Preliminary tests for consolidation treatment

edited by Francesco Piccarreta

These are the first results of the structural survey carried out on the constructed elements within the area of the cloister of the Basilica of Ss. Quattro Coronati in Rome. The investigations carried out concern the following elements: - verticals (masonry walls); - horizontals (masonry vaults and covering structures in wood) whose nature (dimension, composition, structural organization) and state of conservation has been defined.

1. Structural characteristics

1.1 Vertical wall structure

The double perimeter wall structure of the cloister has a quasi-rectangular plan with its longer side facing approximately north-south and its shorter side facing east-west.The external perimeter consists of continuous brick walls with an average thickness of 60 cm., which also extend to the upper floor. The walls are partially plastered and partially exposed. They have few apertures but numerous discontinuous elements (incorporation of pre-existing archeological structures, partially open joins from subsequent phases, etc.) and are interconnected with the wall fabric of the whole complex to which they are linked by numerous perpendicularly positioned walls on the various sides.The latter have an average measurement of about 16 m and 22.5 m, when measured on the side facing the internal perimeter.

The internal perimeter consists of a continuous base wall in brick, about 60 cm thick and 80 cm high, interrupted only at the center of the two longer sides to provide access to the garden enclosed by the cloister. The base wall supports a system of 48 coupled marble columns, 1.30 m high, positioned at intervals of about 75 cm. and divided up by four corner pilasters and six intermediate ones. Two of these are positioned at the center of the two shorter sides and two couples on the longer sides mark the two paths of access into the garden. Columns and pilasters support the small masonry arches and the overhead walls that proceed to the upper floor with a series of pilasters in brick (60 x 50 cm. in section). With reference to the side facing towards the external perimeter, the sides have an average measurement of about 10.5 m and 15 m. Unlike the external perimeter, the four internal walls are not linked to the surrounding masonry fabric (with the exception of a metal "chain" positioned half way down the east corridor and an ogival arch about 50 cm. thick, located at the north-east end of the same corridor). The double perimeter wall at ground level forms a continuous corridor whose width varies along the four sides (averaging approximately 4.20 m, 3.35 m, 3.50 m and 2.60 m along the north, east,south and west sides respectively). On the upper floor the corridor is continuous along the three sides that open onto the garden below; the fourth side (the west one) is instead closed by a continuous wall that incorporates the pilasters.

1.2 Horizontal structure

At ground level the vertical wall structure supports a system of barrel vaults that, together with the four cross vaults positioned in the corners, cover the lower corridor. On the upper floor the wall structure supports the horizontal roof covering.The latter consists of a discontinuous structure in wood made up of:

- rampant trusses (rafters/truss-rods) positioned in correspondence to the pilasters;
- horizontal purlins resting on rafters and sloping overhead beams (the latter positioned at intervals of about 30 cm.);
- lining bricks and overhead covering tiles.

1.3 Summary

The whole organization of the eight walls and masonry vaults constitutes a three-dimensional structure in which the system of 10 pilasters and 48 coupled columns present in the internal perimeter wall assumes special importance. The pilasters and columns, in particular, represent the most interesting structural element both for their very delicate form (12 cm. in diameter with a shaft of about 1 m in height) and their function (they support the vertical and horizontal forces deriving from the weight of the walls and the vaults that rest on them) and finally for their state of conservation (that varies substantially from one to the other depending on the presence of flaking and also on the number of substitutions carried out).

2. Formation of cracks

In the above-described cloister complex, cracking has been verified mainly in the system of barrel and cross vaults situated at ground level. There is an extensive network of cracks visible on the plaster layer at the intrados of the arches, which ranges from slight to marked and spreads over a fairly wide expanse. Many of these can be noted along the former conduits made for the electric wiring (now in disuse) and also where partition walls have been added (and later demolished). The other more marked and widespread cracks are:
- a longitudinal crack in the keystone extending the whole length of the west corridor with subsidiary cracks branching out towards the corner pilasters at the extremities of the system of coupled columns;
- a system of transversal diagonal cracks converging towards the point of attachment of the metal chain positioned halfway down the east corridor;
- systems of fairly numerous and widespread cracking converging on the four corners of the internal perimeter wall, as already mentioned, more conspicuous in the north-west corner. In this corner there are pronounced cracks running at an angle of 45° in the base wall. The latter, together with the marked tilt of the columns and of the wall above on the east side, represent the most extensive evidence of impairment in the vertical walls. Other not so extensive cracks in isolated areas occur along some of the small arches of the internal perimeter wall in correspondence to the keystone spreading vertically on both sides of the overhead wall.