FIELD-ERECTED HOISTING DEVICES
Because of the nature of heavy construction, Steel workers must at times erect heavy structural members when constructing pre-engineered buildings, piers, bridges, and many other components related to Advanced Base Functional Components (ABFC). These members are usually hoisted into position using cranes, forklifts, or other construction equipment. In contingency/ combat operations, however, because of operational commitments this equipment may not be available and structural members must be hoisted without the use of heavy equipment. We will now discuss some of the methods which can be used for the erection process when heavy equipment is not available.
The term field-erected hoisting device refers to a device that is constructed in the field, using material available locally, for the purpose of hoisting and moving heavy loads. Basically, it consists of a block-and-tackle system arranged on a 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 three types of field-erected hoisting devices used are gin poles, tripods, and shears. The skeleton structure of these devices are anchored to 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 types of holdfasts are natural objects, such as trees, stumps, and rocks. When natural holdfasts of suffient strength are not available, proper anchorage can be provided through the use of man-made holdfasts. These include single picket holdfasts, combination picket holdfasts, combination log picket holdfasts, log deadmen, and steel picket holdfasts.
Natural Types of Holdfasts
When using trees, stumps, or boulders as holdfasts, you should always attach the guys near ground level. The strength of the tree, stump, or boulder size is also an important factor in determining its suit ability as a holdfast. With this thought in mind, NEVER use a dead tree or a rotten stump or loose boulders and rocks. Such holdfasts are unsafe because they are likely to snap or slip 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. 6-43). This will provide added support for the guy.
Rock holdfasts are made by inserting pipes, crowbars, or steel pickets in holes drilled in solid rock. Using a star drill, drill holes in the rock 1 1/2 to 3 feet apart, keeping them in line with the guy. Remember to drill the holes at a slight angle so the pickets lean away
from the direction of pull. Make the front hole about 1 1/2 to 3 feet deep and the rear hole 2 feet deep (fig. 6-44). After driving pickets into the holes, you should secure the guy to the front picket. Then lash the pickets together with a chain or wire rope to transmit the load.
Pickets used in the construction of picket holdfasts may be made of wood or steel. A wood picket should be at least 3 inches in diameter and 5 feet long. A single picket holdfast can be provided by driving a picket 3 to 4 feet into the ground, slanting it at an angle of 15 degrees opposite to the pull. In securing a single guy line to a picket, you should 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, the single picket is strong enough to stand a pull of about 700 pounds.
Combination Picket Holdfast
A combination picket holdfast consists of two or more pickets. Figure 6-45 gives you an idea of how to
arrange pickets in constructing a 1-1-1 and a 3-2-1 combination picket holdfast.
In constructing the 1-1-1 combination (fig. 6-46), drive three single pickets about 3 feet into the ground, 3 to 6 feet apart, and in line with the guy. For a 3-2-1 combination, drive a group of three pickets into the ground, lashing them together before you secure the guy to them. The group of two lashed pickets follows the first group, 3 to 6 feet apart, and is followed by a single picket. The 1-1-1 combination can stand a pull of about 1,800 pounds, while the 3-2-1 combination can stand as much as 4,000 pounds.
The pickets grouped and lashed together, PLUS the use of small stuff secured onto every pair of pickets, are what make the combination picket holdfasts much stronger than the single holdfasts.
The reason for grouping and lashing the first cluster of pickets together is to reinforce the point where the pull is the greatest. The way small stuff links each picket to the next is what divides the force of pull, so the first picket does not have to withstand all of the strain. Using 12- to 15-thread small stuff, clove hitch it to the top of the first picket. Then take about four to six turns around the first and second pickets, going from the bottom of the second to the top of the first picket. Repeat this with more small stuff from the second to the third picket, and so on, until the last picket has been secured. After this, pass a stake between the turns of small stuff, between EACH pair of pickets, and then make the small stuff taut by twisting it with the stake. Now, drive the stake into the ground.
If you are going to use a picket holdfast for several days, it is best to use galvanized guy wire in place of the small stuff. Rain will not affect galvanized guy wire, but it will cause small stuff to shrink. If the small stuff is already taut, it could break from overstrain.
Figure 6-46.-Preparing a 1-1-1 picket holdfast.
Still, if you must use small stuff, be sure to slack it off before leaving it overnight. You do this by pulling the stake up, untwisting the small stuff once, and then replacing the stake.
Combination Log Picket Holdfast
For heavy loads or in soft- or wet-earth areas, a combination log picket holdfast is frequently used. With this type, the guys are anchored to a log or timber supported against four or six combination picket holdfasts. (See fig. 6-47.) The timber serves as beam and must be placed so that it bears evenly against the front rope of the pickets. Since the holding power of this setup depends on the strength of the timber and anchor line, as well as the holdfast, you must use a timber big enough and an anchor line strong enough to withstand the pull.
A deadman provides the best form of anchorage for heavy loads. It consists of a log, a steel beam, a steel pipe, or a similar object buried in the ground with the guy connected to it at its center. (See fig. 6-48.) Because it is buried, the deadman is suitable for use as a permanent anchorage. When you are installing a permanent deadman anchorage, it is a good idea to put a turnbuckle in the guy near the ground to permit slackening or tightening the guy when necessary.
In digging the hole in which to bury the deadman, make sure it is deep enough for good bearing on solid ground. The less earth you disturb in digging, the better the bearing surface will be. You should undercut the bank in the direction toward the guy at an angle of about 15 degrees from the vertical. To increase the bearing surface, drive stakes into the bank at several points over the deadman.
A narrow, inclined trench for the guy must be cut through the bank and should lead to the center of the deadman. At the outlet of the trench, place a short beam or logon the ground under the guy. In securing the guy to the center of the deadman, see that the standing part (that is, the part on which the pull occurs) leads from the bottom of the log deadman. Thus, if the wire rope clips slip under strain, the standing part will rotate the log in a counterclockwise direction, causing the log to dig into the trench, rather than roll up and out. See that the running end of the guy is secured properly to the standing part.
Figure 6-47.-A combination log picket
Figure 6-48.-A deadman anchorage for a heavy load.
Steel Picket Holdfast
The steel picket holdfast shown in figure 6-49 consists of steel box plates with nine holes drilled through each and a steel eye welded on the end for attaching the guy. When you are installing this holdfast, it is important to drive steel pickets through the holes in such a manner that will cause them to clinch in the ground. You will find the steel picket holdfast especially useful for anchoring horizontal lines, such as the anchor cable on a pontoon bridge. The use of two or more of the units in combination provides a stronger anchorage than a single unit.