Supertall
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Supertall is a term that refers to a skyscraper exceeding 1000 feet or 300 meters.[citation needed] It also applies to proposed structures over 1 kilometer (1000m/3281ft) or 1 mile (5280ft/1609m) in height — designs which never advanced beyond the concept or planning stages.[citation needed]
Of course the term supertall can be also used for every other kind of structures taller than 300 metres as free-standing towers, guyed masts, chimneys or other structures like bridge pillars exceeding the 1000 feet (304.8 metre) mark.
It should not be forgotten, that from the approximately 1150 supertall (definition: height of pinnacle >= 300 metres) structures built so far, most of them are guyed masts for FM- and TV-broadcating in the United States. The reason for this is that the USA is a big country with large flat areas, in which each TV station built its own tower ( it is not incommon, that there are several supertall TV transmission mast nearby in the USA). The few other supertall structures are guyed masts for TV-broadcasting in other countries, grounded and guyed masts for longwave, mediumwave and VLF-transmission, free-standing TV-towers (in most cases with observation deck open for tourists), free-standing chimneys and last but not least skyscrapers. Further there are two supertall partially guyed TV-towers ( Gerbrandy Tower and Zendstation Smilde, two supertall electricity pylons ( those of Yangtze River Crossing), one supertall bridge ( Millau Viaduct) and one supertall dam (Nurek Dam).
Almost every current supertall structure is among the top 10 in a particular category of height. Conceptual supertall designs, such as The Illinois by Frank Lloyd Wright (image, right), X-Seed 4000, and Sky City 1000 are probably technologically and architecturally feasible, but are considered by some to be simply too tall to be of any particular use. Critics suggest that the current world economic situation does not require such a massive concentration of people and capital and, after the occurrence of the September 11, 2001 attacks, fear that such towers will almost certainly become terrorist targets.[citation needed]
Others, however, consider these designs to be bold and visionary, seeing them as portending a shift in the nature of urban living towards multilayered cities, and reflective of the natural human impulse to always build bigger or taller structures.[citation needed]
[edit] Requirements, advantages and disadvantages of supertall structures
Although building supertall structures is often a way of showing prestige, the construction of a supertall brings many technical challenges.
Supertall radio masts and towers for UHF-/VHF-transmission allow a wide area of reception. Because of reflections, which disturb radio wave propagation, their erection is only sensitive in flatter areas, without single higher mountains. Therefore nearly all very high radio towers (height > 300 metres) are situated in flat areas.
Supertall mast radiators, e.g. half-wave radiators for longwave radio allow larger areas of fading-free reception, because in opposite to shorter mast radiators, the ratio of skywave and groundwave radiation is much smaller, so fading occurs in areas much further away from the transmission tower than using a shorter mast radiator. However, because nowadays the range of longwave transmitters is more determined by jamming from other stations working in the same channel, using halfwave mast-radiators do not embetter much longwave reception in greater distance of the transmission antenna. The only realized half-wave mast radiator for longwave was the Warsaw Radio Mast. The usage of supertall mast radiators and antenna structures with heights between 300 and 450 metres is however very sensitive for high power longwave radio stations with greater bandwidth requirement like longwave broadcasting or LORAN-C, because these antenna structures require for the realization of a quater-wave length radiator no or only few electrical lengthening, which reduces beside bandwidth also antenna effiency and which increases antenna voltage.
For VLF transmission the usage of supertall masts either as mast radiator ( electrical enlengthed as an umbrella antenna) or as carrier of wire antennas is perhaps the best choice. However the erection costs grow taller than the height of the masts, which is also the case for the maintenance. So one tries to find out, if it is possible by using electrical lengthing to build the masts not unnecessarily tall. Because the bandwidth of most VLF-transmitters is very low, the usage of excessive electrical lengthing is possible without bigger problems. At VLF there is also nearly no skywave propagation, so there is no requirement for skywave surpression characteristics of the antenna. In opposite to the skywave propagation the groundwave propagation of VLF is very well and one can easy increase the range of the transmitter by increasing the power. It may be in this frequency range often cheaper to run a high power transmitter with a not extremly effective antenna using lower masts, than building and maintaining an antenna using extreme tall masts. So VLF transmitters use often very tall masts ( height between 200 and 400 metres) for their transmitting antennas, but their masts belong not to the tallest ever built. In the Western hemisphere the masts of VLF transmitter Lualualei on Hawaii are the tallest structures used for transmission in this frequency range.
Nevertheless satisfactorily working VLF and LF transmitters can be built according to the works of the radio engineer Alexanderson without using supertall towers. Such antennas use Kalundborg longwave broadcasting transmitter and the SAQ-VLF transmitter in Grimeton, Sweden. There towers are approximately 120 metres tall.
For mediumwave-transmission supertall mast radiators are a bad choice, because they show high skywave emission, which result in excessive fading at night. However there are some special high-effective fading reducing antenna types for mediumwave broadcasting, which require supertall towers. Because of the high erection costs for the required towers, their usage is only sensitive for high power stations working on nearly interference free channels at the lower end of the mediumwave band. So only a few broadcasting stations like RKS Liblice 2 uses or used such antennas.
Supertall chimneys improve the dispersion of gases from a factory. However their usage does not eliminate toxic substances out of the smoke, it only distributes them over large areas. In many countries pollution laws are nowadays very strictly and just dispersing the exhausts by huge chimneys does not require their efforts in most cases. So special installations for cleaning the smoke must be installed, which purify the exhausted gas so, that there are not extremely tall chimney are necessary. Nevertheless their usage can be sensitive for a factory or a conventional thermal power station situated in a valley. A good example for this is Trbovlje Chimney of Trbvolje Power Station in Slovenia, which is situated in a deep valley.
For powerline crossings of sea narrows and wide rivers at flat terrain building supertall electricity pylons are possible. However only one of such crossings, the Yangtze River Crossing in China was realized, because in most cases building an underground cable may be more sensible for several reasons. At long spans the usage of bundle conductors make problems, because they show easily wind-induced oscilitations, which can result into short circuits. Single conductors however, thick enough to transport as much electricity as bundle conductors show more corona losses. A further, more serious problem at such crossings, is that for long spans, conductive materials of high tensile strength are requires. Unfortunately these materials show a higher specific resistance than standard conductors, which results in higher losses.
Bridges with supertall pillars may be built only in special cases and than only as suspension bridge or other types of bridge carried by cable. The only so far realized supertall bridge is Millau Viaduct, a suspension bridge for a motorway across a valley in France. A proposed supertall bridge is Strait of Messina Bridge over Messina Strait would have qualified for supertall bridge, had it not been cancelled, because Messina Strait is deep, making a tunnel under it or a bridge with pillars in Messina Strait uneconomic. However further bridges with supertall pillars will be only built, if wide deep valleys (as in Millau) or deep, narrow sea straits and rivers (as Messina Strait) have to be crossed, because supertall bridge towers have to stand enormous forces: they have to be designed to carry the bridge with an enormous additional weight of vehicles crossing the bridge and the dynamic loads caused by them, which are especially for railway bridges considerable.
Spans and bridges without supertall support structures crossing a valley higher than the height level defined as criteria for supertall structures at the point, where the valley is deepest, should not be classified as supertall structures at all, because at these constructions the maximum height above ground has nearly no influence of the constructive design.
Supertall dams require a deep canyon-like valley as a construction site. So far only one such dam ( Nurek Dam in Tajikistan)was built and two further are under construction.
Solar chimneys in opposite have to be built as supertall structures, because this improves the efficiency of the facility.
A future use for supertall structures may be special towers with maglev tracks for launching spacecraft and space elevators.
Supertall skyscrapers save land. However, problems with fire safety and quick evacuation rapidly grow with the height of the building and especially the number of persons for which it is designed. In addition the water supply and sewage, becomes much more expansive, the taller the construction is, especially if it must be as in case of a skyscraper designed for the requirements for many people.
Building supertall structures of any kind is expensive, because the erection costs do not grow in direct proportion to height of structure, but grow faster. So a 1000 ft (305 m) radio mast in the USA costs between $0.7 and $1.1 million to build, while a 2000 ft (610 m) radio mast in the USA costs $2.4 to $4 million to build. In opposite to guyed masts, the erection costs for free-standing structures escalate almost exponentially with their height, which set their height in most cases an economical limit.
Supertall structures of all kinds have to withstand greater wind forces than smaller structures of the same type, requiring various structural engineering measures to handle them. Further, the construction site must be approved by flight safety authorities.
[edit] See also
- Megastructure
- World's tallest structures
- List of world's tallest structures
- List of masts
- List of towers
- List of chimneys
- List of skyscrapers
- Skyscraper
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| Antennas: | Alma-Ata Tower, Azeri TV Tower, Emley Moor, Europaturm, Gerbrandy Tower, Kiev TV Tower, Mumbai Television Tower, Saint Petersburg TV Tower, Sumida Tower (proposed), TV Tower Yerevan, WITI TV Tower, Zendstation Smilde |
| Bridges: | Millau Viaduct, Strait of Messina Bridge (proposed) |
| Dams: | Rogun Dam (construction), Nurek Dam, Jinping 1 Hydropower Station (construction) |
| Solar towers: | Solar Tower Buronga (proposed), Ciudad Real Torre Solar (proposed) |
| Electricity pylons: | Yangtze River Crossing |
| Oil platforms: | Petronius Platform, Baldpate Platform, Bullwinkle Platform, Troll Platform, Gullfaks C |
Borj-e Milad • Central TV Tower • CN Tower • Eiffel Tower • Fernsehturm • Guangzhou TV & Sightseeing Tower (construction) • KCTV-Tower • Kuala Lumpur Tower • Liberation Tower • Macau Tower • Oriental Pearl Tower • Ostankino Tower • Riga Radio and TV Tower • Sky Tower • Stratosphere Las Vegas • Sydney Tower • Tallinn TV Tower • Tashkent Tower • Tianjin Radio and Television Tower • Tokyo Tower • Torrena • Vilnius TV Tower
