Seismic Vulnerability Of Churches

The vulnerability of the churches is generally associated to the overturning of the main façade and the perimeter walls, as well as to the excessive unconstrained rocking of the transverse diaphragm arches.

In the churches surveyed following the 2004 earthquake, other mechanisms, such as the excessive and differential rocking of the neighbouring diaphragm arches and the differential deflection of the thin vaults, were acknowledged as further causes of the severe damages observed.

The seismic vulnerability of the churches was assessed by reference to the limit analysis approach.

Table 1 shows the vulnerability coefficients estimated for some significant case studies.

2.1 Overturning of the main façade

Overturning of the main façade was often observed during the survey campaign (Figure 1, Table 1). This collapse mechanism was caused by the seismic action of the wall and by the possible lateral seismic thrust of the roof beams. Provided that wooden roofs are usually unconstrained against lateral movements, the latter contribution to the toppling action was quite relevant in most of the analyzed churches.

The onset of the overturning mechanism is underlined by the development of cracks extending at the wall edges and progressively reducing their width from the top to the footing of the building. The cracks can be vertical or angled, depending of the degree of anchorage to the perimeter walls. However, it is worth noting that, the measured crack angle was always very small, thus the stabilizing contribution of the lateral walls was almost negligible in most cases.

2.2 Excessive rocking of the diaphragm and triumphal arches and existing tie over-tension

Excessive, unconstrained rocking of the diaphragm and triumphal arches was recognised as another source of vulnerability of many churches in the Benaco area (Table 1).

In rest condition, when only vertical loads are applied to the structure, the arch lateral thrust can be either largely or even entirely resisted by the buttress action developed by the abutments, depending on their shape and massiveness (Figure 2a, Giuriani and Gubana 1993 and 1995). As a result the possible ties are required to confine the arch thrust portion, which exceeds the buttress action. The proportion of the confining contribution mainly depends on the structure geometry and tie pretension. Conversely, in the case of rocking, the resisting mechanism significantly modifies (Figure 2b, Giuriani et al. 2007 and 2008). The crack pattern is characterized by two large horizontal cracks developing at the abutment bases and two cracks opening at the diaphragm arch springing (Figures 2b, 3 and 4). The location of the cracks depends on both the structure geometry and the local resistance.

The abutment buttress action dramatically diminishes as the crack penetration progresses. In the case

Table 1. Seismic vulnerability assessment of some churches hit by the 2004 earthquake with respect to some failure mechanisms.

Façade overturning

Façade overturning

Lateral wall overturning

Lateral wall overturning

Diaphragm arch excessive rocking

Diaphragm arch excessive rocking

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