Transmission tower

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A transmission tower

A transmission tower (electricity pylon in the United Kingdom and parts of Europe, and a hydro tower in certain provinces of Canada where power generation is mainly hydro-electric) is a tall structure, usually a steel lattice tower, used to support an overhead power line. They are used in high-voltage AC and DC systems, and come in a wide variety of shapes and sizes. Typical height ranges from 15 to 55 metres (49 to 180 ft),[1] though the tallest are the 370 m (1,214 ft) towers of a 2700-metre-long span of Zhoushan Island Overhead Powerline Tie. In addition to steel, other materials may be used, including concrete and wood.

There are four major categories of transmission towers:[1] suspension, terminal, tension, and transposition. Some transmission towers combine these basic functions. Transmission towers and their overhead power lines are often considered to be a form of visual pollution. Methods to reduce the visual effect include undergrounding.

Naming[edit]

A line worker working on a tower

"Transmission tower" is the name for the structure used in the industry in the United States, and other English-speaking countries. The term "pylon" comes from the basic shape of the structure, an obelisk-like structure which tapers toward the top, and is mostly used in the United Kingdom and parts of Europe in everyday colloquial speech. This term is used infrequently in the United States, as the word "pylon" is commonly used for many other things, mostly for traffic cones.

High voltage AC transmission towers[edit]

Single-circuit three-phase transmission line

Three-phase electric power systems are used for high voltage (66- or 69-kV and above) and extra-high voltage (110- or 115-kV and above; most often 138- or 230-kV and above in contemporary systems) AC transmission lines. The towers must be designed to carry three (or multiples of three) conductors. The towers are usually steel lattices or trusses (wooden structures are used in Canada, Germany, and Scandinavia in some cases) and the insulators are either glass or porcelain discs or composite insulators using silicone rubber or EPDM rubber material assembled in strings or long rods whose lengths are dependent on the line voltage and environmental conditions.

Typically, one or two ground wires, also called "guard" wires, are placed on top to intercept lightning and harmlessly divert it to ground.

Towers for high- and extra-high voltage are usually designed to carry two or more electric circuits (with very rare exceptions, only one circuit for 500-kV and higher). If a line is constructed using towers designed to carry several circuits, it is not necessary to install all the circuits at the time of construction. Indeed, for economic reasons, some transmission lines are designed for three (or four) circuits, but only two (or three) circuits are initially installed.

Some high voltage circuits are often erected on the same tower as 110 kV lines. Paralleling circuits of 380 kV, 220 kV and 110 kV-lines on the same towers is common. Sometimes, especially with 110 kV circuits, a parallel circuit carries traction lines for railway electrification.

High voltage DC transmission towers[edit]

HVDC distance tower near the terminus of the Nelson River Bipole adjacent to Dorsey Converter Station near Rosser, Manitoba, Canada — August 2005

High-voltage direct current (HVDC) transmission lines are either monopolar or bipolar systems. With bipolar systems a conductor arrangement with one conductor on each side of the tower is used. On some schemes, the ground conductor is used as electrode line or ground return. In this case it had to be installed with insulators equipped with surge arrestors on the pylons in order to prevent electrochemical corrosion of the pylons. For single-pole HVDC transmission with ground return, towers with only one conductor can be used. In many cases, however, the towers are designed for later conversion to a two-pole system. In these cases, often conductors on both sides of the tower are installed for mechanical reasons. Until the second pole is needed, it is either used as electrode line or joined in parallel with the pole in use. In the latter case the line from the converter station to the earthing (grounding) electrode is built as underground cable, as overhead line on a separate right of way or by using the ground conductors.

Electrode line towers are used in some HVDC schemes to carry the power line from the converter station to the grounding electrode. They are similar to structures used for lines with voltages of 10 – 30 kV, but normally carry only one or two conductors.

Railway traction line towers[edit]

Tension tower with phase transposition of a powerline for single-phase AC traction current (110 kV, 16.67 Hz) near Bartholomä, Germany

Towers used for single-phase AC railway traction lines are similar in construction to those towers used for 110 kV-three phase lines. Steel tube or concrete poles are also often used for these lines. However, railway traction current systems are two-pole AC systems, so traction lines are designed for two conductors (or multiples of two, usually four, eight, or twelve). As a rule, the towers of railway traction lines carry two electric circuits, so they have four conductors. These are usually arranged on one level, whereby each circuit occupies one half of the crossarm. For four traction circuits the arrangement of the conductors is in two-levels and for six electric circuits the arrangement of the conductors is in three levels.

Towers for different types of currents[edit]

AC circuits of different frequency and phase-count, or AC and DC circuits, may be installed on the same tower. Usually all circuits of such lines have voltages of 50 kV and more. However, there are some lines of this type for lower voltages, for example, towers used by both railway traction power circuits and the general three-phase AC grid.

Two very short sections of line carry both AC and DC power circuits. One set of such towers is near the terminal of HVDC Volgograd-Donbass on Volga Hydroelectric Power Station. The other are two towers south of Stenkullen, which carry one circuit of HVDC Konti-Skan and üne circuit of the three-phase AC line Stenkullen-Holmbakullen.

Towers carrying AC circuits and DC electrode lines exist in a section of the powerline between Adalph Static Inverter Plant and Brookston the pylons carry the electrode line of HVDC Square Butte.

The electrode line of HVDC CU at the converter station at Coal Creek Station uses on a short section the towers of 2 AC lines as support.

The overhead section of the electrode line of Pacific DC Intertie from Sylmar Converter Station to the grounding electrode in the Pacific Ocean near Will Rogers State Beach is also installed on AC pylons. It runs from Sylmar East Converter Station to Southern California Edison Malibu Substation, where the overhead line section ends.

In Germany, Austria and Switzerland some transmission towers carry both public AC grid circuits and railway traction power in order to better use rights of way. .

Tower designs[edit]

Support structures[edit]

Guyed "Delta" transmission tower (a combination of guyed "V" and "Y") in Nevada.

Towers may be self-supporting and capable of resisting all forces due to conductor loads, unbalanced conductors, wind and ice in any direction. Such towers often have approximately square bases and usually four points of contact with the ground.

A semi-flexible tower is designed so that it can use overhead grounding wires to transfer mechanical load to adjacent structures, if a phase conductor breaks and the structure is subject to unbalanced loads. This type is useful at extra-high voltages, where phase conductors are bundled (two or more wires per phase). It is unlikely for all of them to break at once, barring a catastrophic crash or storm.

A guyed tower has a very small footprint and relies on guy wires in tension to support the structure and any unbalanced tension load from the conductors. A guyed tower can be made in a V shape, which saves weight and cost.[2]

Materials[edit]

Tubular steel[edit]

Steel tube tower next to older lattice tower near Wagga Wagga, Australia

Poles made of tubular steel generally are assembled at the factory and placed on the right-of-way afterward. Because of its durability and ease of manufacturing and installation, many utilities in recent years prefer the use of monopolar steel or concrete towers over lattice steel for new power lines and tower replacements.[citation needed]

In Germany steel tube pylons are also established predominantly for medium voltage lines, in addition, for high voltage transmission lines or two electric circuits for operating voltages by up to 110 kV. Steel tube pylons are also frequently used for 380 kV lines in France, and for 500 kV lines in the United States.

Lattice[edit]

See also: Lattice tower

A lattice tower is a framework construction made of steel or aluminium sections. Lattice towers are used for power lines of all voltages, and are the most common type for high-voltage transmission lines. Lattice towers are usually made of galvanized steel. Aluminium is used for reduced weight, such as in mountainous areas where structures are placed by helicopter. Aluminium is also used in environments that would be corrosive to steel. The extra material cost of aluminium towers will be offset by lower installation cost. Design of aluminium lattice towers is similar to that for steel, but must take into account aluminium's lower Young's modulus.

A lattice tower is usually assembled at the location where it is to be erected. This makes very tall towers possible (up to 100 meters—in special cases even higher, as in the Elbe crossing 1 and Elbe crossing 2). Assembly of lattice steel towers can be done using a crane. Lattice steel towers are generally made of angle-profiled steel beams (L- or T-beams). For very tall towers, trusses are often used.

Wood[edit]

wood and metal crossbar

Wood is a material which is limited in use in high-voltage transmission. Because of the limited height of available trees the maximum height of wooden pylons is limited (approximately 30 metres). Wood is rarely used for lattice framework; they are instead used to build multi-pole structures, such as H-frame and K-frame structures. The voltages they carry are also limited, such as in other regions, where wood structures only carry voltages up to approximately 30 kV. In countries such as Canada or United States wooden towers carry voltages up to 345 kV; these can be less costly than steel structures and take advantage of the surge voltage insulating properties of wood.[2] As of 2012, 345kV lines on wood towers are still in use in the US and some are still being constructed on this technology.[3][4] Wood can also be used for temporary structures while constructing a permanent replacement.

Concrete[edit]

A reinforced concrete pole in Germany

Concrete structures can be used for transmission and distribution systems at a range of 25kv to 230-345kv. In exceptional cases concrete pylons are used also for 110 kV lines, as s below 30 kV.well as for the public grid or for the railway traction current grid. In Switzerland, concrete pylons with heights of up to 59.5 metres (world's tallest pylon of prefabricated concrete at Littau) are used for 380 kV overhead lines. Concrete poles are also used in Canada and the United States.

Concrete pylons, which are not prefabricated, are also used for constructions taller than 60 metres. One example is a 66 metres tall pylon of a 380 kV powerline near Reuter West Power Plant in Berlin. Such pylons look like industrial chimneys. In China some pylons for lines crossing rivers were built of concrete. The tallest of these pylons belong to the Yangtze Powerline crossing at Nanjing with a height of 257 metres.

Special designs[edit]

The two main pylons of "Elbe Crossing 2", crossing the German river Elbe
One of the Pylons of Cádiz, Spain
A transmission tower in the shape of Mickey Mouse in Celebration, Florida

Sometimes (in particular on steel lattice towers for the highest voltage levels) transmitting plants are installed, and antennas mounted on the top above or below the overhead ground wire. Usually these installations are for mobile phone services or the operating radio of the power supply firm, but occasionally also for other radio services, like directional radio. Thus transmitting antennas for low-power FM radio and television transmitters were already installed on pylons. On the Elbe Crossing 1 tower, there is a radar facility belonging to the Hamburg water and navigation office.

For crossing broad valleys, a large distance between the conductors must be maintained to avoid short-circuits caused by conductor cables colliding during storms. To achieve this, sometimes a separate mast or tower is used for each conductor. For crossing wide rivers and straits with flat coastlines, very tall towers must be built due to the necessity of a large height clearance for navigation. Such towers and the conductors they carry must be equipped with flight safety lamps and reflectors.

Two well-known wide river crossings are the Elbe Crossing 1 and Elbe Crossing 2. The latter has the tallest overhead line masts in Europe, at 227 metres (745 ft) tall. In Spain, the overhead line crossing pylons in the Spanish bay of Cádiz have a particularly interesting construction. The main crossing towers are 158 metres (518 ft) tall with one crossarm atop a frustum framework construction. The longest overhead line spans are the crossing of the Norwegian Sognefjord (4,597 metres (15,082 ft) between two masts) and the Ameralik Span in Greenland (5,376 metres (17,638 ft)). In Germany, the overhead line of the EnBW AG crossing of the Eyachtal has the longest span in the country at 1,444 metres (4,738 ft).

In order to drop overhead lines into steep, deep valleys, inclined towers are occasionally used. These are utilized at the Hoover Dam, located in the United States, to descend the cliff walls of the Black Canyon of the Colorado. In Switzerland, a NOK pylon[vague] inclined around 20 degrees to the vertical is located near Sargans, St. Gallens. Highly sloping masts are used on two 380 kV pylons in Switzerland, the top 32 meters of one of them being bent by 18 degrees to the vertical.

Power station chimneys are sometimes equipped with crossbars for fixing conductors of the outgoing lines. Because of possible problems with corrosion by the flue gases, such constructions are very rare.

A new type of pylon will be used in the Netherlands starting in 2010. The pylons were designed as a minimalist structure by Dutch architects Zwarts and Jansma. The use of physical laws for the design made a reduction of the magnetic field possible. Also, the visual impact on the surrounding landscape is reduced.[5]

A clown-shaped pylon appears in Hungary (GPS coordinates of the location: 47.235548,19.389177). [6]

Assembly[edit]

Cable riggers atop a pylon engaged in adding a fiber optic data cable wound around the top tower stay cable. The cable (SkyWrap) is wound on by a travelling machine which rotates a cable drum around the support cable as it goes. This travels under its own power from tower to tower, where it is dismantled and hoisted across to the opposite side. In the picture the motor unit has been moved across but the cable drum is still on the arrival side.

Before transmission towers are even erected, prototype towers are tested at tower testing stations. There are a variety of ways they can then be assembled and erected:

  • They can be assembled horizontally on the ground and erected by push-pull cable. This method is rarely used, however, because of the large assembly area needed.
  • They can be assembled vertically (in their final upright position). Very tall towers, such as the Yangtze River Crossing, were assembled in this way.
  • A jin-pole crane can be used to assemble lattice towers.[7] This is also used for utility poles.
  • Helicopters can serve as aerial cranes for their assembly in areas with limited accessibility. Towers can also be assembled elsewhere and flown to their place on the transmission right-of-way.

Markers[edit]

A typical tower identification tag

The International Civil Aviation Organization issues recommendations on markers for towers and the conductors suspended between them. Certain jurisdictions will make these recommendations mandatory, for example that certain power lines must have spherical markers placed at intervals, and that warning lights be placed on any sufficiently high towers.,[8] this is particularly true of tranmission towers which are in close vicinity to airports.

Electricity pylons often have an identification number or code placed on the pole in the form of a sign, an identification plate, painted numbers, or anything else the electric company chooses. These tags are usually marked with the names of the line (either the terminal points of the line or the internal designation of the power company) and the tower number. This makes identifying the location of a fault to the power company that owns the tower easier.

Transmission towers, much like other steel lattice towers including broadcasting or cellphone towers, are marked with signs which discourage public access due to the danger of the high voltage. Often this is accomplished with a sign warning of the high voltage; other times the entire access point to the transmission corridor is marked with a sign. Some countries require that lattice steel towers be equipped with a barbed wire barrier approximately 3 metres (9.8 ft) above ground in order to deter unauthorized climbing. Such barriers can often be found on towers close to roads or other areas with easy public access, even where there is not a legal requirement. In the United Kingdom, all such towers are fitted with barbed wire.

Tower functions[edit]

Three-phase alternating current transmission towers over water, near Darwin, Northern Territory, Australia

Tower structures can be classified by the way in which they support the line conductors.[9] Suspension structures support the conductor vertically using suspension insulators. Strain structures resist net tension in the conductors and the conductors attach to the structure through strain insulators. Dead-end structures support the full weight of the conductor and also all the tension in it, and also use strain insulators.

Structures are classified as tangent suspension, angle suspension, tangent strain, angle strain, tangent dead-end and angle dead-end.[2] Where the conductors are in a straight line, a tangent tower is used. Angle towers are used where a line must change direction.

Cross arms and conductor arrangement[edit]

Generally three conductors are required per AC 3-phase circuit, although single-phase and DC circuits are also carried on towers. Conductors may be arranged in one plane, or by use of several cross-arms may be arranged in a roughly symmetrical, triangulated pattern to balance the impedances of all three phases. If more than one circuit is required to be carried and the width of the line right-of-way does not permit multiple towers to be used, two or three circuits can be carried on the same tower using several levels of cross-arms. Often multiple circuits are the same voltage, but mixed voltages can be found on some structures.

Remarkable electricity pylons[edit]

A list of electricity pylons, which are remarkable either of their enormous height, unusual design, unusual construction site or their use in artworks.

Tower Year Country Town Pinnacle Remarks
Zhoushan Island Overhead Powerline Tie 2009 China Damao Island 370 m Tallest pylons in the world
Jiangyin Yangtze River Crossing 2003 China Jiangyin 346.5 m
Amazonas Crossing of Linhão de Tucuruí 2012 Brazil Almeirim 295 m
Yangtze River power line crossing of Shanghai-Huainan Powerline 2013 China Shanghai 269.75 m
Nanjing Yangtze River Crossing 1992 China Nanjing 257 m Tallest reinforced concrete pylons in the world
Pylons of Pearl River Crossing 1987 China Pearl River 253 m + 240 m
Orinoco River Crossing  ? Venezuela Caroní 240 m Tallest electricity pylons in South America
Pylons of Messina 1957 Italy Messina 232 m ( 224 m without basement) not used as pylons any more
HVDC Yangtze River Crossing Wuhu 2003 China  ? 229 m Tallest electricity pylons used for HVDC
Elbe Crossing 2 1976–1978 Germany Stade 227 m
Chushi Powerline Crossing 1962 Japan Takehara 226 m Tallest electricity pylons in Japan
Daqi-Channel-Crossing 1997 Japan Takehara 223 m
Overhead line crossing Suez Canal 1998 Egypt 221 m
Kerinchi Pylon 1999 Malaysia Kerinchi 210 m Tallest strainer pylon in the world, not part of a powerline crossing of a waterway
Huainan Luohe Powerline Crossing 1989 China Huainan 202.5 m
Balakovo 500 kV Wolga Crossing, Tower East  ? Russia Balakovo 197 m
LingBei-Channel-Crossing 1993 Japan Reihoku 195 m
400 kV Thames Crossing 1965 UK West Thurrock 190 m
Elbe Crossing 1 1958–1962 Germany Stade 189 m
Tracy Saint Lawrence River Powerline Crossing  ? Canada Tracy 174.6 m tallest electricity pylon in Canada
Doel Schelde Powerline Crossing  ? Belgium Antwerpen 170 m one pylon situated in Schelde River
Lekkerkerk Crossing 1 1970 Netherlands Lekkerkerk 163 m Tallest crossing in the Netherlands
Bosporus overhead line crossing III 1999 Turkey Istanbul 160 m
Balakovo 500 kV Wolga Crossing, Tower West  ? Russia Balakovo 159 m
Pylons of Cadiz  ? Spain Cadiz 158 m
Maracaibo Bay Powerline Crossing  ? Venezuela Maracaibo 150 m Towers on caissons
Aust Severn Powerline Crossing  ? UK Aust 148.75 m
132 kV Thames Crossing 1932 UK West Thurrock 148.4 m demolished in 1987
Karmsundet Powerline Crossing  ? Norway Karmsundet 143.5 m
Limfjorden Overhead powerline crossing 2  ? Denmark Raerup 141.7 m
Saint Lawrence River HVDC Quebec-New England Overhead Powerline Crossing 1989 Canada Deschambault-Grondines 140 m dismantled in 1992
Pylons of Voerde 1926 Germany Voerde 138 m
Köhlbrand Powerline Crossing  ? Germany Hamburg 138 m
Bremen-Farge Weser Powerline Crossing  ? Germany Bremen 135 m
Pylons of Ghesm Crossing 1984 Iran Strait of Ghesm 130 m One pylon standing on a caisson in the sea
Shukhov tower on the Oka River 1929 Russia Dzerzhinsk 128 m Hyperboloid structure, 2 towers, one of them demolished
Tarchomin pylon of Tarchomin-Lomianki Vistula Powerline Crossing  ? Poland Tarchomin 127 m
Skolwin pylon of Skolwin-Inoujscie Odra Powerline Crossing  ? Poland Skolwin 126 m
Enerhodar Dnipro Powerline Crossing 2 1977 Ukraine Enerhodar 126 m
Inoujscie pylon of Skolwin-Inoujscie Odra Powerline Crossing  ? Poland Inoujscie 125 m
Bosporus overhead line crossing II 1983 Turkey Istanbul 124 m
Duisburg-Wanheim Powerline Rhine Crossing  ? Germany Duisburg 122 m
Lomianki pylon of Tarchomin-Lomianki Vistula Powerline Crossing  ? Poland Lomianki 121 m
Little Belt Overhead powerline crossing 2  ? Denmark Middelfart 125.3 m / 119.2 m
Little Belt Overhead powerline crossing 2  ? Denmark Middelfart 119.5 m / 113.1 m
Pylons of Duisburg-Rheinhausen 1926 Germany Duisburg-Rheinhausen 118.8 m
Bullenhausen Elbe Powerline Crossing  ? Germany Bullenhausen 117 m
Lubaniew-Bobrowniki Vistula Powerline Crossing  ? Poland Lubaniew/Bobrowniki 117 m
Swieze Górne-Rybakow Vistula Powerline Crossing  ? Poland Swieze Górne/Rybaków 116 m
Ostrówek-Tursko Vistula Powerline Crossing  ? Poland Ostrówek/Tursko 115 m
Bosporus overhead line crossing I 1957 Turkey Istanbul 113 m
Riga Hydroelectric Power Plant Crossing Pylon 1974 Latvia Salaspils 112 m
Bremen-Industriehafen Weser Powerline Crossing  ? Germany Bremen 111 m two parallel running powerlines, one used for a single phase AC powerline of Deutsche Bahn AG
Probostwo Dolne pylon of Nowy Bógpomóz-Probostwo Dolne Vistula Powerline Crossing  ? Poland Nowy Bógpomóz/Probostwo Dolne 111 m
Daugava Powerline Crossing 1975 Latvia Riga 110 m
Nowy Bógpomóz pylon of Nowy Bógpomóz-Probostwo Dolne Vistula Powerline Crossing  ? Poland Nowy Bógpomóz 109 m
Regów Golab Vistula Powerline Crossing  ? Poland Regów/Golab 108 m
Orsoy Rhine Crossing  ? Germany Orsoy 105 m
Limfjorden Overhead powerline crossing 1  ? Denmark Raerup 101.2 m
Enerhodar Dnipro Powerline Crossing 2 1977 Ukraine Enerhodar 100 m Pylons standing on caissons
Reisholz Rhine Powerline Crossing 1917 Germany Düsseldorf  ? Under the legs of the pylon on the east shore of Rhine there runs the rail to nearby Holthausen substation
Strelasund Powerline Crossing  ? Germany Sundhagen 85 m Pylons standing on caissons
380 kV Ems Overhead Powerline Crossing  ? Germany Mark (south of Weener) 84 m
Pylon in the artificial lake of Santa Maria 1959 Switzerland Lake of Santa Maria 75 m Pylon in an artificial lake
Zaporizhzhia Pylon Triple  ? Ukraine Zaporizhzhia 74.5 m Two triple pylns used for a powerline crossing from Khortytsia Island to the east shore of Dneipr
Aggersund Crossing of Cross-Skagerrak 1977 Denmark Aggersund 70 m Tallest pylons used for HVDC-transmission in Europe
Eyachtal Span 1992 Germany Höfen 70 m Longest span of Germany (1444 metres)
Leaning pylon of Mingjian  ? Taiwan Mingjian  ? Earthquake memorial
Carquinez Strait Powerline Crossing 1901 United States Benicia 68 m + 20 m World's first powerline crossing of a larger waterway
Pylon 310 of powerline Innertkirchen-Littau-Mettlen 1990 Switzerland Littau 59,5 m Tallest pylon of prefabricated concrete
Anlage 2610, Mast 69  ? Germany Bochum 47 m Pylon of 220 kV powerline decorated with balls in Ruhr-Park mall.
Colossus of Eislingen 1980 Germany Eislingen/Fils 47 m Pylon standing over a small river
Pylon 24 of powerline Watari-Kashiwabara  ? Japan Uchihara, Ibaraki 45 m Pylon standing over a public road with two lanes
Source  ? France Amnéville les Thermes 34 m / 28 m 4 pylons forming an artwork
Huddersfield Narrow Canal Pylon  ? UK Stalybridge, Greater Manchester  ? Pylon standing over a waterway shipable by small boats

Gallery[edit]

See also[edit]

References[edit]

  1. ^ a b "Environmental, Health, and Safety Guidelines for Electric Power Transmission and Distribution" (PDF). International Finance Corporation. 2007-04-30. p. 21. Retrieved 2013-09-15. 
  2. ^ a b c Donald Fink and Wayne Beaty (ed.) Standard Handbook for Electrical Engineers 11th Ed., Mc Graw Hill, 1978, ISBN 0-07-020974-X, pp. 14-102 and 14-103
  3. ^ http://www.spta.org/pdf/Reisdorff%20Lam%20%209-11.pdf
  4. ^ http://www.mainepower.com/winterport.htm
  5. ^ "New High Voltage Pylons for the Netherlands". 2009. Retrieved 2010-04-24. 
  6. ^ "Clown-shaped High Voltage Pylons in Hungary". 
  7. ^ Broadcast Tower Technologies. "Gin Pole Services". Retrieved 2009-10-24. 
  8. ^ "Chapter 6. Visual aids for denoting obstacles" (PDF). Annex 14 Volume I Aerodrome design and operations. International Civil Aviation Organization. 2004-11-25. pp. 6–3, 6–4, 6–5. Retrieved 1 June 2011. 6.2.8 ... spherical ... diameter of not less than 60 cm. ... 6.2.10 ... should be of one colour. ... Figure 6-2 ... 6.3.13 
  9. ^ American Society of Civil Engineers Design of latticed steel transmission structures ASCE Standard 10-97, 2000, ISBN 0-7844-0324-4, section C2.3

External links[edit]