23/11/2006
While the
strengthening works were being carried out on the portico vaults, a
problem was detected in the diagonal joist of the vault belonging to
the centre of the porch. This problem had been caused by uncontrolled
water leakage from the roof over the centuries. This water contains
soluble salts that are absorbed by the stone. When this water
evaporates it increases in volume and breaks down the stone. This is a
slow process but continues until the stone has literally disappeared.
The vaults of the Gothic fan-vaulting of the portico were erected with
great structural stone arches that served as a frame for the stonework
in the arches placed between them. The ogival joists form part of these
main arches, which run diagonally across the covered space and fulfil a
fundamental structural role.
To avoid the possible destruction of the vault that would have occurred
in the mid-term, it was decided the complex job of substituting the
joint would need to be undertaken.
The technical complexity of this operation consisted of preparing and
installing a brace, or prosthesis, that would stimulate the structural
role being carried out by the damaged joist and would act as a
substitute.
This would then allow the portion of damaged joist to be removed so
that prosthesis would hold up the vault, opening it up and leaving the
arch stones uncovered.
Once the joist had been removed, the damaged arch stones were taken out
and replaced with others made from natural stone similar to the
originals.
The work was made even more complex due to the small amount of space
available, and because in the central part of the damaged joint the
vault has a carved keystone, which must be partly preserved (the carved
part) and replaced at the end of the work.
Once the five arch stones have been substituted the joist must be
replaced and the prosthesis must gradually be taken down.
Lastly, there is a check to make sure that the part of the joist that
has been replaced is functioning correctly and the prosthesis is
removed along with all the complex scaffolding necessary for carrying
out the operation.