Strand jack
Strand jacks (also known as strandjacks) are jacks used to lift very heavy (e.g. 20,000 tonnes or more) loads for construction and engineering purposes. They 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 strandjack 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: climibing (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. Although a jack may lift only 1,000 tonnes or so, there exist computer control systems that can operate 40 jacks simultaneously, offering fingertip feel movement control over extremely massive objects.
Strand jacking is a construction process whereby large pre-fabricated building sections are carefully lifted and precisely placed. The alternative would be to do all assembly in situ, even if expensive, technically risky, or dangerous.
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.
