Hooping as an Ancient Remedy for Conservation of Large Masonry Domes

ABSTRACT

The insertion of metal tie rods all around masonry domes, in order to hoop them and to absorb their horizontal thrusts, is the most ancient traditional strengthening technique for these fascinating structures. This article presents some general considerations for this primeval remedy, starting from the historical and structural analysis of the large 16th-century octagonal dome of Madonna dell’Umiltà in Pistoia (Italy). Several hooping systems had been inserted around this dome in different periods and with different techniques in order to reinforce this weak masonry structure, and their tensile stresses have been measured by means of dynamic tests. The results have shown the large differences in the tensile stresses among the different ties, allowing an understanding of the real contribution of each hooping systems. These data were then used to calibrate a finite element model, which allowed researchers to retrace and quantify from a structural point of view the passage through the centuries of this daring construction and to understand the efforts made to preserve it up to now. The final results presented here are new suggestions on the most efficient way to ensure, once again with hooping ties, the preservation of this monument for the future, starting from empiricism.

KEYWORDS:

• ancient masonry domes
• dynamic tests
• empiricism
• hooping tie rods
• Madonna dell’Umiltà in Pistoia

Acknowledgement

The present study has been carried out in the frame of a protocol (signed in March 2008) between the Curia Bishop of Pistoia and the Foundation of “Cassa di Risparmio di Pistoia and Pescia” for the restoration of Madonna dell’Umiltà in Pistoia, in cooperation with the Superintendence for Architectural Heritage and the Tuscan provinces of Florence, Pistoia and Prato.

Within this study, the dynamic tests on the tie-rods have been carried out by the research group leaded by Prof. R. Garziera, Parma University, while the accurate geometrical survey, has been realized by Prof. G. Tucci, Florence University. Heartfelt thanks go to both of them, for their precious work.

Notes

¹ Hagia Sophia collapse mechanism had to be a lesson for Sinan, who then circled with sturdy tie-rods of iron—thanks to the exceptional skill of the Ottoman in steel industry—all his great domes, up to the great one of Edirne Mosque (Blasi 2003).

² In particular, the cracks along the corners—which are also present in the Brunelleschi dome—are the physiological consequence of the elasto-plastic deformation of the cylindrical webs, generating tensile stresses in the inner surfaces that are incompatible with the low tensile strength of masonry. In the same way the vertical cracks on the three sides with the portals are the consequence of the well-known tensile stresses that develop in the lower meridians of domes and produced similar cracks in all the masonry domes.

³ The inserted tie rod has similar shape and dimensions (about 4.5 x 4.5 cm) to the one inserted by Vasari some years later (1570 and 1572) around the dome of the Umiltà church in Pistoia.

⁴ Actually, St. Peter’s dome was completed by Giacomo della Porta, who partially modified the profile of the dome, originally designed by Michelangelo, who was also the responsible of the high tambour, which represented one of the dome main structural problems. Despite some recent acquisitions that testify the presence of some metal connection elements at certain levels of the dome (Bussi and Carusi 2009)—probably inserted by G. Della Porta during the construction—the dome was built without the insertion of proper chains.

⁵ Certainly, Soufflot knew well that the slender pillars would have not endured an overly thrusting dome, and the first solution to this problem was geometric, designing a truncated cone dome that invoked the tholos and recalled the Wren’s design for Saint Paul in London.

⁶ Perhaps also in this operation Rondelet actually followed Soufflot’s indication. Actually Gauthey underlined that the iron hooping around the dome would had given a solid consistency to the dome itself, in this way anticipating the modern concept of membrane (Gauthey 1798).

⁷ Rondelet could not foresee that the inclusion of iron into the stones would have provoked in time, due to the hygrothermal oxidation, damages to the structures. Indeed, the current crack pattern of the monument testifies two different phenomena: subsidence problems and oxidation phenomena of the iron inserted into the stone blocks (Blasi et al. 2008).