A great new work of French Structural Engineering and Spanish/American Hydraulic Technology
Customer:Eiffage Group TP
Location:Millau Viaduct, France
In June 2002 Enerpac (Hydraulic Technology) was awarded the contract to supply the hydraulic system hydraulic system for lifting the temporary piers and pushing the bridge decks for the Millau Viaduct Project. The erection of the highest bridge in the world started in October 2001. The construction took about 39 months ending in January 2005. In January 2004 six intermediate temporary piers have been completed. After considering various route options, on 28th June 1989 CETE (the Center for Technical Equipment Studies1) of Aix-en-Provence selected the 'middle' route running to the east of Millau over the River Tarn. This 'high solution', included a great viaduct, which would 'soar' over the Tarn valley without descending into it, thus avoiding the necessity for a tunnel. This has been the preferred option since 1991 because it scarcely affects the environment and offers better safety than the other option. The detailed studies commenced in 1993 and in 1994 the restricted competition was called, in which five teams of architects participated, the winning alternative being that submitted by the team comprising French engineers Sogelerg, EEG, SERF and Foster, in 1996.
From design to construction
Foster's design, impressive due to its aesthetics and size, was not exactly easy to construct without getting into cost difficulties. Supported by two abutments and seven piers, it flies the 2460 m above the Tarn valley at a central height of 245 m, with 204 m spans between the abutments and the first and last piers, and 342 m spans between the remaining piers, the heights of which range from 70 m for the first and 340 m for the third pier. The structure is multi-stayed with vertical hollow concrete members in the shape of tuning forks which support the two carriageways from the center, the carriageways having a total width of 27.35 m, sufficient for three lanes in each direction (of which only two will be put into service at the beginning) and hard shoulders on both sides. From the driver's point of view, the viaduct has a gentle slope (3.035 % from north to south) and a gentle curve (radius 20 000 m). It is 270 m above ground level in the middle, although the central pier with its stays exceeds 340 m in height, meaning that it is 14 % taller than the Eiffel Tower.
Two types of deck were investigated, of concrete or of steel, the latter being decided on as it would be slimmer which not only leads to better aesthetics (the concrete deck would have required a height of 4.6 m), but also to greater safety, both during the period of construction and in service.
27 000 cubic meters of concrete, 19 000 tonnes of concrete-reinforcing steel, and 5000 tonnes of reinforcing steel for cables and coverings were required for its construction. It was decided to use B60 high-specification concrete for the piers and self-climbing metal shuttering of variableshape.
Once the decision of the final configuration of the works had been made on 9th July 1996, it remained to determine who would execute it and how. Several companies participated in the competition for the concession, however the French Department of Transport and Public Works opted for the Eiffage Group TP (third in size in France and fifth in Europe), which created a company specifically for its operation, the Compagnie Eiffage du viaduc de Millau. This company was awarded a 75-year operating concession in exchange for financing the works, whose cost was estimated (at the commencement of construction) at 300 million euro, plus a further 20 million for the toll station located 6 km further north.
The works has been designed to withstand the most extreme seismic and meteorological conditions, its faultless operation being guaranteed for at least 120 years. The greatest constructional problems lie in the building of the deck, with a mass of 36 000 tonnes and which will be pushed out from both ends. The elements will be prefabricated at the Eiffel, Lauterbourg and Fos-sur-Mer sites, and an assembly of 64 hydraulic jacks will be used for pushing. The 'travel' involved in each of the six central spans, 342 m, made the installation of five temporary piers necessary, for the construction of which the Spanish division of Enerpac was turned to.
Hydraulic system lifts intermediate temporary piers
When the Millau Viaduct was being designed, Eiffel, a subsidiary of the Eiffage Group and dedicated to steel construction, estimated that seven intermediate temporary piers were required between the definitive piers in order to be able to 'launch' the deck during its construction.
Once a pier has been raised the machinery including the hydraulic system, is disassembled and moved to the location for installation of the following pier.
Telescopic system lifting the temporary piers.
The telescopic system exists of two parts: The first is a cubic structure of 12m bases, containing the entire system, fitted with 'toothed racks' graduated in meters at its vertices. The second comprises the hydraulic cylinders and hydraulic control system forming the lifting mechanism. The hydraulic cylinders are installed at the four vertices of the cube, anchored to supports linked to the toothed rack and which, thanks to the successive insertion of locking chocks in the toothed rack, permit the vertical displacement of both the pier structures and the hydraulic machinery, guided by the structure of the machine.
1000mm lifting steps
The operating process is simple; the supports for the cylinders are locked in the toothed rack by means of chocks, whilst the structure of the pier is free. The operators, using controls provided with comprehensive software that incorporates all kinds of safety options, start/s pumping oil to the cylinders, thus raising the rams that thrust against the structure of the pier. In this way cylinders raise the structure of the pier to the next perforation in the toothed racks.
The cylinders have a stroke of 1100 mm and the 'toothed rack' has notches every 1000 mm, such that there are 100 mm available to compensate for possible unforeseen circumstances.
Each hydraulic cylinder has its own control, with the option of immediate locking, and sensors of all kinds in order to take cognizance of any unforeseen circumstance (wind, temperature, etc), which makes an adjustment in the raising of the pier structure necessary, each cylinder rising independently. Once the desired height has been attained, the structure of the pier is locked with chocks and then the cylinder support chocks are freed. The rams are withdrawn and the bodies of the cylinders are raised together with their supports to the toothed rack perforation immediately above, where they are then locked with chocks. In this manner both the structure of the pier and the hydraulic machinery are raised by 1 m, the process then being repeated until the first element projects beyond the structure of the machine, being locked underneath.
Once it has concluded its function and because it now weighing less, the hydraulic system is lowered to the bottom position by crane; once it is on the ground a second element of the pier is mounted on it and one proceeds in the same way until the entire temporary pier has been completed, at which point the deck can be pushed out over the new pier.
Controlling the process
The hydraulic climbing system; climbing accuracy 3mm.
This lifting process must be very strictly controlled and thus the hydraulic cylinders are fitted with an internal position transducer. Similarly the pressure lines have pressure transducers, all being located internally such that they are protected from inclement weather, dirt, humidity, etc.
All the information is brought together at a control panel, which, by means of a PLC, manages the data and sends orders to the electro valves, the raising of the cylinders being executed from within an established program.
The control panel allows the operators to be aware at all times of the load and position of each of the cylinders and if necessary, to stop their advance if any of the system variables exceeds the maximum limits laid down.
The system has been designed not to permit a deviation at any time in excess of 3 mm in height, or a maximum 5% load difference between each of the cylinders.
Each cylinder has its own hydraulic pump such that, if necessary, each cylinder can be operated individually, always provided that an exhaustive protocol of request for, and granting, permission is carried out from the central application.
The operators, located at each end of the structure, have controls connected to the central control by means of which they validate, at all times during the process, the insertion or withdrawal of the chocks. Once the signal has been received, the person in charge of the central control will give the order to continue the process.
In addition there are oil level and temperature monitors and alarms that stop the advance should there be any unforeseen circumstance, such as pressure drop, cable breakage, etc.
The hydraulic portion of the machine comprises four cylinders, each of which is fed by its own pump, all centralized at a central control panel. Each assembly has a 511 tonnes thrust capacity, thus overall they have a maximum thrust capacity of 2044 tonnes. A 420 tonnes requirement is not expected to be exceeded during normal operating cycles, in this way the assembly has been designed with a generous safety margin. The nominal pressure is 700 bar and the stroke of the cylinders, as already mentioned, is 1100 mm. An overload of 675 tonnes is acceptable with a ram extended and 1500 tonnes with a withdrawn ram.
The entire control system (cabling, control panel, visual display units, etc) is protected against the elements and electromagnetic disturbance and against accidents and possible impact during installation and operation.
Enerpac, a unit of the US based Actuant group of companies, has a long history in the field of the manufacture of all types of high pressure hydraulic equipment and systems for industry and construction, since its beginning of manufacturing water pumps for the legendary Ford T.