Design of Steel Lattice Telecommunication Tower - Literature review Example

?Literature Review Steel lattice telecommunication towers are coming out of style due to a variety of factors, replaced by free standing concrete towers. “Whereas until the mid-1950’s, telecommunication towers were normally built as stayed steel lattice towers, currently almost all television towers and communication towers are designed as free standing concrete towers” (NISEE, 1997). Nonetheless, they are still part of the picture for radio and television transmission, telephone calls, telegrams and telexes, and for the transmission of special information like navigation data (NISEE, 1997). History Steel lattice towers have a pedigree of more than a hundred years of operation (Packer and Willibald, 2006). “In the very beginning, more than 100 years ago, the first steel lattice towers for telecommunications were produced of flat-sided profiles like the angular profiles since it was easy to produce and... assemble” (Packer and Willibald, 2006, 45). Since the phone was invented in the 1870s, with the twang on June 2, 1875 and the famous “Mr. Watson, come here. I want to see you” issued on March 10, 1876, this means that steel lattice towers have been used almost since the start of modern telecommunications, only thirty years or so after the invention of the phone (Bellis, 2011). The early steel lattice telecommunications towers were things of pure efficiency, designed purely for cost-saving and speed (Smith, 2007, 75). Transmission lines make any cost-saving useful because the line requires numerous standardized elements, so the steel lattice towers were often used as a line to run cable (Smith, 2007, 75). However, even in the earliest days of steel lattice construction, extensive testing was used, with testing stations pushing the towers to the point of destruction (Smith, 2007, 75). This helped lead to design curves on angle sections. Early towers were far from precarious, but were vulnerable to wind stress. Modern free-standing steel lattice towers have tended to adopt a tubular profile design because of wind stress and material costs, particularly in Northern Europe (Packer and Willibald, 2006, 45). They tend to “vary in face width from top to bottom” and use different bracing patterns (Smith, 2007, 75). Heights have varied from a mere 10 meters to 200 meters. Steel lattice towers are one of the more common low-lying telecommunication towers design, eclipsing guyed masts until around 150 meters (Smith, 2007, 75). This is because, below 150 meters, the cost “increases more rapidly with height” since there is a large ratio of height to base width which guyed towers do not need (Smith, 2007, 75). Modern steel free-standing lattice towers have fallen out of favor in developed countries because of environmental impacts (Urbano, 2001). “Currently available design solutions with acceptable appearance are not employed in the developing countries, mainly for cost reasons. In the developing countries the use of the traditional lattice transmission towers will continue employing steel angles” (Urbano, 2001, 36). This is not to say they are not in use, but that other alternatives, particularly concrete, have pushed them out of the way. Currently, the tallest free-standing steel lattice construction in the world is the Kiev TV Tower, which was built in 1974 while the Iron Curtain was in full force (Construction Week, 2010). “The tower weighs 2,700 tons and is unique in the fact that no mechanical fasteners were used in its construction”; every single piece is welded together (Construction Week, 2010). The tower rests on a 100 meter four-legged base, after which is the microwave transmission equipment; at 200 meters, TV and FM transmitting equipment begins. Ironically, the Kiev TV Tower could have been even larger, but it was decided not to be placed in Moscow, and the Moscow tower uses prestressed concrete, though it remains free-standing (Construction Week, 2010). The Kiev tower would have been 30% larger had it been built in Moscow. Another classic of steel lattice design is the Crystal Palace of London, which is designed in a tier fashion with Bands I, II, III, IV and V being carried each at higher points on the tower, with the section of the tower shrinking to accommodate each new band (Smith, 78, 2008). Types of Telecommunications Towers 2.1. Types of towers  As noted, the steel lattice tower, the concrete tower and the guyed mast are each common tower designs. Timber towers were used even before steel lattice towers, and while they certainly are not built as often anymore, they can still be seen in many locations in the UK and US (Smith, 71, 2008). Timber towers are made of wood almost exclusively, perhaps reinforced with steel. The advantage of timber is that it is transparent to radio signals, but even with chemical treatment, timber towers are vulnerable to rot and vandalism. Another advantage was that there was a steel shortage, and perhaps as metal prices go up and mines begin to be tapped out, timber will become more popular as a renewable tower design. Steel lattice towers are still in vogue in the UK and are built almost exclusively of steel (Smith, 78-82, 2008). Concrete towers are constructed primarily of reinforced concrete, which does mean that steel and other materials are part of the construction (Smith, 2008, 71-75). They can include micro-concrete as well as regular concrete (Spencer and Hu, 2001, 413). Guyed lattice masts tend to use steel as well, but are different in design from traditional steel lattice masts (Smith, 2008, 81-86). They are based on the logic of bridge and beam design, so they use bracings, joints, cables, concrete bases, etc. They tend to be square or triangular in section. One major advantage is that the cross-section is small enough to allow uniformity in the construction of radiation patterns, vital for UHF. Concrete and aluminum have also been used and proposed (Smith, 2008, 90). There are other types of towers that have been adopted for aesthetic purposes, but these, while unique, do not merit special review because they are adapted for unique circumstances and thus they do not provide insight outside of their region (Smith, 2008, 91-92). Note that all of these towers, if they are visible to large groups of people, are likely either to be designed with aesthetics in mind or be disguised. 2.2. Materials As noted, materials for these towers vary. They can include timber, aluminum, steel, concrete, fiberglass, plastic, etc. However, there are constant experimentations for new material designs like micro-concrete (Spencer and Hu, 2001). Rubber soiling is often used to prevent seepage of water (Satyanarayana, 2009). Aggregates and sand are also part of the construction process, particularly for foundations (Satyanarayana, 2009). It is important to note that concrete towers are reinforced by steel rebar and also include timber components. 2.3: Parameters affecting the construction of a telecommunication tower – Construction issues Angerer (2008) suggests that there are several core parameters affecting how a telecommunication tower is constructed and which type is preferred. First, there are laws and regulations. In the US, the federal 1966 Communications Acts, state laws like Tennessee's T.C.A.  § 13-24-304 and  § 13-24-305, and local laws either directly regulate the placement and location of telecommunications towers or indirectly regulate them by requiring permits, limiting material uses in an area, creating ecological restrictions or aesthetic restrictions, etc. (Angerer, 2008). Then there are the, far more important, physical and scientific constraints. Antennae ideally want to be placed high in the air in order to transmit efficiently. Locations thus are preferred on top of hills or built into existing high-rises. Towers are chosen to get above a certain point on the ground, which is determined not least by the geography of the area. Economic constraints also impact the construction choice (Angerer, 2008). As we've seen, guyed towers supported by guy wires tend to be more expensive than self-supporting towers (Angerer, 2008). However, as towers get larger and larger, the impracticality of self-supporting towers and the low cross-section of guyed towers make them more and more palatable of alternatives, with diminishing returns hitting roughly at the 150 meter mark (Smith, 2007, 75). Of course, guyed towers can cover more area when counting the guy wires and can create safety issues, so that can further complicate matters. The safety issues are primarily ice loads, wind loads, geological forces and unit strength (Angerer, 2008). Units have to be sufficiently strong to withstand the strongest ice storms, wind storms, and geological activity native to the area. 2.4 Steel lattice telecommunication tower's morphology  As noted, steel lattices tend to have morphologies that are triangular, square or rectangular and have either tubular or angular designs (Efthymiou, 2009). “Steel lattice masts are tower structures of triangular, square or rectangular plan form which in the structural design of these structures is conventionally taken as the section of the mast. In view of morphology, steel lattice masts have a vertical, a truncated cone system or a combination of the two systems where truncated cone base continues beyond of a specific height level as a prism” (Efthymiou et al, 2009, 3). Legs and columns usually use types X, K, or V, with cross-sections of the mast members with angle sections L single or double for legs and L and U for other mast elements (Efthymiou et al, 2009, 3). “. In cases where the stresses are low enough to allow relatively simple connections, tubular legs and bracings offer appear to be an economic solution, since masts with tubular members may be less than half the weight of angle towers because of the reduced wind load on circular sections. The disadvantage of this solution is that the extra cost of the tube and the more complicated connection details often exceed the saving of steel weight and foundations” (Efthymiou et al, 2009). 2.5: Research – Description of loading – Applied Loads (actions on structures) Steel lattices face several primary load types. They do not typically suffer the way timber towers do from vandalism or construction damage (Smith, 2007). They do need to account for ice, wind and geological disturbance (Angerer, 2008). Wind loads as calculated by Efthymiou et al (2009) follow: A/A s Load Combination 1 1.35G+1.5Q 2 1.35G+1.5W0 3 1.35G+1.5W0+0.9Q 4 1.35G+1.5W0S+1.5S 5 1.35G+1.5W0S+1.5S+0.9Q 6 1.35G+1.5S+0.9Q 7 G+0.3S+0.3Q±E Wind storms tend to go across the cross-section, though complex wind storms might behave differently or have different shears. Ice storms, on the other hand, can batter the structure at many different angles and then cause stress damage by freezing and thawing. McClure et al (2000, 336-337) proposed simple seismic equations for the effects on steel lattices, but comprehensive testing and design has yet to be done. What is clear is that the equations assume “relatively regular shapes and mass distributions”, and do not account for ice and wind effects. Nonetheless, geological forces are manageable: Only fundamental axial modes are likely to be threatened. 3. Antenna Characteristics 3.1. Types of antennas used in telecommunication towers  Telecommunications towers are built to handle an immense array of antenna types. These include parabolic reflectors, dual-band horn antennas, dipole antennas, log-periodic dipole arrays, Yagi antennas, VHF or unidirectional antennas, 8-dipole reflectors, and numerous others (Nist, 2003). After HDTV became the norm, different types of antennae became used. This paper will focus only on parabolic antennae and unidirectional antennae. 3.2. Parabolic antennas  Parabolic antennae are one of the most commonly used, particularly in radar engineering but also in numerous other types (Wolff, 2011). They are usually constructed out of metal with metal mesh on the inside, though many consumer parabolic antennae like for satellites have plastic shells. “The circular parabolic (paraboloid) reflector is constructed of metal, usually a frame covered by metal mesh at the inner side. The width of the slots of the metal mesh has to be less than ?/10. This metal covering forms the reflector acting as a mirror for the radar energy. According to the laws of optics and analytical geometry, for this type of reflector all reflected rays will be parallel to the axis of the paraboloid which gives us ideally one single reflected ray parallel to the main axis with no sidelobes. The field leaves this feed horn with a spherical wavefront. As each part of the wavefront reaches the reflecting surface, it is shifted 180 degrees in phase and sent outward at angles that cause all parts of the field to travel in parallel paths” (Wolff, 2011). A pencil beam is produced. The beam width is very narrow, so parabolic antennae are mostly used for target-to-target signaling. They have beam widths of as low as 6.5 degrees (Gast, 2009, 2005). Thus, they typically need to be put very high off the ground to avoid any signal degradation, and they have to be mounted incredibly securely lest a wind blow them even slightly off course (Gast, 2009, 2005; Smith, 2007). Parabolic Radiation Pattern Horizontal Cross Section of Radiation Pattern in Log Scale 3.3 VHF or unidirectional antennas VHF or unidirectional antennas typically have reliable coverage of 65 to 70 miles (Sterling and Kittross, 2002). VHF antennae are far more forgiving for placement (Sterling and Kittross, 2002, 352). 4. Maintenance Weather issues are a major problem, but in fact, equipment damage is the most serious (Bedell, 2005). Failures begin in the legs of the towers, then monopoles buckle or compress due to steel failures. Corrosion from weather is still serious, though. Maintenance diagnosis should be done with ultrasound test for interior corrosion. Repainting and avoiding overstressing should make a tower “last for decades with good maintenance”. By following these rules, maintenance is nominal. This means that maintenance is mostly about careful planning: Choosing the right materials and design at the beginning, making sure the tower doesn't get pushed beyond its performance specifications by additional antennae or use, etc. Works Cited Angerer, D. 2009, “SITING TELECOMMUNICATIONS TOWERS: SUGGESTIONS FOR PROTECTING THE PUBLIC INTEREST”, MTAS Tennessee, April 15. Bedell, P. 2005, Wireless crash course, McGraw-Hill Professional. Bellis, M. 2011, “The History of the Telephone – Alexander Graham Bell”, About.com. ConstructionWeek. 2010, “Top 10 world's tallest steel buildings”, September 27. Efthymiou, E., Gerasimidis, S. and Baniotopoulos, C.C. 2009, “On the structural response of steel telecommunication lattice masts for wind loading and combined effects”, EACWE 5. Gast, M. 2005, 802.11 Wireless Networks: The definitive guide, O'Reilly Media. Hu, Y.X. And Spencer, BF. 2001, Earthquake engineering frontiers in the new millennium: Proceedings of the China-US Millennium Symposium on Earthquake Engineering, Beijing, 8-11 November 2000, Taylor & Francis. Nist, Ken. 2003, “Common TV Antenna Types”, HDTV Primer. Packer, JA. And Willbald, S. 2006, Tubular structures 11, Taylor & Francis. Satyanarayana, KN. 2009, “Construction of telecommunication towers - Presentation Transcript”, Available at: http://www.slideshare.net/snookala/construction-of- telecommunication-towers Smith, BW. 2007, Communication structures, Thomas Telford. Sterling, CH and Kittross, JM. 2002, Stay tuned: a history of American broadcasting, Psychology Press. Tirro, S. 1993, Satellite communication systems design, Springer. Urbano, CA. 2001, “Steel transmission towers: some current topics”, Progress in Structural Engineering and Materials, Volume 3, Issue 1, pages 36–47, January/March 2001. Read More