R E V I E W A N D A N A L Y S I S 33 structures lined with timber board. In the 2012 earthquake the general lack of ties and structural rigidity led to out of plane overturning affecting the slender walls, causing the vaults and roofs of many of them to fail. In almost all cases the bell towers suffered severe damage to the columns constituting the bell chamber. In the case of San Lorenzo (illustrated above), the bell tower was damaged by the earthquake, leaning and twisted on the main axis and the church roof vaults were in danger of collapse. Stabilisation work was therefore executed immediately. In the restoration phase that followed, the tower was dismantled, carefully recording the position of each brick and decorative element as work proceeded. Then the tower was reconstructed with a cement-free, high-strength lime mortar, and with the bricks returned to their original position. The bell chamber was strengthened with a new bell support and a new timber roof. The external columns were re-assembled and strengthened by filling the internal cavity with a reinforced grout compound. Where the structural elements of a church have failed, thorough analysis of the mechanical properties of the structural materials must be carried out before reconstruction commences. Furthermore, in-depth analysis of the dynamic behaviour should be the basis for the specification. Since the motion of a structure induced by seismic activity is dynamic not static, kinematic analysis is needed to explain the internal forces, and to make sense of any visible cracks (or ‘yield’ lines) where masonry had started to hinge and fold. A variety of strengthening techniques can be used in the reconstruction works, but some techniques can only be used where the structural elements are free from coatings of artistic value, such as frescoes, paintings and timber panelling. In any case, the missing parts need to be reconstructed using materials to match those used originally, to avoid changing the overall structural behaviour. Then, the design needs to consider the reduction or removal of the vulnerability factors, starting with the most dangerous. Where yield lines are likely to spread systematically through masonry elements, the loss of structural continuity and restraint can lead to a total loss of stiffness in the structure as a whole. Elements prone to separation and fragmentation are generally dealt with by using a system of ties and lintel beams at the crown of the masonry box, or at levels that do not affect the architecture of the monument. In this way even the arch and vault thrusts can be eliminated, increasing the resistance of the walls to overturning. Ties are normally formed with round steel bars, plates and coupler nuts (illustrated opposite). The tie end restraints require local reinforcement of the masonry around them. Lintel beams can be formed by using steel profiles running on the top of the masonry box, but they can also be designed with timber or reinforced masonry sections. In general they are positioned in the space above the last ceiling and under the roof, but for churches with parts of different height the beams can be run on different levels. However, if the beams are to be connected together to form a ring, the structure will function more effectively if all the components are in the same plane. How the beam is designed depends on the works needed to the roofs, since the upper face of the walls is usually the ideal location for constructing a beam, and this area is most easily accessed when roof coverings have been removed. A general measure which is of great importance is the enhancement of the connection at the walls’ corners. In the past, this work would have required the partial demolition of the walls and the introduction of new connection keys, which is certainly The collapsed lantern of the Anime Sante church in L’Aquila (Photo: from the Italian Civil Protection Agency portfolio on stabilisation works after the L’Aquila earthquake) The damaged bell tower of San Lorenzo and, below, one of its columns being rebuilt with a reinforced core (Photos: Andrea Benedetti)
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