PSI - Issue 11

Mariateresa Guadagnuolo et al. / Procedia Structural Integrity 11 (2018) 444–451 Author name / Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction

Most of historical structures has deteriorated over time by natural and environmental effects, because of inadequate preservation, which is considered a fundamental issue in the cultural life of modern society. Therefore, if the actual behavior of structures is known, protective measures can be supplied. The uniqueness of the cultural patrimony requires the processes aimed at the prevention of the degradation phenomena and finalized to the structural safety. The fulfillment of the foregoing target needs a thorough understanding of the masonry building, from the constructive rules and the modifications and restorations (sometimes inadequate) suffered during the centuries to the inevitable degradation due to age and damage caused by natural disasters, such as earthquakes or landslides. Furthermore, a thorough mechanical characterization of materials and an exhaustive knowledge of masonry walls and structural layout are also needed. Such information can be currently obtained through several non-destructive in-situ tests that allow to assessing composition and integrity of masonry walls, quality and mechanical parameters of stones and mortars, location and size of cracks, voids and defects within wall structures, location, size and condition of tie rods and metal elements, state of degradation, presence of moisture, etc. Nevertheless, in spite of the recent advances in non-destructive testing (NDT) technology, at the present time, there is no single technique appropriate in all situations. Then, the careful application of complementary techniques often provides the most reliable information (Faella et al., 2012; Pérez-Gracia et al., 2013; Lignola and Manfredi, 2010; Bosiljkov et al., 2010). Furthermore, the results of the experimental investigations regarding geometrical data, the in situ evaluation of the materials mechanical characteristics, the properties of the structural elements like masonry walls, arches, vaults and floors, the complex response of the construction to different possible actions, as well as the results of eventual previous monitoring can be crucial for reliable designing interventions on ancient structures (Monaco et al., 2014; Gesualdo and Monaco, 2015). Sometimes the structural conception of the building or an unsuitable realization of retrofit interventions (Guadagnuolo et al., 2014b) is the major cause of collapse. In bell towers, the systematic observation of damage caused by recent earthquakes highlighted the high seismic vulnerability of belfries, located in their upper part. This is due to the presence of top mass and wide openings that imply slender pillars. The vulnerability consequently rises from their modest vertical-load bearing (related only to dead weight) that does not ensure stabilizing effect with respect to overturning (Gesualdo et al., 2014; Gesualdo et al., 2017). Quite frequently, bell towers are in contact with other lower structures. Customary cases are towers built as part of or next to churches, towers incorporated in various ways within the urban setting and towers built into city walls. In these structures, the presence of horizontal constraints at different heights can deeply modify the seismic behavior: limiting the slenderness, introducing localized stiffening elements and producing stress-concentrations, thus the vulnerability is greatly increased (Faella et al., 2010). Then seismic behavior of towers depends on certain specific factors: structure slenderness, degree of connection between walls, presence of adjacent structures in the lower portions (which may create horizontal constraints), presence of slender architectural elements at the top of the structure (steeples, towering gables, battlements, etc.) or in any case belfries (Giordano et al., 2008). In the Italian National “Guidelines for evaluation and mitigation of seismic risk to cultural heritage” (MIT, 2011), different simplified mechanical models (LV1) are identified for the most diffuse types of historic structures: buildings, palaces and other structures with bearing walls and horizontal diaphragms (Guadagnuolo and Paolillo, 2012), churches and other structures with large halls (De Matteis et al., 2016; Corlito et al., 2017), without intermediate diaphragms (Faella and Guadagnuolo, 2007), towers, bell towers, and other tall and slender structures (Guadagnuolo et al., 2014a). For towers propose a simplified model based on failure hypothesis due to combined axial force and bending moment. The model considers towers as cantilevers which, if loaded by lateral forces in addition to their dead loads, may be subject to crises in a generic section due to crushing in the compressed zone, after the reduction of the effective un-cracked area due to non-tensile-strength. This paper presents the restoration project and the results of seismic analysis, applying the LV1 model, carried out on a seventeen-century masonry tower in Italy, the “Corpus Domini” bell tower in Maddaloni (Italy).