; Konstantinidis, A.A.; Karayannis, C.G.; Aifantis, E.C. Assessment and Fragility of Byzantine Unreinforced Masonry Towers. Infrastructures 2021, 6, 40. https://doi.org/10.3390/ infrastructures6030040 Academic Editor: Maria Pina Limongelli Received: 18 February 2021 Accepted: 5 March 2021 Published: 9 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Laboratory of Engineering Mechanics, School of Civil Engineering, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece; ekouris@civil.auth.gr (E.-G.K.); akonsta@civil.auth.gr (A.A.K.); mom@mom.gen.auth.gr (E.C.A.) 2 School of Civil Engineering, Democritus University of Thrace, GR-67100 Xanthi, Greece; karayan@civil.duth.gr * Correspondence: lakouris@civil.auth.gr; Tel.: +30-2310-996042 Abstract: The seismic response of five cultural heritage towers erected between the 10th and 19th century AD are investigated herein. Firstly, their architectural and modal characteristics were studied in the light of seismic events that hit the monuments. There exist several historical reports of strong earthquakes, as well as damaged structures and collapses. The limit analysis is adopted to examine the post-elastic behavior of the towers up to collapse due to out-of-plane failure. Recurrent damage modes were collected from recent earthquakes and a classification of four possible collapse mechanisms in towers and slender masonry structures is here proposed: overturning, separation of perpendicular walls, diagonal cracking, and dislocation of the belfry. A thorough examination of the towers under investigation verified the proposed damage classification. The capacity curves were derived combining the capacity curves of each of the collapse mechanisms. Damage thresholds were defined on these curves in correspondence with damage states. The studied group of structures is representative of a wider typology. A statistical approach was adopted to describe damage with seismic intensity, and vulnerability curves were generated. The results of this study will improve the understanding of the performance and the collapse mechanisms of slender masonry structures under seismic loading and provide a characterization of seismic vulnerability for the studied cultural heritage types of towers. Keywords: cultural heritage towers; campaniles; out-of-plane collapse; limit analysis; displacement spectral capacity; vulnerability 1. Introduction Unreinforced masonry (URM) structures excited by seismic actions most often form at the ultimate response state an out-of-plane partial collapse mechanism due to weak connections and in-plane cracking [1–4]. It is common knowledge that the main defect of URM walls is their low tensile strength; as a consequence, URM structures tend to create cracks perpendicular to the principal tensile stresses. This inefficiency had been known since ancient times and traditional reinforcement techniques have been developed involving timber and iron ties to mitigate it [3,5–8]. In historic structures, a number of additional critical features, such as high weight/strength ratio, insufficient connection with timber diaphragms, inhomogeneous nature of material, complex constructive stages, lack of rigid diaphragms, presence of vaulted systems, and progressive material degradation due to high compressive stresses, ageing, and environ- mental influence, dramatically increase their vulnerability. As a result of the inertial forces, the initially monolithic structure is cracked and cut into parts; these parts define the col- lapse mechanism [9,10]. The interaction among the cracked parts is often merely simple Infrastructures 2021, 6, 40. https://doi.org/10.3390/infrastructures6030040 https://www.mdpi.com/journal/infrastructures