Trussed pylons and towers

Many masts and towers are built in the form of trusses, combining stiffness with lightness. The Tour Eiffel is a famous example. Built as a temporary structure for an exhibition in 1889, it has stood ever since.

A common form of trussed towers is the pylon which holds up the wires which carry electric current across the land. Although they all solve similar problems, their detailed designs differ from place to place.

Some masts are held in place by guy wires: these masts often taper to a pivot at he bottom, since there is no requirement there for stiffness. Masts and towers without guys usually spread to a wide base for stability against buckling and wind. At the base, where the tower is widest, the foundations can be of two different kinds. In the Tour Eiffel, the foundations continue the lines of the four legs, taking the vertically acting weight and the outward thrust. The lowest storey of the tower is more or less a propped beam, the arches being purely decorative. On the other hand, the foundations of many electricity pylons act vertically, because the outward thrust of the four legs is held by ties above the ground. This observation is a reminder that what lies below the ground is as important as what is seen above it.

Pylon1.jpg (30696 bytes)This picture shows an exceptionally tall pylon, one of two that hold the cables that cross the Severn estuary near Aust. Backing up these two pylons are two smaller ones that are nevertheless bigger than the usual ones. At the bottom of the tall pylon there are eight diagonal ties that handle the outward thrust of the four legs. Thus the supports handle only vertical forces. As is so often the case with simple statements, this one is not quite true. When a wind blows, not only is there an overturning moment at the base, but also a transverse force equal to the total force of the wind on the whole pylon. A better statement would be to say that the self loads at the base are vertical. It is a good principal in all technical writing to examine every statement in order to test its absolute veracity. If you do that, you may discover something you haven't even thought about before. A  typical statements with limits is this - 

"The behaviour is linear." (It may be linear only up to a certain point, or it may be slightly non-linear from the start. The end of linearity may be marked by yielding, breaking or buckling in structural members. The behaviour of most ferromagnetic materials is highly non-linear. People who want linear behaviour, as in a tape-recorder, add a strong high frequency signal to their own signal. This extra signal sweeps the system backward and forward through the non-linear region, resulting in an average behaviour that is much more linear. Such a system could be used to overcome cross-over distortion in hi-fi amplifiers, but there are better ways.)

"Lift increase with angle of incidence for a given speed." (Only up to the stalling angle: after that it falls suddenly. Incidentally, people sometimes speak of stalling speed. This isn't quite correct. In a pull-up manoeuvre, the aircraft is effectively heavier than in straight flight, and will require a larger angle of incidence for a given effect. Thus it is possible to stall at very high speed, if the manoeuvre causes the angle of incidence to reach the limit.)

There are many physical rules and laws that have limitations, though some are less limited than others. Laws with no limits at all are highly prized, as being universal laws of nature. Are there any at this time? Almost any equipment that you buy has limitations in terms of input voltage, current, force, power, bending moment, tension, compression, pressure, speed, etc, and also limitations to any available outputs. Exceeding the rated limits will shorten the working life of the equipment, often catastrophically. Trying to fly a glider with a pilot weighing less than the placarded minimum would be very foolish indeed. Weights are provided for small people to bolt on to the airframe.

Pylons often generate strong emotions. Many people dislike them for disfiguring the landscape, though buried cables are far more expensive than pylons and wires.

Some people believe that the electric or magnetic fields emanating from wires can cause illness in people who live nearby. The field from a very long straight wire is inversely proportional to the distance. For a pair of wires with opposing currents, the field is proportional to the inverse square of the distance. Many high voltage lines are in three-phase mode, with no return wire, but the field still falls off faster than for a single wire. Perhaps the problem, if it exists, is caused by electromagnetic radiation rather than simple fields. Since the currents are oscillating with frequencies of either 50 Hz or 60 Hz in most cases, the wavelength of the radiation would be very long - 6000 km for 50 Hz and 5000 km for 60 Hz. Therefore people near a set of wires are in the near-field of the radiation. Furthermore, the distance between the wires is very small compared with the wavelength, the effects of the wires will almost cancel each other. Nevertheless, there may be some small effect from the wires that is not understood. Who knows?

Links

Tour Eiffel

Tokyo Tower

Pylons

Objection to pylons

Objection to pylons

Illness research