The Staircases Of Caffa And Metellino Palaces

The staircases of Caffa e Metellino palaces are made up of stone elements (pink granite of Sardinia Island) and show a particular constructive technique that allows the monolithic steps to be linked together, providing an overall behaviour among steps of a flight and among flights.

The lack of indications, in current technical building rules, about how to model and verify the static behaviour of stairs made up of monolithic stone elements rules led to perform in depth analyses.

Numerical FEM simulations through a detailed non-linear solid model of the flights of steps and an exhaustive diagnostic campaign, aimed to characterize the material mechanical parameters and to identify the presence of defects in the stone elements, were carried out. Besides that, an accurate historical investigation about the ancient building technology in case of monolithic stone stairways seemed to be very important.

3.1 Historical and technological aspects

In many ancient building manuals, technical characteristics and constructive details of the monolithic stone stairways are described. Generally, they are made up of large freestone elements. In his treatise "Wooden, stone and brick stairs" (1885), Breymann wrote "Using freestones, one may obtain very strong, durable and nice stairs and also very easy to build". In a few words, he highlighted the main features of this typology. In Figure 5, the original drawings are shown.

The section shapes of the step are various: starting from the simplier rectangular profile, up to more elegant ones, characterised by moulding, carving or special intrados outline, aiming to give a more aesthet-ical sight from below, but also to facilitate the flight construction.

The way through which the steps are laid one upon the other depends on the support typology of the step edges: they can be fixed in the masonry walls on each side or they can be simply supported by beams, walls or arches.

According to Breymann, in case of fixed supports of the edge, having at least an insertion depth equal to 10-12 cm, the step overlapping could be only necessary to hide the back of the element and ensure the laying clearance; the overlapping distance may be minimal (4-6 cm, as in Figure 5, image 466). If, on the contrary, one or both the edges of the step are free-standing, the element superposition is needed to avoid relative displacements among them. In this case, an almost 2 cm moulding has to be trimmed. The shape can be various: the profiles in Figure 5, images 464 ans 465, aim to avoid the movement of the step, by means of a of 2-3 cm backing. The same scope

Figure 5. Monolithic stone stairs: examples of step profile -reprinted from Breymann (1885).
Figure 6. Lateral view of the stairs: moulding.

may be identified in the 6 cm trim shown in Figure 5, image 467.

The geometry of the staircases in the interior of Caffa and Metellino palaces is ascribable to this last typology. However, in the examined stairs, the superposition of the shaped steps, regards elements having their edge fixed in the walls for about 20 cm (Fig. 6).

So it can be presumed that those structures, relying on both the housing in the masonry walls and the overlapping of the steps through the moulding, show a good behaviour in relation to loads applied on them.

According to what specified in the "Traité théorique et pratique de l'art de bâtir" (Rondelet, 1802), the behaviour of the open staircases, that are stairs only supported by the moulded shape of the steps, is independent from the presence of a beam propping up

Table 1. Geometry of the stone step.

Dimensions

Total height H (m) Total depth B (m) Moulding height h (m) Moulding depth b (m) Total length (m) Free span L (m)

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