Lifting Millau's Temporary Support Towers
The deck of the world's highest bridge is being launched over a distance of 2460 metres over the Tarn Valley in the south of France. It is impossible to move the deck from pier to pier without intermediate support towers.
Halfway between each concrete pier support towers enable the bridge deck to reach a temporary resting point. These huge steel cages are raised by telescopic hydraulic lifting systems from Enerpac metre by metre from ground level up to 163,7 metres for the highest temporary tower to allow insertion of towers segments to extend it's height.
Raising temporary towers to support the launching of the 36.000 ton steel deck
The Millau viaduct will cross the Tarn Valley at a maximum height of 245m, with 204m spans between the abutments and the first and last concrete piers and 342m between the remaining five concrete piers with heights from 77m up to 245m. 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.
Once a pier has been raised the machinery - including the hydraulic system - will be disassembled and moved to the location for installation of the next temporary support tower.
The telescopic system exists of two parts:
The first is a cube structure of 12m bases, containing the entire system, fitted with 'toothed racks' graduated in meters at its vertices.
The second part of the system includes 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 racks. As the system is raised, the successive insertion of locking chocks in the toothed rack permits the vertical displacement of both the pier structures and the hydraulic machinery, guided by the structure of the machine.
Lifting in steps of 1000 mm
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 software incorporating all kinds of safety devices, starts 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 slots 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
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 metre, the process then being repeated until the first element extends beyond the structure of the machine and is locked underneath.
Once the element is in place the hydraulic system can be lowered to the ground by crane; a second element of the pier is then mounted on it and the procedure begins again, until the entire temporary pier has been completed.
Controlling the process
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 received 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 is carried out in accordance with an established procedure.
The control panel allows the operators to be constantly aware of the load and position of each of the cylinders and they can stop the lifting procedure if any of the system variables exceeds the maximum limits laid down.
The system has been designed to restrict deviation at any time to less than 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.
Operators at each end of the structure have a connection to the central control, by means of which they confirm 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 and so on.
The hydraulic portion of the machine consists four cylinders, each of which is fed by its own pump, all linked to a central control panel. Each assembly has a 511 tonnes thrust capacity, so overall the system has a maximum thrust capacity of 2044 tonnes. A requirement of 420 tonnes is not expected to be exceeded during normal operating cycles, so the assembly has 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 it withdraw. The entire control system – cabling, control panel, visual display units and so on – is protected against the elements and electromagnetic disturbance and against accidents and possible impact during installation and operation.
The Millau progress
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 and the highest one is being constructed.