Bridges in Venice - Architectural and Structural Engineering Aspects
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Bridges in Venice - 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, Italy 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: arch 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. Arches 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 Rialto 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 ‘Ponte delle Guglie’ 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.