Strand jack

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 400 tons, strand jacks were invented in the 1970s.Fact|date=March 2008

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.

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 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 computer control is 20, and the largest jacks on the market today lift 850 tonnes (937 tons), with a safety factor of 2.5 to 1. Therefore loads of up to 17,000 tonnes can be lifted with utmost precision and safety.

In 2006 Bigge Crane & Rigging, a US heavy lifting specialist, used strand jacks to lift a pair of transition spans, each weighing 2000 tons (1800 tonnes), on the New San Francisco Oakland Bay Bridge.These transition spans connect the concrete Skyway bridge on the Oakland side and a suspension bridge to Yerba Buena Island.

The strand jacks, made by Dutch company Hydrospex, were controlled by computer to precision lift the deck sections to a height of 200ft within tolerances of 1/32 of an inch. Computer controls ensured that the bridge sections, which were 200ft long by 85ft wide, were kept horizontal to within an eighth of an inch throughout the lifts.

The strand jacks were mounted on special lifting frames at either end of the transition spans. At the Oakland side (to the east), where the Skyway bridge deck had already been constructed, four strand jacks were mounted on a mobile jacking platform that was supported by a pair of 60ft girders, cantilevered 28ft off the bridge deck. Each jack had a lifting capacity of 365 tons. Each jack pulled on a bundle of 31 strands. Each rope was 0.62 inches (15.7mm) in diameter.

At the San Francisco side (to the west), where construction of a suspension bridge would later follow on, two steel lifting towers were, founded on piles driven into the Bay for this purpose. Bigge assembled two girders to span 125ft across the top of this lifting tower and mounted strand jacks on top of a mobile jacking platform that sat on the girders. At the San Francisco End, six jacks of 235 ton lifting capacity were used, each with 19 wire rope strands of 0.62 inch diameter.

Each lift took approximately ten hours, as the strand jacks raised the loads at speeds of up to 30ft per hour.

The first span was lifted overnight on 7th/8th February 2006; the second was lifted in a similar overnight operation on 29th/30th August 2006.

The project had particular resonance for Bigge president Weston Settlemier because it continued a family tradition of involvement in San Francisco’s great structures. His grandfather, Henry Bigge, who had founded the company in 1916, hauled the steel that built the Golden Gate bridge in the 1930s.

Other structural applications have been the raising of the 316 metre span, roof-supporting arch over Wembley Stadium, London, UK and the raising of the London Eye observation wheel from the horizontal to the vertical position.

Strand jacks have also been used in other lifting applications, such as marine salvage. For example : salvage of the submarine Kursk.

External links

* [http://www.wbequipment.com/strandjacks-bigpic.htm Diagram of a strand jack]

* [http://www.enerpac.com Enerpac Strand Jacks]
* [http://www.ttfijnmechanica.nl TT Fijnmecanica]