How Does a Mast Stay Up Without Being Thick? Tie It Down at an Angle

A tower standing on its own must resist wind by acting as a vertical beam fixed at its base, which means the base carries the entire turning effect of the wind acting along the whole height, and that requires a structure that is broad, heavy and expensive. Adding wires from points up the tower to anchors some distance away changes the problem completely, turning the tower into a column held in position at several heights, so it no longer has to resist bending on its own and needs only to carry compression. That allows a mast a metre across to stand hundreds of metres tall.

The arrangement trades one kind of load for another:

• •Each wire can only pull, since a wire cannot push
• •So wires are needed on every side, usually three or four sets
• •All are tensioned so none goes slack when the tower leans
• •Wind increases tension on one side and reduces it on the other
• •Every wire's pull has a downward component along the tower
• •That adds substantially to the compression the tower must carry

Choosing where to put the anchors is a direct trade between land and load. An anchor far out gives a shallow wire angle, which pulls mostly sideways and holds the tower well while adding little downward force, and it requires a large area of ground. An anchor close in gives a steep angle, which pulls mostly downward, adding heavily to the compression while restraining the tower poorly, and needs little land. Practical designs settle near forty five degrees or shallower, and the ground required is why guyed masts stand in open country while self-supporting towers are used in cities where land is expensive.

Setting the tension is a specialist operation and getting it wrong causes trouble in both directions. Too little tension lets the tower sway further than designed, which fatigues joints and can allow a wire to go slack in high wind, at which point it whips and can be damaged. Too much adds unnecessary compression to the tower and can pull it out of straight. Tension is measured by plucking the wire and reading its frequency, exactly as a musical string is tuned, since a stretched wire's pitch depends on its tension in a known way. Temperature changes the tension as the wires expand and contract, so measurements are corrected for it.

The arrangement has one serious weakness, which is that it depends entirely on every wire remaining intact and tensioned, so the failure of any single one can bring down the whole structure. Several very tall masts have collapsed after a wire was severed, in one case by an aircraft striking it and in another during maintenance when a wire was disconnected in the wrong sequence, and the collapse is sudden and total rather than gradual. Ice loading is a further hazard, since ice builds on wires and adds enormous weight and wind area, and shedding ice unevenly leaves the tower briefly unbalanced. Anchors also corrode below ground where they cannot be inspected easily.

Wires holding a tower at several heights turn it from a beam resisting bending into a column carrying compression, which is why a mast a metre wide can stand hundreds of metres tall. Every wire pulls partly downward, adding to that compression, and the anchor distance trades land against load. Because a wire can only pull, sets are needed on all sides, and severing one has brought whole masts down.