Hydraulic Strand Jack

 
 
Strand jacking is a construction process whereby large pre-fabricated building sections are carefully lifted and precisely placed. Strand jacks are jacks used to lift very heavy (20,000 tons or more with multiple jacks) loads for construction and engineering purposes.
 
 
The Octamec Group has pioneered the use of the Hydraulic Strand Jack technology in India when it constructed the largest span space frame aircraft hangar for Air India in Mumbai. Ever since then, Octamec has successfully used the Strand Jack technology for many of its projects including the new terminal building at Chandigarh airport, the aircraft hangars at Shamshabad airport for Air India, Mas GMR among others.
 
 
Strand 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. Strand jacks give the engineer more flexibility 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.
 
 
 

What is a Strand Jack?

 
 
Strand jacks were invented in Europe in the 1970s as a development of post tensioning systems and are now used all over the world to erect bridges, offshore structures, refineries, power stations, major buildings and other structures where the use of conventional cranes is either uneconomic or impractical.
 
 
A strand jack is a hollow hydraulic cylinder with a set of steel cables (the "strands" in the name) passing through the open centre, each one passing through two clamps - one mounted to either end of the cylinder. The jack operates in the manner of a caterpillar's walk: climbing (or descending) along the strands by releasing the clamp at one end, expanding the cylinder, clamping there, releasing the trailing end, contracting, and clamping the trailing end before starting over again. The real significance of this device lies in the facility for precision control. The expansion/contraction can be done at any speed, and paused at any location.
 
 
 

How is it used?

 
 
Strand jacks can be used horizontally for pulling objects and structures and widely used in the oil and gas industry for skidded loadouts, speeds of up to 40m/hr and Oil rigs of 38,000T have been moved in this way from the place of construction on to a barge.
 
 
As strands can only act in tension, a strand jack cannot push a load and is primarily used 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.
 
 
 

Working principle of the Hydraulic Strand jack:

 
 
The high tensile strands used in strand jacks are of the same solid wire used in pre and post tensioned concrete. These strands are often run through an additional manufacturing process to reduce their diameter further by flattening/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 modified strand  
     
   
  Figure 2:  Principle of operation  
     
  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.  
  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.  
  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 flow requirements to each jack. During operation, all jack loads and lifting point positions are displayed on the screen of the control unit.  
     
 

CROSS SECTION OF A HYDRAULIC STRAND JACK

 
     
 
  Used for Octamec Projects:  
        1. Chandigarh Airport
        2. GMR Hangar
        3. NACIL Hangar
 
Model no. HSL700
SWL: 700 kN @ 205 bar
Test pressure:  245 bar
Strands: 7 x Ø 15.7mm
Weight: approx. 640 kg
 
Supplier: Freight Wings Pvt. Ltd, Mumbai
 
 
     
 

STRAND JACKS USED IN BUILDING CONSTRUCTION BY OCTAMEC

 
  1. Erection of Chandigarh Airport Space frame terminal building  
 
     
Assembly before lifting Strand jack with power pack
   
Control panel Structure after lifting
   
 
  2. Erection of Truss for Air India Hangar, Shamshabad Airport  
 
     
Erection of assembled truss Strand jacks
   
 
 

STRAND JACK USED FOR RAISING/LOWERING NON-VERTICAL LOADS

 
 

The construction methodology adopted for the Silleda viaduct in Spain incorporated the use of strand jacks. As shown in figure below 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

 
  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.  
     
 
     
   
   
 


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