For more than a year after the fire, it was impossible to ascertain the state of the vaults of the transept, nave and choir: scorched by flames, drenched with water, littered with the vestiges of the partly collapsed framework, the state of the vaults gave rise to considerable anxiety, so much so that no one was allowed to pass under or over them.
It took many months to secure the vaults and to assess their state of disrepair.
Consolidating the extrados (outer side) of the vaults
The work of clearing away the remains of the framework that littered the extrados of the vaults of the central nave began in the summer of 2020, while the damaged scaffolding was being dismantled. Roped to girders attached to the provisional platforms installed at the top of the side walls of the cathedral, rope workers removed fragments of beams and metal one by one. The same system enabled the architects to descend to evaluate the state of the masonry.
Once the vaults had been cleared, the rope workers, stone cutters and masons covered them with a layer of plaster reinforced with a net to act as an armature. This bound the stones together to prevent them from falling inside the edifice. From then on, it became possible to move about under the vaults without any danger in order to secure them from the inside.
Supporting the intrados (inner side) of the vaults
In the nave, the choir and the transept, the scaffolders first installed scaffolds extending from the floor to the base of the vaults. They then installed a platform on the scaffolding at a height of 27 meters.
The provisional arches made of wood and metal in the shape of the vaults were then placed on this platform. This relieved the walls of the weight of the vaults by resting it on the scaffoldings, and through them, on the floor below.
Each span of the vault is different: each arch thus had to be made to measure. Engineers and draftsmen from a framework company in Jarny en Moselle calculated the weight of the loads and modeled the structures.
Once the arches and their metal keystones had been made, they were dispatched to the work site, one half-arch at a time, and crane-lifted to the outside platforms created at the level of the oculi, the circular openings in the upper bays. To enable the arches to be introduced into the cathedral, the saddle bars (metal parts usually supporting stained glass windows) first had to be removed from the oculi, which have a diameter of 2.30 meters.
Measuring more 6 meters long, each half-arch introduced into the cathedral was positioned under the arch it was destined to support, then raised up with an original system of hydraulic jacks that enabled each half-arch to be positioned without a hitch. The half-arches were then assembled together using metal joints, and fixed to the platform. Due to the confined space, extreme precision was required. Finally, wooden lagging, planks a few centimeters thick, ensured a perfect fit between the wood and stone and thus the transfer of the load.
At Notre Dame de Paris, as in the majority of Gothic constructions, all the vaults rest on a system of concave arches with crossed diagonals known as crossed ribbing, or Gothic arches. The wooden arches correspond to this principle: they were placed two by two under each pointed arch and thus support the whole vault!
Once the weight-bearing arches had been raised into position, they were equipped with secondary arches to support the "wall" arches positioned along the bays of the cathedral.
Carpenters also attached struts to the weight-bearing arches. These lighter pieces of wood were in direct contact with the vaultings, or stone veils that constitute the vaults between the arches. They thus consolidate the arches and hold in place any stones that may be likely to fall.
Lastly, the weight-bearing arches were connected by metal tie-beams to prevent them from moving apart or sagging.
Once all the arches and struts had been positioned in August 2021, there was no longer any danger of the vault of Notre Dame collapsing: it could now be restored and the whole inner space of the cathedral could be accessed in complete safety.
Main visual ©David Bordes / RNDP