CHAPTER 6 FIELD RIGGING AND HOISTING SYSTEMS

This chapter presents information on how to rig and erect field hoisting systems used within the Naval Construction Force (NCF).

Formulas are given on how to determine or find the safe working load (SWL) of fiber/ synthetic line and wire rope. These formulas are important when constructing a field hoisting system and also when lifting by any other means. In addition, the breaking strengths of fiber line and wire rope are covered.

FIELD-ERECTED HOISTING DEVICES

The term FIELD-ERECTED HOISTING DEVICE refers to a device, generally of a temporary nature, that is constructed in the field, using locally available material, for the purpose of hoisting and moving heavy loads. Basically, it consists of a block-and-tackle system arranged on some form of skeleton structure consisting of wooden poles or steel beams. The tackle system requires some form of machine power or work force to do the actual hoisting. The skeleton structure with attached tackle is held in place and supported by means of guy lines anchored to holdfasts in the ground.

HOLDFASTS

Gin poles, shear legs, and other rigging devices are held in place by means of guy lines anchored to HOLDFASTS. In fieldwork, the most desirable and economical holdfasts are natural objects, such as trees, stumps, and rocks. When natural holdfasts of sufficient strength are not available, proper anchorage can be provided through the use of man-made holdfasts. These include single-picket, combination-picket, combinationlog-picket, and log deadman holdfasts.

Natural Types

When using trees or stumps as holdfasts, always attach the guys near ground level. Of course, the strength of the tree or stump is also an important factor in determining its suitability as a holdfast. With this thought in mind, NEVER use a dead tree or a rotten stump for this purpose. Such holdfasts are unsafe because they are likely to snap suddenly when a strain is placed on the guy. Make it a practice to lash the first tree or stump to a second one (fig. 6-1). This will provide added support for the guy.

Single-Picket Holdfast

Pickets used in the construction of picket holdfasts may be made of wood or steel. A wood picket should be at least 3 inches (76.2 millimeters) in diameter and 5 feet (1.5 meters) long. A SINGLE-PICKET holdfast can be provided by driving a picket 3 to 4 feet (0.9 to 1.2 meters) into the ground, slanting it at an angle of 15 opposite to the pull. In securing a single guy line to a picket, take two turns around the picket and then have part of the crew haul in on the guy as you take up the slack. When you have the guy taut, secure it with two half hitches. In undisturbed loam soil,

Figure 6-1.\Use of trees as natural holdfasts.

the single picket is strong enough to stand a pull of about 700 pounds (317.5 kilograms).