Glossary term
Jacking Force
The controlled force applied by a hydraulic or mechanical jack to lift, push, tension, align, or preload a structure or component.
Definition
quantityJacking force is the force delivered by a jack or stressing device to move, lift, push, align, or preload an engineering element.
Jacking force is used in bridge lifting, structural bearing replacement, tunnel and pipe jacking, prestressing operations, heavy machinery alignment, excavation support, load testing, and staged construction. In hydraulic systems, it is commonly estimated from pressure multiplied by effective piston area, corrected for friction, losses, jack calibration, geometry, and load path. Its safe use requires control of force, displacement, stability, support reactions, sequencing, and redundancy.
Jacking force is the controlled force applied by a jack to a structure, component, or tendon. In hydraulic jacks, the ideal relation is:
where p is hydraulic pressure and A is effective piston area. In practice, calibration factors, friction, seal losses, hose pressure drop, jack orientation, eccentricity, and load transfer through bearing plates or fixtures all affect the actual force delivered to the structure.
Applications
In bridge maintenance, jacks lift girders to replace bearings or adjust support elevations. In post-tensioning, stressing jacks apply force to tendons before anchorage. In pipe jacking and tunnelling, hydraulic rams push pipe segments or shield machines through ground. In heavy machinery installation, jacks align equipment, level bases, or preload supports. In excavation support and retaining systems, jacks may preload struts to reduce wall movement.
The jacking force is not an isolated number. It creates reactions elsewhere. Lifting one bearing changes load distribution in the whole structure. Pushing a pipe through soil generates ground resistance and friction. Prestressing a tendon introduces losses from anchorage seating, friction, wobble, elastic shortening, creep, shrinkage, and relaxation.
Control and measurement
Force may be inferred from hydraulic pressure, measured with load cells, or verified by displacement response. For critical work, pressure gauges alone are not sufficient unless the jack has been calibrated and the setup matches the calibration conditions. Displacement monitoring is often required because equal force does not guarantee equal movement, especially when supports have different stiffness or friction.
Staged jacking plans specify jack locations, force increments, displacement limits, hold points, communication protocol, contingency actions, and acceptable differential movement. Temporary works must be designed for the same seriousness as the permanent structure because jacking often creates unusual load paths.
Common mistakes
A common mistake is assuming that the calculated jack force is exactly the force in the structure. Misalignment, eccentric contact, soft bearing pads, friction, locked-in stress, pressure gauge error, and uneven stiffness can make the real load path different. Another mistake is focusing on maximum force while ignoring stability, lateral restraint, local bearing stress, deflection compatibility, and what happens if one jack loses pressure.