FAQ >> Guy Derrick

Guy Derrick

Guy derricks at one time were the only practical means to erect steelwork on high-rise structures, but that work is mostly done now with tower cranes. Guy derricks may also be used for general rigging, stone quarrying, and construction of refineries and chemical plants. A guy derrick can be described as a Chicago boom with its own integral column, called a mast, held vertical by six or more guy ropes, as shown in Figure 2.3. The guys radiate in a horizontal circle about the derrick and are spaced as evenly as site conditions permit. A common configuration would include a mast of about 125 ft (38 m) and a boom of about 100 ft (30 m), although both larger and smaller derricks may be found. Capacities can range to 200 tons (181 t) or more. Both boom and mast are almost invariably latticed.

The boom of a guy derrick is usually made shorter than the mast so that at close-in radii the boom can swing under the guys, enabling the derrick to work through an arc of 3600. Narrow loads such as steel beams are easily handled in this way, but care must be exercised to prevent loads from striking the boom. Some derricks have steel plates on the load side of the boom to prevent this.

When a guy derrick swings, the boom and mast move together; a pivoted fitting is provided at the mast top, and a ball-and-socket joint is mounted at the bottom. Above the top pivot is the fitting to which the guys are attached, called a spider. The top pivot itself is called a gudgeon pin. Swinging, or s/ewing, is accomplished by using a large horizontal wheel, called a bull wheel, which is fitted at the bottom of the mast. Wire rope is run around the bull wheel, and winch power is used for swinging. Alternatively, on small derricks, an arm, called a ball pole, can be used to provide leverage for pulling the derrick around manually. Another method, sometimes used on steel erection derricks, makes use of a handwheel and gearing so that one person at the base of the derrick can swing the boom.

The topping lift is attached to the mast just below the gudgeon pin. The topping and load lead lines come into the mast at the top and above the boom foot, respectively, and run down the center of the mast. The lead lines run into the derrick base, which is fixed in position, over sheaves and out to the winch. Because the lead lines run through the center of rotation, they do not encumber the swing motion.

The mast must be held close to plumb by the guys; excessive lean will make swinging difficult to control, akin to running up and down a hill. Even when tensioned to remove excess sag, the guys hang in a catenaryv shape. Under load, those opposite to the boom are stressed; this causes rope elastic stretch and a reduction in catenary sag, which make the mast lean toward the boom. In order to control mast lean, the size, construction, and initial tension of the guy rope must be taken into consideration, together with guy configuration and load levels.

Guy derricks are practical for steel erection work because it is possible for the derrick to lift itself, or jump, as the height of the work increases. The winch is left at base level as the derrick jumps; the winch operator works blind, receiving all instructions by signal or voice communication.

A guy derrick must be supported at the mast base, and anchorage points are required for the guys. On a building, a steel grillage is usually placed under the mast to transfer the loads to the host structure. The guys are then anchored to the building frame. Inasmuch as steelwork first proceeds with only partial bolting at the connections, analyses must be made to assure that the partially connected structure is adequate to support the derrick reactions together with structure dead weight, construction loads, and wind. This study must be done for each derrick-mounting level.

The initial erection of a guy derrick requires use of another lifting device such as a crane. The grillage and its supports, the guy anchorages, and thejumping operation are all cost factors that need to be considered. But in addition, in order to swing a load past a guy, the derrick must be boomed-in until the boom can pass under the guy, or alternatively the guy must be lowered and then replaced after the boom passes. This cyclical operation is both awkward and time-consuming; its relative inefficiency should be factored when comparing the guy derrick to alternative erection means.

Though at one time they were stock tools of the lifting trade, guy derricks are now unfamiliar to most ironworkers. and the skill set needed to operate them has been largely lost. For industrial work, they have been replaced by other equipment types. In high-rise construction, they are not easily adapted to the large bays and lighter floor framing of contemporary structures. Thus guy derrick utilization is a rarity nowadays.

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