20.04.2004

Enerpac continues support for world's highest bridge

In the latest phase of construction of what will be the world’s highest bridge, the Millau viaduct, hydraulic technology specialist Enerpac has raised seven enormous telescopic steel towers for temporary support of the bridge’s 36,000 t deck.

The deck will eventually span 2,460 m over the Tarn Valley in the south of France, and rest on seven fixed concrete piers up to a maximum height of 245 m above ground level. With distances of up to 342 m between each concrete pier, however, Enerpac was called upon to construct and raise seven temporary steel towers at intermediate points between each fixed tower to guide and support the deck on its journey across the Tarn Valley.

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Hydraulic launch systems have been installed on all concrete pillars and and temporary support piers to push the Millau bridge deck out into space.

The first two 10 and 12 m high temporary support towers nearest the two end pillars were erected using tower cranes, while the five remaining support towers were erected using a hydraulic system developed by Enerpac.

The telescopic system comprises, two parts, the first of which is 12 m base cube structure, which contains the entire system, fitted with ‘toothed racks’ set at metre increments at its vertices. The second part is a hydraulic mechanism that hugs the first part of the tower structure comprising hydraulic cylinders, each with a 511 t thrust capacity, installed at the four vertices.

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Temporary support pier showing Enerpac's two-part, telescopic, hydraulic lift system.

As the system is raised by operators using a controlled PLC procedure to pump oil into the system’s cylinders, the successive insertion of locking chocks in the toothed rack enables the vertical displacement of both tower and hydraulic machinery, which then raises the tower section at 1000 mm intervals. This process is repeated until the desired height has been reached, at which point the structure is locked from underneath and the hydraulic system is lifted to the ground by crane. A second tower section is then added and the process is repeated until the entire tower is complete.

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On completion, scheduled for January 2005, the Millau viaduct will span 2460 metres across the Tarn Valley in southern France.

For maximum control during the lift, each hydraulic cylinder is fitted with an internal transducer, while the pressure lines are fitted pressure transducers, which provide the capacity for constant load monitoring through the PLC system, which in turn is responsible for monitoring the data at all times during the lift and sending the correct orders to electro-valves in accordance with a preordained procedure. Each of the four hydraulic cylinders is fed by its own pump and produces a 511 t thrust capacity, creating an overall thrust of capacity of 2044 t.

Construction of the Millau viaduct began in October 2001, and scheduled for completion by the end of 2005.