JACKING AND CRIBBING

The oldest and most rudimentary method for lifting and setting equipment and machinery is the jack and crib method. It is conceivable that this was the method used to construct the pyramids of Central America and Egypt. The method consists simply of raising one end of a load (a stone or piece of machinery) a small distance with a jack or lever, then stuffing some cribbing blocks under that end. The jacks or levers are then removed and moved to the opposite side, which is lifted and cribbed. This procedure continues, alternating from side to side, until the crib pile is built to the desired height. Once it has reached its desired height, the load can be slid or rolled off the pile to its intended destination. Jacking and cribbing is still common on today’s construction sites because it offers the advantage of requiring no expensive equipment other than a set of jacks and an ample quantity of cribbing material. It is, however, a time-consuming and labor-intensive procedure. Do not be misled by the simplicity of the method. Jacking and cribbing requires a skilled labor force to sequence the jacking procedure and to build safe crib piles. A knowledgeable rigging engineer must design the cribbing piles and supporting foundation to ensure that it is suitable for the required jacking operation. Generally, the cribbing is made of wood. Hardwood cribbing such as oak is preferred over softwood cribbing such as Douglas fir because of its higher compressive strength. All wood cribbing should be in new or like- new condition with no dry rot or splits.

The size of the jack selected for use will, of course, depend on the weight of the load. It is recommended that the jack be selected so that the load to be lifted by the jack is not more than 75 percent of the jack’s rated capacity. For example, if it is desired to lift 75 tons per jack, use a jack of not less than 100-ton capacity. Usually, hydraulic jacks, also commonly referred to as hydraulic cylinders, are used over lever-operated mechanical jacks. They come in capacities ranging from 1 ton to more than 800 tons, with maximum operating hydraulic pressures of 3,000 to 10,000 pounds per square inch (psi). Jacks with a maximum hydraulic pressure of 10,000 psi are the most common. The hydraulic jack or cylinder consists of a hollow cylinder body and a piston that moves inside the body. The height of each cribbing layer depends on the stroke of the piston. For this type of operation, the cylinder stroke typically ranges from 6 to 12 inches. The smaller cylinders can be operated with

hydraulic hand pumps, but the larger cylinders require a hydraulic power unit because of the large quantities of oil required. Construction time and stability of the crib pile are the only limitations on the final lift height.

Once the final lift height is achieved, rollers or skids may be placed under the load to move it off the crib pile. Typically, a steel runway is slid under the load to facilitate rolling or skidding. Machinery rollers produced by manufacturers such as Hilman or Multi-Ton are preferable to skidding the load because much less force is required to move the load. If skidding is to be performed, the force required can be reduced by using lubricants such as grease, dry graphite spray, or silicon. Skidding can also be performed using low-friction materials such as Teflon or UHMW on the skidding surfaces. Jacking operations require a check of the foundation against which the operators are jacking. A firm base consisting of steel plates or wood mats is typically required so that the jacking force does not damage the foundation. A piece of equipment should only be jacked at locations approved by the equipment manufacturer. Typically, manufacturers of heavy equipment provide jacking lugs or flat- bottomed trunnions on which to jack. These same precautions also apply to cribbing and roller placement. Always set the first layer of cribbing on a solid, level surface. A rigging engineer must check the bearing pressure under the cribbing. Place cribbing and rollers only at approved support points (foot pads) under the equipment being lifted. Otherwise, the equipment may be damaged.

5.2.1 Detailed Jacking and Cribbing Procedures

The first step in the jacking and cribbing procedure is to set up a safe jacking arrangement. This arrangement consists of two cribbing piles and two identical, hydraulically interconnected

(hydraulically interconnected jacks are jacks that share a common hydraulic supply line from a single hydraulic manifold and pumping unit in a configuration so that the lifting pressure in both jacks is always equal) jacks placed at one end of the piece of equipment at the manufacturer-approved equipment support points. The cribbing piles at the jacking end provide safety in the event that the hydraulic jacks fail to support the load. These piles will also support the load when the jacks are not used or removed. The other end of the equipment rests hard on cribbing to provide stability. This is very important because it prevents the load from rolling during jacking. This arrangement provides predictable loads in the jacks and at the cribbing points and prevents two-pointing. During the jacking operation, the cribbing piles adjacent to the jacks should be continuously shimmed to minimize the distance that the load will settle in the event that a jack fails.

The next step is to extend the two interconnected jacks high enough to install the cribbing layer. The jacks will extend equally because the opposite end is resting hard against its cribbing. Install the layer of cribbing under the jacked end. Shim the cribbing tight with steel shims or hardwood.

Next, slowly release the pressure in both jacks using a common needle type valve. This method will allow the jacks to slowly retract and gradually load the cribbing pile. Because the cribbing is shimmed tight, the load will not move downward other than to slightly compress the cribbing pile and shims. However, the load should be lowered slowly while watching the cribbing pile to ensure that the cribbing is adequately holding the load. Repeat this jacking and cribbing procedure at this end.

5-5

As stated before, the stroke of the jack determines the height of each cribbing layer. However, the height must not be so excessive that the jacks tip over and “shoot out.” Jacking up to a one-or two- degree tilt angle is reasonable. With this in mind, the jack tops must be equipped with swivel bearings (tilt saddles) to allow for this tilt.

Another word of caution,”never place metal against metal. Use a thin piece of wood or plywood to increase friction between the jacking point and the top of the jack. This approach will reduce any chance of the jack slipping and will also improve load spreading and prevent point loading of the jack’s piston and cylinder housing. Following the above procedures will result in a safe jacking and cribbing operation.

Consider the consequences of not following the proper procedures:

•

If four hydraulically interconnected jacks are used, one at each corner, there is the possibility of

the load rolling. If the jacks are not all placed symmetrically about the center of gravity of the equipment, the jack with the lightest load will extend quicker than the others (Pascal’s Law). The load will then tilt or roll unpredictably.

•

If four independent jacks are used, there is no way of controlling them so that they all extend at

the same rate. Consequently, the load will two-point diagonally on the two highest jacks. This could possibly overload those two jacks or damage the equipment.

•

If one end is hard cribbed and the two independent jacks are used at the other end, two-

pointing may occur.

•

If a thin piece of wood is not used between metal surfaces, the jack could slip out or the

jack piston or cylinder housing could be point loaded and damaged.

In document Bechtel Rigging Handbook (Second Edition) (Page 167-169)