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Structural analysis of Palladio’s timber bridge in Bassano del Grappa

22 May, 2019 13 min reading
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Based on the thesis of: Francesca Porta, Building Engineer at Yellow Room Engineering Genova, Liguria, Italy

Structural analysis of Palladio’s timber bridge in Bassano del Grappa

The Bassano’s bridge is a covered timber bridge 65m long and 8m wide located in the Veneto Region in Italy. During its history it was destroyed several times and then reconstructed following the idea of Palladio’s original project, which dates back to 1570. Reconstructions and refurbishments provoked some c hanges in the structural behaviour, especially in the foundation system; further study is required in order to understand the current structural behaviour of the foundations. Moreover, the structural degradation and the heavy weight led to significant settlements that required urgent consolidation.

 

This thesis by Francesca Porta is a continuation of the previous studies of the bridge and it aims at analysing the stress levels of the foundation elements with respect to different loads configurations and at understanding the role of the piers' geometry in the out of plane behaviour.

 

Location 

The timber bridge called 'Ponte degli Alpini' is located in Bassano del Grappa, a town it the Vicenza province, in the Veneto region, in the northern Italy. The first news regarding the existence of this medieval city dates from 998, several families succeeded in the government of the town, such as the Ezzelini in the 13th century, for a long period it was part of the Republic of Venice who had an important role in the bridge reconstructions.

The Bridge is located on the river Brenta, it runs from Trentino to the Adriatic Sea just south of the Venetian Lagoon. The river in the area of Bassano is still stormy and floods are frequent.

 

Two hydraulic structures are present in the portion of the river inside the town: 400m north there is the Arcon weir where a secondary channel starts, instead 250m south there is the derivation of the Medoaco channel.

Morphological Description 

It is important to describe the complex structure of the timber bridge before the historical overview of the main phases. The timber bridge of Bassano is a unique example in Italy, its structure and style is not similar to any existing case. The actual structure is a result of a long history of almost one thousand years: several reconstructions and repairs were necessary and the river force has been a problem through all these years because of its energy and the frequent floods. The main project is from Palladio, who construct the bridge in the 1567 (paragraph 3.1).

 

The bridge present five spans of 13.2m; the structure is loading on the two abutments and on four central piers.

In the Four Books of Architecture, written by Andrea Palladio, there is the description of the bridge and the drawings defining the different parts (Figure 4). The Palladio's drawings present five spaces between the columns for each span, instead the actual bridge just four.

+ is the water level

A is the longitudinal view of the bridge
B are the piles inserted in the river
C are the heads of the transversal beams
D are the beams posted in the longitudinal direction (supporting beam)
E are the oblique elements (strut)
F are the columns
G is the transversal section of the bridge
H is the plan
I is the referring measure of ten feet

 

The structure consist on five spans resting on four piers in the riverbed. Each pier has eight piles of the same length (1.1m) supporting the structure above, as well as 6 more columns of decreasing length on both sides, protecting the bearing columns; these triangular lateral elements are called Rostri in Italian. Two transversal beams connect the columns together on the top and at the base. The columns are covered by a planking structure with wooden elements (10x17cm) with a spacing of 25cm. The connection between columns and beams is done by threaded bars of 16mm diameter; instead, the planks are not connected horizontally and their main aim is to protect the internal columns. In the 1991, a survey was done by the engineer Ugo Bonato, his drawings are a great source of information. Each span presents the same type of structure: longitudinal beams loading on the cantilever beams which is supported by transversal continuous beams. The latter distribute the loads on the eight pile constituting each of the eight pier.

The oblique elements (struts), taking load from the longitudinal beams, are connected by the longitudinal supporting beams. The connection between those two elements is done by metallic plates 8mm thick and 9 bolts of 16mm diameter. The other connections are made with bolts of 25 or 30mm diameter with the use of Appel connectors.

History of the Construction 

The timber bridge in Bassano del Grappa was built between 1124 and 1209 over the Brenta river, it was designed in order to connect Bassano to Angarano, whose nowadays are part of the same municipality. The bridge was constructed for economical, social, political and military reasons, it was the connection between the town and the road direct to Vicenza; because of its position, it was fundamental in the goods transportation. Due to its strategic importance, two towers were built in order to protect it, even dough it was destroyed many times during its history. The bridge felt down five times and it was destroyed by men other three.

 

The first information regarding a bridge in this position is from the XII century, when the city wall was built. The Ezzelini family wanted the construction for economical reasons, the connection of Bassano with Vicenza was important due to their alliances. The bridge was built in accordance with the city wall and two defensive towers were located on the two rivers (Figure 9). Other than due to the severe and frequent floods, the bridge was damaged by the materials, such as wood, transported by the water flow. The timber trunks, coming from Trentino, were used as a building material and as combustible and they were transported to the port of Brenta, southern than Bassano; this type of commerce represented a source of richness for the main families of the town.

In the De Pontatico Pontis Brentae (1259), it is stated that in order to cross the bridge people had to pay a tax; moreover, in the De lignis Pontis Brentae, it was said that whoever steal wood from the bridge have to pay 40 coins, these documents testify the importance of the bridge at the time.

 

In the Middle Ages the bridge was protected by the two over mentioned towers, even thou adjustments and reconstruction succeeded over the years. After the 14th century, the town of Bassano was under the domination of Venice, the 'Serenissima'. In the 1406, the bridge was destroyed by floods and in the following year the reconstruction took place, mainly founded by Venice and other close towns. However, in the 1450 the bridge collapse again and the doge Francesco Foscari ordered the city of Bassano the re-construction. In the 1452, Jacopo Barbano was in charge for the reconstruction, the following year the bridge was opened again. Due to a flood, in the 1493, the bridge was severely damaged and it was necessary another reconstruction. It took a few year to decide which was the most suitable material to be used; the idea of a stone bridge was rejected and the bridge was built as it was, in timber. In the 1506, an inspection regarding the safety and the conditions of the bridge was done as requested by the doge of Venice; at the time the bridge was well built and the roof was already present. The bridge was already a symbol in the landscape of Bassano.

 

After a war between the Lega dei Cambrai and France, in the 1509, the powerful French army conquered Bassano. One year later, the latter burned the bridge to stop Bassano troops' advance. In the 1522, the bridge was rebuilt, and again in the 1524, but using stones token from Priara. The new bridge was composed by two arches with one pile in the center, it was wide 28 feets and high 27. The masonry bridge lasted only a couple of years, in the 1526 there was a collapse due to a flood of the Brenta river. As it happens before, when the bridge was under construction a ferry was used to move the goods and the people. Six years later the Potestà Alvise Grimani ordered to destroy the bridge and rebuild it in timber with the original shape, in the meantime the occidental access was restored. The power of the river strike the bridge again in the 1567.

 

  • The bridge from Andrea Palladio

The following year the Council of Bassano ordered a new bridge, and in the 1570 the project was awarded to Andrea Palladio. The architect proposed a three-arches bridge based on the classic roman model; this project is close to the one designed for Rimini and present in the third of the four books about architecture written by Palladio (Figure 10).

However, the population of Bassano strongly request a timber bridge, it was a symbol of the city from years and a stone one would not be appreciated. The city council of Bassano ask Palladio to design the new bridge as the previous but with all the necessary modifications:

 

'esso ponte sia rifatto e costrutto nel modo et forma che era il precedente menato via dalla Brenta, con quelle aggiunte che parer alli proti che lo costruiranno'.

 

The city of Venice allows Bassano to cut of 200 oak trees for the new project, before the project approval the material had already been set up.

It is hard to say how free Palladio was in the project, he was asked to build a timber bridge but it is not clear if he entirely designed it o if he could just make some adjustments. According to Francesco Memmo, the result is a bridge similar to the previous one, there was probably not so much space for the architect choices. In addition, Guglielmo Marchesi, not Palladio, supervised the operations since July 1569. In the Four Books of Architecture (Venice, 1570) Palladio describe and present the project of the bridge for , he affirm he was asked to realize a timber bridge, but he never say that he also designed it. Palladio, anyway, applied his concepts about architectural orders and proportions to the bridge that was previously an historical element of the city with a more 'spontaneous' design.

Palladio built the scaled model and the building phase was assigned to Marchesi and the bridge was ready in the 1570. After a survey done by Giovanni Piccoli in the 1593, the dimensions of the columns result different from the Palladio's drawings; the poles should have been on one piece but it was hard to find the trees of this dimensions (30 feets long and 1.5 wide), for this reason they built them in two pieces. A few repairs and substitutions were needed but the bridge lasted until the 1748, when a flood destroyed it.

 

The following year, the rebuilt was done by Bartolomeo Ferracina exactly as the previous one. The project of Ferracina was cheaper than the other proposed by Giovanni Miazzi and Tommaso Temanza. Ferracina used a machinery of his invention in the building phase, it was an innovative machine to beat the poles using the water energy and it was supporting the building at the same time. Nevertheless, some modifications has been done: a new balcony is present at the middle of the bridge and the distance between the columns is different, from 5 to 4 column-span each pile-span; however the balcony was already present before Palladio's design. The result was even more similar to the original bridge than the one of Palladio.

 

Miazzi built a temporary structure in the 1749 to link the two riverside during time of the bridge construction, this structure was destroyed by another flood a few time later. The 30 of September 1751 the bridge designed by Ferracina was opened.

The new bridge was resistant, in the records there are just a few repairs in the following years. However, between the 1796 and the 1813 it was damaged by military campaigns. From the 1805 Bassano is part of the Regno d'Italia but during the Napoleonic time it was damaged by many battles such as the one of May 1811 (cannon ball holes are still visible on the façade of the distillery building next to the bridge). In the 1813, during the war between Austria and France, the French troops burn the bridge. Between the 1819 and 1821 the bridge has been rebuilt by Angelo Casarotti da Schio. The new architect made some modifications respect to the project of Ferracina: he modify the supporting structure adding a new longitudinal beam in the foundations in order to have a better distribution of the forces, in addition he took off the additional beams and the balconies in order to reproduce the Palladio's project. A drawing from the 1852 describe a intervention to straight the columns by Sebastiano Mocellin but it is hard to say if it was realized or not.

 

  • The Bassano Bridge in the 20th century

At the beginning of the 20th century the traders wanted a wider bridge for the movement of goods but this plan was interrupted by the First World War. During the conflict the bridge was largely used as the main supply route for alpine troops, the vehicular and pedestrian traffic was intense; the bridge was damaged by the bombing and repaired in the 1923.

 

At the end of the Second World War, in the 1945, after the destruction of the new Bassano bridge the Partigiani blew up the bridge in order to stop the German invasion, but that was not too effective and it was still possible to pass. A few months later the German army destroy the occidental part the bridge as a revenge (Figure 14).

In the 1948, the president of the Italian government, Alcide the Gasperi, inaugurated the restored bridge. The expensive works were partially founded by the National Association of the Alpine for this reason the bridge is nowadays known as 'Ponte degli Alpini'. The repair was done by the local construction company Giulio Tessarolo e Figli, during the works six overflows caused damages. The wood used was of larch, oak, chestnut and locust tree, over 400 cubic meters. The image of the bridge we have nowadays is almost the one of the 50s, even if in the 1966 there was another flood and the pavement was changed in the restoration. During this event the central part start to bend in the south direction.

 

In 1968-69 there was an intervention to restore the bridge, some parts were substituted, in the 90s new works were necessary.

 

STRUCTURAL ASSESSMENT

The aim of the project is to achieve a high level of knowledge of the timber bridge of Bassano in terms of structural behaviour and specifically on the safety of the foundation system. The understanding of out of plane behavior is one of the main issues, for this reason two different bridge configurations are analysed: the actual state of the structure and a second configuration without the Rostri elements. The second configuration is analysed because historically those elements are the most damaged by floods and, therefore, vulnerable. The safety level of the structure is analysed with three main load states representing the conditions the bridge may suffer.

 

The Floods condition, this configuration is the one that caused the worst damages in the structure provoking the collapse many times, this is the case concerning the municipality and the community the most. A static analysis is proceeded taking in account the river load corresponding to the river flow of the 1966.

 

The daily state, this load configuration represent the daily life of the structure, a static analysis is proceeded. The daily loading is a good instrument to relate the effect of the exceptional actions to the every day stress level of the bridge.

 

The earthquake condition, in this case the seismic dynamic analysis is proceeded to evaluate the risks connected with this type of natural disaster. From the 2008, all the Italian regions result to have a seismicity hazard, therefore it is necessary to evaluate the structure response to this action. In this project the response spectrum analysis is proceeded following the Italian code.

Conclusions

After the damage survey and the monitoring performed in 2015, it was evident that a structural assessment of the Palladio's timber bridge was required. The high level of deformation and decay is a concern for both the safety of the pedestrians and for the conservation of the historical monument, which is a symbol of the town of Bassano del Grappa.

 

This thesis project is focused on two fundamental aspects: the assessment of the foundation system, which was modified many times during the bridge's history; and the analysis of the role of the Rostri elements, which affects the structure especially in the out of plane behavior of the structure.

 

Based on the analysis carried out in this thesis, the Bridge was determined to be unsafe because of the high stress levels at the foundations. Even if temporary structures are present, a retrofitting plan is urgent and necessary.

 

The results of the three analyses, performed with different loads-states, shows that the minimum safety level is not reached. Applying the quasi-permanent combination, the foundation elements are overstressed, meaning that the daily state is not safe. In addition to that, it has to be accounted that the 90’s heavy floor has been already removed decreasing the daily state stress. The flood state resulted in high levels of shear stresses at the base of the structure; unfortunately, this accidental load is not so rare: many times the structure was damaged by the river flow.

 

Finally, performing the linear dynamic analysis, the earthquake's effect was analysed. Comparing the stresses at the foundations, this condition resulted in lower stress levels than the flood case, but still many elements are not safe in respect to the code's limits.

 

The reason why the foundation system is nowadays in these unsafe conditions may be found in the decay process, in the natural riverbed modification and, furthermore, in the changes of the structure done during past reconstructions and refurbishments. The results of the analysis, showing the unsafe level of stresses, are comparable with the strong level of damage of the foundation timber elements.

 

The second main aspect of the project is the role of the triangular-shaped portion of the piers, called Rostri. In order to understand their structural role two FEM model were analysed: one with and one without these elements. Performing the same analysis for both the cases, it turned out that in the second case the stress levels at the base are higher, especially in the out of plane direction (Y). In the case of vertical static loads, the role of the Rostri is negligible; on the other hand, if the structure is horizontally loaded (such as in floods or earthquakes) their role becomes evident. The presence of the Rostri makes the structure stiffer in the out of plane direction; therefore, the displacements are much lower.

 

The analyses of the two models, with and without the Rostri elements, represent the two limit cases, where all possible damage states will fall in-between the two extremes. Therefore, it is possible to conclude that it is necessary to include the Rostri elements in the seismic analysis; this because a higher stiffness correspond to a higher seismic force. In case of flood state analysis, instead, it is safer not to include the Rostri in the model and take in account the possible damaged configuration of the bridge.

 

The flood and the earthquake states are compared, but it is important to notice the difference in their load distribution and in the nature of the load itself: the flood load is mass-independent and applied only at the piers, instead the earthquake action is mass-dependent and applied to the entire structure.

 

To conclude, it has been demonstrated that the worst scenario, in terms of stress levels at the foundation, is the flood-state in the absence of the Rostri elements at the piers. The performed analyses and element verifications, together with the understandings on the structural behavior of the bridge are one step in the direction for conservation, in the endeavour to maintain our cultural heritage and preserve it to the future generations.

 

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