# An Overview of the Historical Retrofitting Interventions on Churches in Central Italy
## 3 The Main Interventions Observed in Central Italy
### Buttresses  
Among the most widely used strengthening techniques aimed at resisting and counter-balancing seismic forces in masonry buildings, buttresses are worthy of being mentioned. In fact, their use was rather common in ancient churches. These elements were conceived as massive structural counterforts and were placed at critical locations, such as the corner or mid-span of the aisle wall of the churches, to counter the lateral thrust arising from the vaulted roofing system of the nave(s) (Figure 17a). According to the \"line of thrust\" theory [73], the path of the resulting compressive forces passing within a retaining wall or an arch must lie entirely inside the wall thickness of the masonry to avoid the formation of  
Figure 15: Positioning of the tie-rods: (**a**) longitudinal (parallel to the naves); (**b**) transversal (perpendicular to the naves); (**c**) in the counter-facade (in adherence to the wall thickness). hinges; hence, the buttresses constituted one of the best solutions to provide massive local additions without increasing the area of the lateral walls (Figure 17b). These elements, already in use during the Romanesque period, progressively evolved over time, becoming indispensable in the Gothic architecture due to the increasing ambition for lighter structures, lifting the vaults higher and higher [80]. Yet, despite the advantages inherent in their use, the construction/addition of buttresses in churches with several naves did not represent a desirable solution, given the necessity for these elements to be in contact with the wall at ground level, thus obstructing the space of the aisles. Hence, a new solution consisting of \"flying buttresses\" was conceived to convey to the ground the thrusts of the vaulted ceilings of churches (Figure 18). This optimal buttress system was composed of two parts: a large external masonry pier rising from the outer wall of the aisle, and an arch element bridging the span between this pier and the upper portion of the central nave wall. Although this system was seldomly applied in the churches of Central Italy, it is possible to observe one example in the famous single nave church of Santa Chiara in Assisi. The functional efficiency of flying buttresses boosted their use as static supporting systems in many important churches of the time. Yet, their employment was less notable in ordinary churches. Figure 16: Metallic tie-rods: (**a**) transversal ties in S. Antonio church in Cascia; (**b**) longitudinal ties in S. Antonio church in Norcia. Figure 17: (**a**) Possible locations of buttresses in church; (**b**) theory of “line of thrust”. With reference to the churches of Central Italy taken into consideration within the scope of the present work, two main types of buttresses can be distinguished: the oldest ones, built by widening outwards the base of masonry walls to contrast the slope of the ground as well as prevent out-of-plane mechanisms, as observed in the church of Santa Maria Argentea in Norcia (Figure 19a); and the most common and \"recent\" ones, namely buttresses inserted with purely anti-seismic purposes, after the expansion of the existing structure. For example, this is the case of the San Francesco church in Terni, Umbria, where unconventional semi-cylindrical buttresses were built against the lateral walls following the enlargement of the church (Figure 19b), or the case of San Agostino church in Cascia, in the same region, where triangular retaining buttresses projecting from the lateral walls were probably added following the increase in height of the structure (Figure 19c), as the different masonry textures suggest. The main problem with the latter type of buttresses is their connection to the original masonry. In fact, if a buttress is not well anchored to the pre-existing masonry, in the event of facade overturning, its complete detachment could occur, making it completely ineffective.