Under wooden poles all forms wooden constructions are summarized in the first place, which serve as carriers of current or communication lines. Furthermore, wooden masts are also used in shipbuilding (masting wood), as lighting masts and flagpoles. As a rule, a straight, healthy trunk made of pine , spruce , Douglas fir , larch or fir wood is used . In Australia, eucalyptus strains are also used for this. In the production process, this is brought to the required approximately round shape by peeling and then impregnated. The maximum height of wooden poles is around 30 meters because of the limited height of trees in Europe. Wood withdraws more CO 2 from the atmosphere during growth than is released during processing. In addition, the forest cleans the air, secures drinking water resources and offers people and animals protection, living space and relaxation.
There are different variants of the construction of masts as supports for power lines. In the medium-voltage range, the triangular arrangement of the insulators is mainly used, while the staggered arrangement of insulators is mainly used for low-voltage lines. Basically, the construction of wooden poles can be divided into two types. On the one hand there are masts with post insulators and on the other hand overhead line masts with chain insulators are used. The basic design of wooden mast arrangements are single mast, double mast, face mast and A mast. So -called A-masts are used for cable end masts and generally as guy masts in the line. Double masts are used at slight angles in the course of the line, as they are able to absorb higher forces than single masts.
H-masts offer space on cross struts to set up a transformer and switchgear.
In addition to slightly conical round masts, conical masts with a square or right-angled cross-section are also made from plywood.
The double mast consists of two parallel single masts with little space between them (e.g. 5 cm). The masts are connected to one another using bolts and spacers, usually made of hot-dip galvanized steel.
Face masts consist of a vertical mast on which z. B. an inclined mast was mounted at a depth of 1.5 meters. This mast should absorb the horizontal forces in the event of a diagonal or end pull.
A masts consist of two inclined mast elements that meet at the top and are connected again halfway up by a crossbar, so that the shape of a capital A. They can absorb high forces in the direction of the two inclined elements.
A-masts are assembled from two single masts on the construction site. The masts are screwed at the top with a bolt and a steel part with claws, which prevents movement, and at a depth of approx. Three meters with a bolt, e.g. B. a steel pipe with welded brackets mounted. There is also a 1.5 meter long bolt running in the tube. Under the ground, the two masts are again connected with two pieces of wood - usually dismantled wooden masts in good condition are used for this purpose - frictionally connected by bolts.
In Russia were occasionally in 1930 for cables with operating voltages of 100 kV and lattice towers built of wood.
Corolle is the name of a type of overhead line mast made of laminated pine wood with three cross members . The special thing about the Corolle wooden mast is that it is suitable for cables with higher voltages than normal wooden masts. However, the mast is very expensive and complex to process.
The dimensions of wooden poles vary depending on the area of application and use. The length dimensions are usually between 7 and 16 m. Longer masts can no longer be transported with ordinary semi-trailers . In the case of the low-voltage line, which represents the majority of the wooden masts, the mast length is between 9 and 10 m. The length dimensions in the medium voltage range are in the interval between 11 and 12 m. The diameter of the mast foot is in the interval between 13 and 33 cm and decreases towards the plait to 10 to 22 cm. However, these dimensions depend on the length of the masts. In the case of communication masts, the length is 7 to 8 m. In order to ensure the stability of overhead line masts, the minimum diameter of the pigtail and the base of the trunk are prescribed. DIN 48350 assigns different "nominal sizes" to the wooden masts according to their length, base and spigot dimensions. The maximum mast spacing in Germany is 160 m.
The slightly conical shape of masts allows the conical growth shape of tree trunks to be used well and also meets the mechanical-static requirements for high buckling strength below and great thickness for anchoring in the ground.
Wooden poles are typically 1.6 to 2.5 m deep (in the ground or in-situ concrete) embedded; shorter ones by 10 m total length more like 20% of their total length, longer ones by 15 m about 15% of their length.
Precast concrete foundation
In Austria, the procedure is that wooden masts (A masts per strut) are lengthened by a reinforced concrete leg. This standard molding, about 3 m long, has a diameter of about 20 cm, is thickened a little hammer-like at its lower end and is buried about 2 m deep here. The upper end is shaped as a spherical cap and has been covered with a black polyethylene cap since around 1990 in order to drain off rainwater well. On the above-ground meter there are two cross bores on one side, one above the other, with contact surfaces protruding about 3 cm on one side. With an M20 screw made of galvanized steel each , the correspondingly drilled wooden mast is screwed on twice with arched shims. The wooden mast runs entirely above ground, which means it lasts longer and can be exchanged quickly and without excavation if necessary.
Use and displacement
The use of wood as a construction element for masts offers a number of advantages. In addition to its low investment price and its low weight, wood is easy to climb, has high insulating properties, which is of particular importance for power lines, and high elasticity. Furthermore, the transport costs are low (low dead weight) and the installation and removal costs are time-saving and inexpensive. Wooden poles are mainly used in the area of low-voltage lines from 110 to 950 V and for medium-voltage lines from 5 to 30 kV; in the USA, wooden poles are even used for 345 kV lines. However, the use of wooden poles for high-voltage lines is rather rare. However, the wooden masts, as carriers of power lines, are in strong competition with steel and concrete masts and are largely displaced by them. Wooden masts are also used to direct communications. However, the need for communication lines on wooden poles is falling sharply due to the displacement by underground cabling systems. Occasionally wooden masts are and were also used as antenna supports. Because of the limited maximum height, the application is limited to low constructions such as radial masts of shield antennas, support masts of trap antennas for shortwave, support mast as transmission antennas for NDBs or as support mast for television and VHF transmitters with low power. The use of wood for ship, lighting and flagpoles or street / local railway catenary masts only play a minor role today.
The number of standing wooden poles in use in Germany is 3 to 4 million. Wood is a renewable raw material and, like every wooden structure, binds carbon over a long period of time and thus helps to keep the CO 2 content in the atmosphere low.
In order for wooden poles to have the longest possible service life or to remain competitive with other low-maintenance substitute raw materials, adequate wood protection treatment is necessary. To protect the loss of stability of the wooden poles, especially against biological degradation by fungi, the poles are treated with chemical wood preservatives. The toxic active ingredients of the wood preservatives minimize the susceptibility of wooden poles to attack by fungi. This is done through impregnation with coal tar oils or through full impregnation with biocidal protective salts in the context of pressure impregnation . The prerequisite for impregnation is the removal of the bark and bast . This processing step is also known as white peeling. The disadvantage of tar oils is that they are harmful to the environment and health, so the use of tar oils in Germany is very limited. Copper salts make up the largest proportion of protective salts. Copper salts are mainly used for wooden components that are in direct contact with the ground, as they have a toxic effect on soft rot fungi . In order to be able to prevent and combat copper-tolerant brown rot fungi , copper salts were previously combined with additives with arsenic and fluorine salts and later with boron salts . However, these are now being replaced by other additives due to their harmful effects on health and the environment. The most important copper preparations include a. Triazoles and copper HDO, which are effective against both mildew and brown and white rot . Modern, fixing and ecologically comprehensively tested products for salt impregnation, such as B. Tanalith, Wolmanit CX 8 and Korasit KSM / KS2, offer a safe and permanent protection of the masts.
DIN EN 14229 prescribes a minimum amount of approx. 25–33 kg protective agent per cubic meter of sapwood, whereby the wood preservative can be brought into the wood using vacuum pressure technology, for example . The previous standard VDE 0210 expired at the end of September 2007. For Great Britain, masts are manufactured according to the British BS standard using vacuum pressure technology with a minimum of 95–140 kg protective agent WEI type "B" or "C" per cubic meter using the Rüping process.
The life expectancy of impregnated wooden poles depends on the wood preservatives used. But the service life can also be strongly influenced by other influences, such as storage time before installation, location, quality of the wood used and assembly influences. A service life of 20 to 30 years is calculated for impregnation with protective salts, while for treatment with coal tar oils a service life of 40 years can be achieved. In contrast, there are untreated masts, which have an average service life of 7 years. To ensure that the use of wooden masts is profitable for economic reasons, early failures should be avoided in the first 10 to 20 years. In addition, a long life expectancy or service life is also a guarantee for ensuring the long-term road safety of wooden poles.
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