In the months after the fire of 15 April 2019, it did not seem as though anything good could soon emerge from the badly wounded cathedral of Notre-Dame.1 The broken vaults, with a web of melted and welded scaffolding erected for the restoration of the spire hovering above them, presented a series of formidable challenges. The fire caused severe damage to the mortar and limestone of the surviving vaults, where portions suffered thermal shock from intense heat followed by quick cooling from the water used to douse the flames (Figure 1). The possible destabilization of the equilibrium between the vaults and the flying buttresses posed the threat of further collapse: Did the surviving vaults and upper walls retain enough of their compressive strength to remain in place? Would the greater weight of the vaults—which had absorbed massive amounts of water and were loaded with debris from the incinerated...

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