The dynamic behaviour of the roof interventions in the Basilica San Francesco in Assisi

Abstract

<p>During the 1997 Umbria-Marche earthquake, two of the vaults of the Basilica of San Francesco in Assisi collapsed as a result of two earthquakes with 5.6 and 5.8 magnitudes. The structure had endured stronger earthquakes for centuries before these events, and there are likely many reasons for the 1997 collapse, such as damages cumulated from previous earthquakes or the retrofitting made to the structure over its lifetime. A full description and history of the different retrofit interventions of the Basilica are presented. In particular, the Basilica's roof has been subjected to three major restorations in its life. Besides, this study considers the effects of the different roof interventions on the global dynamic behavior of the structure. A simple analytical model is employed to show trends in the structure's seismic response to the 2nd shock of the 1997 earthquake given the changes in stiffness and mass of the roof over the centuries. The results of the analytical model suggest that the roof interventions have had the effect of decreasing the structure's fundamental period, and therefore, attracting more seismic loads to the façade and the transept. These were overloaded, leading to a probable nonlinear behavior that might explain the collapse of the two Gothic vaults.</p>

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“…To consider the impact of embedment in the perimeter walls and stiff abutment, the model was truncated at the base. The failure of the vaults began at a somewhat higher level, as various studies of the damage to historic buildings during seismic events have shown, as was noted in [6], effectively being embedded into support parts to offset the outward thrust [28], [29]. Thus, this approximation is frequently used in the literature to understand and forecast how masonry vaults would react to seismic activity.…”

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“…Studying damage of historic buildings in seismic events reveals that failure of vaults does not initiate at their springings, but at the key-stone zone which is essentially embedded into support elements to counteract the outward thrust [37], [38], [39]. Examples are displayed in Figure 1.…”