STRAND JACKS IN THE CONSTRUCTION INDUSTRY

Colin ChapmanIntegrated Solutions ManagerEnerpac Australia (Actuant Australia Ltd)

AbstractVery heavy loads can be positioned by synchronising multiple strand jacks with the aid of a computerised control system. Strand jacks fi nd applications in the erection of bridges, offshore structures, refi neries, power stations, major buildings and other structures where the use of conventional cranes is either uneconomical or impractical.

This article features the installation of six 800t ball mills and six 1,200t autogenous mills used to process magnetite at the Sino Iron Ore project in Western Australia.

IntroductionModern structures are placing increasing demands on designs, construction materials, construction processes and ongoing maintenance activities. Structures such as bridges are increasing in dimensions with spans lengths continually pushing the boundaries of constructability. Often these structures cannot be constructed economically by using traditional construction methods. What was needed to meet these construction pressures was a heavy lifting system that could precisely lift heavy loads over extended distances.

To fulfi l this need, strand jacks were invented in Europe in the 1970s as a development of post tensioning systems in use at that time. They are currently manufactured by a small number of companies based in Europe. The aim of this article is to describe the basic operation of strand jacks and briefl y explore two case studies. It is the intention that as the subject of strand jacks is given increased exposure, more designers will hopefully add strand jacks to their construction toolkit.

Strand jacks are jacks used to lift very heavy loads for construction and engineering purposes. They are now used all over the world to erect bridges, offshore structures, refi neries, power stations, major buildings and other structures where the use of conventional cranes is either uneconomical or impractical. The Enerpac strand jack technology has been used on many notable projects such as the raising of the 10,000 tonne Russian submarine, Kursk, from the ocean fl oor and construction of the Viaduc de Millau in France.

The strand jack lifting technique originates from equipment used for concrete pre and post tensioning. A strand jack utilises a number of individual high tensile stressing strands. In a strand jack, a bundle of steel strands is guided through a hollow hydraulic cylinder. Mounted above and below the cylinder is an anchor system with a number of collet anchors. For each strand there is a single collet assembly above and below the hydraulic ram. By stroking the ram in and out and engaging or disengaging the collets in the appropriate sequence, the strand jacks can either lift or lower a load. As strands can only act in tension, a strand jack cannot push a load. In the preceding paragraphs strand jacks have been described primarily as a lifting device in the vertical direction. However, a strand jack can be used in any direction as long as tension is maintained in the strands.

The high pressure oil to operate the hydraulic ram is supplied by hydraulic power packs which can be either electric or diesel powered. A sophisticated software program controls the motion of each individual jack to provide an overall synchronised lift. Integrated within each strand jack is a displacement transducer which measures the position of the hydraulic ram piston. This allows the computer system to monitor the fl ow requirements to each jack. During operation, all jack loads and lifting point positions are displayed on the screen of the control unit.

In recent years, computer control has added to the versatility and safety of strand jack technology. In theory, any number of strand jacks can be used simultaneously to achieve unlimited lifting capacity, with computer-controls to keep the motion of all jacks synchronized. In practice, the maximum number of jacks that can currently be used simultaneously and kept under existing computer control systems is 80, and the largest jacks on the market today can lift 1022t with a safety factor of 2.5. Therefore loads of up to 81,760t can be lifted with utmost precision and safety using a single computer control system.

Working principleThe high tensile strands used in strand jacks are of the same solid wire seven piece construction as used in pre and post tensioned concrete. These strands are often run through an additional manufacturing process to reduce their diameter further by fl attening/rounding slightly the outer wires to give the strand more circumferential contact area (Figure 1). The reduction of diameter means more strands can be utilised within a given area although the sectional areas are the same. Additionally the increased circumferential area allows more contact area with the collets and hence less wear.

Figure 1. Cross section standard and modifi ed strand

Figure 2 demonstrates the principle of operation of a strand jack when raising a load. For clarity only one strand has been shown in the diagram. The capacity of a strand jack is directly proportional to the number of strands used. For example, strand numbers can range from a single strand to 55 strands in a 6600kN jack. Strands are usually 15.2mm in diameter although 18mm diameter strands are sometimes used.

LoadLoad Load

Load

Retract Raise Retract Raise

Collet closed

Collet closed Collet closed

Collet closed

Collet open

Collet open Collet open

Collet open

1 2 3 4

Figure 2. Principle of operation

With respect to Figure 2, the yellow and green sections indicate a hollow hydraulic ram while the black shaded area represents high pressure hydraulic oil. The collets are shaded red and the mating collet housing is shaded blue.

Strand jack operation

Step 1. The bottom collets are closed and hold the load while the main ram is retracted. The hydraulic oil is supplied to retract the ram while the top collets are open and do not grip the strand. The load is stationary during this part of the cycle.

Step 2. The top collets are engaged and the hydraulic oil is supplied to the other side of the piston causing the load to advance. While the load is being raised the bottom collets are open and do not grip the strand.

Step 3. Repeat step 1.

Step 4. Repeat step 2.

When raising or lowering a load, a built-in displacement transducer measures the movement of each strand jack. The central control system keeps track of all strand jack displacements and corrects for an errors that may accumulate. While the operation demonstrated in Figure 2 relates to raising a load, the lowering operation is very similar except that the phasing of the collet operations is reversed.

Case study 1 Sino Iron OreThe Sino Iron Ore project is located near the north-west coast of Western Australia approximately 100km from Karratha, and is expected to become one of the worlds biggest iron ore developments. The project, owned by CITIC Pacifi c Mining, will employ 10,000 workers to build it and 800 to run it. This will involve extensive technology transfer, with Chinese and Australian design teams developing the mills that will process the magnetite ore into fi ne concentrate and produce value-added products that will help pave the way for the magnetite industry in the state.

The Sino Iron Ore project involves twelve of the biggest iron ore processing mills ever built. The mills are manufactured in China and transported in sub-assembly form for precision positioning onto their bearings twenty-one metres above the ground. Figure 3 shows one of the installed ball mills. An initial production target for the mine is 27.6 million tons a year from a resource estimated at more than two billion tons of magnetite ore. It is the fi rst time iron ore processing mills have been fabricated in China, transported by sea and road more than 7,000km distance and in the process achieving huge time savings over on-site fabrication. Mills are off-loaded at the Cape Preston wharf, then transported by road at night by multi-wheel transporters at about 1km/h. The transport takes approximately three days. Figure 4 shows a ball mill after arriving at the job site supported on multiple cradles.

VDM Group is constructing the magnetite mining and processing operation. This involves installing six 800t ball mills and six 1,200t autogenous mills using Enerpacs strand jack technology. The original mill installation program extends from January 2010 to January 2011 (additional mills may be added later). The PLC-controlled synchronous strand lift system specifi ed for the lifting of the ball mills involved four - 4440kN strand jacks with each jack utilising 37 strands. Each strand jack has its own dedicated cable reeler (recoiler) to dispense and rewind the strands (Figure 5). A mill is delivered and positioned beneath specially constructed steel towers with the strand lifting equipment overhead. After the four strand jacks synchronously raise the mill, the multi-wheel road transporters are removed, a rail mounted transporter is moved into position from the side and the mill lowered onto the rail transporter. Figure 6 shows a mill being raised from the road transporter utilising two of the support cradles; the other support cradles are for road transport only. The processes to install the mills onto their trunnion bearings (mounted off the ground on large reinforced concrete towers) are complex and technically challenging. As the strand jacks can only lift and lower, not slew, the positioning entails constructing temporary structures to support the mills between transfer processes.

Figure 4. Ball mill and support cradles arriving on the multi-wheel transporters

Figure 3. Installed ball mill

People

Cable reeler Upper collet assembly

Displacement transducer

Lower collet assembly

Hydraulic ram

Figure 5. 4440kN strand jack with cable reeler

Final position

Rail transporter

Figure 6. Night time transfer from road to rail transporter

Figure 7. Mill prior to lowering onto rail transporter

The rail transporter subsequently transfers the mill laterally to a position adjacent to the foundation towers on which the trunnion bearings are located. Strand jacks are used on additional prefabricated jacking towers which facilitate the fi nal placement of the mill. The strand jacks can raise and lower at speeds up to 110mm/min, with one-person controlling the operations via a touch screen interface which synchronises the action of the four jacks.

When the ball mills are close to their fi nal position, surveyors perform the fi nal measurements to allow accurate positioning. During the fi rst ball mill installation, the ball mill was positioned over the bearings to within 0.5mm of the target position as

required by the mill manufacturers specifi cations. This was considered to be quite an achievement considering the mill was nearly 800t and winds at the time were estimated to be 10m/s at the top of the mill foundation. The fi nal seating of the mill was the most delicate part of the operation as there was only 0.5mm clearance on each side between the thrust blocks and the groove in the trunnion of the mill. Four ancillary manual jacks were used in the fi nal transverse positioning operation.

At the time of publication of this article, one ball mill and one autogenous mill have been successfully installed.

Figure 8. Silleda viaduct Spain

Case study 2 Silleda viaduct

The construction methodology adopted for the Silleda viaduct in Spain incorporated the use of strand jacks. As shown in Figure 8 the main span is supported by two reinforced concrete arches which span a river valley below. The two arches were constructed in a near vertical position adjacent to the main span piers with a hinged joint constructed in the base of each arch. After completion of both arches, they were then lowered/rotated into place with strand jacks (Figures 9,10).

Figure 9. An arch being lowered into position

Figure 10. Control cabinet uppermost on pier controlling lowering

The slender main piers could not sustain the bending forces induced during the lowering process. To overcome these forces the piers were back stayed with pre-tensioned multiple strands. These backstays are more evident in Figure 11.

Backstays

Figure 11. Backstays counteract lowering forces

Figure 12. Arches meet at mid span position

ConclusionStrand jack technology can not only raise or lower heavy loads but can do so within tight operational tolerances and over long distances. The use of synchronised heavy lifting with strand jacks is now becoming a standard tool for construction of projects where the use of cranes is impractical and uneconomical. Strand jacks give the engineer more fl exibility in the design of projects which otherwise may not have been possible. Designs that were previously discounted for constructability reasons can now be reconsidered as viable options.