Strand jack
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Strand jacks (also known as strandjacks) are jacks used to lift very heavy loads for construction and engineering purposes. Capable of lifting loads of over 1,000 tons, strand jacks were invented in Europe in the 1970s.[citation needed] as a development of post tensioning systems in use at that time. They 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.
The strandjack lifting technique originates from the concrete post tensioning principle. A strandjack can be compared to a single-part winch. In a strandjack a bundle of steel cables is guided through a hydraulic cylinder. Above and below the cylinder is an anchor system with a number of grips. Those grips can clamp the cable bundle. By stroking the cylinder in and out while the grips are engaged in the anchors a lifting or lowering movement is achieved. The motion of the cylinders is driven by hydraulic power packs. They can be electric or diesel powered. A sophisticated software program synchronizes the motion of the strandjacks and adjusts the motion to the loads per lifting point. This program can operate and control dozens of strand jacks simultaneously. During operation all loads and anchor positions are displayed on the screen of the control unit. A useful Powerpoint presentation of how a strand jack works can be downloaded from Dorman Long Technology's web site - see the external link below.
Strand jacking is a construction process whereby large construction elements (such as concrete slabs and steelwork) are lifted into position with computer-controlled hydraulic jacks. Because the multiple jacks can be moved in unison and with great precision, heavy structures can be assembled at ground level (with increased safety and reduced cost)and then lifted into position, rather than having to be built in the air. Traditional cranes and other lifting methods cannot provide this level of precision.
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 40, and the largest jacks on the market today lift 930 tonnes (1023 tons), with a safety factor of 2.5 to 1. Therefore loads of up to 37,200 tonnes can be lifted with utmost precision and safety.
Strand jacks for heavy lifting and skidding operations are owned and operated by a large number of construction and heavy lifting companies around the world. They are currently manufactured by a small number of companies based in Europe.
