Mario DE MIRANDA Consulting Engineer Studio DE MIRANDA Associati Milano, Italy

Umberto BARBISAN Associated profesor Università Iuav di Venezia

Marko POGACNIK Research professor Università Iuav di Venezia

Luka SKANSI Assistant teacher Università Iuav di Venezia

Brief considerations on the Accademia Bridge in Venice Between 1852 and 1854 Alfred Neville completed the new Accademia bridge, a reticular iron structure very similar to the Railway station bridge (Ponte degli Scalzi), which was concluded just a few months later, right after the first bridge proved its stability. The structure was formed by reticular iron beams, assembled following a scheme that would later be known as the “Neville beam”: a structure of great relevance, but at the same time with low static efficiency, especially for the compressed beams, their evident undersizing and the following problem of load peak.

- Neville’s Accademia Bridge. Its length was less than 50 m, not very common at the time. In particular, the straightgirder structure proved inappropriate because, although it was raised 4.2 meters above the water level, during the periods of high tides, sailing boats and, especially, the “vaporetti” could not pass under the bridge: a transfer of passengers had to be organized from one boat to another. Neville – with his building company – “landed” in Venice in the first half of the nineteenth century, after a succesfulsuccessful experience as a “caster” and builder of bridges in Europe (France, Piemonte, Belgium, Austria and Slovakia, etc.). In this period Venice was part of the Austrian Empire. Neville proposed a bridge with a 50 m span. After a long controversy between supporters and detractors of the reticular bridge, the Commissione dell’Ornato (the “Commission for ornament”), approved the project, even if with some doubts about the iron decorations. In May 1852 Neville signed a contract with the Venetian Municipality – and the Austrian administration – that permitted him to erect a bridge in front of the Accademia at his own expense; in exchange Neville would benefit from a pedestrian toll, valid for thirty years starting from 1854, the supposed date of the delivery of the bridge (the construction continued till 1859). Several problems extended the building process. One was that Neville, engaged in other building sites in different European countries, sent doubtful construction plans and details from England where he was temporarily living. There were also continuing controversies

on the height of the bridge above the water (the austrian military authorities demanded no less than 4.57 m above water level). Furthemore, bridge components were built elsewhere (presumably in England): it is documented that a ship called “Aletta” had crossed, in 1854, the Strait of Gibraltar, with bridge components. The bridge was opened in November 1854. The only evidence of the calculation proceedings that certifies the validity of the structure are contained in a report signed by the engineer G. Bianco, of the municipality of Venice, written in November 1854, and subsequently in the analysis made by the engineer M. Ballarin, published in the journal Ingegneria in October 1926. Over seventy years divide these two studies: Bianco’s report testifies that Neville’s structure weighed aproximately 145,500 kg, downloading (vertically) 72,750 kg to supports, and that each element of the reticular structure was adequately sized.

- Detail of the Bianco report (1854), with the indication of the structure weight, 145.409,25 kg and 72.706,62 kg.

Ballarin, viceversa, an active supporter of the demolition of the two Neville bridges, supposed even higher numbers, in both the compressed (iron) and tensed (in “wrought iron”) shafts, and the bolts (more than 2,000 kg/cm2). From recent investigations it was observed that the compressed shafts supported just under 2000 kg/cm2 weight.

- Supposed foundation scheme of Neville bridge at Accademia.

In fact, after about eighty years of service, and partially because of the corrosion caused by the Venetian environment, in the 1930s the Accademia bridge, as well as the Scalzi bridge, were declared precarious, to the point that the Municipality had to reduce the width of the pedestrian walkway to just one meter, and to continuously verify the safety of the structure. The older Venetians, till a few years ago, remembered how the iron bridge was “trembling”, when crossed. In those years a long debate started, several interesting projects were proposed, some based on the idea of renovation, and and some on the idea of reconstruction of the bridge. What in the end prevailed was the hypothesis of a temporary wooden bridge, designed by the senior official of the Technical Office of the City of Venice, the engineer Eugenio Miozzi. Miozzi proposed a solution of a single 48 m free span bridge, consisting of two arched reticular ribs, made by 4 cm thick wooden (larch) boards, held together by bolts and plates welded to obtain the necessary continuity. A mixed wood-steel structure of a great interest, especially because the lamellar techniques were in this period still being established. A technique used by Miozzi also for the bridge of Tre Ponti (now replaced). The arches were connected by a reticular system of wooden diagonals and metal tie-beams with turnbuckles. The large ribs were 2 m “high” at the supports, decreasing to 1 m at the keystone, also connected by a wooden reticular system and, again, tie-beams. To withstand the resultant of the bridge (about 260,000 kg the horizontal component, 200,000 kg the vertical component), Miozzi had to use both concrete foundation piles, and a crown of wooden poles (vertical and inclined). The poles could not be inserted by “hammering” them, due to the vicinity of ancient buildings, so Miozzi invented a special process, unique to Venice, using “screwed piles” – wooden piles with screw-like metal tip.

- E. Miozzi, section of foundations, Accademia bridge (1932).

Basically because the dimensions of the bases, of the concrete and wooden piles, Miozzi managed to obtain a very low solicitation on the ground: 0.2 kg/cm2. This was an optimal result to avoid the sagging/sinking of the bridge abutment, especially important for the extremely low consistency of Venetian grounds. After he settled the foundations problems, Miozzi worried about the failure of the wooden system and its joints. Miozzi proposed oak wedges – that were hammered “a forza” in keystones – in relation to the disarmament of the provisional ribs.

- Detali of Miozzi’s structure, 1932.

The bridge was assembled in a few days – from 10th December 1932 to 15th January 1933. On February 15th of the same year the bridge was tested (Miozzi had provided an overload of 400 kg/m2 for the crowd and 140 kg/m2 for the crosswind). Loaded with nearly two thousand sand bags, equal to 400 kg/m2, the maximum deflection was just 9.5 mm, which returned to zero after removing the load. The absence of permanent deformation was attributed to the state of coercion triggered by wedges forced into keystones, and to the tension caused by tie-beams. A first series of maintenance work was done already in 1934 (re-painting with linseed oil and re-tightening of the nuts). In the following years the bridge needed no major maintenance. Only after the Second World War, was it decided to implement an important structural alteration: 4 steel arches – with the “wood around them”, connected by wooden diagonals, were realized in 1948 by Breda company (shipyards specializing in metal carpentry). An imitation of the original structure designed by Miozzi, but less efficient in terms of lateral bracing. We still do not know how much the new structure increased the weight of the bridge, and the subsequent weight on the abutments (we assume a modest increase, however).

- Works made in 1948. Between 1963 and 1965, professors Jogna and Creazza (from University IUAV in Venice) were involved to create a substanial stiffening of the structure. In 1982 the bridge had new difficulties regarding its stability and, between 1984 and 1986, Jogna and Creazza, with Ballio and Turrini as consultants, designed a “new” temporary bridge that became an arch structure in metal and wood.

- Accademia bridge in 1986. Summarizing, the first Miozzi bridge (the temporary bridge) remained “active” for 15 years, in a very difficult period and without maintenance. The intervention of 1948 by the Breda company did not improve the structure, particularly because adequate stiffeners were not included: put in place only afterwards (1965) by Jogna-Creazza, while the bridge of the 80’s is essentially a mixed steel-wood structure. What remains questionable is the load-stress on the land: the Neville bridge unloaded on each side about 72,750 kg (and only in vertical), the first Miozzi’s structure 200,000 kg and 260,000 kg (vertically and horizontally), the second is supposed to be bigger, but within the acceptable limits set by Miozzi’s foundational works. If we search for a moral, thinking of the loads of Calatrava bridge, we cannot but reflect that, in the event of a new bridge at the Accademia, we will have to respect the modest capacity of the ground and follow the great lessons of Miozzi, Jogna and Creazza. Bibliography - Aa.Vv., Boschi della Serenissima, Archivio di Stato, Venezia, 1987. - Albenga G., I ponti, Torino, 1958. - Alessandri A., “Fondazioni per edifici civili e industriali in Venezia e Marghera”, in Atti del Convegno di Geotecnica, Cagliari, 1967. - Ballarin M., “Perché dovrebbero essere demoliti i ponti in ferro sul Canal Grande a Venezia”, in Ingegneria. Rivista Tecnica Mensile, n. 10, ottobre, 1926. - Barbisan U., Lovelli S., “Il ponte reticolare in legno sul fiume Cismone”, in Tetto e Pareti in Legno, periodico dell’edificare con il legno, n. 14, settembre 2009. - Barbisan U., “Venezia, il legno e i ponti”, in rivista Tetto e Pareti in Legno, periodico dell’edificare con il legno, n. 13, giugno 2009. - Barbisan U., Il legno: tradizione e innovazione, Forlì, 2008. - Barbisan U., “Ponti in legno: si rinnova la sfida”, in rivista Tetto e Pareti in Legno, periodico dell’edificare con il legno, n. 10, settembre 2008. - Barbisan U., “Il legno lamellare: una storia complessa”, in rivista Tetto e Pareti in Legno, periodico dell’edificare con il legno, n. 5, settembre 2007. - Barbisan U., “Attraverso le Alpi: il legno della Serenissima”, in Economia Montana, Linea Ecologica, n. 2, marzo aprile 2002. - Barbisan U., Guardini M., I ponti, immagini per una storia, libro su Cd Room edizioni Tecnologos, Cavriana, Mantova, settembre 2002.

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