Freyssinet Flat Jacks
The Freyssi Flat Jack is a thin hydraulic jacking device used in Civil and Structural Engineering work to exert large forces simply and economically. Originally developed by Eugene Freyssinet as a means of prestressing concrete under unusual circumstances, it's field of application has now extended far beyond this sphere.
Simple and compact, Freyssinet Flat Jacks are often used for remedial measures or structural extensions as well as for new construction. The Freyssi Flat Jack can be used to solve a large variety of engineering problems:
- Control of thrust forces
- Prestressing between abutments
- Adjustment of support reactions
- Structural pre-loading
- Structural lifting
- Underpinning
- Load measurement
- Thrust maintenance
The Freyssi Flat Jack is a hydraulic capsule in the form of a flat double saucer. It is usually made of two circular, soft grade steel stampings welded and heat treated to produce the required internal pressure strength and flexibility. Each jack has an inlet for inflation and may have an outlet. The inlet is normally supplied with an extension piece terminating in a needle valve and, if required, a pressure gauge. When inflated hydraulically the upper and lower plates of the jack are forced apart. In doing so, the jack can exert an extremely large thrust force depending on the hydraulic pressure, which can be raised in normal service to 2200psi (15N/mm2).
Standard Sizes
| Reference Number |
Outside Diameter (inches) |
Normal Thickness (inches) |
Minimum access gap (inches) |
Max force Jack closed (tons) |
Max force Jack fully open (tons) |
Maximum stroke (inches) |
| 12 | 4.7 | 1 | 1 1/2 | 9.8 | 8.5 | 1 |
| 15 | 5.9 | 1 | 1 1/2 | 18 | 15 | 1 |
| 22 | 8.7 | 1 | 1 1/2 | 46 | 40 | 1 |
| 25 | 9.8 | 1 | 1 1/2 | 62 | 56 | 1 |
| 27 | 10.6 | 1 | 1 1/2 | 74 | 66 | 1 |
| 30 | 11.8 | 1 | 1 1/2 | 94 | 87 | 1 |
| 35 | 13.8 | 1 | 1 1/2 | 127 | 120 | 1 |
| 42 | 16.5 | 1 | 1 1/2 | 197 | 189 | 1 |
| 48 | 18.9 | 1 | 1 1/2 | 262 | 250 | 1 |
| 60 | 23.6 | 1 3/8 | 1 3/4 | 427 | 418 | 1 3/8 |
| 75 | 29.5 | 1 3/8 | 1 3/4 | 690 | 680 | 1 3/8 |
| 87 | 34.2 | 1 3/8 | 1 3/4 | 940 | 930 | 1 3/8 |
| 92 | 36.2 | 1 3/8 | 1 3/4 | 1075 | 1065 | 1 3/8 |
Notes
Standard Shape
Freyssi Flat jacks are normally of circular form, as indicated in the table.
However, specially shaped jacks, e.g. oblong, rectangular, and even centerhole, may be custom-made for a large variety of applications.
Minimum Access Gap
The table gives the minimum access gap required to place flat jack and thrust membranes between the surfaces to be jacked.
Additional shims should be used to fill the access gap if necessary.
Methods of Installation
The shape of the Freyssi Flat Jack demands the use of an intermediate thrust membrane between itself and the structure. A number of methods are used:
Cement or Epoxy Mortar
This is the method used when the jack is to remain in position permanently.
The dished surfaces are filled and levelled with a cement mortar or a hard setting epoxy mortar which has the advantage of being lighter, easier to handle and noncorrodable.
Ground Steel Plates
This method is preferred when the jack is to be removed on completion of duty.
The jack is used with loosely fitted steel plates in each saucer of the jack. Apart from their robustness, the steel plates enable attachments to be made to the assembly for ease of handling and for extraction after inflation. This is particularly important with the larger jacks.
Pre-cast Concrete Blocks
Used as a permanent installation where the concrete block is to remain part of the visible structure, it can be designed so that there is a facility for future adjustment.
It is a convenient methond to use where a number of flat jacks are compounded in series to give larger movements. The concrete block is usually reinforced above and below each flat jack with a nominal reinforcement mesh.
Methods of Inflation
Temporary
When Freyssi Flat Jacks are to be used for a limited period and subsequently removed, inflation is normally done with hydraulic fluid using normal pumping techniques and a circuit designed according to the structural balance or "failsafe" requirements.
Single or multiple jacks can be inflated individually or on manifolded circuits. Each jack has a needle valve for locking off the pressure, and may be fitted with a hydraulic gauge if required.
Permanent
Where Freyssi Flat Jacks are to remain permanently installed, they are inflated under pressure using a hard-setting epoxy grout. This hardens with negligible shrinkage over a short controlled period to leave the force permanently applied. This type of application is particularly suitable for underpinning work.
Often in these cases, temporary oil inflation is required for periods of up to six months for structural stability to be achieved or to allow for the effects of a continuing construction program.
This can be done easily where each jack in turn may temporarily be depressurized for replacement of the oil by the grout. Where this cannot be permitted a more advanced method involving transfusion techniques may be used.
It should be noted that while epoxy resin is the preferred medium for permanent jack installation, under certain controlled conditions cement grout may be used for this purpose.
