Delicate hydraulic crown jacking procedure shapes the concrete bowstring arch of Zaragoza’s Third Millenium Bridge

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As the deadline loomed for the Expo from June 14 - September 14 2008 – and with only three days in which to complete the job - a PLC-controlled Enerpac synchronous lifting system was employed to delicately and precisely ease apart a 12000 ton load to make space in the crown of the arch for the bridge’s final concrete casting.

Just as the Romans used advances in the technologies of concrete and hydraulics to build their aquaducts and triumphal arches, so construction company Dragados turned to the advanced Enerpac PLC-controlled hydraulic technology to perform the world record in the crown jacking of the arch of The Third Millennium Bridge (which has a total length of 270 m, a 216 m span, a 48 m wide deck and a total 68 m overall width, including 6 traffic lanes and 2 bicycle lanes. The entire structure is made of high-strength concrete).

"Concrete is an unusual choice for such a large bridge with such a unique configuration, and that presented the challenge of performing a world record crown jacking operation carried out by Dragados using the hydraulic solution supplied by Enerpac,” said Jesus Gonzalez, technical director of Enerpac in Spain. "The most crucial operation of the construction process, which took place in the first week of April 2008, was the jacking apart of the crown of the bowstring arch. This was done using the Enerpac synchronous hydraulic system with six double-acting lock nut cylinders, each with a lifting capacity of 2000 tons and with all six jacks monitored by a single PLC-control unit. This integrated hydraulic system was used initially to move the arch and push the cantilevers apart to make space for the final casting. Then it was employed to provide hydraulic jacking to tension the cables and raise the deck to its final position.”

Absolute precision, reliability and safety

The success of the project depended crucially on the smooth and safe implementation of delicate jacking procedure as part of the project’s critical path. With three days to complete the crown jacking of the arch, engineers needed absolute precision, reliability and safety as they struggled against the Expo 2008 deadline in the northern summer. The custom-designed synchronous system was engineered to push apart and hold the two parts on the top of the arch, leaving the arch totally un-swung. This precision operation involved an electronic programmable system that synchronized three pairs of cylinders with a precision of half a millimetre between leading and trailing points of the jacks, and which tolerates a disalignment of loads of 30 tons among them. The system imposes a load on the arch of slightly more than 12.000 tons to permit the jacking and closing operation 36 meters above the deck of the bridge, he explains.

Two phases of the bridge construction used the Enerpac Synchronous System. First, the deck of the bridge was built with a pushing system that slid the structure on provisional pivots. This involved a system of electronic monitoring of eight lines of cylinders, of 150 tons each. The arch was constructed in the second phase, with the three pairs of cylinders involved governed by a 1600 bar pressure-transducer and a race sensor. A computer-based synchronization was carried out with specific software developed to take account of roll as the arch opened, while controlling individual loads per cylinder as well as pairs of cylinders. The system was designed with automatic failsafe functions to automatically halt the operation and hold the load if its synchronisation was interrupted.

As soon as the key-closing was completed, the cables supporting the deck returned to normal tension, imposing additional load on the cylinders, which, having performed their function, become part of the mechanical structure. The cylinders were covered with concrete and remain inside the arch.

"The work has been a challenge and a credit to everyone involved, because it is a monumental undertaking bearing the architectural signature of Juan Jose Arenas de Paul, the hallmarks of Dragados design, and Enerpac's guarantee of precision and safety in its execution. The teamwork has produced a landmark bridge of extremely complicated construction in white concrete, which involves added difficulty because of its manipulation and control. Further challenges presented by the placing of transverse girders, making this one of Europe’s most challenging bridge engineering tasks in recent years. Featuring the highest arch in the world of a fluvial bridge, execution of the project was made possible in large part by the synchronized systems developed by Enerpac's Integrated Solutions centre of Spain. The lessons in control and safety are applicable on other projects worldwide”, says Jesus Gonzalez.

Synchronous lifting system – controlled hydraulic movement

Enerpac’s Synchronous Systems have been used in many major projects, including the construction of the world’s highest bridge, the 343 metre high Millau Viaduct, in France. Their prodigious lifting capacity and safety has been employed globally, from the construction of North Sea oil rigs to the maintenance of a 3500 tons coal mine dragline at in Queensland, Australia, as well as lifting of bridges and structures during construction and maintenance. Their delicate precison was also used in lifting, weighing and manipulation system of ship hull segments in the construction of UK Royal Navy’s anti-air warfare destroyers.

Available in configurations from 4-64 lifting points, synchronous lifting systems electronically control and monitor movement during the hydraulic raising, lowering, positioning or testing of very heavy objects such as manufacturing machinery, motors, manufactured structures, buildings, bridges, oil platforms, ships, turbines, generators, mills, mining equipment and heavy but delicate computerised/electrical equipment.

A major benefit is load balancing and removal of internal stresses. With manual control, differences arising between the lifting points are unavoidable, because measurement of the movement and control of the lifting points are never optimal. Internal stresses resulting can ultimately cause hidden damage compromising serviceability and safety. Synchronous lifting overcomes this problem.