Strand jacks approach the problem of lifting large loads in a unique way. Like the jacks used to tension steel for post-tensioned concrete slabs, they work by pulling strands through a pair of gripping collets set into a conical anchor. Indeed, the similarity is no mere coincidence.
The pioneering company was Freyssinet, set up by the French structural engineer and father of modernist architectural materials Eugene Freyssinet, who had developed post-tensioning technology for his 1934 repair of Le Havre station. In the 1970s, Freyssinet and other companies, including VSL, took the simple step of turning post-tensioning jacks on end and using them to lift loads, rather than to tension concrete slabs. The relationship between post-tensioning, lifting and cable-stayed suspension continues to be seen in bridges around the world.
However, a simple single strand jack, as used in post-tensioning, lacks the power needed to raise significant loads. Instead, in modern strand jacks, forty strands typically pass through each unit. Capacities per unit are determined by the power that can be delivered through the hydraulic power pack, and the pressure the hydraulic chambers can withstand. Capacities range from 15t, for Fagioli PSC’s single-strand L15/1, through to 750t for the same company’s 50 strand L750, or even as much as 850t for Hydrospex’s HSL 8500. Hydraulic pressures can reach as high as 350 bar (5,100 psi) µfor the largest jacks. The power packs used to push fluid through the system operate at up to 325hp (242kW).
A strand and collet – as the three-section collet is pulled into the conical anchor plate, it grasps the strand
Andrew Duncan, project engineer for John Gibson Projects, a UK strand jack and heavy lifting specialist, explains how they work. “Each strand has a three-segment gripping collet within the top and bottom ‘assembly’ of the strand jack. A gripping collet works in a similar fashion to a thumb lock. It sits in a tapered cup in the assembly – as the load pulls down, it keeps the collet gripped on the strand. When the hydraulic jacks lift the upper assembly mounted on the piston of the strand jack’s hydraulic cylinder, the upper collets grip, and lower collets are pulled up, releasing them.”
A cutaway view of a Fagioli PSC jack, showing strands passing through the top anchor plate
In lifting, sections of strand are pulled from the lower anchor plate to the upper, taking the load up in increments: a process similar to a person climbing a rope hand over hand. As the v-shaped holes in the anchor plates hold the conical gripping collets in place using the pull of the load, lowering is more difficult. In this case, the main hydraulics move up by an inch or so, and a system of mini-jacks lift the top collets out of the anchor plate in sequence, transferring the load to the lower anchor plate. The main hydraulics then extend again, allowing the strands to be gradually played out.
On its own though, the lifting power of a single strand jack is of little value. Instead, project engineers must find ways to attach them to a load, and to a support structure. This structure can take a range of forms. For lifting a refinery vessel into position, a modular mast system may be used, with the jacks either mounted on an arch and lifting the vessel from the top, or in a cantilever configuration, with two towers either side of the load, and the strands attached to the centre.
Dorman Long Technology used a novel approach to bridging the Boyne river, in Ireland. One set of strands run vertically across the span, and a second set run diagonally from the top of the bridge tower to the tip of the span, temporarily taking the place of the final cable stays. As the first set of jacks pull the bridge across the span, the second set are loosened and the angle of the jacks adjusted, to keep the load level.
The lining of this refinery in Taiwan needed to be replaced. However, there was limited access for cranes and the surrounding structures could not be dismantled. Dorman Long Technology cut a space at the bottom of the refinery and pulled sections of the lining in using a temporary structure. Jacks mounted inside the refinery then lifted each section of lining into place.
On a bridge construction project, jacks may be mounted on a mobile gantry and used to lift span sections up from barges, or across a span and used to pull an entire span out from one side of a crossing to another. On a roof erection, jacks can be mounted on permanent columns that will later form part of the finished structure, or on temporary gantries that support external walls until the roof is in place.
As the jacks are powered by a hydraulic system, they can easily be used in groups: the fluid is delivered evenly across the system, meaning that the jacks will move in synchronisation, rather than having to be controlled separately as cranes used in tandem would be.
Manufacture and sale
While the concept behind strand jacks is relatively straightforward, their design requires detailed knowledge of the behaviour of the materials and parts used. The parts themselves need to be machined to tight tolerances. For this reason, the manufacturing process tends to be split between specialist heavy lifting companies that design, assemble and test the complete jacks, and machine shops producing pieces to the tolerances required.
Hydrospex’s innovative electronic control system makes it easy to monitor and control multiple jacks
David Dyer, managing director of Dorman Long Technology, explains: “There’s around a dozen strand jack companies working internationally. We design our own jacks, but like everyone else in the industry, we use parts supplied by third-party machine shops. We then assemble and test the jacks ourselves.” While this comment was supported by other heavy lifting companies, Hydrospex takes a different approach. The Dutch company focuses on manufacture of complete lifting systems, rather than using jacks itself. Owner Tjerko Jurgens explains, “We don’t outsource anything, we machine and assemble everything ourselves.” For Hydrospex, it makes sense to build jacks from scratch, and sell them as a complete unit with the control system and power.
Dorman Long Technology’s control system uses a simplified display, with lines representing each jack
Hydrospex works with both established heavy lifting firms and newcomers to the sector. This means building jacks to different designs. Jurgens notes, “Our designs and those of our customers, such as Fagioli PSC or Mammoet, are often incompatible. In those cases, we’ll work to their design. For newcomers to the sector, we can manufacture jacks to our own designs.”
The immense lifting power of strand jack systems make new methods of construction possible. At the Al Jubail refinery, a vessel has been built on the ground – a much safer, and cheaper, option than building it in place. Fagioli PSC then built two supporting towers, with jacks mounted on an arch at the top. With the help of a tailing crane, the jacks lift the vessel into its final, vertical, position.
At a visit to Fagioli PSC’s UK base in Hertfordshire, sales and marketing director Martin Haynes explained that between jobs, each strand jack hired on rental contracts is disassembled, inspected, and tested, before being rebuilt and sent out again. Jacks are tested using compression testing: two jacks are placed together, bottom anchor plate against bottom anchor, and jacked up.
In the past, strand jack manufacturers have generally kept the equipment to themselves, using their jacks on contract lifts, either directly for the main contractor, or with a steel sub-contractor. Recently though, there have been moves for crane rental and other heavy lifting firms to make use of strand jacks, and for contractors to own their own jacks.
John Gibson Projects’ background in ballasting has given them a lead in barge load outs. Here, the 19,200t Panyu 30-1 Jacket is pulled onto a barge in Shekou, China. The tension of the strands pulls the barge tight against the quayside. The powerful, 325hp, power pack used means the load out can be performed more quickly, cutting the risks posed by the possibility of the barge moving during the job. As with the vessel lift, above, the capacity of the jacks used meant that structure could be built safely on land, before being mounted on the barge and taken out to sea.
Duncan of John Gibson Projects says:“It’s changed over the last five or six years. It used to be that maybe up to half a dozen businesses like John Gibson Projects built, owned and operated strand jacks, on the basis of engineering, supplying and rental with supporting supervision on a contract lift basis, like in the crane rental industry. Now, we have noticed a trend for many companies to buy their own – there are also some erection and crane companies that offer them along with cranes and other lifting solutions such as gantries or powerlifts.
“They’re often rented out with cranes, and included in the larger multi-crane supply contract that supports a stadium or bridge erection. Chinese contractors have witnessed strand jacks owned and operated by European firms over the last ten years on projects in the country for bridge building and they, like the Koreans, have taken to buying and operating their own – they like to keep the whole job in-house,” Duncan says.
One crane rental company that has taken a lead in the use of strand jacks is Mammoet, based in the Netherlands. As well as using strand jacks in their traditional manner, attached to the structure they are lifting or to a temporary gantry, the company has developed a series of very high capacity cranes, the MSG 50 and MSG 80, which use strand jacks in place of winches. These lattice boom cranes can be moved between, and around, sites like a standard crawler, but can lift loads that few cranes can match: in a record breaking lift in Qatar in 2005, the MSG 50 installed two reactors at a refinery, each weighing 2,016t.
Control
A key problem with making use of the theoretically limitless capacity of jacks has been controlling them in large groups. In the past, strand jacks would be monitored and controlled via gauges and levers on the front of a hydraulic power pack. Even with only a few jacks on a lift though, it rapidly becomes difficult for an operator to monitor the entire lift system properly. Haynes comments, “Electronic controls are important because of the complexity of information on large lifts – sometimes up to 80 different jacks need to be monitored on a lift. This would be impossible with traditional gauges.”
The first company to develop such asystem was Hydrospex. Jurgens describes the system’s development: “We developed our first electronic controls in 1996. Our inspiration was the rapid development in office and home computing – by 1996, I’d given up my fax for email, but strand jacks were still controlled with gauges and levers. Look at Windows in computing: anyone can use a PC now. We wanted to do the same for heavy lifting.
“A monitoring system just supplies information, and leaves the decisions to the operator. A control system makes the decisions for him: if you’re a contractor, facing damage liabilities, who do you want making the decisions, an operator or a computer? If you take one of the most complex lifts in the world, raising the Kursk, we only needed to use 7% of the processing power of an Intel Pentium 4. Could any operator have handled that lift without an electronic control system?
Hydrospex’s control system continues to dominate the market. However, it is not the sole player in this field. While the Hydrospex system uses detailed graphical representations of each jack to show their state, Dorman Long Technology has developed a user interface using a simplified system of lines to represent the jacks more symbolically. David Dyer, Dorman Long Technology MD, explains, “We designed the system from the GUI backwards, working closely with engineers. We felt that the existing system could be simplified. Rather having an image of each jack, we use a system of vertical lines, with each line representing one jack.”
Hydrospex’s success has been helped by its ability to offer jacks, controls and training together. Jurgens explains, “A Chinese operator can come to our offices in Holland, speaking only a few words of English, and I only have to explain the basics of the program. As long as he can input the lift numbers, the system can control the lift for him.” If strand jacks are to be adopted by new markets, and particularly if they are to be operated by construction workers who lack the immersion in the technology that heavy lifting specialists enjoy, electronic control systems will become increasingly important.
Growth
Currently, while strand jacks are used on projects worldwide, their ownership and manufacture is focused on Europe. Demand for increased heavy lifting capacity, whether from a general growth in industries such as oil and gas extraction, from a surge in infrastructure projects, or from developments in construction technology, with large structures such as rigs increasingly being built on land and then assembled and loaded onto barges, is spurring their increased uptake around the world.
Jurgens notes the disparity between European and US ownership and use of jacks: “In Europe there are 600 strand jacks. In the US, with a similar population, there are only 100. We see this as an area of future growth – there’s no reason why the market should be smaller. The US has been slower to adopt new technology, and resistant to buying equipment manufactured in Europe. We think that by offering a simple control system, we will be able to get past this.”
Jeff Latture, senior vice president of sales and marketing at USlifting contractor Barnhart, says that the US is slow to adopt strand jacking. “Relative use of all types of alternative lifting in the US is lower than in many parts of the world. The US construction industry is very resistant to change. Traditionally, large structures such as stadiums or rigs are piece-built, employing large numbers of workers. Whether this conservatism comes from union power, or a general aversion to change is hard to say.”
However, things are slowly picking up, as Latture explains: “In the last five to six years, alternative construction techniques, such as modularisation, have begun to take off. It will take time though: it’s only as people gain familiarity with the techniques that new ways of lifting will be accepted. Overall, in terms of the quantity of jobs using strand jacks, we’re seeing growth of maybe 25-30% a year, but this is from a very small base. At the moment, Barnhart, Bigge, Mammoet and Fagioli PSC account for almost all the strand jack jobs taking place in the US.”
With demand for strand jacks in the US still low, the prospects of any US manufacturer moving into their development look slim. “We have a fleet of jacks that has come through a variety of acquisitions over many years. We’ve recently bought some new jacks from Hydrospex, which we’re very happy with. Hydrospex and Fagioli PSC are likely to continue to be the main suppliers to the US market: people are looking for suppliers with a strong record. There’s little space for any US manufacturers to enter the market, and I haven’t heard of anyone with the capabilities needed to build strand jacks,” Latture says.
Across the Pacific, China is also a growing market for strand jacks. As the vast country’s brisk development continues, joining its massive population together is hugely important. A string of bridge building projects have made use of strand jacks, while preparations for the 2008 Olympics, and iconic architectural projects such as the Beijing Ferris wheel, have showcased their power. However, the Chinese aren’t going to look to merely bring in outside specialists.
Jurgens notes, “Three years ago, strand jack sales in China were zero. Today, we have orders and potential for a couple of hundred sales. On the London wheel, we used 20 jacks. The new wheel being built in Beijing will need 25. We think this will mark a tipping point for strand jack sales in China. Chinese contractors will always want to buy their own jacks. It’s unthinkable to them to rent: they don’t want to export money.”
Dyer from Dorman Long agrees that the Chinese are looking to buy their own equipment: “Owning your own equipment is particularly popular in China. The Chinese are increasingly moving over to running lifts themselves. OVM in southern China, and two joint ventures out of Tonji University in Shanghai, are now running heavy lifts. There are around 50-60 projects using strand jacks in China each year. At the moment, around 10% of these are put out to international tenders. We’ve set up our own company in Shanghai (as a
wholly-owned foreign enterprise) to compete with domestic Chinese heavy lifting companies. We’ve also started a joint venture with Tiong Woon in the Singapore. They have a background in cranes, trailers and barges. We’re working on a 3,000t capacity tower system, which we will use in the oil and gas industry. Uniquely, it operates without guy wires.”
Demand for heavy lifting in the Middle East is having a dual effect on the strand jack market. On one hand, oil and gas projects in the region demand strand jacks for jobs that are beyond the capacity of even the largest cranes. On the other, overall demand for heavy lifting is draining global supplies of cranage. Dyer comments, “The Middle East is possibly key to the future of the market. Development in the oil and gas industry is driving massive demand for heavy lifting equipment. This has led to high demand for strand jacks in the region. Because it’s also sucking up world supplies of heavy cranes, it means there’s more demand internationally in all sectors. As strand jacks are very fast to market, compared with the highest capacity cranes, we’ve been able to respond to this demand.”
Conclusion
While there may be little scope for anything but incremental development of the jacks themselves, it seems reasonable to expect that control systems will continue to evolve. Most important in growing the market, perhaps, is the added value that the innovation of project engineers brings to the deployment of the technology. Over the last thirty years, strand jacks have moved from being used merely for vertical lifts, and are now used for everything from pulling entire bridges across gorges, skidding 20,000t rigs on to barges, to closing sections of immense steel structures together like the two halves of a clam shell. How will engineers of the future make use of these powerful and flexible tools? How will their projects be planned and controlled?
