is year the two 130m-long main spans of the old Carquinez Bridge were lowered down on to pontoons. This very elegant steel truss bridge was built in 1927 and was being demolished because it does not meet modern seismic safety requirements and has been superseded by the new bridge. Strand jacks were installed between the permanent suspension members on the bridge, which allowed for a very economical dismantling method in regard to the small amount of temporary steel work required. This was only possible because of the small size of the equipment compared to its capacity. A VSL strand jack SLU 330 with a SWL capacity of 330t is, with an outer diameter of only 550mm, not much bulkier than a grown man; its weight of 1200kg also makes it only slightly heavier.

In the early days of strand jacking it was always regarded as necessary for safe and proper operation that the strands should be aligned perfectly with the strand lifting unit. These days, with more demanding projects being carried out, it has become necessary to find ways around this. A very interesting application took place last year on the Miraflores Viaduct, which is part of the high-speed railway link between Madrid and Valladolid. The 132m-long central span is supported by a concrete arch, and the two halves of the arch were constructed vertically and then lowered into their final position. One of the reasons for this procedure was to minimise the environmental impact of temporary works in this protected area. At various stages in the lowering operation, the strands had to be deviated by up to 30 degrees. This was achieved with a system of funnels and guide plates through the top of the piers and the arch itself. During the lowering operation the load in the strand cables increased gradually to almost 600t. The whole operation of pulling and lowering each 1400t half arch as well as retaining the supporting pier was controlled from a small platform high up in the air.

For heavy lifting work, quality, safety and price are important criteria, but now the actual lifting speed is becoming a decisive factor, for example where lifts have to take place above busy shipping channels. When lifting components of bridges for example, the shipping traffic may be hampered or might even have to be stopped and it is very important to keep closures and restrictions to an absolute minimum.

Strand-jacking equipment is extremely suitable for such lifting work simply because of its small self weight compared to its lifting capacity, which suits the design of today's long and slender structures. The nature of operation means that lifting speeds of 10m to 30m per hour are achievable before impact loads become a controlling factor. As such, the lifting cables can be lowered down for the next lift with the same speed and this to a large extent defines the time a certain area has to be closed to ships.

VSL has now developed a hydraulic strand coiler which reduces the time for lowering or lifting of strand cables including attachments by 20 times. The company offers a range of coilers for cables of up to 580t capacity and more than 100m strand length, capable of pulling or releasing speeds of up to 8m/min. Eight of these coilers were used with great success most recently on two cable-stayed bridges on the Industrial Ring Road in Bangkok (Bd&e issue 43).

The foundations of a bridge, although mostly hidden from the human eye when completed, offer a lot of room for optimised methods of construction, and this clearly applies to large pile caps in rivers. Being able to build them to a large extent above the water level can provide various advantages and benefits. Such an application was seen recently at the sixth bridge in Rouen, France, which is slowly rising from the waters of the river Seine. This bridge has all the attributes to become a landmark of the region. Its massive 100m-long main span is designed to be raised to a height of 55m, enabling even the tallest ships to pass underneath.

Six large concrete caissons, to form the two main tower foundations and the four units for ship impact protection, were built above the water level. VSL's strand-jacking equipment took the load as soon as the bottom slabs were assembled. In this suspended position, the walls were slipformed and then the shells, with its weight of 3,000t, were lowered with millimetre accuracy to the river bed. Meticulous planning and the use of a reliable level control system for the 12 support points ensured the success of each operation.

In the near future the bridge industry will see more interesting projects where strand jacking will be used to fulfil even greater demands. In November this year the 120m-long steel truss of a bridge over the Nord Ostsee Kanal in northern Germany will be replaced. In a period of just three days, the 94 year old structure will be lowered by 40m and a new one lifted in its place. Thirty years ago when the new central span of the Milford Haven bridge was erected, an average lifting speed of 3.3m/h was acceptable. This time the lowering and lifting of the 1400t steel trusses will take place at 15m/h to be able to maintain the very tight schedule.

And early next year the back-spans of the Stonecutters Bridge in Hong Kong will be lifted some 70m to their final position. During this operation the 3600t twin deck will be supported from 20 strand lifting units. These will be located on supports, some of which will be cable-stayed, and hence of various stiffnesses. A sophisticated level survey system will control the whole lifting operation, integrating the deflection data of the supports as well as the lifted load itself.

Finally, another major project in 2007 will be the lifting works of the steel arches on the Sheikh Zayed Bridge in Abu Dhabi (Bd&e issue 43). The 22 steel segments weighing up to 600t each will be delivered to the site by pontoon. Each will then be lifted by four strand lifting units from a temporary tower system. To reach their final position each one will have to be moved in three directions and rotated around three axes, a new challenge in terms of speed and accuracy.

Daniel Junker is marketing manager for heavy lifting at VSL

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