Milo Is Known As The Tallest Cable-Stayed Bridge In The World. A Bridge In France That Crosses A Huge Valley And Connects Two Plateaus. How Did They Build The Tallest Bridge In The World?
Milo Bridge is a cable-stayed bridge in France, built in a valley of the Tarn River near the city of Milo. The town of Milo is located at the confluence of the Tarn and Dorbi rivers. These two rivers have created two profound valleys on southern France’s old plateau of the Central Massif.
Milo is known as the tallest bridge in the world. One of the towers of Milo Bridge is 343 meters high. It was designed by French structural engineer Michel Virlojo and British architect Norman Foster.
The story of the bridge and, in fact, the feeling of the necessities for creating such an expensive and unique structure on a bed of different soils and rocks, which we will talk more about in the following sections of the article, began when each year on the Paris-Spain axis near the city of Milo.
However, during the busiest and busiest days of the year, there were massive traffic jams and delays. Officials in those years sought a sustainable way to solve this problem.
The first idea to solve this problem was proposed in 1987, and in 1991 decided to build a bridge over the Tarn River valley.
From 1993 to 2001, the French government consulted with numerous architects and engineers, conducted various descriptive studies by the government, and organized an engineering competition to find the best design for such a bridge. As mentioned, the French authorities intended to connect the 600-meter-high North Plateau to the 720-meter-high Larzak Plateau via a suspension highway.
Therefore, choosing the exact route of this highway was not easy, mainly since the lower parts of the region’s hills were primarily composed of unstable clay. They crossed Various courses to cross the bridge.
Proposed Four main routes, and finally, one of them (the red route in the picture below) was chosen as the appropriate route for the project. After setting aside other impractical options, I finally decided to connect the Milo Bridge on two plateaus directly on either side of the valley at 275 meters above the Tarn River.
In 1996, the Soglerg Consortium proposed a cable bridge designed by Michel Virloge and Norman Foster.
Their plan was finally accepted and approved. The construction process officially began on October 16, 2001, and in November 2003, the first phase of construction, which included the installation of the first main pillars of the bridge, was completed.
The carriageway or bridge section of the bridge was built in May 2004, with towers or towers and restraints in the second half of 2004, and finally, this great crossing over the Milo Valley on December 16, 2004, 25 days ahead of schedule. The planned event was inaugurated by the then President of France, Jacques Chirac.
The construction of this bridge broke several records in those years. As mentioned earlier, the Milo Bridge has the tallest mast or columns in the world on both sides (245 meters and 221 meters) as well as the tallest bridge tower in the world (343 meters) and the tallest deck among the road bridges built in Europe with a height of 270 meters.
The French government outsourced the bridge to contractors after a design phase.
The main idea of the designers was to build a narrow but fantastic bridge, so designing a cable-stayed bridge with the same decks was on the agenda. Milo Bridge is so high that it can be easily seen from Milo city.
Following the decision of the French Ministry of Public Works to propose the construction and operation of such a bridge, they sought to have it contracted out to a contractor. Therefore, an international call for tenders was issued in 1999, with five consortiums bidding. Finally, the Compagnie Eiffage du Viaduc de Millau team, in collaboration with architect Norman Foster, succeeded in bidding for the bridge project.
As the French government had already completed the design work, it significantly reduced the technical uncertainty for the project contractor. Another advantage of this process was the facilitation of contract negotiation, the reduction of public costs, and the acceleration of construction while minimizing the remaining design work for the contractor.
Where is Paul Milo?
Milo Bridge between two plateaus is made of limestone. The bridge’s foundations are located in a deep valley, a valley formed by erosion caused by the Tarn River. Limestone plateaus are a type of sedimentary bed that began in the Middle Mesozoic. This plateau has remained unchanged even after the construction of the Milo Bridge and has been well protected so far. Here are some tips for describing the geography of the Milo Bridge:
As mentioned earlier, the construction site of Milo Bridge is composed of sedimentary rocks. These sedimentary rocks are primarily dolomitic limestone and Khark -man (a mixture of clay, sand, and limestone) with low adhesion.
The study of the site’s historiography shows that the old faults of the region have affected the ancient strata. These old faults are located in the northern part of the bridge and have not affected the newer rocks at the bridge construction site on the high plateau of the southern table. There are also newer inactive faults at the bridge site that have altered the arrangement of the rock layers. These faults pass through the P4 base and the area between the P7 floor and the C8 flank wall (Figure below).
Continuous landslides caused by faults near the P4 foundation construction caused construction problems that required a change in the design of the foundation.
On the other hand, the rock mass rating (RMR) varied between 0 and 150, meaning that the values recorded at the bridge construction site Milo were 65 for limestone and 53 for marl.
There are three different types of foundation stones in the construction of Milo Bridge. One of these rocks is Bajocin dolomitic limestone in the northern support wall, which is a hard rock with unlimited compressive strength of 110 MPa, But there are also clay karsts in it. The RMR was detected between 70 and 80 at the platform’s top and where the raft bridge is located.
The second type of stone is compact marl, located between the bases of P7 and P6. Due to a pebble layer between the soft clay and the marls, continuous landslides are visible in this area. The average shear strength for the 15-meter layer of marl is equal to:
RMR = 45
C = 0.1MPa
φ = 300
Hetangin limestone forms the third type of rock on both sides of the Tarn River between the P4 base and the retaining wall. The layering of this stone in the southern part is below horizontal, and in the northern part with an angle of 150 degrees. The values of shear strength for this type of bedrock are:
RMR = 65 to 70
C = 2.5 MPa
φ = 370
The figures mentioned here may be too confusing for some readers. But the summary of the comparison of the three mentioned beds for the bridge is that limestone has more resistance than marl soil. Therefore the builders made deeper piles in the beds containing marl than in the limestone bed.
Cross-section of Milo Bridge
Milo Bridge is 2460 meters long and is made of 8 decks. Each side deck is 204 meters long, and the six middle decks are 342 meters long. Due to design and construction considerations, the bridge’s central section comprises optimized orthotropic girders with two vertical grids.
Triangular crossbeams with longitudinal distances of 417 m are preferred to use full diaphragms. Boxes support two incoming lines. The shoulder width on both sides of the bridge is 3 meters to increase the distance of the line from the edge of the bridge to reduce the effect of dizziness.
In addition to the usual side fences, the bridge girders are equipped with windshields to limit the wind speed so that the wind speed on the bridge is equal to the ground level. It prevents air shock to vehicles entering the bridge, improves aerodynamic flow, and enhances the bridge’s beauty.
Details about the Paul Milo Foundation in France
Michel Virlogs designed milo Bridge, and the authorities created the foundation system for the foundations and side walls of the bridge (Cole Bridge) based on his designs.
Although the design of the entire bridge foundation system is based on the same principles, the foundations of the bridge foundation vary depending on whether the bed is limestone or marl. Marl soil not only has poorer mechanical properties than limestone but also has a surface slip and affects the upper layers.
Extensive foundations have been used to build the side walls C0 and C8, which are located on the limestone bed. The foundation system of each front wall is a wide raft with a thickness of one meter, which is connected to the heels of the two rear walls by different platforms.
The foundation system of each of the seven piers of the bridge consists of four reinforced concrete piles, each with a diameter of five meters and a drilling depth of 10 to 15 meters inside the bedrock. In the upper part, these piles are connected by a reinforced concrete heel with a thickness of 3.5 meters, and the concrete heel is also connected to the base of the bridge.
In Marl soil bed, piles are thicker and more profound, and their base diameter reaches 7 meters.
Pillar No. 2, with a height of 245 meters, is the bridge’s tallest pillar, built on a limestone bed. It is while the base number 6 is made on the Marl bed and has a medium height.
The behavior of this type of foundation system is complex. The foundation is of the pile raft type; part of the bridge load is transferred to the heels. Simplifying the behavior of the foundation has its limitations. The first assumption is that the heel of the bridge between the foundations does not withstand any load, and the second is that there is no frictional friction along the shaft except for longitudinal stress.
The design of foundations and piles has become quite reliable despite some challenges.
Simplification of the foundation behavior leads to the assumption that the bridge seating capacity depends only on the absolute bedrock pressure under the shafts; Therefore, the final subsidence of the bridge is due to the deformation of the rock under the post, and this makes the foundation of the bridge more flexible than it is.
Several pile loading tests have been performed on Marl soils to evaluate the surface frictions along the shaft. One of the tests performed on an in-situ spark plug with a diameter of 0.8 m showed that the critical load of the spark plug is 5200 kN, and the subsidence rate is 5.6 mm.
Despite the doubts about the estimation of the mechanical properties of the bedrock and the computational methods used, the design of the foundation and piles seems to be reasonably reliable.
Bases of Paul Milo
In the design of the bridge, primary structural considerations have been anticipated. Several different cable openings have been used to balance the asymmetric loads and to consider the effects of temperature on the bridge girder. The box cross-section design has been used. Also, the upper part of the foundations (90 meters end) has been divided into two flexible shafts.
The bridge deck is connected to the piers by prefabricated cables with two fixed seats in each shaft. Also, the bridge towers above the foundations are designed in an inverted V-shape. Due to the change in live load (load due to vehicle traffic) and load due to strong winds, the vertical load of each dwelling may reach 100 megawatts.
Spherical bearings are used to coat a new type of composite material that increases their stress strength up to 180 MPa under critical loads.
The cross-section of the bridge piers is variable; changing the cross-section of the ports is designed so that their construction of them is not too complicated. The four panels have fixed dimensions, but the size and orientation of the other four panels change slightly in each section. Used External self-lifting forms to build the foundations, and tower cranes moved internal shutters.
The two high piers of the bridge, P2 and P3 are 245 and 223 meters high, respectively. Each bridge pier is built on four wells with diameters of 4 to 5 meters and a depth of 9 to 16 meters. The tallest tower crane used to make the bridge was the P2 base, which eventually reached a height of 275 meters. Therefore, at each stage of the foundation construction, it was necessary to attach a tower crane.
Milo Bridge Towers
After the blockade of the Tarn River upstream on May 18, 2004, the Milo Bridge towers, various built-in factories assembled in the area behind the side walls of the bridge, were pulled separately into the bridge deck by two crawlers. During the bridge’s construction, the weight of each load of the towers reached eight meganonatons, which was a difficult test for the structural strength of the bridge.
The tower towers, which stretched horizontally to their location, were lifted by cables attached to a temporary support tower. The construction of the buildings was completed after the installation and extension of the retaining wires by the Freising system.
Launching system used in the construction of Milo Bridge
The girder deck of Milo Bridge is made by two launch systems (horizontal displacement system of openings) on both sides of the bridge; The final connection of the craters took place above the Tarn River and between the P2 and P3 piers. In each of the openings, except for the last door, temporary support structures in trusses were used.
To reduce the length of the launch opening in the middle doors, these temporary support structures, each measuring 12 by 12 meters, were located right in the middle of the space and next to two lines of launch equipment. Support structures at the side openings were more straightforward, smaller, and had only one line of launch equipment.
Six support cables connected the two launch structures to the front towers to reduce the bending moment during the launch operation. To minimize the effect of wind on the opening operations of the openings, the height of the buildings was limited from 87 m to 70 m without connecting their ends.
Each launch operation involved a 171-meter displacement of the openings.
It took five days to move the first part, which was the most complicated part of the operation. For other details, this time was reduced to three days, provided the weather was favorable. If the meteorological stations forecast winds of more than 37 km / h, the launch of the launch operation would be delayed.
The Milo Bridge launch system was innovative in its way. Due to the very high height of the foundations, it was necessary to ensure the balance of friction forces in each supporting structure.
Therefore, each supporting structure was equipped with an active launch seat. Horizontal hydraulic jacks in the seats moved the facilities forward at the command of a central computer. Sensors were also used to monitor the displacement of temporary support structures because the removal of the supporting structures had to be equal during the launch operation.
Milo was an excellent example of a project with a calculated purpose.
In addition to its transit functions and traffic junctions, this bridge also became one of the region’s tourist attractions. A large number of tourists stop at this bridge at different times of the year and spend some time watching the surrounding scenery or taking pictures. Tourist stops on the bridge eventually reduced the speed limit on the bridge road from 130 km / h to 110 km / h.