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HYDRAULIC FLAT JACKS
Temporary Flat Jacks • Permanent Flat Jacks • Complex Flat Jack Systems
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HYDRAULIC FLAT JACKS
Hydraulic Flat Jacks
WB Equipment offers a range of low-profile Hydraulic Flat Jacks spanning from 8 to 1000 tons in capacity. These ultra-compact jacks boast remarkable thrust capabilities, making them indispensable tools in the civil and construction sectors. Their versatility extends to various applications, including lifting heavy structures, conducting stressing operations, transferring loads to temporary or permanent supports, and pre-loading buildings, steelworks, or bridge bearings for long-term stability.
Hydraulic Flat Jack
Advantages of Hydraulic Flat Jacks
- Known Forces
- Precise Control
- Low Height For Limited Access
- Low Bearing Stresses
- Out Of Parallel Tolerance
- Between Jacking Surfaces
- Temporary, Removable Jacks
- Permanent, Sacrificial Jacks
- Structural Lifting
- Underpinning
- Control Of Thrust Forces
- Maintenance Of ConstantThrust
- Adjustment Of Support Reactions
- Pretressing
- Structural Pre-Loading
- Loading Of Seismic Isolators
- Arch De-Centering
- Control Of Settlement
- Measurement Of Forces
- Weighing Of Offshore Structures
- Permanent Shimming
Hydraulic Flat Jack Chart
HydraulicFlat Jack Detailing Guidlines
- Bearing Area – The whole surface of the flat jack thrust plate must bear upon a jacking surface. Local stiffening, distribution steel or concrete may be necessary.
- Access Gaps – The selection chart shows the thickness and minimum access gap requirements which must be provided. If the full travel of the jack is required then packing shims must be fitted to fill any remaining space before inflation.
- Tandem Jacks – Where the required jacking distance exceeds the travel of a single jack they may be stacked one on top of another and then inflated in sequence. Only the larger sizes of flat jacks may be stacked more than two high.
- Sheer Resistance – Flat jacks must not be subjected to shear loads. Where necessary the flat jack must be protected from shear forces by housing in a shear box or by another means.
- Inlet/Vent Positioning – Flat jacks can be made with suitable inlet positions where access is especially difficult.
- Oil-Resin Transfusion Jacks – When jacks are to be oil inflated and later transfused with epoxy resin, they are manufactured with an internal pipe to ensure satisfactory oil displacement. This requirement should be specified at the time of detailing.
- Safety Packs – It is frequently advisable to make provision for safety packs when temporary oil flat jacks are intended to be in position for extended periods and at risk from mechanical damage from other activities in the jacking area
Temporary flat jacks
Temporary flat jacks are essential for bridge jacking applications, providing hydraulic jacking and monitoring equipment for lifting purposes. These flat jacks, designed for temporary use and subsequent removal, are typically inflated using water or hydraulic oil.
Key advantages of temporary flat jacks include:
- Functioning as load cells when connected to pressure gauges or transducers, equipped with digital read-outs.
- Pre-loading steelwork with temporary support systems.
- Facilitating continuous load monitoring and adjustment.
- Utilizing multiple flat jack circuits for uniform loading.
- Ability to transition seamlessly from temporary to permanent applications.
- The flat jack consists of a mild steel capsule, circular in plan, with a cross section that is initially dumb-bell shaped. The dished portions are fitted with ground steel thrust plates before the jack is placed between the surfaces to be jacked apart. When the jack is inflated with a liquid the hydrostatic pressure is transmitted by the flat portions of the capsule through the thrust plates while the toroidal rim of the flat jack deforms to allow the flat faces to move apart.
- The maximum operating pressure is only 150 bar (2,200psi). Flat jacks thereby exert low bearing pressures on the jacked faces.
- The maximum recommended opening stroke of a flat jack is equal to the depth of the toroidal rim – about 1in.
- Two pipe connections are welded into the jack rim usually at 90 degrees to one another. One provides the inlet for inflation and the other serves as a vent during filling and is subsequently capped off.
Calibrated Flat Jacks & Weighing
A flat jack which has been pre-opened, fitted with a pressure gauge or transducer, sealed and then calibrated in a press can be used as a robust, accurate load cell. This load cell can be placed in tandem with a standard flat jack of the same size.
The twin jacks are then installed beneath a structure at weighing positions. The weight of the structure taken by inflation of the standard flat jack registers a pressure in the calibrated flat jack which, by reference to its calibration graph, provides an accurate weight.
Using this principle, offshore structures ranging from modules of several hundred tonnes to complete integrated decks of over 30,000 tonnes to an accuracy of +/- 1%. Where necessary for lighter structures, an arrangement is available with greater accuracy.
Temporary Flat Jacks
Oil Inflation – When flat jacks are to be used for a limited period and subsequently removed inflation is usually done with hydraulic fluid. For small numbers of jacks, requiring relatively small volumes of oil, simple hand pumps may be used for inflation. Larger jacks or large numbers of jacks may require electric or diesel powered pumps.
Hydraulic Circuits – Single or multiple flat jacks can be inflated individually or on inter-connected circuits. Each jack has a needle valve for locking off and may be fitted with a pressure gauge if required.
Permanent Flat Jacks
Where a force is required to be permanently applied to a structure, flat jacks are inflated under pressure with an epoxy resin by means of a special resin injection cylinder. The epoxy resin hardens over a period of time to leave the force permanently applied. This type of application is suitable for pre-loading new or additional steelwork in structures, underpinning works and structural bearing installation.
This method is often used where sufficient room is not available for wedging or grouting the jacking gap.
In certain circumstances, it may be necessary to carry out the initial inflation on hydraulic oil with resin inflation being carried out at a later date. An oil to resin transfusion system is then required.
Complex Flat Jack Systems
In applications using multiple flat jacks the hydraulic pumping circuitry can be arranged appropriately using pipe and compression fitting or flexible hoses and quick release couplings.
Individual jacks are usually fitted with manual needle valves. Pressure gauges may be fitted to individual jacks, downstream of their needle valves, instead of or in addition to a common ring main gauge. Differing loads in neighboring jacks may thus be locked in and monitored as necessary.
For especially complex operations solenoid valves, electronic pressure transducers and computer control, monitoring and data logging systems may be used.
Calibrated Flat Jacks & Weighing
A flat jack which has been pre-opened, fitted with a pressure gauge or transducer, sealed and then calibrated in a press can be used as a robust, accurate load cell. This load cell can be placed in tandem with a standard flat jack of the same size.
The twin jacks are then installed beneath a structure at weighing positions. The weight of the structure taken by inflation of the standard flat jack registers a pressure in the calibrated flat jack which, by reference to its calibration graph, provides an accurate weight.
Using this principle, offshore structures ranging from modules of several hundred tonnes to complete integrated decks of over 30,000 tonnes to an accuracy of +/- 1%. Where necessary for lighter structures, an arrangement is available with greater accuracy.
HydraulicFlat Jacks — Technical Specifications and Uses by Industry
Hydraulic Flat Jacks are hydraulic load-transfer devices developed for applications where large, controlled forces must be applied in confined spaces. They are widely used in civil, structural, and geotechnical engineering where conventional hydraulic cylinders cannot be accommodated due to height or access limitations.
Technical Specifications
Operating Principle
Hydraulic Flat Jacks consist of two welded steel plates forming a thin internal cavity. When pressurised with hydraulic fluid, the jack expands uniformly, converting pressure into a controlled compressive force distributed over a defined bearing area.
Operating Pressure
Nominal operating pressure: ≈ 2,175 psi
Low operating pressure results in:
Reduced bearing stresses on concrete or masonry
Improved control and safety
Compatibility with standard hydraulic pumping systems
Force Capacity
Force output = Pressure × Effective Area
Typical force ranges:
50,000–500,000 lbs
Larger units can exceed 1,000,000 lbs, depending on size
Capacity selection governed by allowable bearing pressure and structural limits
Thickness and Geometry
Unpressurised thickness: typically 0.4–1.2 in
Typical stroke: 0.2–0.6 in
Extremely low profile allows installation in minimal access gaps
Bearing Area Requirements
Requires flat, parallel bearing surfaces on both sides
Bearing pressure must be verified against:
Concrete compressive strength
Masonry or steel contact limits
Steel spreader plates often used to distribute load and reduce local stress
Hydraulic Connections
Inlet and vent ports positioned to:
Ensure complete hydraulic filling
Prevent air entrapment
Multiple flat jacks can be connected in parallel or tandem circuits for synchronized loading
Shear and Stability Considerations
Flat jacks are designed for axial compression only
They do not resist shear unless additional measures are provided:
Shear keys
Mechanical guides
External restraint systems
Lateral loads must always be carried by the structure, not the jack
Tandem and Multi-Jack Use
Multiple flat jacks may be:
Installed side-by-side
Stacked with separation plates
Enables uniform lifting, load redistribution, or staged structural adjustment
Pressure equalisation is critical to prevent uneven loading
Uses by Industry
Civil & Structural Engineering
Structural lifting and lowering
Load transfer during refurbishment or strengthening
Correction of differential settlement
Bridge bearing replacement
Stress redistribution in concrete and masonry structures
Geotechnical Engineering
Foundation underpinning
Load compensation during excavation
Preloading of foundation elements
Controlled adjustment of ground–structure interaction
Seismic Engineering
Installation and adjustment of seismic isolators
Load tuning of isolation systems
Controlled force application without shock loading
Infrastructure & Transportation
Rail and metro structure realignment
Tunnel lining correction
Temporary load support during staged construction
Industrial & Energy
Alignment and support of heavy machinery
Load redistribution in power plants
Foundation retrofits for rotating or sensitive equipment
Key Engineering Advantages
Known and measurable forces (pressure × area)
Extremely low height for restricted-access locations
Uniform load distribution over large bearing areas
Fine, incremental control of applied load
Adaptable to both temporary works and permanent installations
Suitable for emergency interventions and planned construction
Engineering Design Considerations
Engineers specifying FHydraulic Flat Jacks should evaluate:
Required force (lbs)
Allowable bearing pressure on the substrate
Available access gap and required stroke
Load path continuity and redundancy
Need for shear restraint or guidance
Hydraulic circuit layout, monitoring, and safety provisions
Summary
Hydraulic Flat Jacks provide a low-profile, high-capacity solution for applying large, precisely controlled compressive forces. Operating at approximately 2,175 psi, they offer predictable performance, reduced bearing stress, and exceptional adaptability—making them indispensable for structural lifting, underpinning, seismic isolation, and load redistribution across multiple engineering industries.
