Strengthening Masonry Cross Vaults Damaged by Geometric Instability

Cross vaults can be easily destabilised when their thrusts are not sufficiently contained by the stiffness of their lateral walls or systems of buttresses. A quarter-scale model from the aisles of Holyrood Abbey church in Edinburgh, which collapsed in 1768 due to excessive load from diaphragm walls that substituted the original roof trusses, demonstrated earlier the pattern of cracks that leads to failure under horizontal spread of supports. A recent model of this vault aimed to study the effects of applying Aramid fibre reinforcement against such failure exactly at the critical cracks, compared to other tests that studied arches or vaults under point load, reinforced continuously. The paper presents how the quality of certain areas of the fabric diverted failure from the longitudinal vertex merging with the detachment of the ribs, as originally observed. Moreover, the repair allowed the vault to resist 50% more spread of its supports, and failure occurred ultimately when new cracks formed in parallel to the repaired ones. The tests and repairs validate qualitative observations on crack patterns and failure of real cases and highlight the benefits and limitations when specific repairs are made instead of wholesome application of a reinforcing mesh at the extrados of vaults.