2019 Yearbook

34 Y E A R B O O K 2 0 1 9 not possible for walls with artistic decorations. One alternative is to use metal fasteners screwed into the corner from outside the building, and it is increasingly common to use helical bars of stainless steel inserted into drilled holes. This technique is particularly suitable where the walls are plastered and frescoed because it does not require any fluid injection. Because traditional timber roof systems lack sufficient stiffness to withstand distortion of the masonry box, a rigid roof diaphragm should be formed by introducing bracings connecting the timber frames or beams. One way to achieve this is by sheathing the roof with a further layer of timber boarding which has been engineered to provide the cross- bracing required, using two crossing planks fitted with metal strips. The connection of the timber frames to the external masonry walls is a point of paramount importance, and the beam head needs to be protected against environmental degradation (illustrated above left). The internal masonry vaults normally are executed with thin layers of in folio bricks and weak mortars or even gypsum. In the earthquakes they were normally cracked or experienced local losses of bricks. In the reconstruction they can be strengthened in the upper face by adding carbon fibre reinforced polymer strips (C-FRP). This is often suitable even where the underside is decorated with decorative plasterwork or frescoes. The seismic capacity of the vaults can be significantly enhanced if reinforced ribs are added to the vault. In general, they preserve the curved shape of the vault which is the main factor of the large geometric resistance of the curved shells (illustrated above right). As a summary, it is possible to compile a checklist of the necessary interventions, starting from the most effective and least costly, and ending with the most extensive and most costly. Typically, these might include: • strengthening the roof to create a rigid diaphragm and adding ties – this protects the walls from out of plane overturning • strengthening cracked vaults with connected ribs and perimeter ties – this protects the walls from vault thrusts causing flexural failure, and allocates more geometric strength to the vaults themselves • enhancing the grip at the corner and tee connections of the masonry box – this can improve the 3D stiffness of the building significantly • reinforcing openings with perimeter steel or timber frames – this can avoid local masonry edge failures • the execution of grout injections and transversal ties inside masonry walls with internal rubble layers – this can increase considerably the compression and shear capacity of weak masonry walls with multiple leaves • the inclusion of fibre reinforcements in mortar courses when repointing and the reconstruction of any degraded masonry sections – this can restore the continuity and effectiveness of masonry walls. Seismic events with a magnitude greater than 6.5 can result in large structural losses in church heritage. Therefore, some trade-off must be made between the need for greater structural capacity and the need to conserve the original nature of the monument. Where the artistic content of the monument is of particular importance, this aspect of its significance may be considered more important than structural originality, so significant alterations which resolve the monument’s structural drawbacks without affecting its artistic content would be desirable and appropriate. In any case, the basis of any intervention needs to be excellent communication between all those involved, including heritage and structural experts, in order to ensure that seismic mitigation measures achieve the desired level of protection in the most sympathetic manner. Today the use of building information models (BIM) can help designers to predict how the required structural additions will interact with the existing original and artistic textures, and to minimise the loss of the monument’s identity. Andrea Benedetti (andrea.benedetti@ unibo.it ) is professor of structural engineering and leader of the University of Bologna’s structural engineering laboratory (LISG). His international collaborations include research projects with UWE in Bristol and the ERA- funded SHAPE project on damage detection in bridge structures. He is author of more than 60 journal papers and 120 conference contributions, particularly on non-destructive testing techniques for concrete and masonry. Timber roof substitution of the Dosso (Ferrara) church (Photo: Andrea Benedetti) Vault strengthening with C-FRP strips and a brick reinforced rib (Photo: Andrea Benedetti)