Mario DE MIRANDA Umberto BARBISAN Marko POGACNIK Luka

Mario DE MIRANDA
Consulting Engineer
Studio DE MIRANDA Associati
Milano, Italy
Umberto BARBISAN Marko POGACNIK
Associated profesor Università
Iuav di Venezia
Luka SKANSI
Research professor Università Iuav Assistant teacher Università
di Venezia
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 tiebeams 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.
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