Bridges in - Architectural and Structural engineering aspects

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

Summary With its hundreds of bridges built over the course of centuries, most of which are st ill in use today, Venice probably has more bridges than any other city in the world. It is also a city where the culture of bridges and bridge-building is closely linked to the culture of the town. From an engineering point of view, it is of particular interest to study certain aspects of Venetian Bridges, specifically the problems that Venetian artisans, artists, engineers and architects encountered over the centuries, and how they overcame these problems. The aim of this paper, together with its companion paper on the historical background, is to illustrate and discuss certain engineering and structural aspects of the traditional Venice Bridge.

Keywords: bridges; stone bridges; construction history.

1. Introduction Over the centuries, many different building techniques and materials have been used in the construction of Venice’s bridges, and it is interesting to study how these techniques came about and developed over time, following a sort of “natural selection”, both from an engineering and architectural point of view. This natural selection process has left us today with a heritage of over 400 bridges [1], all of which are well integrated into their urban context, and are often both elegant and suggestive. The aim of this paper is to look at the interaction between the design and construction techniques employed in the building of these bridges, and their formal characteristics, i.e. the relationship between structural engineering requirements and Architectural considerations, with specific reference to the bridges of Venice. We begin with an analysis of the “boundary conditions” in which the Venetian engineers worked, i.e. soil conditions and the urban context. We will then look at the main structural types employed, typical dimensions, materials used and construction techniques. Lastly, we shall consider how the master builders, engineers and architects of Venice tackled and resolved the theme of interaction between technical and engineering constraints and formal- architectural requirements.

2. Boundary conditions 2.1 Soil conditions and foundation work

1 The subsoil of Venice is not uniform: it is in fact characterised by a certain variability of soil types and alternating strata (fig. 1). Generally speaking, however, the following formations can be identified [2], [3]: - a first stratum of fill, 1÷5 m thick, with poor load-bearing capabilities; - a second stratum, 2÷5 m thick, of clay-loam soil with a low-medium consistency, and a high degree of deformability; - alternate strata of clay and loamy clay with a medium consistency, and sandy silts and fine sands; - in some areas, typically at a depth of between 5 and 8 m, there is a formation of over-consolidated loamy-sandy clay, with a good consistency, known as “Caranto”; in several areas, this ‘Caranto’ is also found at greater depths. Fig.1 – Geological cross-section of the subsoil of Venice (from [3])

In general, therefore, below depths varying between 5 and 15 m, the subsoil has enough good load- bearing characteristics, and given modern construction techniques, it can be said to present no particular problems for laying foundations, even for large loads, after having taken into account the problems given by the soil deformability. In the past, however, technology was not yet advanced enough to enable the necessary depths to be reached to ensure solid foundations. The technique used involved driving wooden piles until check was reached. Up until the sixteenth century, the pile-driving hammer was manoeuvred by hand; then along came the first pile-driving machines. The work was done in water, and began with the erection of containment screens; the water was removed, and wooden piles, typically in oak or larch measuring 3÷6 metres long, with diameters of 20÷25cm, were driven into the soil. From the sixteenth century, with the introduction of pile-driving machines, pile lengths, which previously did not exceed 2-3 m, gradually increased. Fig.2 – Layout of a typical foundation for a Venetian building [4]

Typical pile density was 9 piles per square metre, which could be increased in cases where greater soil compaction was required. After squaring the tops of the piles, they were overlaid with wooden (timber) beams with a simple or double beam layout, on top of which the vertical ‘scarp’ foundation work was laid. (fig. 2) For smaller constructions, piling was not used, and foundations were formed of wooden timber beams laid on compacted ground and reinforced with rockfill and brick. In conclusion, the subsoil of Venice is characterised by low load-bearing capabilities on the surface, and medium capabilities at depths of 6-7m; and, since it is prevalently ‘cohesive’, its deformability 2 has evolved over time. It is therefore well suited to support non-excessive loads, and requires deep foundations and structures not overly sensitive to subsidence.

2.2 The urban context Bridge construction in Venice began only after the ninth century. Prior to this, goods and passengers were transported mainly by boat, and, although piers and small wooden bridges existed, pedestrian traffic was still rather limited. Up until 1200, there were only about a dozen bridges in Venice. In the second half of the thirteenth century, however, coinciding with Venice’s burgeoning trade, construction work in the city also began to increase. A growing need for mobility led to the development of a network of pedestrian ways, the construction of “fondamenta” walkways, i.e. the roads running alongside the “rii” or canals; and the construction of several bridges as an essential element of this new urban fabric. For the most part, these new bridges crossed the numerous canals, with spans of 5÷12m. The Canal Grande, whose width varies between 30 and 50 metres, was crossed for the first time in 1180 by a pontoon bridge. By the mid-1500s, the number of bridges in Venice was similar to the number found today: over 400. These bridges permitted pedestrian traffic, while at the same time allowing navigation below, which remained a fundamental system of transport and mobility, and a mainstay of Venetian life.

3. Structural types and materials There are two main types of bridge in Venice: arch bridges and girder bridges. Girder bridges generally have a horizontal deck between two imposts, and therefore require longer access ramps than arch bridges, whose ramps are inclined from the keystone to the abutments; in addition, generally speaking, girder bridges need to have higher structural depth than arch bridges. Against these disadvantages, truss bridges have the undoubted advantage of generating mainly vertical reactions on the foundations, making them highly suitable for the city’s soil characteristics. Arch bridges are much more prevalent, as they successfully integrate the need for a continuous pedestrian walkway with the need to leave sufficient space underneath for boats to pass. are designed according to various formal types: semi-circular, horseshoe, segmental, equilateral pointed and elliptical. (fig. 3)

Fig.3 - Types of arch profile, from 1 to 6: semi-circular(L/f=2); horseshoe (L/f<2); segmental (L/f>2); low segmental (L/f>≈5); equilateral pointed; multi-centric or elliptical Typological analysis [4] reveals a strong prevalence of segmental arches, but only rare cases of low segmental arches. (fig. 4) These bridges are thrusting structures that transmit large horizontal forces to the

3 foundations, which posed a not-inconsiderable technical problem given the poor quality of the superficial sub-soil in Venice. Venetian engineers solved this problem with two principle technical solutions: - by lightening the central part of the arch, thus reducing the horizontal thrusts; - by increasing the weight of the arch near the imposts, and of the piers and foundations; this resulted in a strong vertical component in the forces transmitted to the foundations, and straightened the axis of thrust. Fig.4 – Histogram showing the frequency of structural types found in Venetian bridges It is also interesting to note that in many cases (e.g. the bridge), buildings adjoining the bridge, with their vertical load, actually help to stabilise the foundations. The materials used in the construction of the bridges of Venice are stone and brickwork for the arch bridges, and iron and wood for the truss bridges, but also often for arch bridges. Wooden bridges The first Venetian bridges were built of wood, using raw materials from the forests of Istria and drawing on the skills and techniques of the navy carpenters of the Republic of Venice. The first wooden bridges were flat, or only slightly arched, without steps, and suitable for horse-drawn traffic. Up until the first half of the thirteenth century, they were built mainly by private companies and individuals, with the authorisation of the local authorities. In 1484, the Venetian Senate passed a law calling for the modernisation of all the bridges in Venice. This provided the opportunity to replace most of the city’s wooden bridges with stone structures. However, wood continued to be used for the city’s main crossing points, such as the Canal Grande (with the Rialto), the Cannaregio Canal (with the ‘’ and the ‘Tre Archi’ bridge), and the bridge over the Giudecca Canal. Fig.5 - The Rialto Bridge preceding the current structure, built in 1500, in a drawing made from a painting by Vittore Carpaccio (1465-1525). The central section of the bridge opens, with a stayed structure. The lateral spans already house small shops, a precursor to the current ‘inhabited bridge’ configuration. It is interesting to note how the wooden bridge remained in use for over 90 years.

From 1264 to 1591, various vicissitudes saw the Rialto Bridge built and rebuilt in wood: it was destroyed in 1310, and collapsed under the weight of the crowd during a festival in 1444. It was rebuilt with an opening central section, supported by two piers and two pairs of mobile stays, (fig. 5) as can be seen in the “Miracolo della Reliquia della Croce” (Miracle of the Relic of the Cross) painted by Carpaccio fifty years later and now preserved in the Galleria dell’Accademia in Venice. Up until 1854, it remained the only bridge to cross the Canal Grande. Subsequently, however, these bridges were also rebuilt in brick, maintaining the arch structure. Stone bridges Stone and brick bridges, which had been built, albeit sporadically, since 1200, became the norm following the law passed by the Senate in 1484.

4 The most important example was the new Rialto Bridge, built between 1588 and 1591 to replace the then ageing wooden structure, and not without controversy, as it was to be the largest bridge in Venice. (fig. 6) Although the 29m span and construction techniques used were nothing unusual for the time, there were still considerable technical problems to overcome, not least of which was the laying of foundations that had to withstand large thrusts on yielding soils. Fig.6 – The Rialto Bridge, in a drawing by Rondelet from 1841. The span of the bridge is 28.83m; the rise is 6.39m, with a rise-to-span ratio of 1:4.5; keystone thickness of 1.25m, with a slenderness ratio of 1:23. Construction of the bridge was directed by Antonio da Ponte, who has also been credited with the overall design, although many indications point to the bridge actually having been designed by . It appears that the single-span design adopted was preferred over a more ‘prudent’, three-arch solution simply because it was much cheaper: in all probability, the cost of the foundation work proved decisive. For more than four hundred years, the stone bridge has been part of the infrastructure, an architectural icon, and a symbol of the unity of the city of Venice. Metal bridges Bridge-building techniques using iron and cast iron are the most recent to be employed in Venice. Towards the middle of the nineteenth century, a number of foundries, particularly the Neville foundry in San Rocco, directed by the engineer Alfred Neville, and the Collalto foundry in Mestre, proposed the use of cast iron, and later iron, for the construction of numerous small bridges, prefabricated in their workshops and quickly laid on site. Above all, however, they proposed the construction of two bridges over the Canal Grande: the “Ponte dell’Accademia” (fig. 7) and the “Ponte degli Scalzi”. The first “Ponte dell’Accademia” was built over a period of two and a half years, and was completed in 1854. The bridge had an isostatic design, transferring only vertical forces to the ground. The weight was kept to a minimum by the lightness of the materials used and the structural system adopted. The engineering solution was therefore well adapted to the characteristics of the Venetian subsoil. The structure consisted of two open reticular beams and a through bridge deck. However, the intrados was only four metres above the water’s surface, which led to strong criticism as it limited the height of boats that could pass underneath on their way to the Canal Grande. Fig. 7 - The first “Ponte dell’Accademia”, built in 1854, in a drawing from the period. The appearance of the reticular structure, which was statically efficient, but certainly not in keeping with Venetian tradition at the time, was also the subject of controversy. Nevertheless, in 1857 a very similar bridge was built near the railway station, where the “Ponte degli Scalzi” stands today. In any case, by the first decades of the twentieth century, the two iron bridges were already in a poor state of repair. It is reported that the “Ponte dell’Accademia” in particular was far from stable, and that in its last years, “the whole structure shook”, so much so that the authorities had to narrow the deck to prevent it being overloaded with people.

5 Eighty years apart, both iron bridges were finally replaced, the “Ponte dell’Accademia” by a wooden reticular arch, and the “Ponte degli Scalzi” by a stone vaulted structure, thus returning – symbolically – to the two main construction techniques traditionally used in Venice.

4. The “Ponte degli Scalzi” The new “Ponte degli Scalzi”, crossed the Canal Grande near the railway station, between the Chiesa degli Scalzi and the Chiesa di San Simeon Piccolo. The bridge was built between 1932 and 1934 to the design of Eugenio Miozzi, civil engineer, and is an outstanding example of formal elegance, architectural consistency and daring engineering.

Fig.8 - Profile of the bridge, with a profile of the arch and the plan of the “Ponte degli Scalzi”, in a drawing by Eugenio Mozzi. To a greater extent than perhaps even the Rialto and Accademia structures, this bridge also represents an example of the successful integration of architecture and engineering, and is an emblematic example of the art and tradition of the bridges of Venice. The bridge has a total length of 55 m, a span of 40.40m, a rise of 6.75m, and a rise-to-span ratio of 1:6; it is 7m wide, and the thickness of the arch varies from 80cm at the keystone to 1.30m at the imposts. (fig. 8) The slenderness ratio at the keystone is 1:50, a figure that would be high for an arch bridge built in reinforced concrete. And yet, the vault of the “Ponte degli Scalzi” is built entirely of stone! And, as the designer himself states “…the whole structure is made of solid blocks of Istria rock, from the Orsera quarry, the same rock used in the construction of the Libreria Vecchia in San Marco’s Square; it has no supporting structure, either in reinforced concrete, or iron, or bronze… it is simply made of stone”. Stone was chosen ahead of reinforced concrete, which was then not very durable, for two reasons: “An arch in reinforced concrete would in all probability have been subject to the action of sea salt, in much the same way as similar structures in the lagoon. Secondly, the bridge also plays a monumental role, standing as it does next to buildings of enormous architectural importance, and a masked structure would undoubtedly have detracted from the appearance of the whole” [9]. It can be noted that the Istria Stone (fig. 9), due to its very low porosity, low pore size and high superficial hardness, is considered to have a durability of 500-1000 years. And the preservation of so many Venetian monuments is largely due to the high quality of this stone.

Physical and Mechanical properties of the Istria Stone Classification mudstone/micrite 6 Density kg/m³ 2670÷2690 Compression Strength, cut parallel to bedding N/mm² 69÷180 (average = 129) Tension - Bending Strength N/mm² 17 Porosity % 0.5÷0.6 Typical pore dimension micron 9÷90 Hardness ( Knoop scale) N/mm² 1510 Fig. 9 – Properties of the Pietra d’Istria according to various authors [5 ], [ 6] , [7 ]

The construction system was described by its designer as the “compensatory systematic lesion” method, and consisted in the creation of three kinematic joints, which were open when the voussoirs were laid and gradually closed as the formwork was removed (fig. 10). This resulted in a structure that was isostatic during the deformation phase as weight was transferred from the falsework to the actual arch, without causing any bending stresses during these phases. Fig.10 – Schematic illustration of the “systematic lesion” system, as described by Mozzi in [9]: three wedge-shaped opening were left at stone segments placing; they closed during the lowering of the centering that is during the load transferring to the abutments. The top figure shows the geometry after the abutments displacement; the bottom one shows how the arch would open if the abutments should get closer, and, dually, shows the opening shapes before the abutments movement.

Using this system, bending moments that could have arisen when the falsework was removed, had the structure been rigid and hyperstatic, were eliminated right from the start of the falsework removal process. (fig. 11) In conclusion, once the falsework had been removed, the pressure curve for the arch was well centred, bending moments were eliminated, and each section of the arch was almost uniformly compressed. The thrust exercised by the arch under its own weight was around 700t. Note that the problem of how to eliminate initial hyperstatic effects in segmental arches was tackled in the past using various methods: either by applying a counteracting force on the keystone or by pushing the supports using hydraulic jacks, as Freyssinet did on various projects. In such cases, it was necessary to apply considerable forces by means of operations that were both delicate and risky. Compared to these methods, the Miozzi system displays a truly admirable conceptual and operational simplicity.

Fig.11 - Diagram of the longitudinal top and bottom stresses present in the arch in the absence of (two top diagrams) and in the presence of “ systematic lesions” (from [9]).

The vault falsework structure was made of steel, with a

7 reticular arch structure, weighing 90t, and articulated at its apex to allow it to be lowered when the time came to remove it, with a kinematic motion. (fig. 12) This falsework also meant that construction on the bridge could proceed without hindering navigation. The foundations were laid on piles. Fig.12 - Assembly of the reticular arches that would form the load-bearing structure of the falsework for construction of the stone arch. The main piles are of concrete, with a diameter of 30cm, made using a technique similar to that used today for micropiles, with concrete injected at a pressure of 6-7 bar, and they are in part vertical and in part inclined (fig. 13). Around 200 wooden piles were used to lay the provisional structures, and constitute a further load-bearing component. To check the stiffness and linearity of the piles’ response to horizontal forces, load tests were conducted by applying horizontal forces to the piles when they were sunk. The bridge was completed in 29 months, and required for its foundations 3411 cubic metres of concrete and 223,900kg of steel reinforcements; also required 531 cubic metres of Istria rock, 324 of which were used for the arch (fig. 14); the total construction cost was 2,550,000 Italian Lire, equivalent to around 4.7 million Euro today. The bridge didn’t require special maintenance or monitoring in its 76 years life.

5. Conclusions The history of the bridges of Venice is both rich and interesting, and this paper, which deals with only a very small part of that history, is intended as an introduction to that history from the point of view of the interaction and dialogue between the various architectural and structural–construction considerations involved. This interaction has always been a part of the construction of the bridges of Venice. Engineers and designers have always been required to solve technical and engineering problems while at the same time producing structures that were in keeping with the city’s architectural context and tradition. It is interesting to note how this dialogue has always taken account of economic constraints, the resources available to build new structures, and the problems associated with the duration of the bridges themselves. The problems associated with the presence of deformable soils, and thus the recurrent problem of countering thrusts and the horizontal yielding of the abutments, initially led to a preference, especially for the larger bridges across the Canal Grande, for isostatic beam structures: in wood for the Rialto and in steel for the Accademia and the Station. Subsequently, the arch bridge became prevalent, built using brick or, preferably, stone. Rise-to-span ratios were high, but always within 1:7, considering this value as a traditional limit for the venetian soil conditions; with gradually increasing slenderness ratios. And, as we have seen, these slenderness ratios were often the result of precise structural and construction solutions, designed to pursue those objectives of formal elegance that the bridges of Venice display to such a considerable extent.

Fig.13 - Executive phases for the foundation work on one of the two ends of the bridge.

8 Fig. 14 – The “Ponte degli Scalzi” just after its completion, seen from the Piazzale della Ferrovia

6. References [1] Tiziano Rizzo - “I Ponti di Venezia” - Newton Compton - 1983. [2] P. Colombo e F. Colleselli - “Preservation problems in historical and artistic monuments of Venice” - Balkema - 1997. [3] Fulvio Zezza - “Geologia, proprietà e deformazione dei terreni del centro storico di Venezia” - Second Convention “La riqualificazione delle città e dei territori” - Venice - 2007.

[4] Doranna Murat, Gianluca Samaritani, Stefano Uccelli “Venezia e i suoi ponti” - “Il ponte e l’architettura” - Città Studi Edizioni - IUAV - 1995. [5] Floriano Calvino , “Lezioni di litologia applicata”, Padova, 1967 [6] Lorenzo Lazzarini, “Pietra d’Istria: genesi, proprietà e cavatura della pietra di Venezia” – Atti del Seminario di Studio Iuav Venezia – 8 ottobre 2003 [7] R.Geometrante, D.Almesberger, A.Rizzo “Characterisation of the State of compression of Pietra D'Istria elements by Non Destructive Ultrasonic Technique”– 5° WCNDT– Roma 2000 [8] Eugenio Miozzi - “Venezia nei secoli” - Volumes 1-2, Libeccio, Venice - 1957. [9] Eugenio Miozzi - “Dal ponte di Rialto al nuovo ponte degli Scalzi” Roma - Stabilimento tipografico del Genio Civile - 1035.

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