The Strengthening Intervention

Figure 8. Strengthening of the stone tomb of Cansignorio della Scala, detail of the interventions.

supports (horse's hooves) of the equestrian statue of Cansignorio by means of CFRP strips; (2) strengthening a cracked capital with hoopings in high resistance stainless steel cable.

4.1 Introduction

In parallel with the studies previously reported, and beneficiating from their outcomes, a light strengthening intervention was carried out. New structural elements were introduced as precautionary measures, e.g. by providing redundant confining systems, to collaborate with existing deteriorated elements and acting in case of sudden structural deficiency of the original material.

In general the stone tomb does not present indications of worrying structural problems. Interventions mainly consisted in hooping the monument at different levels. With reference to Figure 8, interventions included: A) hooping the base of the canopy with a stainless steel cable; B) hooping the capitals with a couple of stainless steel cables; C) repair of the junctions of existing tie beams; D) hooping the tabernacles with high resistance stainless steel cable. Local interventions consisted in: (1) binding the damaged

4.2 Interventions description

Widespread cracks were noted in the cornice above the pointed arches, and in the same arches close by their keystones (Fig. 9a, b).

Parts of the cornices tended to separate, being this however not a recent damaging process, since iron clamps of previous interventions were found. A hooping device (hooping A) consisting in a stainless steel 7 mm diameter cable, connecting 6 corner steel plates and tensioned by turnbuckles, was positioned above the cornices. The size of the steel elements was minimized in order not to be invasive with respect to the monument (Fig. 9c, d).

The existing iron tie beams, spanning between the capitals of the upper order columns and locally damaged by oxidation (Fig. 10a), were complemented with a couple of 3 mm diameter stainless steel cables (hooping B). In fact, a strengthening intervention on the original tie beams was not feasible without heavily

Figure 9. Hooping A, details.

Figure 9. Hooping A, details.

Figure 11. details.

Figure 10. Hooping B, details.

intervening on the columns capitals. Cables were fixed to the capitals through stainless steel bushes, moulded following the shape of the capitals (Fig. 10c). An even contact between steel and stone was provided by means of lead sheets.

The iron tie beams connecting the tabernacles of the fence to the spiral columns, likely positioned during the XIX c. interventions, manifested marked decay at the connection with the original copper tie beams anchored to the stone. The tie beam strengthening intervention (C) aimed at the restoration of the original elements avoiding the onset of a new oxidation process on the copper anchoring elements. Two titanium studs were placed to join the existing iron and copper tie beams (Fig. 11a, b).

The deterioration or lack of the original iron tie beams in the same tabernacles required the

Figure 12. Strengthening of the horse's hooves: a) copper bandage; b) deteriorated stone conditions; c) application of CFRP strips; d) final appearance.

introduction of new elements (high resistance stainless steel 1.6 mm diameter cables - hooping D) to restore the original layout (Fig. 11c, d). To minimize the dimensions of the clamps, the fixing methodology was tested (tensile strength of cable and connection) by means of laboratory experimental activities.

The removal of the copper "bandage" provided to the equestrian statue of Cansignorio during past interventions for the strengthening of the supports of the statue, highlighted the presence of material decay, with severe cracks and voids (Fig. 12a, b). Damaged supports were strengthened by the application of high resistance CFRP strips, subsequently covered with a plaster facing (Fig. 12c, d).

Figure 13. Strengthening of one of the stone capitals of the spiral columns: (a) damage induced by the tie beam oxidation; (b) intervention layout.
Figure 14. Above: a almost full-scale picture of the used displacement transducers; below: displacement transducer PZ2 prior (left) and after (right) the restoration intervention.

The expansion of the iron tie beams due to material oxidation caused the cracking of the capitals of the upper order columns, in some cases with severe effects (Fig. 13a). The strengthening intervention required the sealing of the crack and the positioning of hoop-ings on the capital, on 3 levels (1.6 mm diameter high resistance stainless steel cable). Purposely shaped titanium elements (Fig. 13b) were employed to allow the clamping of the cable.

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