IntroductionWith its hundreds of bridges built over the course of centuries, most of which are still 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.

Soil conditions and foundation workThe subsoil of Venice is not uniform: it is in fact characterised by a certain variability of soil types and alternating strata. Generally speaking, however, the following formations can be identified: 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; 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.

Structural types and materialsThere 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 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. Arches are designed according to various formal types: semi-circular, horseshoe, segmental, equilateral pointed and elliptical. Typological analysis reveals a strong prevalence of segmental arches, but only rare cases of low segmental arches, as is shown in the figure. These bridges are thrusting structures that transmit large horizontal forces to the foundations, which posed a not-inconsiderable technical problem given the poor quality of the superficial subsoil in Venice.

The “Ponte degli Scalzi”The “Ponte degli Scalzi”, crosses the Canal Grande near the railway station, between the Chiesa degli Scalzi and the Chiesa di San Simeon Piccolo. The bridge was built, replacing a steel girder truss, 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. 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 key-stone to 1.30m at the imposts. 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! More precisely the material is “Pietra d’Istria”, the white, strong and compact limestone used in venetian buildings since 14th century. 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. 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. 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; the total construction cost was 2,550,000 Italian Lire, equivalent to around 4.7 million Euro today, that was a good result when considering the high cost of the valuable material used for the main structure. The bridge didn’t require special maintenance or monitoring in its 76 years life.

ConclusionsThe 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.

BRIDGES IN VENICE. ARCHITECTURAL AND STRUCTURAL ENGINEERING ASPECTS

Università IUAV Venezia. Unità di ricerca “Arte del costruire” Mario de Miranda, Marco Pogacnik, Luka Skansi*

References P. Colombo e F. Colleselli, “Preservation problems in historical and artistic monuments of Venice”, Balkema, 1997. Fulvio Zezza - “Geologia, proprietà e deformazione dei terreni del centro storico di Venezia” - Second Convention “La riqualificazione delle città e dei territori” - Venice - 2007. Doranna Murat, Gianluca Samaritani, Stefano Uccelli “Venezia e i suoi ponti” - “Il ponte e l’architettura” - Città Studi Edizioni - IUAV - 1995. Floriano Calvino , “Lezioni di litologia applicata”, Padova, 1967 Lorenzo Lazzarini, “Pietra d’Istria: genesi, proprietà e cavatura della pietra di Venezia” – Atti del Seminario di Studio Iuav Venezia – 8 ottobre 2003 R.Geometrante, D.Almesberger, A.Rizzo, “Characterisation of the State of compression of Pietra D’Istria elements by non Destructive Ultrasonic Technique”– 5° WCNDT– 2000 Eugenio Miozzi - “Venezia nei secoli” - Volumes 1-2, Libeccio, Venice - 1957. Eugenio Miozzi - “Dal ponte di Rialto al nuovo ponte degli Scalzi” Roma - Stabilimento tipografico del Genio Civile - 1035.

Fondo Miozzi, Archivio Progetti dell’Università Iuav di Venezia.*M. de Miranda is a Consulting Engineer DE MIRANDA Associati and professor of Structural Design at Università IUAV di Venezia. M. Pogacnik is a research professor and L. Skansi a teaching assistant in History of Architecture at Università IUAV di Venezia.

IABSE34th IABSE SYMPOSIUM - VENICE 2010

LARGE STRUCTURES AND INFRASTRUCTURES FOR ENVIRONMENTALLY CONSTRAINEDAND URBANIZED AREAS

Geological cross-section of the subsoil of VeniceLegend: 1. fill; 2 clay and silty clay; 3 clay with remains of shell-fish; 4 sandy silt; 5 fine sand; 6 caranto; 7 clay and silty clay; 8 sandy silt and silt; 11 medium sand;12 sandy silt; 13 clay and sandy silt.

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 pre-cursor to the current “inhabited bridge” configuration. It is inter-esting to note how the wooden bridge remained in use for over 90 years. Subsequently, however, these bridges were also re-built in brick, maintaining the arch structure.

Antonio Da Ponte, project for the Rialto bridge (see the asimetrical solution for the foundations on three different levels). AS Venezia.

Histogram showing the frequency of structural types found in Venetian bridges.Types of arch profile:- 1. semi-circular (L/f=2); - 2. horseshoe (L/f<2); - 3. segmental (L/f>2;- 4. low segmental (L/f>≈5); - 5. equilateral pointed; - 6. multi-centric or elliptical

From above: Assembly of the reticular arches that would form the load-bearing structure for the stone arch.View of the completed reticular arches.

Schematic illustration of the “systematic lesion” system, as described by Miozzi: 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.

The “Ponte degli Scalzi” just after its completion, seen from the Piazzale della Ferrovia. In foreground, Neville’s iron bridge before the removal.