PSI - Issue 44

Michele Mirra et al. / Procedia Structural Integrity 44 (2023) 1856–1863 Michele Mirra et al. / Structural Integrity Procedia 00 (2022) 000 – 000

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1. Introduction Existing or historical monumental buildings belonging to the architectural heritage of several countries, often feature masonry walls as vertical loadbearing structural components, and timber floors or roofs as horizontal elements. With reference to the Italian context, building typologies with these characteristics are very frequent, and have highlighted significant vulnerabilities from the seismic point of view, as demonstrated by several local or global collapses observed after recent earthquakes. The main causes for these failures proved to be the poor characteristics of masonry walls, the lack of adequate connections among vertical and horizontal structural components, as well as the flexibility and insufficient capability of timber floors to transfer and redistribute seismic loads. Hence, the improvement of these characteristics is crucial for preserving monumental constructions and the architectural heritage in general, by preventing as much as possible the structural damage caused by earthquakes. However, seismic retrofitting methods for such buildings should also account for their historical value, and have thus to be reversible, not invasive, and enable their architectural conservation. In this context, timber-based techniques constitute a promising and effective opportunity for reversible and seismic strengthening and restoration of exist ing buildings (Gubana 2015, Pozza et al. 2021, Mirra et al. 2021b, Mirra and Ravenshorst 2021). With reference to the improvement of the response of timber floors to earthquakes, research studies on wood-based retrofitting techniques such as the overlay of cross-laminated timber (Branco et al. 2015), oriented strand board (Gubana and Melotto 2018), or plywood panels (Peralta et al. 2004, Brignola et al. 2012, Giongo et al. 2013, Wilson et al. 2014, Mirra et al. 2020), demonstrated the excellent performance and high potential of these strengthening methods. In particular, an overlay of plywood panels fastened around their perimeter to the existing sheathing can greatly increase not only the in-plane strength and stiffness of a wooden floor, but also its energy dissipation, providing additional benefits for the whole masonry building (Mirra et al. 2021a,c,d, Mirra and Ravenshorst 2022). In this work, the application of this strengthening technique to the timber roof of an existing stone masonry church is discussed, highlighting the advantages of a seismic wood-based retrofitting, also from the professional and practical point of view. The case- study building consists of St. Andrew’s Church in Ceto (Province of Brescia, Italy), and is shown in Fig. 1. After describing the church and its vulnerabilities in detail (Section 2), the applied reversible, wood based retrofitting interventions will be discussed (Section 3). The contribution of these strengthening solutions to the seismic improvement of the building was analyzed, in agreement with the Italian Building Code (NTC 2018), by means of a numerical model, presented in Section 4. The results from the analyses are discussed in Section 5, followed by the conclusions of this study. 2. Description of the case-study building 2.1. General The Church of Ceto (Figs. 1-2), built 1708 – 26, is a stone masonry building consisting of a single nave measuring 21×14 m, with an average height of 14.5 m, and covered with a barrel vault. The apse has dimensions of 8.2×8.2 m, with an average height of 13 m, and is covered with a cross vault. The building rests on sloping rocky ground and is located on top of a retaining wall. With regard to the main structural components of the church, all walls are composed of stone masonry, reaching an average thickness of about 220 cm in the location of the buttresses, which also feature existing metal ties at the vault height. Elsewhere, the thickness of the walls varies from 70 cm to 100 cm. The roof entirely consists of wooden structural elements (spruce), arranged as follows: • 38×38 cm struts, resting on the buttresses, and connected by two 20×20 cm wooden ties (Figs. 2-3) fastened to the struts by means of a passing metal pin. Only in the location of the apse, 32×32 cm struts are present. • Primary 26×26 cm longitudinal rafters, and a 32×32 cm ridge beam; at the supports a 24×24 cm wooden wall plate is also present, resting for 75% of its thickness on the masonry walls. For the roof portion covering the apse, 24×24 cm rafters and a 28×28 cm ridge beam are present.

• Secondary 16×16 cm (15×15 cm above the apse) joists at 80 cm spacing. • Existing sheathing, having a thickness of 20 mm, covered with roof tiles.