General Description
Freyssi Flat Jacks are simple, practical devices for the civil or structural engineer wherever the application or control of large forces is required. They are frequently used in thesolution of unforeseen problems such as in remedial measures or structural additions.

They are as often used in new constructions in which they form part of the structural concept.

In the years since their invention by Eugene Freyssinet in 1934, many techniques
have been evolved for many applications:
Custom hydraulic systems designed to raise or lower any type of structure. 
Give our engineers a call to discuss your applications.

Freyssi Flat Jack Chart

Freyssi Flat 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  
  • 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 25mm (1in).

  • Two pipe connections are welded into the jack rim usually at 90o to one another. One provides the inlet for inflation and the other serves as a vent during filling and is subsequently capped off.

Advantages of Freyssi Flat Jacks

– Known Forces
– Precise Control
– Low Height For Limited Access Situations
– 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

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
Freyssi Flat JacksA 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.

Freyssi Flat JacksUsing 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.

Freyssi Flat Jacks

Bolt Testing” generally describes using our hydraulic load cells i.e. HS, J, K, M, MZ, S etc. to measure fastener performance as follows:

Pre-Installation Verification Tension Testing & Rotational Capacity Testing of structural fasteners per AISC/RCSC specifications.
General fastener testing to determine torque/tension relationships.
The types of components that can be tested include but are not limited to Hex Bolts, Tension Control Bolts, Anchor Bolts, Studs, Lock Nuts, Direct Tension Indicating Washers, etc.

Skidmore Wilhelm’s hydraulic tension calibrator, or bolt tension calibrator, has been the industry standard for testing high-strength bolts since we invented the technology in the 1950’s. Our units are required to be used on steel buildings, bridges and other structures using fasteners in tension critical joints.

Our units can safely be used with hand torque wrenches, impact guns, electric wrenches and hydraulic wrenches.

Know What You Want?

A large inventory of Enerpac, Power Team, Simplex, Freyssi Flat Jacks, Skidmore and Calibore equipment enables W.B. to answer any need rapidly and effectively. To discuss the best system for your bridge lowering, raising, or moving, call W. B. Equipment at 866-522-5464 or email or

For more information about WB Equipment sales, rentals and service consult our website at



Custom hydraulic cylinders and hydraulic jacks made to order. Huge inventory readily available. With loads from 1 – 2,000 tons and pressure rating of 100 – I0,000 psi we can make any hydraulic cylinder or hydraulic jack to your exact requirements. Whether it be single acting, double acting, lockout or centerhole let us provide you with the right jacking system to perform your task.

Custom power units and hydraulic pumps made to order. Electric, Gas, Diesel or Air Operated power units. Flow rates range from 1 GPM to 100 GPM. Our hydraulic pumps are available in over 1000 different configurations. They have a pressure rating of 10,000 psi.

Freyssi Flat Jacks are simple, practical devices for the civil or structural engineer wherever the application or control of large forces is required. Flat jacks are frequently used in the solution of unforeseen problems such as in remedial measures or structural additions.

The sensor extension is different from the Cylinder stroke value. The sensor extension is hooked to the load, and it has a value even when the Cylinder is collapsed. The potential applications of this system are numerous synchronized lifting levering, pushing, pulling and positioning using computer technology.of a synchronous lifting system 

The hydraulic press depends on Pascal’s principle: the pressure throughout a closed system is constant. One part of the system is a piston acting as a pump, with a modest mechanical force acting on a small cross-sectional area; the other part is a piston with a larger area which generates a correspondingly large mechanical force. Only small-diameter tubing (which more easily resists pressure) is needed if the pump is separated from the press cylinder.

Power Team Monostrand Stressing Jacks are the most durable in the industry.

Monostrand Stressing Jacks suited for work on slab-on-grade where dirt, heat and high volume use take their toll.

Stressing jacks are mainly used on construction sites for stressing mono-strand anchors, and for the separate stressing of the strands of multistrand tendons.

We can provide customized jacking equipment to test driven piles from 1 -2000 tons. The systems can be manually, electrically or automatically controlled.

WB Equipment has delivered components for multiple jacking systems. Our knowledge of structural design, hydraulics, and automation means that we are capable of designing a complete jacking system with controls that allow for fully automated jacking and monitoring.

A hydraulic torque wrenchis a power tool designed to exert torque on a fastener to achieve proper tightening or loosening of a connection through the use of hydraulics. A torque wrench is applied to the nut either directly or in conjunction with an impact socket. Hydraulic torque wrenches apply a predetermined, controlled amount of torque to a properly lubricated fastener.

strand jack (also known as strandjack) is a jack used to lift very heavy (e.g. thousands tons or more with multiple jacks) loads for construction and engineering purposes.

Strand jacks can be used horizontally for pulling objects and structures, and are widely used in the oil and gas industry for skidded loadouts. Oil rigs of 38000 Te have been moved in this way from the place of construction on to a barge. Since multiple jacks can be operated simultaneously by hydraulic controllers, they can be used in tandem to lift very large loads of thousands of tons. Even the tandem use of two cranes is very difficult operation.

hoist is a device used for lifting or lowering a load by means of a drum or lift-wheel around which rope or chain wraps. It may be manually operated, electrically or pneumatically driven and may use chain, fiber or wire rope as its lifting medium.

The basic hoist has two important characteristics to define it: Lifting medium and power type. The lifting medium is either wire rope, wrapped around a drum, or load-chain, raised by a pulley with a special profile to engage the chain. The power can be provided by different means. Common means are hydraulics, electrical and air driven motors.

A jack, screwjack or jackscrew is a mechanical device used as a lifting device to lift heavy loads or to apply great forces. A mechanical jack employs a screw thread for lifting heavy equipment.


A hydraulic jack uses hydraulic power. The most common form is a car jack, floor jack or garage jack, which lifts vehicles so that maintenance can be performed. Jacks are usually rated for a maximum lifting capacity (for example, 1.5 tons or 3 tons). Industrial jacks can be rated for many tons of load.

Quick acting re-bar grips are available for pull out testing of reinforcing bars in concrete, (re-bar/reinforcement bars) used in construction of highways, bridges, and tunnels as well as rigging and other applications.


When used with hydraulic jack, user slips jack and grip over grouted re-bar and applies pressure to load. These re-bar grips come in sizes No. 4-18 and accommodate construction industry’s focus on repair, maintenance, and building of public infrastructure.

Custom made cylinders made to your specifications. All cylinders calibrated and traceable back to NIST. Centerhole,  locknut,  low height, single acting,  double acting,  long stroke.

  • 0-1500 tons
  • Centerhole sizes .75 inch to 15 inches
  • Stroke ½ inch to 10 feet

Enerpac is a global market leader in high pressure hydraulic tools, controlled force products and solutions for precise positioning of heavy loads. 

Work site solutions for stressing, lifting and surfacing for railroad maintenance.

  • Track Jacks
  • Rail Pullers
  • Lifting Jacks
  • Truck Jacks

Wide range of high strength mechanical and hydraulic pullers

  • Master Puller Sets
  • Grip Puller Sets
  • Cross Bearing Puller Sets
  • Bearing Pullers
  • Bearing Cup Pullers
  • Posi Lock Pullers