Extrados strengthening of single-leaf vaults against seismic actions

Single-leaf vaults are acknowledged among the most vulnerable components of historical masonry constructions with respect to earthquake loads, particularly when featuring large span to thickness ratios, as in the case of single leaf covering the main nave of churches. These elements often require structural strengthening against seismic actions. In this paper, two different extradostechniques are tested: lightweight plywood restraining elements and FRP laminates embedded in a lime mortar layer. The techniques are tested on single leaf vaults having a very unfavorable span to thickness ratio. A previous study on less slender vaults, showed that lightweight plywood centerings, applying passive confinement to the vault extrados, inhibit the onset of the typical four-hinges failure mechanism. This lightweight, dry solution can be easily prefabricated, transferred and assembled at the construction site. The technique is reversible and fully compliant with the major preservation principles. FRP is also effective against the onset of the failure mechanism but entails larger deformations of the retrofitted vault, which may be detrimental in the case of possible decorations. The solution requires special man labor to ensure correct smoothening and cleaning of the vault extrados and to trigger effective bond between the mortar and the vault extrados. Both solutions are shown to enable small relative displacements of the vault springing, which may follow the deformation of possible internal ties. The effectiveness of these retrofit techniques was comparatively verified through experimental tests on single-leaf barrel vault stripes at 1:2 scale subjected to cyclic distributed unsymmetrical loads and through comparison with the seismic response of a reference unreinforced single-leaf vault.