Jacking Systems

After it has been decided to slip-form a portion or all of the work, the details necessary to put the system into operation must be worked out. Of course, the initial decision must be that of selecting which jacking system to use. At present there are four primary systems:

hydraulic, pneumatic, electrical and manual.

Of these, the hydraulic and pneumatic systems seem to be the most common, and we shall discuss their details.

The hydraulic jacking system is a patented system employing a network of hydraulic jacks connected by oil lines to a central reservoir and powered by an electric pump. The jacks usually climb on pipe that has a diameter of 1 inch and a wall thickness of V inch. Each jack is calibrated to climb approximately 1 inch each time the pump is activated. It is necessary to set the pump pressure high enough to make sure all jacks have raised before the pump is turned off. If this is not done, that portion of the form which is lightly loaded will progressively gain on the balance of the form and throw the deck out of level. The yokes that are used with this system are made of steel and are furnished by the slip form equipment supplier as part of the equipment leased to the job. The clearance from the working deck to the underside of the yoke is approximately 12 to 14 inch, depending on the location of the bottom waler. Normal spacing between the yokes is 6 to 8 ft. with 7 ft considered to be optimum. If this spacing has to be increased because of architectural or structural requirements, special steps should be taken to ensure that the deflection of the walers between the yokes is kepi within acceptable limits.

The pneumatic system of slip-forming is also a patented system employing a network of jacks connected by air lines to an air compressor generally located near the base of the slip form. The control is simply a pressure exhaust valve located on the form and operated manually to raise the form in in increments. The rods that these jacks climb on are most often a solid 1 in in diameter, drilled and lapped at each end for a coupling stud. The yokes are made of wood and fabricated on the job, which gives an added degree of flexibility in the form designing. Occasionally, jacking rods are withdrawn for reuse as the slip progresses. The expense of this process is seldom warranted by the material cost of the jacking rods.

As mentioned before, some systems employ manual and electric jacks, but these are not employed to any great extent in building construction work.

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