Launching technique for the Viaduc de Millau

Transcription

1 Launching technique for the Viaduc de Millau Introduction The Viaduc de Millau is part of the A75 linking Paris with Barcelona (Figure 1). It is currently the highest bridge in the world at 343m high with a construction cost of approx 310 million Euros (AUD $525 million). It was opened to traffic on 17 December 2004, just 39 months after work began. While this engineering feat set a number of benchmarks in construction, the purpose of this article is to describe how the viaduct deck was launched across such a wide gap between supports. There are a number of articles about the various aspects of construction of this structure, however the focus will be on the equipment used to launch the structure as this method has a broader application to bridges and structures generally which lends itself to the launching technique. Traditionally, one method of launching involves sliding the bridge deck over PTFE 1 /polished stainless steel bearings. However in this case, specially designed hydraulic lift and transfer units, called translators, controlled the deck launch. While this technique has occurred before, it has never been attempted on a project of this size and complexity. Tony Brooks Regional Manager Australia and New Zealand Enerpac Abstract The 2460m long Viaduc de Millau in France is a continuous, eight span cable stayed viaduct symmetrically supported from seven pylons. The deck has a lightweight steel trapezoid shape with component parts that were prefabricated offsite with final assembly on site at the bridge approaches. The deck was launched in two halves from the north and south abutments, supported by the permanent piers and temporary piers. The 717m northern half of the deck and the 1743m southern half, once welded together, formed a continuous deck structure. The decks were launched in small 600mm increments with the total distance of each launch stage being 171m. Maurer Söhne and Enerpac manufactured specially designed translators for the project. Under demanding environmental and structural conditions, these translators allowed the launching of a deck structure to be synchronized in such a way to produce minimal side loads on the very slender support piers. This launching system is applicable to a wide variety of industry applications. Figure 1 Viaduc de Millau under construction showing red temporary support piers. Background This 2460m long cable stayed bridge crosses the Tarn River and is located 5km west of the town of Millau in France. The roadway, with two lanes on each side, crosses the river at 270m above ground. At its highest point, the viaduct is higher than the Eiffel Tower. Conceived by British architect, Norman Foster, and designed by French structural engineer, Michel Virlogeux, this suspension bridge is supported by seven pylons, instead of the two that are common for this kind of construction. 1 PTFE stands for polytetrafluoroethylene and is characterised by very low friction properties. 41

2 Figure 2 Viaduct elevation showing deck spans. Figure 3 Typical deck cross section. The Millau viaduct is a multi cable-stayed structure, slightly curved in plan on a radius of 20,000m and with a constant 3.025% slope. The structure is continuous along its eight cable-stayed spans - two end spans of 204m each and six central spans of 342m each (Figure 2). The deck is a lightweight steel structure which is prefabricated and assembled on site. The cross-sectional profile of the motorway consists of a dual carriageway, each carriageway bordered by a 3m emergency lane and a 1m shoulder next to the central reservation (Figure 3). The width of the central reservation has been determined by the size of the stay-cables, which are arranged in a single plane along the centre of the viaduct. The cross-sectional profile resulting from these constraints gives an overall deck width of 27.75m. In addition, the structure is equipped with heavy-duty security barriers and screens to protect users against side winds. The high complexity of the construction site such as steep slopes in various areas, led to the number of piers being limited and their position being restricted to the top or bottom of the slopes. The deck consists of an inverted trapezoidal profiled metal box girder with a maximum height of 4.2m at the axis with an upper orthotropic decking made up of metal sheets 12 14mm thick on the greater part of the main spans. To ensure resistance to fatigue, a thickness of 14mm was adopted for the whole length of the structure under the traffic lanes. This thickness is increased around the pylons. The longitudinal stiffening of the upper orthotropic decking is provided by longitudinal trapezoidal stiffeners 7mm thick which are generally 600mm apart. These stiffeners continue through the transverse diaphragms. The sloping base plates of the bottoms of the side box girders consist of 12mm sheet steel on the greater part of the spans, and mm sheets around the pylons. Longitudinal trapezoidal stiffeners, 6 mm thick, are fitted at variable centres. The bottom of the box girder consists of metal sheets of between 25 and 80mm thick. Rigidity is provided by three longitudinal trapezoidal stiffeners 14 or 16mm thick. Two vertical webs 4m apart and consisting of metal sheet between 20 and 40mm thick run the entire length of the structure in order to spread out the localised forces of the temporary piers during the launching of the deck. These webs are stiffened on their lower part by two longitudinal trapezoidal stiffeners. The transverse stiffening of the deck is provided by lattice diaphragms at 4.17m spacing on the spans. 42

3 Assembly The cross-sectional profile of the deck was designed by Eiffel Constructions métalliques so that it may be factory prefabricated, transported to site, assembled on-site and then launching. The cross section of the profile of the deck is broken down into a number of manageable components as shown in Figure 4. The component members arrive on site in various stages of assembly where they are finally welded together. Behind each abutment on the north and south ends of the viaduct, a factory was set up. Each factory consisted of three 171m work zones, each with its own specific activities: The first 171m zone, farthest from the abutment, joined together the pieces of the central box girder. The second 171m zone was used to assemble the other elements of the deck and to join them to the central girder. The third 171m zone was where the completely-assembled deck was painted, and the remaining mouldings, brackets and the uprights of the wind screen with their protective mesh were assembled. The welding work on the site necessitated about 75 welders for each assembly area. The complete assembly of a 171m deck section required the use of approximately five tonnes of welding metal with a total consumption of welding metal for the whole structure estimated at 150t. After an initial bedding in period, the assembly time for each 171m section was reduced to approximately four weeks. Launching the deck During the initial setup before the launching operations began and while on firm supports, one single pylon was erected onto the deck. The support cables from the pylon to the deck were then partially tensioned. The purpose of this pylon and cables was to prevent excessive sag of the cantilevered end during the launch operation. A lightweight lattice frame launch nose was also fitted. The function of this nose is explained later in the article. The steel deck was positioned by launching 171m long sections. Each launch operation consisted of moving the leading edge of the deck over the 171m span which separates each support (pier or temporary pier) from the next. At the southern end, 1743m of deck was launched while at the northern end, 717m of deck was launched. The two sections met mid span between piers P2 and P3. Figure 5 shows the launching operation during the early part of the project. Translators Traditionally a launched bridge would be pushed or pulled into position with the bridge deck sliding over low friction plain bearings or rollers. However in this case because of the bridge s length and weight, weather conditions, geometry and height of the piers, a mechanical launching device called a translator was developed for this project by Maurer Söhne of Germany and Enerpac in Spain (Figure 7 and 8). By referring to Figure 7 the operation of a translator is explained in detail. Figure 4 Typical deck cross section. Figure 5 Launching from the southern end with a single pylon in place. 43

4 Figure 6 Docking of the deck onto temporary pier T2 on the northern end. 1. Initial position while it is difficult to see the small clearance in the diagram, the deck s weight is supported by the orange cradle through to the balance jacks to the pier. The deck weight is not supported by the top sliding advance plate at this stage. The advance cylinder is in the extended position and the raise cylinders are in the retracted position. Per wedge there is one raise cylinder and two advance cylinders. One advance cylinder is mounted on either side of the wedge. 2. Raising the raise cylinder is extended causing the blue wedge to force the two adjoining plates apart. The result of this is the top advance plate is raised a small distance which in turn raises the deck clear of the orange cradle. Either side of the wedge is a low friction treatment of PTFE on one surface and polished stainless steel sheet on the other. The raising capacity of each translator is 250t. The lifting forces are transferred into the deck directly through the vertical web of the central box core of the deck. 3. Advancing while the raise cylinder is still extended, the advance cylinders are slowly retracted, advancing the deck by 600mm. The advance cylinders have a combined force capacity of 120t. 4. Lowering the advance cylinders remain fully retracted and the raising cylinder is slowly retracted. As the blue wedge is retracted, the top advance plate and deck are lowered. As soon as the deck rests on the orange cradle, the weight is removed from the top advance plate as there is now a small clearance between it and the underside of the deck. 5. Return to start the advance cylinders are extended, returning the unloaded top advance plate to the initial starting position. The hydraulically powered translators have an overall launch rate of 10 m/h or 16 cycles per hour. 44 Figure 7 Operation of a hydraulically powered translator. Each pier is equipped with four translators together with their single acting balance jacks (except T1 and T7 which have two translators). Two translators are mounted into a single assembly as shown in Figure 8. A number of translators are also installed in the deck assembly area on the bridge approaches. Apart from T1 and T7, each permanent pier and temporary pier is fitted with two translators assemblies mounted symmetrically about their centreline and spaced longitudinally apart by approximately 21m. In the highly loaded areas during the launching process, the balance jacks had an individual capacity of 600t and a 500mm vertical stroke. Hence with 24 jacks the absolute lifting capacity was

5 Figure 8 Two blue translators form part of an assembly during installation (advance cylinders missing). 14,400t. Other less loaded locations used 280t jacks with a stroke of 300mm. Hydraulic valves are used to make the different groups of jacks independent of each other to enable control of the height and angle of the deck. During the launch, the balance jacks of the two translators on the same side of the deck are hydraulically linked to ensure equal pressure is maintained by all jacks on that side of the deck. This is to allow for variations in longitudinal rotation of the deck due to deck deflection. This load sharing feature ensures that the supporting load was evenly distributed over each pair of longitudinally aligned translators. The translator jacks on either side of the deck s longitudinal centreline were not normally hydraulically linked as this would create a virtual pin joint and allow the deck to roll about its longitudinal axis. As a safety measure, the balance jacks could be mechanically locked off by a large nut mounted to each jack s piston. The outer advance cylinders on each translator have a positional transducer that indicates the amount of travel. This allowed the synchronised control of the translators at all locations via a central computer. The horizontal deflection of the top of each pier was monitored by survey methods to ensure the piers were not subject to unexpected bending forces. During the final phase of launching from the southern end, the total pushing capacity was 5280t and from the northern end a pushing capacity of 2400t. A total of 3280 individual 600mm launches were made from the C8 end and 1540 were made from the C0 end. Launch nose operation During the launch of the cantilevered deck, the end of the deck sagged under it own weight. This deflection was not fully controlled by erecting the Py2 pylon to the northern end launch and Py3 pylon to the southern end launch. The leading extremity of the cantilevered section of the deck was fitted with a launch nose whose purpose was to facilitate docking onto the different supports and to stabilise the leading edge in case of an emergency stop in the launch owing to high wind. The rotation of the launch nose could be controlled with two hydraulic cylinders. To make the final height adjustment due to the cantilevered deflection, a pair of special bar jacks was fitted. These jacks 45

6 Figure 9 Pictorial view of the lifting and advance of the launch nose. function in a similar manner to a strand jack (1). This system is comprised of four 270t cylinders with hollow pistons and 205mm of travel. The cylinders work in pairs and transfer the load to pull bars that fastened to the structure. In conjunction with Figure 9, a brief summary of the operation of the launch nose is given below: 1. The launch nose and deck stop short of the translator assembly on top of a pier (or temporary piers as the case may be). The translator is in the start position and the orange column is above the translator. 2. The bar jacks advance and lower an orange column onto each translator. Further advancement of the bar jacks raise the launch nose so that its underside and the underside of the deck are above the advance plate. 3. The raise cylinder on the translator is extended advancing the blue wedge plate. The full weight of the launch nose is now taken on the advance plate. 4. The advance cylinder on the translator is now retracted advancing the deck. This operation must occur synchronously with all other translators to launch the deck by 600mm. 5. The bar jacks retract the orange column until the launch nose sits on the launch plate. The orange column is then lifted clear. The weight of the advance nose is now taken by the translator. 6. The lift cylinder is retracted which lowers the launch nose onto the translator cradle. The advance cylinder is 46 extended back to the cycle start position. The launch nose/ deck is now on the translator so the deck can now be advanced in the normal cycle as shown in Figure 7. PLC control system All hydraulic systems for pushing each deck are operated from an individual control centre on the bridgehead. Although all hydraulic systems operating during a launch are controlled from this central control centre, each individual hydraulic system has its own local control panel. This allows local movement of the translators 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 synchronised pushing movements from all the pushing cylinders of all the piers. Movement of the deck can be made in three modes: manual, semiautomatic and automatic. The manual mode is used for adjusting the system and, if necessary, to make instant corrections. In semi-automatic mode, each movement is made step by step: raise, push, lower, withdraw cylinders. Automatic mode completes the entire cycle. Hydraulic system integration of high force hydraulics and advanced control technology played a critical role in the controlled movement of such a large-scale civil engineering project the Viaduc de Millau.

7 References 1. Chapman C. Strand jacks in the construction industry, Queensland Roads, Edition 9. Sept Figure 10 Launch nose fitted to leading edge of deck. Figure 11 Launch nose close-up prior to the initial launch. Figure 12 General view of viaduct before the last launch. 47