MILAU BRIDGE


1. Introduction

 

The Millau Bridge provides the final missing link in the A75 highway ultimately connecting Paris to Barcelona. Prior the bridge construction traffic would have had to descend the Tarn Valley causing a bottle neck in the town of Millau. The multi-span cable stayed bridge passes over the Tarn valley at its lowest point between two plateaus. In order to do this it had to become the tallest road bridge in the world creating the world's tallest bridge piers standing at 244.96m, the structure rising to 343m at the top of the pylon. The bridge also holds the title of the world's longest multi-span cable stayed bridge with a total length of 2460m. There is a slight gradient of 3% from North to South as well as a slight curve about a radius of 20,000m. The piers are of post tensioned reinforced concrete and the deck and pylons are of steel.

 

Bridges are often considered to belong to the engineer's realm rather than the architect's. But the architecture of infrastructure has a powerful impact on the environment. The Millau Viaduct, designed in collaboration with engineers, illustrates how the architect can play an integral role in bridge design.

 




 

2460m
Alt    

204m    342m     342m     342m     342m     342m     342m 204m

601m
    675m

 

         245m     230m    
145m P6 P7


P1     P4 P5      P3

             P2    Tarn River    

 


 


 


 


 

2. Aesthetics


 
Analysis of the aesthetics of the bridge will be categorized according to the ten different areas highlighted by Fritz Leonhardt.


 

2.1 Fulfillment of Function


 

The huge concrete piers help to portray the magnitude of the construction and the huge task of building across the Tarn Valley at such height. These also help to make it clear how the bridge is supported and the importance of the piers being strong, rigid structures. It is clear when looking at the bridge which members are most important from the sizing of them. Nothing has been hidden and load path is obvious throughout the structure.

 

2.2 Proportions of the Bridge


 

Looking perpendicular to the bridge it appears that the pylons and abutments are of identical width at the deck with the abutments and pylons both splitting and tapering out to meet there. However looking almost parallel along the bridge it can be seen that this is not the case with the concrete abutments being considerably wider than the steel pylons.

 

The outside of the piers remain constant and all that occurs is an increase in the distance between the two pier 'halves' as they rise. Where the piers join the deck is well proportioned especially with the wind barrier seemingly giving increased depth to the deck

 


 

2.3 Order within the Structure


 

Although the tallest bridge in the world, the bridge appears simple with good order. The repetition of the pylons across the bridge is easy on the eye as is the constant height at which piers 'split' regardless of their starting height. The distances between piers appear equal as does the effective spans between cables. The way, in which the piers and pylons seem to flow as one, when reaching the deck is good continuity.

 

2.4 Refinements of Design


 

When looking longitudinally along the bridge in the presence of sunlight it appears that the piers are of equal thickness to the pylons. The piers have been deliberately made hexagonal in shape to produce this effect with the leading face of the hexagon reflecting the sunlight and the other sides in shadow.

 

2.5 Integration into the Environment


 

It is extremely unusual for a cable stayed bridge to span across an entire valley so integration with the environment is very important. By choosing to have a cable stayed bridge serious consideration had to be given to make the bridge look as natural as possible.

 

The morning mist and low lying cloud hide the concrete piers and gives the impression that the deck is delicately floating on them. This natural feature would have been noted and taken into consideration in the design to produce this effect. The apparent thickness added to the deck by the windshield helps to enhance this.

 

This area of aesthetics varies considerably with the direction from which the bridge is viewed. Looking perpendicular to the bridge on both clear and cloudy days the cables merge into the sky resulting in a rather elegant appearance. On a sunny day the smooth finish of the pylons cause them to sparkle and glisten and as a result make the presence of the bridge even more noticeable.

 


 

2.6 Surface Texture


 

The Millau viaduct follows the general rule whereby the piers have been given a rough finish and the deck and pylons a smooth one. The reasons for this have been touched upon in the previous integration into the environment section. The texture also helps to make it clear the materials used with the concrete left untouched and the steel all given a glossy white finish. The rougher appearance of the concrete helps to give the piers a slightly more organic appearance as well as giving a sense of strength and rigidity to assure users. The smooth texture of the steel deck, cables and pylons help it to seemingly float above the mist and clouds.

 

2.7 Colour of Components


 

The colour plays an important role depending on the viewpoint from which the bridge is observed. When viewing perpendicular to the bridge the white of the cables blend into sky behind. The pylons, also being white in colour remain visible as the thickness means that some part is always in shadow. For the piers it is their geometry which is the key factor determining the appearance and not the colour. They are shaped such that different areas are in shadow depending on the viewpoint. The colour plays very little part here except for it being light such to exaggerate this effect when contrasted against dark shadow.

 

2.8 Character


 

The design itself is fairly unique where the norm is that each pylon has stays anchoring the pylons to the ground on one side and supporting the deck on the other. This is not the case with Millau with cables either side of the pylons acting to support the deck.

 

Despite potentially complicating design and construction, the in plane curvature (on a radius of 20km) adds character. When driving over the bridge you can see each of the pylons as you progress helping you to see exactly how the bridge works. This would not be the case had the bridge been straight.

 

Holding several world records and being known for this also adds character as there is no ridge higher/longer etc. The most important feature of the bridges is its height, obviously it is this which gives the bridge most of its character.

 

2.9 Complexity in Variety


 

Complexity has very much been kept to a minimum in the Millau bridge with it obvious exactly how the bridge works. There is no structural confusion and it is clear what role each component has to play. Concrete piers support the deck at 342m intervals (204m at end spans). Additional support is provided by cables attached to steel pylons located above the concrete piers. The cables are of a semi fan arrangement whereby they are fixed at equal distance over a certain area of the pylon, neither all at the top nor equally spaced over the entire height of the pylon.

 


 


 

The slight curve gives some apparent complexity when crossing the bridge but is measured to perfection as none of the cables appear to cross therefore avoiding unnecessary confusion. As mentioned the piers are of hexagonal shape. This mildly introduces a subtle complexity as from some viewpoints the piers appear to twist as the light catches the different faces together with the taper in the longitudinal direction.

 

2.10 Incorporation of Nature

    
The bridge does have a slight organic feel about it. The slight taper of the columns is almost tree like with the much organized cables acting as branches picking up the deck.

 

3. Loadings


 
The initial study for the bridge was undertaken using French standards. The final structure was also designed to French standards as specified within the contract. The temporary steel supports and steel deck were designed and checked for instability according to Euro code 3. The loadings used in the actual design process are therefore likely to be different to those about to be considered. The loadings will be discussed according to BS 5400. As well as the basic loads applied to all bridges the geometry and design of the bridge leads to other loads and effects which need to be considered. The constant curvature introduces horizontal centrifugal loading and the single plane of cables requires consideration to be given to torsion effects.

 


 


 


 


 


 


 


 


 

3.1 Dead Loads


 

The dead load is primarily just the steel deck. The cornice and wind screen can also be considered as dead load as removing these will seriously affect the aerodynamics of the deck so will never happen. The fixings for the cables and also the cables themselves may be considered as dead loads as well as the pylons.

 

3.2 Super Imposed Dead Loads


 

The black top surfacing (a surface developed especially for this bridge), concrete and steel crash barriers, handrails and all drainage can be considered as super imposed dead loads. These are all considered permanent but can potentially be removed. When the bridge was constructed the loads just mentioned were added after the main structure (dead load) had been completed.

 


 

3.3 Live Traffic Loads


 

The bridge currently has two lanes of traffic and a narrow hard shoulder in each direction. The total width of carriageway is 32 meters including the steel crash barriers on the outside. It is therefore appropriate to take the number of notional lanes to be six. As previously mentioned there will be centrifugal loading generated by the curvature of the bridge.
This is given by:

FC = 30000/(r + 150)
The curvature of the bridge is on a radius of 20,000m so the horizontal force associated with this is 1.49kN. Braking from trucks can cause horizontal loading on the deck with a force of 8kN/m being assumed as acting along one notional lane. Accidental skidding from vehicles is considered as causing a point load of 250kN.

 

The height of the bridge introduces another collision loading in the form of impact from aircraft. There is potential for this to occur on any part of the structure. This is something obviously not covered in the British Standards but will be of similar principle to vehicle collision with a single horizontal load being designed for. This would have been a hot topic around the time construction began in January 2002.

 


3.0m 3.5m 3.5m 1.0m 3.5m 3.5m 3.0m


 


 


 

3.0m

 


 


 


4.2m


 


 


 


14.025m 4.0m 14.025m


 


32.05m


 


Bridge steel deck section


 


 


 

3.4 Wind Loading


 

The British standards apply to the bridges spanning up to 200m. Designing to these or probably to any other standards is unlikely for a bridge of this size. The deck of the bridge relies on aerodynamics to resist the wind loads. Comprehensive wind tunnel testing was carried out to gain an understanding of the decks response to the applied wind loads.

 

The standards may be of some use when considering the effects on the piers. Standard drag coefficients apply to various cross sections, for an octagon (the closest thing to a hexagon) the drag coefficient would be 1.3. These may have been used for an initial analysis before using an advanced computer model in conjunction with wind tunnel results. The importance of wind tunnel testing is crucial as in terms of dynamics it may prove impossible to successfully model the interaction of the whole structure.

 




 


 


 


 


 


 


 


 


 


 


 


 


 




 


 


 


 


 

 
 


 


 


 


 


 


 


 


 


 


 


 


 


 

3.5 Temperature Loading


 

As with wind loading the British Standards are unlikely to be of much use as all maps and data apply to the British Isles only. With the deck 2460m in length temperature effects are extremely important. The design process would have into account the stresses Induced with the expansion joints clogged. With the effective temperature range for the design process taken to be from -35ºC to 45ºC these stresses will be substantial and will considerably increase compression in the deck. Another issue is the temperature difference between the upper and lower surface of the deck. This will introduce bending into the deck for which the effect will vary depending on the time of day.

 


 

3.6 Other Load Effects


 

With substantial amounts of concrete involved in the design one of the most important loads to be considered is that associated with creep of concrete. For Millau the highest bridge pier in the world was being constructed so any changes in height, particularly if uneven across the 7 different piers would lead to adverse effects as well as potentially aesthetic problems.
The construction technique used probably generated worse loading as the deck continuously spanned 171m between piers and temporary piers unsupported by any cables as it is in its final state. The deck is likely to have experienced more adverse tension and compression than can be expected from the various load combinations during its serviceability lifetime. When looking at pictures of the bridge during its construction the undulations caused from these forces are obvious.

 


 


 


 


 


 


 

4. Structural Assessments


 

The bridge takes the form of a multi-span cable stayed bridge. Having multiple spans there are no back stays as with most cables stayed bridges to anchor the pylons to a rigid support. Instead adverse loads on one span directly interact with the next as the pylons bend to accommodate this.
Due to the height of bridge it is important that the pylons have a relatively low bending stiffness compared to the piers. If this is not the case and large bending moments may be transferred to the pier, huge bending moments would result at the base of the piers. Considering the poor bedrock of limestone containing significant cavities the piers are founded on, this would potentially cause problems.
The shapes of the pylons seem to be significant in reducing the bending moment transferred to the piers. The longitudinal A frame appears to encourage the resolution of moments into vertical forces. With the cables inducing a bending moment in the pylon, one 'leg' of the pylon will go into tension and the other compression. These forces can be transferred to the ground by the split piers.
The steel deck is placed into compression by the cable stays. The expectancy here would be to use a prestressed concrete deck due to its good compressive strength. However the chosen launching method dictated that the deck is of steel. During the launch effective spans where 171m so the ductility of steel was taken advantage of. A concrete deck may have been susceptible to cracking under its own weight which may have lead problems during its serviceability lifetime. Preventing such cracking during the launch would mean pre-stressing the deck in advance using tendons and also completely erecting the pylons and cables prior to launch, effectively pre-stressing the deck superstructure. This would prove time consuming and the steel deck was considered the more efficient option. The steel deck was seen to undulate during construction but due to its high ductility this did not result in any lasting structural problems.

 




 


Top Section

 


 


 


 


88.92m Cabled-stayed zone section


 


 


 


 


 


Bottom Section


 


3.75m


 


 

                            4.75m


15.50m


 

The temperature difference is likely to be more of a problem. This effect is likely to be greater in the day time due to the surface finish of the deck. With the road surface being black asphalt and the underside having a white finish the effect of the sun heating the deck may be exaggerated. This will cause a sagging moment as greater compression will be induced on the upper surface of the deck as it tries to extend more. The design of the bearing above the piers is very important here. The deck must be allowed to expand as necessary and excessive restraint at the piers can cause increased compression. A transfer of moment to the pier will occur if the connection is stiff which will then in turn have to be resisted by the foundations as previously mentioned. The likelihood is that the bearings allow a limited amount of rotation to prevent this transfer of moment A careful balance is required taking into account the various load conditions ensuring the deck remains predominantly in compression but to avoid buckling. Having a steel deck means that if adverse loading causes the deck to go into tension at some point, this is not a problem due to steels ductile properties.

 

5. Construction


 

Constructing the worlds tallest road bridge was always going to be extremely difficult. There are traditionally two methods used for constructing cable stayed bridges, incremental launching and cantilever construction. Working at such height poses significant risks as well as the cost involved in lifting sections of the deck over 200m. The design of the bridge also deems this method inappropriate as a single pier and pylon cantilevering deck from either side would be very unstable and susceptible to wind.
The decision was therefore made to launch the deck incrementally which itself posed many risks. A launch of this size had never been undertaken before and new technologies had to be developed to slide the deck out into position.
Firstly the foundations for the piers had to be constructed. The concrete for these and the piers was produced in newly built plants close to site to minimize transportation costs. This was important as a recently developed concrete was used so anything which could potential impair the quality had to be eliminated.

 

5.1 Foundation


 

The foundations for each pier consisted of four bored piles ranging in depths of 9m to 16m. The piers were then constructed on top of the pile cap. In order to satisfy aesthetic requirements these pile caps where buried and hidden from view after construction.

 


 


 


 


 


 


 


 


 


 

5.2 Pier


 

The piers are constructed of reinforced concrete. The formwork for the piers was a revolutionary self climbing device using hydraulics. This removed such a need for manual work where the only input required was slightly changing the alignment of the formwork after every four meter rise. This system was only used on the outside with a more traditional formwork system used to form the inside of the piers. This was lifted by crane as required and adjusted accordingly for each new section. Obviously self standing cranes could not be used so the pier itself acted as a support for the cranes as they grew in height. When the piers split 90m below the deck and continue effectively as two separate structures additional self climbing formwork was required. The same process of pouring four meter sections at a time then continued as before.

 




 




 


 


 


 


 


 


 


 


 


 


 


 


 


 


 




 


 

5.3 Temporary Support Towers


 

It is impossible to move the deck from pier to pier without intermediate support towers. With the first piers nearing completion the temporary intermediate supports were constructed. Halfway between each concrete pier support towers enable the bridge deck to reach a temporary resting point. Because of the length of the spans seven temporary intermediate steel piers will have to be built to support the deck during launching. The two temporary towers closest to both abutments will be erected with the use of cranes as they are only 12m and 20m high. The five other temporary support towers with heights from 87.5m to 163.7m will be built up by hydraulic stage lifting. The hydraulic technique is developed by specialist Enerpac.


 

5.4 Steel Deck

The steel deck, which appears very light despite its total mass of around 36,000 metric tonnes, is 2,460 m long and 32 m wide. It comprises eight spans. The six central spans measure 342 m, and the two outer spans are 204 m. These are composed of 173 central box beams, the spinal column of the construction, onto which the lateral floors and the lateral box beams were welded. The central box beams have a 4 m cross-section and a length of 15–22m for a total weight of 90 tonnes. The deck has an inverse airfoil shape, providing negative lift in strong wind conditions. To allow for deformations of the metal deck under traffic, a special surface of modified bitumen was installed. The surface is somewhat flexible to adapt to deformations in the steel deck without cracking, but it must nevertheless have sufficient strength to withstand motorway conditions (fatigue, density, texture, adherence, anti-rutting, etc.). The "ideal formula" was found only after ten years of research.
The steel deck was fabricated offsite by one of Eiffage's subsidiary groups Eiffel. The deck was transported to site in sections by road. Sections of the deck were launched from both plateaus and met above the river Tarn, where it was impossible to construct a temporary support. The launching of the deck was undertaken essentially using a lift and push system which occurred all in one motion. The hydraulic system was designed and built by Enerpac's Construction Centre of Excellence in Madrid, Spain. This system was especially developed for the launch as due to the magnitude of the task, no such device existed capable of doing the job.
The technically advanced hydraulic system is designed to push the deck from both sides onto the seven concrete piers during the launching process; the deck will be supported by seven temporary metal piers (pier T7 to T1). The enormous yet at the same time "light" deck is pushed by means of hydraulic launching devices on each pier, which first lifts and then pushes the deck. An adjustable nose structure at the end of the deck, allows the deck to land on each pier as it approaches it.



 


 


 


 


 


 


 




 


 


 


 


 


 




 


 


 


 


 

Basically, each system consists of a lifting cylinder, with a capacity of 250 ton, lifting the deck off the supporting structure of the pier, and two or four skates, each equipped with two 60 tons cylinders, which retract to launch the deck a maximum of 600 mm. All of this rests on a system of single-acting lock nut cylinders supporting both the launching device and the deck.
     The launching process was started on the western slope (C8) with two launching devices, each with two 120 ton cylinders. In total, in the last phase of the launch, there will be 5280 tons pushing capacity from the southern slope (1752m of deck) and 2400 tons from the northern one (708m of deck, making up a total length of 2460m).

The weight of the deck means that, as it is pushed along and gets further from its support, it curves downwards, as to be expected the cantilevered deck sagged and obviously this was at its worse as the deck approached its next support. In order to compensate for this deviation, a nose recovery system is constructed at the end of the deck. This independent system, consisting of a hydraulic group of four 270 tons cylinders, pulls the nose upwards to the level of the skate. Another hydraulic system allows the nose-end to pivot. Without this the deck would have simply impacted with each support. To help reduce this sag the first pylon on each launching deck were constructed and six out of the 11 cables attached.

 

All hydraulic systems for pushing the deck are operated from the Control Centre on the bridgehead. This control centre receives data via a PROFI-BUS cable, where it is automatically handled so that the system can follow the parameters established when programming the cycle. Although all hydraulic systems installed on each pier are controlled from this centre, each single hydraulic system has a local control panel, which allows local movement of the skates to be made from that pier independently, as long as this is allowed by the Control Centre, which in turn must receive the approval of each local control centre in order to make synchronized pushing movements from all the pushing cylinders of all the piers. The outer cylinders on each pier have a positional transducer that indicates the amount of travel, and each hydraulic system has its independent hydraulic control centre.

5.5 Pylons


 

The pylons were each transported onto the deck in a horizontal position using crawlers. Once in position they were raised and connected to the deck. The method for doing this was inspired by the Ancient Egyptian means of raising obelisks. These steel pylons were then connected to the deck by a method of high strength welding. The pylons are of adequate size for welding to be carried out inside the pylon as well as outside to create a good joint.

 


 

While the pylons were being erected the cables would have started to be attached to the already erected pylons including the two in place for the launch. Several steel wires making up the cable would have passed through protective tubing and then anchored onto the prefabricated locations on the deck and pylons. The final stage of construction was to dismantle the temporary supports.

 


 


 


 


 


 


 


 


 


 


 

6. Conclusion

The construction of Millau Bridge in France has proved the ability of the engineer in site and the innovative techniques used by them to overcome the challenges against nature to provide a successful transportation facility for the people of France considering the aesthetic of the structure which in turn reflects the pride of the country. The formwork used for the construction of pier was a new technique which works automatically using a hydraulic machine, saving time and manpower giving precision output. Similarly the technique used for laying deck slab uses the same principle of hydraulics, which is supported by temporary pier gives a new idea for the engineers working in the field of bridge construction. On overall the construction of Millau Bridge proved as one of the efficient way of engineering talent serving the purpose for the country.

 


 


 




 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 

 


 


 

    

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