Bridge suspension replacement

Replacement suspension and widen road deck Aquitaine bridge

Every day nearly 90.000 vehicles use the Aquitaine bridge to travel across the Garonne river, thus relieving the traffic flow passing through Bordeaux's city centre. In 1999 the Highway Authority decided to replace the bridge's entire suspension and to widen the road deck, whilst maintaining normal traffic flow during the construction work. For the realisation of some of this important work the Vinci Construction group, and in particular the Baudin Châteauneuf company, employed Enerpac technology.

France's second largest suspension bridge.

The old suspension needs to remain in service while the new one is being constructed.

The Aquitaine bridge, France's second largest suspension bridge, was inaugurated in 1967. It comprises three spans measuring 143, 394 and 143 metres respectively. Its effective road surface is 20 metres wide and is suspended 53 metres above the river. The deck weighs 11.460 tonnes (metal frame and superstructure). The suspension weighs 1900 tonnes and the support pylons are 104 metres high.

The bridge's suspension system consists of two main cables that stretch from one river bank to the other. They pass over a pylon on each bank and support the bridge deck . The main cables are fixed in solid anchoring consoles, built on the riverbanks. The cables and hangers are made up of steel wire strands. Bearing the entire weight of the bridge, those strands are constantly under high pressure and their rupturing, due to corrosion, was the determining factor in the Highway Authority’s decision to replace the entire suspension system rather than renovate it.

As the roadway is to be extended to 2 x 3 traffic lanes, the new main cables of the Aquitaine bridge have to be moved 2 metres further out than the existing cables. This requires mounting 128 extension structures perpendicularly onto the bridge deck so that the new hangers can be attached to the deck. The heads of the pylons, onto which cranes will be installed for the remedial work, also need to be extended outwards to be able to accommodate the new saddle supports for the cables.

The solid anchors need to be modified or reconstructed as the new cables are further apart and need to support the extra weight caused by enlarging the bridge. On the left river bank a 3,300 tonne reinforcement beam needs to be cast at the foot of the mountain range and hoisted up to a height of 40 metres in order to anchor the beams and apply the necessary tension to the new cables. This lifting operation will be carried out using hydraulic cylinders. Finally some construction work is planned on the surrounding terrain.

The suspension was to be replaced and the road widened.

Enerpac hydraulic system

The old suspension needs to remain in service while the new one is being constructed, i.e. the two need to coexist. An auxiliary suspension is installed to take over the supplementary load resulting from the work to enlarge the roadway. The 61 strands that make up the new cable are installed in cradles alongside the old suspension, and subsequently clamped together by means of collets, thus resulting in one big 500 mm diameter cable. There are 128 two-part steel collets, each weighing over 1000 kg. A crane fits the two semi-collets into place around the strands, where they are supported by a mounting jig. The lower part of the collet is connected to the hanger, which extends down to the bridge deck. The upper part of the collet covers the strands. The two halves of the collets are then bolted together. Tensioning and tightening the screw assembly is done by means of a special hydraulic Enerpac system consisting of 16 hollow plunger cylinders, RCH603 type, with a capacity of 930 tons, powered by a GPEM 3820E central hydraulic pump. This system's high efficiency enables the 128 collets to be tightened in three days using two sets of equipment, thus saving a substantial amount of time.

The weight of the bridge then needs to be transferred gradually, in two stages, from the old suspension to the new one.

Stage one. The new main cable is connected to the side spans of the bridge, without being pulled taut. At the highest part in the central span it will then be 2 metres above its final position. Using two Enerpac CLL1006 lock nut cylinders placed at the foot of each hanger, i.e. a total of 128 cylinders providing a total hydraulic pull of 12,160 tonnes, the extremities of the hangers are gradually pulled closer to the bridge deck in successive stages. When the extremities of the hangers reach their final level they are hooked into place. At the end of this stage, 70% of the bridge's load will rest on the new cables.

Stage two. To be able to disconnect the old hangers, the pressure must be removed by releasing the tension still applied to the old cable. This is done using four Enerpac RCS302 type low height cylinders per hanger. At the end of this second stage, the load of the bridge will be resting entirely on the new suspension. During these operations cylinders control the displacement of the mobile saddle supports capping the pylons.

The dimensions of the Enerpac hydraulic groups are especially designed to allow simultaneous operations on the assembly of hangers. Around 1,400 metres of flexible Enerpac thermoplastic hoses will be used.

Once the transfer of the load is completed, the old cables are jacked up to a height of approx. 4.3 metres on the central span and dismantled, just like the auxiliary cables before. All that then remains to be done is to conduct load tests and finish the work

Especially in the beginning phase where there is no support yet the risk of out of line drilling is high. With the help of 6 hollow plunger jacks and steel hand pumps the machine is anchored to the soil to keep it straight.