Guy-wire

A guy-wire, guy-line, guy-rope, down guy, or stay, also called simply a guy, is a tensioned cable designed to add stability to a freestanding structure. They are used commonly for ship masts, radio masts, wind turbines, utility poles, and tents. A thin vertical mast supported by guy wires is called a guyed mast. Structures that support antennas are frequently of a lattice construction and are called "towers". One end of the guy is attached to the structure, and the other is anchored to the ground at some distance from the mast or tower base. The tension in the diagonal guy-wire, combined with the compression and buckling strength of the structure, allows the structure to withstand lateral loads such as wind or the weight of cantilevered structures. They are installed radially, usually at equal angles about the structure, in trios and quads. As the tower leans a bit due to the wind force, the increased guy tension is resolved into a compression force in the tower or mast and a lateral force that resists the wind load. For example, antenna masts are often held up by three guy-wires at 120° angles. Structures with predictable lateral loads, such as electrical utility poles, may require only a single guy-wire to offset the lateral pull of the electrical wires at a spot where the wires change direction.

Conductive guy cables for radio antenna masts can catch and deflect radiation in unintended directions, so their electrical characteristics must be included in the design. Often the guy wire is divided by strain insulators into isolated sections whose lengths are not resonant with the transmission frequencies.

The guys supporting a sailboat mast are called "standing rigging" and in modern boats are made of stainless steel wire rope, stainless rod or synthetic line such as ultra-high molecular weight polyethylene (UHMWPE) fiber. Guys are rigged to the bow and stern, usually as a single guy. Lateral guys attach to "chain plates" port and starboard attached to the hull. Multiple guys are usually installed with spreaders to help keep the mast straight ("in column").

Temporary guys are also used. A fore-guy is a line (rope) pulling on the free end of a spar. On a modern sloop-rigged sailboat with a symmetric spinnaker, the spinnaker pole is the spar most commonly controlled by one or more guys. Running backstays can also be employed on sailboats when beating to windward to further strengthen the mast in heavy winds.

Utility poles are buried in the ground and have sufficient strength to stand on their own; guys are needed on some poles only to support unbalanced lateral loads from the utility wires attached to them, or to resist ground movement. Guys are particularly needed on dead-end (anchor) poles, where a long straight section of wire line ends, or angles off in another direction. To protect the public against faults that might allow utility guy cables to become electrified, they usually have a ceramic strain insulator ("Johnny ball") or a fiberglass strain insulator inserted near the top, to keep dangerous voltages away from the lower end. The length near the ground is often encased in a yellow plastic reflector to make it more visible, so that people or vehicles do not run into it.

In urban areas with pedestrian traffic around the pole, a variation called a sidewalk guy is often used: the guy line extends diagonally from the top of the pole to a spar brace extending out from the middle of the pole, then continues vertically to the ground. Thus, the bottom length of the guy is vertical and does not obstruct headroom, so a sidewalk can pass between the pole and the guy.

An alternative to guy-wires sometimes used on dead-end utility poles is a push-brace pole, a diagonal pole with one end set in the ground and the other butting up against the vertical pole, opposite to where a guy cable would attach.

Electromagnetic fields from the antennas complicate the design of guys that support mast antennas. Conductive metal guy-wires whose lengths are near to quarter wavelength multiples of the transmitted frequency can distort the radiation pattern of the antenna. This also applies to guy wires of neighboring masts or nearby metal structures. To prevent this, each guy wire is divided by strain insulators into multiple sections, each segment non-resonant at the transmitted wavelength. Cylindrical or egg-shaped porcelain "Johnny ball" insulators (also called "egg insulators") are usually used. Non-conductive guys of Kevlar fiber (Phillystran) or extruded fiberglass rod are frequently used to not disturb the radiation pattern of the antennas. The strength and low stretch properties of Kevlar fiber approaches that of steel. However, Kevlar is very susceptible to ultraviolet degradation, so it is enclosed in a UV resistant plastic sheath.