What is Strand Jacking?

Jack capacity varies widely by model and strand configuration and is stated by the manufacturer; this page does not give a general range. What matters conceptually is that multiple jacks are used in parallel for larger loads, hydraulically synchronised so the lift stays level and load is distributed as the lift designer intended.

The lifting medium is a bundle of individual high-strength steel strands, sized and counted to suit the capacity required. Strand count, strand diameter and the resulting rated capacity are properties of a specific jack model and are stated by its manufacturer; they are not general values that can be assumed across systems.

The principle is cyclic rather than continuous. Each hydraulic stroke advances the load by the jack's stroke length, the strand is held while the ram resets, and the next stroke follows. A full lift is therefore the accumulation of many short, individually controlled movements rather than one sustained pull — which is what gives the method its positional precision. The mechanics of gripping and releasing, the stroke length and the permissible operating envelope are defined by the jack manufacturer and by the lift designer for the specific operation; this page does not set out an operating sequence.

Control systems monitor strand load at each jack and adjust hydraulic pressure to maintain synchronisation. On multi-point lifts — lifting a bridge deck from four corners, for example — the control system ensures that no single jack carries disproportionate load as the structure moves.

Strand jacks can lower as well as lift, giving them a controlled descent capability that cranes at maximum radius cannot always provide.

The method is applied wherever crane access is impractical, load capacities exceed crane limits, or precision control at height is required. On the London Eye, strand jacks rotated the fully assembled 120-metre diameter wheel from horizontal to vertical — a lift in which balance and controlled rotation were the governing requirements, not raw capacity.

On stadium roof erections, strand jacks lift pre-assembled roof structures from ground level to final bearing elevation. On bridge projects, strand jacking is used to thread new cables into cable-stayed structures, to lift precast segments into final position, and to provide controlled lowering of falsework after deck closure.