Lightweight extrados restraining elements for the anti-seismic retrofit of single leaf vaults

Substantial vulnerability of single-leaf vaults has repeatedly been observed in the aftermath of past earthquakes. Major vault damage or even collapse may follow the onset of mechanisms such as the indirect bending of the vault crown caused by the unconstrained rocking of the abutments, the shear failure of the vault lunettes induced by possible differential rocking of the supporting masonries, and the direct differential bending induced by the inertia forces acting as a uniformly distributed horizontal load along the vault crown. Unlike other mechanisms, which can be inhibited by traditional global retrofit interventions aimed at triggering a box-like seismic response of the existing building, limiting direct bending requires targeted measures on the vault crown. In this paper, extrados lightweight plywood restraining structures applying passive confinement actions are conceived to delay the onset of the vault direct bending failure mechanism. The reinforcement is designed as a 3-hinged arch, hinged-constrained at the springing and at the vault key section to enable small relative displacements of the vault springing, which may follow the deformation of any internal ties or roof box structure. The technique is a lightweight and dry solution that does not require specialised labour; it is reversible and minimally impairs the structure's integrity, thus respecting major restoration principles. The effectiveness of the solution is verified through an experimental study on the behaviour of a strengthened single-leaf vault, also in the case of possible relative displacements of the abutments. A special pivoting testing frame is conceived to apply cyclic, uniformly distributed inertia-like forces. The strengthened vault is shown to substantially outperform the response of an unreinforced single leaf vault, tested in a previous research study.