ANALYSIS OF THE RIALTO BRIDGE, VENICE

N M E Tolley1

1Department of Architecture and Civil Engineering, University of Bath

Abstract: This paper will critically analyse The Rialto Bridge in Venice, both in terms of its aesthetics and structural efficiency, with consideration given to elements such as the soil conditions and time of construction. Current concerns such as maintenance and vandalism will also be discussed, as well as the possibility of any future changes or improvements.

Keywords: Single span, masonry, compressed arch, footbridge

Figure 1: The Rialto Bridge in elevation

1 History

The Rialto Bridge, which spans the shortest part of the Grand Canal, began in 1181 as a series of floating pontoons. The structure was built by Nicolò Barattieri, and called the Ponte della Moneta. However, the development of the Rialto market on the eastern bank led to increased traffic and the pontoons were soon worn down. A wooden structure replaced it in 1250, and was renamed the Ponte di Rialto. The bridge was burnt during the revolution in 1310, before collapsing in 1444 under the weight of spectators at the wedding ceremony of the Marchessa di Ferrara, and collapsed again in 1524.

The city decided to rebuild the bridge with stone, and Antonio da Ponte won the contract over prodigious designers such as Michelangelo and Palladio. The Rialto Bridge was designed and built in 1588-1592, and remained the only way to cross the canal on foot until 1853, when the Ponte Accademia was constructed.

2 Aesthetics

The aesthetics will be analysed based on the ten criteria set out by Fritz Leonhardt.

2.1 Fulfilment of Function

The structural system is simple and clearly expressed through the single span marble arch, which lies below the deck. The arch spans 28.8m and reaches a maximum height of 6.4m, giving it a span/rise ratio of 4.5. The shallow, elliptical shape generates large lateral thrust, which is transferred into the abutments. A parabolic arch, with a span/rise ratio of 4, would be more efficient but provide less height over the springing points.

The shape of the Rialto has been criticised in the past but is necessary in fulfilling its function. The bridge was constructed to allow pedestrians to cross the Grand Canal, whilst permitting vessels to pass underneath. Due to the high water level in Venice, the arch rises above the banks on either side, and consequently there is an inclined deck. However, the distinct ‘humpback’ shape

Proceedings of Bridge Engineering 2 Conference 2009 April 2009, University of Bath, Bath, UK

Natalie Tolley, nmet20@bath.ac.uk

of the Rialto arguably adds character, and has certainly helped it to acquire an iconic status.

2.2 Proportions

Footbridges are usually slender structures as they support much lighter loads than those carrying traffic. The Rialto, however, is a market place as well as a bridge and consequently top-heavy. A series of twelve stone arches were built on top of the deck to house the shops, which are constructed from timber and covered by a curved masonry portico. The change in materiality, between the stone galleries and timber shops, has been used to seemingly reduce the mass of the structure above the deck.

The relationship between the amount of structure and size of void is arguably the most important in an arch bridge. The void below the supporting arch of the Rialto is relatively small, due to the high water level. However, the central archway that exists above the deck also contains a void. Together, they provide an almost uninterrupted view through the centre of the structure, and make it appear more lightweight.

The supporting arch ring is slender and can be clearly identified from the orientation of the stonework. The deck is also relatively light, and this is enhanced by voids between the balustrades.

The spandrel walls appear quite small, as they are limited by the sloping deck and low rising arch. Unlike most single span arch bridges, there are no wing walls. Wing walls are normally used to retain the earth fill placed on top of the arch, shown in Fig. 2. This infill strengthens the arch and enables a smooth transition between the bridge and approach spans, particularly in road construction.

The Rialto Bridge is a pedestrian structure and has been constructed above the level of the approach spans. Whilst there may be some earth fill between the spandrel walls, the force will be so small that retaining wing walls are not required.

Figure 2: Features of a single span arch bridge The size of the abutments is disproportionately

large but to a certain extent necessary, due to the poor ground conditions, which will be discussed in depth later in the paper. The size of the abutments is not an

issue aesthetically as they are completely hidden below the water.

2.3 Order

A single span arch bridge is arguably of the simplest order, having only one primary structural member. However, there are a number of other arches, of varying sizes, present on the Rialto Bridge.

The galleries are semi-circular, and therefore a different shape to the supporting arch, which is elliptical. The central arch is larger than the other twelve arches, but is the same shape to give some visual continuity across the structure.

2.4 Refinements

Pedestrians cross footbridges quite slowly, and so have more time to appreciate the refinements. The balustrades have been shaped for aesthetic appeal; and in the superstructure, the stonework in the arches has been clearly expressed. The pitched roof, which sits above the central archway, has been detailed with grooves carved into the stone.

Gondolas also travel relatively slowly, and so the details on the outside of the Rialto Bridge are just as important. There are a number of adornments on the spandrel walls. On the façade facing San Silvestro there are figures of Saint Mark and Saint Theodore. Whilst on the opposite façade is the annunciation; the angel Gabriel, as shown in Fig. 3, the Virgin Mary and the Holy Ghost. It is possible that, at the time the bridge was constructed, many people were afraid of crossing water, and the religious figures are representative of them putting their trust in God to provide a safe passage both over and under the bridge.

There is a series of masonry ‘nodes’ below the parapet, with every fifth node falling below a rectangular baluster to create vertical continuity. The design is repeated below the masonry portico, but the nodes here are smaller and more closely spaced together. Both patterns of stonework are non-structural, but add interest, texture and definition to key components of the bridge.

Figure 3: Detailing on the elevation

2.5 Integration into the Environment

The majority of the bridge is built from Pietra d’Istra; a calcareous rock of bright white colour that blends in with the surrounding buildings. There is some distinction between the paving stones on the bridge to those on the approach spans. The paving stones on the bridge are grey, although the tread has a lighter edge to clarify the position of the steps, which vary in size. On the approach spans the stones are lighter and laid at a different angle, as shown in Fig. 4.

Figure 4: Paving on approach span

Whilst the arch has a fairly low rise, the bridge

itself is quite tall. The deck is inclined at an angle of approximately 15°, and pedestrians are unable to see over the crest, as shown in Fig. (4). This is not permitted in the design of modern bridges, but would not have been an issue at the time of the Rialto’s construction. Furthermore, as many of the surrounding buildings are high rising, the structure does not look out of place.

2.6 Texture

The stone bricks are relatively smooth to reflect the light and draw attention to the primary structure, whilst the timber is much rougher, as the shops are meant to be a secondary feature.

The stones on the spandrel walls have been laid at an angle, which separates them from the arch, and the deck above. The arch ring protrudes slightly, highlighting it as the key component of the structural system, and the stones that protrude from below the parapet emphasise the slender nature of the deck.

In terms of the superstructure, the stones forming the aches can be clearly distinguished, expressing the structural system. The spandrel walls on the other hand, are extremely smooth, to the point where the joints are barely visible. This is to reflect the light, making the bridge appear less top-heavy.

2.7 Colour

The timber shops have been painted a dark colour so that the eye is drawn to the white stone structure, making the bridge appear lighter.

The stone used in the sub-structure is rougher and so appears relatively dark, possibly to make this part of the bridge appear denser and stable.

The base of the bridge has been discoloured by water washing against the surface, and there is also some staining on the underside of the arch. However, given the age of the bridge the damage is fairly small.

Figure 5: Discolouration of the base and underside of

arch

2.8 Character

The engineering of the Rialto Bridge was considered so audacious that Vincenzo Scamozzi, another architect competing for the contract, predicted ruin. At the time it was believed that only bridges built in the Roman style, with multiple, semi-circular arches, would be long lasting.

The Rialto Bridge is an icon of renaissance architecture, and one of only four bridges lined with shops in the world. The unique shape and history of the Rialto have provided it with character and status.

2.9 Complexity

The superstructure is quite complex, and the adornments and carved stone features also make the bridge fairly ornate. However, the Rialto has a simple substructure in the form of the single span arch, which provides balance and prevents the bridge from appearing overly complex.

2.10 Nature

Arches are a man-made rather than organic shape, being too symmetrical to occur in nature. However, as the Rialto is situated in a man-made environment over the Grand Canal, this is an appropriate design choice.

3 Dimensions

The key dimensions of the Rialto Bridge are taken from Ref. [1] and shown in Table 1.

Table 1: Dimensions of the Rialto Bridge Dimension Measurement

Length 48m Width 22.9m Span 28.8m

Height of arch 6.4m

4 Loading

Where possible, the loading will be calculated according to BS 5400.

4.1 Dead Loading

The dead load acting on the bridge will be calculated from the weight of the deck, arch ring, spandrel walls and abutments. To simplify calculations, the substructure will be assumed solid across the width of the bridge; and its area, in elevation, has been calculated from a scale drawing shown in Fig. 6.

Figure 6: Elevation including foundations

The figures that will be used to calculate the dead

load are shown in Table 2. The density of the Pietra d’Istra stone was taken from Ref. [2].

Table 2: Values for calculating dead load Quantity Value

Volume of stone 4466m3 Density of stone 2772kg/m3

Force due to gravity 10N/kg Taking the area of the bridge on plan as 48m by

22.9m, the dead load can be approximated as:

Pdl =4466 × 2772 ×10( )

48 × 22.9( )≈ 113kN m2

This is an over-estimate of the actual dead load, as the substructure will not be solid stone across the width of the bridge. Between the spandrel walls there will be a series of stone beams supporting the deck, and probably some earth infill. This is lighter than stone, with a density of approximately 2240kg/m3. However, the dead load will be taken as 113 kN/m2 in order to be conservative.

4.2 Superimposed Dead Loading

Infill material above the arch would ordinarily be the primary component of the superimposed dead load. However, in this instance the superimposed dead load will be based on the weight of the superstructure only.

The parapets will be taken as 800mm high, and spanning the length of the bridge on plan. They will be treated as two, 200mm thick, stone walls; and the gaps between balustrades will be ignored.

The area of the stone arches and timber shops in elevation has been estimated; and assuming that the stone arches span half the width of the bridge, and the timber walls are 300mm thick, the volumes of these two materials can also be calculated. The masonry roof covering the shops will be assumed as 300mm deep.

The values that will be used to calculate the superimposed dead load are shown in Table 3. The density of the masonry and timber are only approximate values, as the specific materials used are unknown. Table 3: Values for calculating superimposed dead load

Quantity Value Volume of stone arches 1001m3 Volume of stone parapet 15m3 Volume of masonry roof 127m3

Density of masonry 2400kg/m3 Volume of timber 139m3 Density of timber 700kg/m3 The superimposed dead load acting on the bridge

can then be estimated thus:

Psdl =10 × 1016 × 2772 +127 × 2400 +139 × 700( )

48 × 22.9( )≈ 29 kN m2

4.3 Live Loading

Footbridges less than 36m long carry a live load of 5 kN/m2. The Rialto is 48m long, and so the live load can be reduced by a factor k, given by the equation

k =nominal HA udl for bridge length / 30. (1) This gives a reduced live load of 3.7 kN/m2.

However, given that the bridge is frequently crowded with tourists, it would be conservative to ignore this reduction factor, and take the live load as 5 kN/m2.

When a footbridge does not have barriers at its ends, accidental vehicle loading should be taken into account according to British Standards. However, as there are no vehicles and only boats in the centre of Venice this will not be applicable.

The parapets should be designed to withstand 1.4kN/m, in the event of crowd loading or intentional damage; and the piers should be able to withstand a 50kN nominal horizontal impact load up to 3m above ground level. In the case of the Rialto Bridge, any significant impact loading would probably be caused by the collision of a motorized boat with the arch ring. Therefore the area of the arch up to 3m above water level should be checked to withstand this.

The parapets and arch ring would probably be able to resist these loads, as they are constructed from a material with high strength.

4.4 Wind Loading

4.4.1 Horizontal Wind Loading On an arch bridge, the horizontal wind load will act

as a UDL against the spandrel wall facing the oncoming wind. Before this force can be calculated, one needs to find the maximum wind gust vc, given by the equation

vc = vK1S1S2 . (2)

The mean hourly wind speed, v, has been estimated

as 25m/s at the location of the Rialto Bridge, based on the information provided by Ref. [3].

The wind coefficient K1 is based on the height of the bridge above ground and the horizontal wind loaded length. The height of the Rialto will be taken as the distance above water level, which is approximately 15m; and it will be assumed that the entire length of the bridge is loaded. S1 is a funnelling factor, which affects bridges located at the base of a valley or steep slope, and will not apply to the Rialto. The gust factor S2 is simply based on the height of the bridge. The values of these three factors are given in Table 4. Table 4: Values for calculating the maximum wind gust

Variable Value K1 1.58 S1 1.0 S2 1.07

For footbridges spanning less than 30m, a

reduction factor can be applied, which in this instance will be equal to 0.85. Therefore the maximum wind gust can be calculated:

vc = 25 ×1.58 ×1.0 ×1.07 × 0.85

= 36m s

This value can be used to calculate the dynamic

pressure head q, and consequently the horizontal wind load Pt, using the equations

q = 0.613vc

2,

(3)

Pt = qA1CD . (4)

A1 is the solid horizontal projected area, which will

be taken as the area of one spandrel wall, estimated from Fig. 6. The drag coefficient, CD, is taken from BS 5400; and is based on the ratio of the width of the deck, which is 22.9m, to the depth of the deck, estimated as 0.9m from Fig. 6. The final values are given in the table below.

Table 5: Values for calculating horizontal wind load Variable Value

q 794 N/m2 A1 126m2 CD 0.3

The horizontal wind load acting on the spandrel

wall can now be approximated. Pt = 794 ×126 × 0.3

= 30.0kN.

4.4.2 Uplift

As the underside of the arch is exposed, the effects of uplift must be considered. This vertical wind loading is calculated with the equation

Pv = qA3CL . (5)

As before, q is the dynamic pressure head, and A3

will be taken as the plan area of the arch, equal to the span multiplied by the width of the bridge. The lift coefficient CL has a different value to the drag coefficient CD, but is also determined by the breadth to depth ratio. For bridges with a breadth to depth ratio of over 16, as in this case, the lift coefficient has a minimum value of 0.15.

Table 6: Values for calculating uplift Variable Value

A3 660m2 CL 0.15

From Table 6 and Eq. (5), the vertical wind load

can be calculated thus. Pv = 794 × 660 × 0.15

= 79kN

4.5 Temperature Effects

Changes in temperature can cause stresses to build up within a structure. From Ref. [4], the temperature in Venice was shown to fluctuate between -9°C and 34°C,

however the site does not state over what time period this information was gathered.

British standards provides data for the UK over a 120 year period, which can be adjusted to a 50 year period as this is all that is required in the design of a footbridge. This involves reducing the maximum temperature by 2°C, and increasing the minimum temperature by 2°C.

The data taken from Ref. [4] is unlikely to have been gathered over a period as long as 120 years and so will not be altered, in order to be conservative. Therefore, the effective temperature can be estimated at 43°C. The relationship between this effective temperature and the strain is defined by the formula

ε = αΔT . (6)

Ordinarily, this strain would result in a

displacement. However, the arch has a fixed base, preventing any movement at the supports. Therefore, the strain will induce a stress in the structure, which can be calculated using the formula

σ = εE . (7)

The thermal coefficient of expansion, α is

approximately 4-12 x 10-6 per degree Celsius in limestone, and 5-9 x 10-6 in marble Ref. [5]. There has been some debate over whether Istrian stone is a limestone or marble, given that the definition of marble is fairly vague. Century Dictionary states “any limestone, even if very compact or showing only traces of a crystalline structure may be called ‘marble’ if it is capable of taking a polish, or if it is suitable for ornamental and decorative purposes.” A court case in 1899 ruled that Istrian stone is a marble Ref. [6], and for the purposes of this calculation a conservative value of 9 x 10-6 will be taken. The Young’s Modulus, E will be taken as 55 GPa, Ref. [7], which is also a conservative value for marble.

Table 7: Values for calculating stress and strain Variable Value

α 9 x 10-6 °C-1 ΔT 43°C E 55 x 103 N/mm2

From Table 6 and Eqs. (6, 7) the strain and stress

can be calculated as follows. ε = (9 ×10−6 ) × 43= 387µε

σ = (387 ×10−6 ) × (55 ×103 )= 21.3N mm2

The force in the deck can hence be calculated, by multiplying this stress by the cross-sectional area in millimeters.

F = 21.3× 900 × 22900= 439MN

4.6 Oscillation Effects

Footbridges are particularly susceptible to oscillations, as they are subjected to dynamic loads induced by the movement of pedestrians. The Rialto, however, is unlikely to be affected as it is neither lightweight nor flexible, and a simple calculation should confirm this. In British Standards, the fundamental natural frequency is given by the formula;

F0 =C 2

2π l2EIgM

⎛⎝⎜

⎞⎠⎟ .

(8)

C is the configuration factor, and for a single

spanning bridge is equal to π. To calculate the second moment of area at mid-span, the depth of the deck will be taken as 0.9m and assumed to be a solid rectangular section spanning the width of the bridge. The values that will be used in calculating the natural frequency of the Rialto Bridge are summarised in Table 7.

Table 8: Values for calculating natural frequency

Quantity Value Configuration factor, C π Length of main span, l 28.8m Young’s Modulus, E 55GPa

Second moment of area of cross-section, I

1.4m4

Weight per unit length of cross-section, M

1198kN/m

From Table 7 and Eq. (2), the natural frequency

can be estimated as

F0 =π 2

2π × 28.82( )55 ×109( ) ×1.4 ×10

1198

⎛

⎝⎜

⎞

⎠⎟

≈ 48Hz

British Standards requires the natural frequency of

a bridge to be above 5Hz, so as not to coincide with the frequency of walkers or joggers, thereby preventing resonance, and below 75Hz. The value calculated will be an over-estimate, as it assumes a solid, rectangular cross-section; but the natural frequency of the Rialto will still be much greater than 5Hz, as stone is a heavy, inflexible material. Consequently it will not be necessary to check the maximum vertical acceleration of the deck.

5 Analysis

Most single span arch bridges can be analysed using the MEXE method. However, this only applies to bridges spanning 18m or less, and as the Rialto spans 28.8m this method is not appropriate.

5.1 Mechanism Method

The mechanism method is a plastic collapse analysis, and has been utilised in programs such as ARCHIE and RING. This paper will take a simpler approach, but based on the same principles, in order to calculate the maximum vertical load that can be applied to The Rialto Bridge. 5.1.1 Assumptions

The mechanism method assumes that the masonry has zero tensile strength, infinite compressive strength; and that there is no sliding.

In assuming that the masonry has zero tensile strength, one is effectively assuming that the thrust line occurs within the middle third of the bridge. This is because, when the tensile force is zero, the axial and bending stresses are given by the formula

PA+ PeZ

= 0 . (9)

Hence the eccentricity e is equal to one sixth of the

depth of the arch, when viewed in plan. The Rialto’s arch is 22.9m wide, which is relatively large, and so it is reasonable to assume that the thrust line would not wander outside the middle third.

5.1.2 Procedure

The weakest point of an arch bridge is assumed to be at the quarter span position, and the collapse load, W, will cause a hinge to form here. Three further hinges will form, at the approximate positions shown in Fig. 7. The self weight of the arch, and the weight of any infill material, is taken as a series of point loads acting halfway along the length of each arch section.

Figure 7: Collapse mechanism

Assuming w2 and w3 are equal, taking free body

diagrams and moments at hinges C and D gives the following formulae

5.5HA + 3.6w1 = 7.2VA , (10)

11VB = 5.9HB + 5.5w2 . (11)

Equating HA to HB and resolving vertically gives an

equation which can be substituted into the following formula, produced by taking moments about A

29.2VB = 3.6w1 + 36.4w2 + 7.2W . (12)

Thus the collapse load W, is found to be given by

W = −0.5w1 + 0.77w2 . (13) As the amount of infill material is unknown, the

point loads w1 and w2 will be based on the weight of the arch only. Assuming the arch is 500mm deep, estimated from Fig. 6, the self weight is approximately 9141kN. The forces w1 and w2 can be taken as 25% and 37.5% of this weight respectively, giving an approximate value for W of 1500kN. However, this is a conservative estimate as it assumes that there is no infill, and ignores the weight of the deck; both of which would increase the strength of the arch.

5.2 Strength

Based on the loads discussed in section four, the axial force acting through the arch can be calculated. One can then gain an appreciation of the compressive strength of the stone, required to resist this force.

To simplify the calculation, the arch will be assumed to be a parabolic shape, and in pure compression. Impact and wind loading will both be ignored and the ultimate load case, using combination 1 from BS 5400, will be

w = 1.15D +1.2SID +1.2L . (14)

The factor of safety for the dead load is based on a

concrete bridge, as this is more conservative than the factor given for a steel bridge. Using the previously determined values, the UDL under the ultimate limit state will be as follows:

w = 1.15 ×113+1.2 × 29 +1.2 × 5( ) × 22.9= 3910 kN m

In order to determine the compressive force, the

vertical and horizontal forces at the springing points must first be calculated from the formulae

V = wl2 ,

(15)

H = wl2

8 f .

(16)

Where l is equal to the span of the arch, which is 28.8m, and f is taken as the height of the arch, equal to 6.4m. Hence:

V = 3910 × 28.82

= 56.3MN

H = 3910 × 28.82

8 × 6.4= 63.3MN

The compressive force will be the resultant of these

two forces, and is equal to 84.7MN. This is the force acting on the cross-sectional area of the arch; and assuming that the arch is 500mm deep, the compressive stress can be determined.

σ = 84.7 ×106

500 × 22900= 7.4 N mm2

The compressive strength of Istrian stone, given by

Ref. [2], is between 105 and 129 N/mm2, depending on whether the samples are cut perpendicular or parallel to the bedding plane. The Rialto Bridge will be subjected to a higher compressive stress than the one calculated, as the shape of its arch is less efficient than that of a parabola. Nonetheless, the stress will be of a similar order, and consequently significantly lower than the compressive strength of the arch.

6 Serviceability

Arch bridges can support large uniform loads, but have a reduced capacity under point loads, particularly at the quarter span position. Significant point loads can lead to large deflections, and cause the arch to deform.

The nature of the superstructure on the Rialto Bridge means that there will be some point loading acting on the arch. However, the magnitude of these point loads will be insignificant compared to that of the uniformly distributed dead load. Therefore, the arch is unlikely to deflect, and if any displacement does occur then this will be extremely small.

7 Durability

7.1 Chemical Resistance

Limestone and other rocks containing large amounts of calcium carbonate are particular susceptible to chemical erosion. This could be as a result of acidic gases, in industrial areas; sea water; spray from roads treated with de-icing salts; and cleaning agents, such as those used for stone cleaning. The Rialto is unlikely to be exposed to any of these sources of chemical attack, except perhaps the cleaning agents.

7.2 Frost Resistance

Freeze thaw resistance is an important consideration for stones in locations where the temperature will drop below freezing for extended periods. This will cause water in the stone to freeze, leading to the formation of ice crystals and disruption of the structure of the stone. Consequently the compressive strength of the stone may be reduced, and the natural frequency to fall.

Generally stones with a low porosity will be less susceptible to this form of erosion. Marble is particularly resistant, and as the temperature of Venice rarely falls below freezing, the Rialto Bridge is unlikely to be affected by this form of erosion.

7.3 Abrasion Resistance

The bridge is subjected to high levels of pedestrian traffic and the stone will be worn down over time. Wear is defined as “the progressive loss of substance from the surface of a body brought about by mechanical action” Ref. [5]; and is most severe in places where changes of direction occur, or where people are moving in a confined area.

The deck of the Rialto will be particularly susceptible to this form of erosion, due to the large number of people that walk across it daily. In order to move between shops, these people will be changing direction; and due to the limited widths of the three walkways, they will also be moving in a confined space. Therefore, the paving stones on the Rialto are likely to need replacing before much of the rest of the structure.

7.4 Slip Resistance

The slip resistance of natural stone paving is becoming increasingly important as safety in the use of flooring materials becomes more prominent. Most stone has excellent natural slip resistance, but when polished can become hazardous in wet conditions. The paving stones on the Rialto Bridge have a relatively rough texture, and so this unlikely to be an issue.

8 Maintenance

Maintenance was vital for the preservation of the timber bridge. One of the main reasons for the addition of shops in the fifteenth century was so that the profits could be spent on maintenance and repairs.

The current, stone Rialto Bridge is much stronger and any repair work would probably be fairly superficial. The deck is also more durable and hardwearing, but will require cleaning as illustrated in Fig. 7.

Figure 8: Maintenance work on the stone deck

9 Vandalism

Vandalism is unlikely to occur due to the large number of people that use the bridge, and the presence of shopkeepers and their staff.

If any intentional damage were to occur, the structural integrity of the bridge would probably not be compromised due to its high strength. More superficial damage, in the form of graffiti for example, could be difficult to remove but is unlikely to occur.

10 Construction

The precise methods used to erect the Rialto Bridge are unknown; therefore the construction sequence will be based around that of similar arch bridges.

When a bridge is built over a river, a cofferdam or similar structure is often required to divert the water. The Grand Canal is effectively a man-made river, and so it is reasonable to assume that a section would have been drained in order to facilitate the construction of foundations and abutments.

Antonio da Ponte was one of the first architects to recognise that placing the stones perpendicular to the thrust lines of the arch would significantly increase the strength of the abutments. This method has been adopted into the design of arch bridges ever since.

A timber falsework would then have been erected in-situ. This method, known as centring, is used to provide support during construction of the arch. The stones used to form the arch ring and spandrel walls would probably have been lifted into position using a block and timber hoist on a timber derrick. Depending on the size of the stone, horses could have been used to provide the lifting power.

The stone would have been sourced from a quarry on the Istria Peninsula of Croatia, over 100km away. It was probably shipped from Rovinj, a town under Venetian control at the time the bridge was constructed, and the primary exporter of Istrian stone.

Figure 9: Map of Venice and Istria Peninsula Mortar may or may not have been used in the joints

between stones, but it is generally considered unlikely. The arch ring is thick enough, and on sufficiently stiff abutments, to prevent the line of thrust occurring too close to the edge; and so stones are unlikely to break off, even without the use of mortar.

When the arch and spandrel walls were complete, stone and rubble infill would probably have been placed between the spandrel walls. This additional load gives the structure strength, as it helps to prevent excessive deformation.

The deck and superstructure would then have been constructed, and details such as stone carvings and adornments on the spandrel walls completed. The final stage would be removing the falsework.

11 Foundations and Geotechnics

The ground conditions were largely responsible for the repeated collapse of the timber bridge that preceded the Rialto. Figure 10 shows a cross-section of the subsoil through Venice, with the depth given in metres.

Figure 10: Venetian subsoil

Venice is situated on alluvial soil that is still

normally consolidating, and the city is consequently sinking. This can lead to differential settlement, resulting in the collapse of structures.

The position of the groundwater table also affects settlement, as the weight of a submerged object will be

reduced by the weight of water it displaces. Soil that is light will be unable to consolidate under its own weight, and settlement will then ensue.

To prevent settlement, most buildings in Venice are built on piles driven deep into the clay, allowing the vertical load to be distributed over their length. Several long piles are more efficient than multiple short ones; but little was known about foundation design at the time of the Rialto’s construction, and Renaissance engineers did not have the technology to excavate deeply.

They had however, perfected the techniques of spread footings. Six thousand timber piles were hand driven into the soil on either side of the Rialto; and capped by stone grillages so that the abutment stones could be laid perpendicular to the arch. The piles are just 3.3m long, but have been successful in preventing settlement, and distributing large forces transferred into the abutments via the low rising arch.

12 Future Changes

As an historic landmark, any unnecessary changes to the Rialto Bridge will almost certainly be prevented. It is possible that in time the bridge may require some repair work, particularly at its base. One would hope that Istrian Stone would be sourced for this purpose, in order to maintain the bridge’s appearance.

The paving stones on the deck may also need replacing, but they will be less expensive than the marble stones on the facade; and in terms of the aesthetics, it will be less crucial to find an exact match.

The underside of the arch would benefit from cleaning every so often, providing a non-corrosive agent was used.

13 Suggested Improvements

If the Rialto Bridge were being built today, a number of elements would have been constructed differently. Using fewer long piles rather than many short ones would provide a more efficient foundation system.

A parabolic arch would also be more efficient at transferring loads; however, the elliptical arch provides more height at the springing points, allowing larger vessels to pass underneath.

Increasing the amount of infill material on top of the arch would increase its strength, but as a pedestrian bridge, the Rialto carries relatively light loads; and so this would be an unnecessary expense.

The deck would be stepped at a shallower angle, so that pedestrians could see over the crest; and a ramp installed for disabled users. However, this would require the approach spans to be built up quite significantly, which would be expensive.

In terms of the aesthetics, the stone galleries and timber shops within them, make the bridge appear top-heavy. However, they are also an unusual feature with an historic purpose; and the unique shape of the bridge has given it status.

Whilst a number of changes could be made, whether or not these can be called improvements remains debateable. Either way, The Rialto Bridge is certainly a successful piece of engineering for its time.

14 References

[1] Janberg, N. (2009). Rialto Bridge. [WWW] http://en.structurae.de/structures/data/index.cfm?id=s0000461 (28 March 2009). [2] Geometrante, R., Almesberger, Dario., Rizzo, A. (2000). Characterisation of the state of compression of Pietra d’Istria elements by non-destructive ultrasonic technique. In: 15th World Conference on Nondestructive Testing, Rome, October 2000. Italy: AIPnD. [3] Risø National Laboratory. (1989). European Wind Resources. [WWW] http://www.windatlas.dk/Europe/landmap.html (29 March 2009). [4] Research Machines. (2009). Average Conditions. [WWW] http://www.bbc.co.uk/weather/world/city_guides/results.shtml?tt=TT003950 (29 March 2009). [5] Smith, M. R. (1999). Stone: Building stone, rock fill and armourstone in construction. London: Geological Society. [6] Author Unknown. (1899). Fisher v United States. [WWW] http://bulk.resource.org/courts.gov/c/F1/0091/0091.f1.0759.pdf (31 March 2009). [7] Young, P. (2003). Useful information for geomechanics. [WWW] http://www.liv.ac.uk/seismic/links/info.html (28 March 2009). [8] Holgate, A. (2005). Anderson Street (Morell) Monier Arch Bridge. [WWW] http://home.vicnet.net.au/%7Eaholgate/jm/texts/asbhist.html (8 April 2009). [9] Mainstone, R. (2001). Developments in Structural Form. 2nd ed. Oxford: Architectural Press. [10] Herle, I. (2004). History of Geotechnical Engineering. [WWW] http://tu-dresden.de/die_tu_dresden/fakultaeten/fakultaet_bauingenieurwesen/cib/studium/rehabilitationengineering/building_history/20041026_VL_Building_history_geotechnics_Herle.pdf. (10 April 2009).