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Re: flat jack

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Besides being used for masonry tests, a flat jack is also a device which can
be used for leveling a structure or for permanently transfering loads.  

In the case of a load transfer application, the flat jack device is usually
circular in shape and made up of two circular steel plates sealed along the
edge.  A fluid is then injected (often epoxy) inside this flat jack causing
the horizontal surfaces to expand outward.  There is a limit to the amout of
expansion which a flat jack can undergo before it becomes too oval in shape
and there is a reduction in usable bearing surface area.  The other limiting
factor is the bursting pressure of the flat jack since the initial load
transfer is a accomplished by fluid pressure until the epoxy hardens.  The
design load capacity of the larger flat jacks is typically around 2200 psi.
Circular steel shim plates are often placed on the flatjack to help fill the
void spaces, prior to expanding the flat jack.  These flat jacks can be
anywhere from 6 inches in diameter to 48 inches and larger (typically custom

Flat jacks of this sort are often used in Base Isolation retrofits of existing
buildings  when the gravity loads needs to be transfered back to structure
below the plane of isolation, prior to removing of shoring.   They are also
used in heavy lifting and temporary erection.

Michael Cochran