32 Y E A R B O O K 2 0 1 9 EARTHQUAKES AND THE SEISMIC STRENGTHENING OF CHURCHES ANDREA BENEDETTI FROM THE evidence of past repairs to historic structures, it is clear that churches have always been susceptible to damage from earthquakes. However, we are now able to evaluate dynamic properties and load capacity of our heritage, and many new repair techniques are available that can fulfil strength requirements without altering the identity of the monument. The study of the damages identifies some very specific vulnerabilities of these tall, high volume buildings, and suggests that one of the main problems is the lack of floor diaphragms or elements to distribute the horizontal forces across the walls. The quality of the local vernacular construction is also of great importance, affecting the bonding of masonry at corners and the boundary support to the vaults and lintels above openings, all of which are significant in allowing the structural system to perform as a stiff box. Key factors include:   low stiffness of traditional timberframed roof systems, making roofs vulnerable to any distortion of the masonry box below   deficiency of the transverse ties which connect walls (the sides of the nave for example) or which resist arch thrusts   lack of buttresses or ribs able to resist transverse forces, causing the walls to lean (out of plane overturning)   very poor shear resistance of the masonry components. In general, churches are symmetric along the main entrance to altar (east-west) axis with a consistent capacity to resist shear forces along this axis during seismic events. In the orthogonal direction (northsouth), the presence of the apse or the transept, or even the church length can induce a non-uniform seismic response which can lead to local overturning of walls or even to large collapses. The regularity of a church structure is often modified by the presence of a bell tower or steeple, which can be an integral part of the church or it may share some parts of the perimeter walls. Even if a freestanding structure, the tower can still represent a vulnerability factor for the whole church, since falling masonry can have a domino-effect, triggering a collapse in the main body of the building. All these seismic vulnerability factors are easily identified in the damages observed after the Italian earthquakes of L’Aquila in 2009 and of Emilia in 2012. More than 30 important churches were damaged in L’Aquila, while in the Emilia region more than 500 church buildings and monuments were affected. In L’Aquila for example, the church of Santa Maria Paganica had been badly damaged by the 1703 earthquake and then carefully repaired, but in 2009 its transept and dome were almost completely destroyed. A similar situation occurred in Santa Maria di Collemaggio (illustrated above), in which the four stone columns supporting the transept dome collapsed. Its front façade had been damaged in an earthquake in 1915, but in 2009 it escaped damage because it was fully attached to a scaffold used for masonry cleaning. In San Bernardino, part of the bell tower collapsed causing extensive damage to the church structures below. A similar event affected the lantern of the well-known Anime Sante church in L’Aquila (illustrated overleaf ), and special stabilisation works were necessary in order to avoid full collapse. The churches of the Emilia region are more recent and spread across a wide area. Most were built in the 18th and 19th centuries with slender clay brick masonry structures and vaults constructed in either single leaf masonry or gypsum plaster on cane. In almost all cases the roofs are timber The collapsed transept of Santa Maria di Collemaggio in L’Aquila (Photo: from the Italian Civil Protection Agency portfolio on stabilisation works after the L’Aquila earthquake)