Flat jacks utilize internal hydraulic pressure to temporarily load and lift masonry structures. However, access gaps must be available and must not be exposed to shear loads.
Double flat-jack tests are widely utilized to ascertain elastic modulus of cob walls; however, interpretation problems often arise with their results.
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Flat jacks are an invaluable asset to structural engineers, providing a flexible way for them to tackle various projects. Composed of a mild steel capsule initially dumb-bell-shaped that is then distended and fitted with ground steel thrust plates for contact with the masonry being separated apart, these devices use water or hydraulic oil inflation for low bearing pressures that allow independent movement in each case.
Gregorczyk and Lourenco  utilized single flat jack tests to assess the stress state of brick masonry walls, finding variations to be no greater than 7% from test to test. Their new in situ testing technique called “Flat Jacks – Shear Compression Test” (FJ-SCT) method is more reliable, less destructive, and can even be applied retroactively on existing structures.
Flat Jacks are indispensable tools for civil or structural engineers when large forces must be applied and controlled. Commonly found in remedial works or structural additions, but also used when designing new buildings.
Each jack features a manual needle valve which can be closed off once desired pressure has been reached, as well as being equipped with a pressure gauge for monitoring purposes. They’re connected via hydraulic circuitry so as to inflate at various times without incurring large initial investments of hydraulic oil.
Leveling blocks that are cumbersome, hard to use and susceptible to breaking are now being provided with a novel solution: bicycle pumps or air compressors can easily inflate them for use – leaving minimal footprint when deflated.
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Flat jacks can be fitted with pressure transducers to form load cells for accurate weight weighing of structures. When used together with another standard flat jack at different weighing locations on the structure, pressure measurements register against an accuracy graph to provide an accurate weight reading. They may also be integrated with solenoid valves and electronic pressure gauges as monitoring or data logging systems for additional convenience.
Flat jacks have been developed as an in situ testing technique that reduces the impact of tests on buildings, making possible to measure stress, deformability and resistance characteristics of masonry in situ.
Flat jacks are an efficient and cost-effective solution to apply large compressive forces to buildings and structures for remedial jacking or other structural work, whether to address unexpected problems or as part of original designs. They can be used both proactively to address potential problems as well as planned within original plans for construction projects.
They can be filled with either water for temporary loading and hydraulic oil for permanent loading, and function as load cells when connected with a pressure gauge/transducer/read-out that provides continuous load monitoring.
Where the total jacking distance exceeds that of a single jack, they can also be combined and inflated simultaneously to reduce shear loads on buildings and allow more accurate testing. Before and after inflation, an initial distance measurement using strain gauges allows for this.
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Flat jacks are an insulated testing apparatus composed of two circular or square steel plates joined at their peripheries by welding, separated by an air gap filled with hydraulic fluid. When installed into an opening in a structure or rock surface and gradually brought up to pressure by pump, deformation readings from this test device are used to determine its in-situ stress state.