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TOWER DESIGNING

The purpose of installing a Tower is to support different kinds of Antennae at a particular height to receive and transmit communication signals free from any obstructions. These towers are fabricated out of steel angles / tubes (as per the design) and then galvanized to protect from oxidation. All the parts of the tower are manufactured in the factory and then shipped to their respective sites in completely knocked condition and then erected by bolting, over the foundations made (suitable to respective towers and as per the soil conditions).

Introduction 1.0 Types of communication towers 1.0.1 Based on Structural Action

Towers can be classified into three major groups based on the structural action : • Self supporting towers or Free-standing towers • Guyed towers • Monopoles

1.0.1.1 Self Supporting Towers Self-supporting towers are supported on ground or on buildings and they act as cantilever trusses in carrying the wind and seismic loads. Though the weight of these towers is more they, require less base area and are suitable in meet situations. Most of the TV, MW, Power transmission and flood light towers are self-supporting towers.

Steel towers can be constructed in a number of ways but most efficient use of material is achieved by using open steel lattice. The use of an open lattice avoids presenting the full width of structure to the wind but enables the construction of light in weight and stiff structures. Most of the power transmission, telecommunication and broadcasting structures are lattice structures.

1.0.1.2 Guyed Towers Guyed towers provide height at a much lower material cost than self-supporting towers due to the efficient use of high strength steel in the guys. Guyed towers are normally guyed in three directions over an anchor radius of typically 2/3 of the tower height and have a triangular lattice section for the central mast. Tubular masts are also used, especially where icing is very heavy and lattice sections would ice up fully.

These towers are much lighter than self-supporting type but require a large free space to anchor guy wires. Whenever large open space is available, guyed towers can be provided. There are restrictions to mount dish antennae on these towers and require large anchor blocks to hold the ropes.

1.0.1.3 Monopole As the name indicates, it is single self-supporting pole. It is generally placed over roofs of high raise buildings. This is preferable when number of antennae required is less or height of tower required is less than 9m.

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1.0.2 Based on cross section of tower Towers can be classified in to the following groups based on the cross section of the tower :

• Square towers • Rectangular towers • Triangular towers • Delta towers

Lattice towers are typically square or triangular and have low redundancy. Towers with a square cross section in plan are simple from the point of view of fabrication, although the amount of steel required for a tower of triangular cross section is smaller. But such towers have less stiffness in torsion. With increase in the number of faces of the cross section, the weight of a tower with square base increases compared to that of a tower of triangular cross section.

1.0.3 Based on type of material sections Towers can be classified in to the following two major groups based on the sections used for the fabrication of tower :

• Angular towers • Hybrid towers (Legs tubes and bracings angles)

Lattice towers for most purposes are made of bolted angles. Tubular legs and bracings can be economic, especially when the stresses are low enough to allow relatively simple connections. Towers with tubular members may be less than half the weight of angle towers because of the reduced wind load on circular sections. However the extra cost of the tube and the more complicated connection details can exceed the saving of steel weight and foundations.

1.0.4 Based on placement of tower Based on the placement of tower, communication towers can be classified in to the following two groups :

• Green Field Tower • Roof Top Tower

If the tower is erected on the natural ground after excavation of soil and provided with suitable foundation system then this tower is called Green Field Tower. Similarly if the tower is erected on the existing building by raising the existing columns and provided with tie beams as foundation in between the columns, it is called Roof Top Tower. The height of roof top tower varies generally from 9m to 30m where as the height of green field communication tower varies from 30m to 200m depending on the line of sight required. Green field towers are preferable in rural areas where land is available and in urban areas most of the towers are of roof top as it is difficult to acquire open land for green field towers. Roof top towers are more economical as compared to Green field towers as the height of building is utilized in reducing the height of tower required.

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2.0 DESIGN PARAMETERS The following are the design parameters for a communication tower.

• Tower type (Guyed or Self supporting) • Height of tower • Cross section of tower • Type of tower, Green field or Roof top • Types of sections for legs, angles or tubes • Design codes • Base width of tower • Top width of tower • Geographical location of tower ( or Wind speed) • Factor of safety required for the design of tower • Angle of twist and sway acceptable • Type, numbers and location of antennae • Type of climbing facility and safety climb device required • The position of ladder, inside or outside • Location of platforms • Truncations

2.1 Tower type The tower types whether self-supporting or guyed type is to be decided by the user , depending on the area available at the location of tower.

2.2 Height of tower The height of tower is fixed by the user based on the required height of antenna to obtain line of sight. Generally the height of communication tower varies from 9m to 200m. 2.3 Cross section of tower The cross section of the tower should be decided by the designer based on the requirement of client.

2.4 Type of tower The type of tower whether Green field or Roof top should be decided by the user depending on the topographical location of tower. If the location of tower is inside the city or town, the, tower is to be designed as Roof top tower; if the location of tower is outside the city or town the tower is to be designed as Green field tower. While designing the roof top tower, the height of building should be considered in calculating Gust Response Factors (GRF). 2.5 Types of sections

The types of sections to be used for the legs of the tower are to be decided by the designer as per the requirement of the client.

2.6 Design codes

• Generally loads on the communication towers are estimated according to Code of practice for design loads (Other than

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Earthquake) for buildings and structures. Part 3 is for Wind loads.

• The tubular sections shall be designed in accordance with Code of practice for

use of steel tubes in general building construction.

• The foundations for the tower shall be designed .

2.7 Base width of tower The center-to-center spacing between the tower footings i.e., base width at concrete level is the distance from the center of gravity of the corner leg angle to that of the adjacent corner leg angle. The width depends upon the magnitudes of physical loads imposed upon the towers by antennas, wind loads and the height of application of the loads from ground level. Towers with larger base widths result in low footing cost and lighter main leg members at the expense of longer bracing members. There is a particular base width which gives the best compromise and for which total cost of the tower and foundation is minimum.

2.8 Top width of tower Top width of tower is depending on the cross section of tower and position and width of ladder. If the tower is having triangular cross section and the ladder is inside the top width of tower should be fixed so as to accommodate the ladder inside. Generally width of tower varies from 400mm to 500mm. If the tower is having square cross section to accommodate the ladder inside the top width of tower should be minimum 1500mm to 1750mm. If the tower is triangular to accommodate ladder inside top width should be minimum of 2500mm.

2.9 Geographical location of tower Depending on the geographical location of the tower the basic wind speed is to be considered to calculate wind load on the tower. Basic wind speed is based on peak gust velocity averaged over a short time interval of about 3 seconds and corresponds to mean heights above ground level in an open terrain (Category 2). Basic wind speeds have been worked out for 50-year return period.

2.10 Factor of safety required for design of tower members Factor of safety adopted in the design of members have a great bearing on the cost of the structures and they have to be chosen so that the structures prove economical as well as safe and reliable. To account for the reduction in strength due to dimensional tolerances of the structural sections and yield strength of steel used, the following strength factors shall be considered:

i. If steel with minimum guaranteed yield strength is used for fabrication of

tower, the estimated loads shall be increased by a factor of 1.02.

ii. If steel of minimum guaranteed yield strength is not used for fabrication of tower, the estimated loads shall be increased by a factor of 1.05 in addition of the Provision i.

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However, from reliability and serviceability considerations for communication towers it is usual practice to take a factor of 1.2 to 1.5. 2.11 Angle of twist and sway From the serviceability consideration according to TIA/EIA standards the angle of twist and sway should not be greater than ± 0.5 degrees. 2.12 Type, numbers and location of Antennae The type of antenna and their numbers and height at which these antennas are required to be mounted will have great impact in the cost of tower. The wind loads due to antenna shall be considered from Andrew’s Catalogue. 2.13 Type of climbing facility and safety climb device required For communication towers the climbing facility can be provided by two ways. a. By providing climbing ladder with or without safety ring b. By providing step bolts . The exposed area of ladder shall be considered while calculating the wind load on tower. The position of ladder will impact on weight of tower, if ladder is provided internally its effect will be less and if it is provided externally it will have more effect. 2.14 Location of platforms The platforms shall be provided at different levels as rest platforms or working platforms. Rest platforms are required to provide to enable the climbing person take rest while the working platforms are provided to fix and maintain the antennae at that levels. The working platforms may be internal or external. If the platforms are provided externally their exposed area shall be taken into account while calculating the wind load on tower. Hence, size of platforms will also affect the design of tower. The number and location of platforms will depend on the height of tower. 2.15 Truncations In general the height of tower required may not be same for all the sites. The height may vary depending upon line of sight required and height of building available if it is Roof top tower. It is not suggestible to design one tower for each site. It is usual practice to design maximum height of tower required for certain area over which there is no considerable variation in basic wind speed. Then the required height of tower can be achieved by truncating the designed tower. The designer should select the panel heights so as to enable the user to truncate the tower to the required height.

Generally for Roof top towers the truncations vary at 3m intervals, similarly for Green field tower they vary at 5m or 10m intervals.

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TOWER GEOMETRY 3.1 Tower anatomy

The typical communication tower is constituted of the following components:

1. Panels 2. Back marks 3. Legs 4. Diagonals/Bracings 5. Horizontals 6. Diamond bracings 7. Redundant 8. Gusset plates 9. Cover plates 10. Cleats 11. Climbing ladder 12. Cable ladder 13. Step bolts 14. Rest platforms 15. Working platforms or GSM/CDMA platforms 16. Mounts for MW antennae 17. Mounts for GSM/CDMA antennae 18. Template 19. Bolts, Nuts and washers 20. Stub 21. Stitch plates 22. Base plate 23. Anchor bolts 24. Stiffeners 25. Flange plates 26. Pack plates 27. Bracket members 28. Railing 29. Chequered plate 30. Welded wire mesh 31. Flats 32. Anchor plate 33. GSM/CDMA antenna mounts 34. Microwave antenna mounts 35. Aviation lamp 36. Lightening arrester

1. Panels Any lattice tower is the assembly of different panels. The panel is the component of the tower, which consists of legs, diagonals, horizontals and redundants. A panel can be added or truncated from the tower. The height of the panel should be decided based on truncation requirements and economical design criteria. Generally for Roof top towers panels heights will be in multiples of 3m and for Green field towers the panel heights will be in multiples of 5m or 10m.

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2. Back mark Back mark is the distance from the back of an angle to the center of a hole through the leg. This dimension is determined so as to allow the tightening of a bolt with a standard podger spanner, to be as near as possible to the centroidal axis and to allow the required edge distance. 3. Legs The corner vertical or nearly vertical members of the tower are called Legs or Column members and they are main load bearing elements. Generally angular sections are used for legs. If single angle sections are inadequate, starred angles or double starred angles are used. Tubular sections are also used for legs, generally for triangular towers. Tubular sections are economical when the loading on the tower is low. The legs will have compression or tension depending on the direction of wind loads. 4. Diagonals/Bracings The inclined members, which are connected to the legs diagonally, are called Diagonals. The loads from the legs will be transferred to the diagonals by shear force. Diagonals will have both compression and tension. 5. Horizontals The straight members which interconnects the legs horizontally are called Horizontals. The horizontals will carry shear/torsion. These horizontals are also provided to support ladders and where the platforms are to be supported. The legs are interconnected by diagonals and horizontals. The horizontals will also be subjected to bending at the platforms due to imposed live loads. 6. Diamond bracings These are provided to reduce effective length of horizontals and at platform levels to accommodate platforms. 7. Redundants The secondary members which are provided to reduce the slenderness ratio of the main members are called Redundants. The redundant will carry nominal stresses. And generally these are designed from slenderness ratio criteria. The redundants are also verified for bending due to maintenance loads and up to 2.5% of main member force to which it is connected. 8. Gusset plates Gusset plates are the plates used to interconnect the members. For angular towers gusset plates will be provided when it is not possible to accommodate number of bolts required for a connection in the space available on the members. For hybrid towers (legs tubular and bracings angular) gusset plates are required to connect secondary members to the legs. Gusset plates may also required to reduce the secondary stresses introduced due to eccentricity to a minimum. The bracing members should preferably meet at a common point within the width of the tower in order to limit the bending stresses induced in the main members due to eccentricity in the joints. To satisfy this condition, it is required to provide gusset plates. But it is economical to avoid gusset plates as far as possible.

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9. Cover plates Cover plates provided in butt joints on outside. The width of the cover plate should be kept same as or 20mm less than the width of the connecting members. The length of the cover plate should be provided so as to accommodate the bolts required for that connection. 10. Cleats Cleats are small angular sections, which will be provided inside of butt joint. The length of the cleat should be provided so as to accommodate bolts and to satisfy maximum pitch requirements. Heel grinding should be done for cleats. The total cross sectional area of cover plates and cleat should be 10% more than that of main members. 11. Climbing ladder Climbing devices are essential in communication towers as persons are required to climb the tower to maintain antennas or tower or tower appurtenances. Generally the climbing ladder is provided with safety cage. Climbing ladders may be provided inside for square towers and outside for triangular towers or narrow based towers. Protection ring is a safety requirement and may be replaced by fall arrest safety system 12. Cable ladder Cable ladder is provided to support the cable wave-guide running from antenna to the equipment shelter. The cable ladder will be provided on inside of the tower along the slope of tower. The width of the cable ladder depends on dia. & number of wave-guides and their spacing. 13. Step bolts Step bolts are seldom provided for communication towers. Step bolts are provided only for narrow based towers of smaller heights as per user’s requirements. Step bolts usually adopted are of 16mm dia and 175mm length at a vertical spacing of 450mm from 3.5m above ground level to the top of the tower. The bolts are provided with two nuts on one end to fasten the bolts securely to the tower, and button heads at the other end to prevent the foot from slipping away. The step bolts should be capable of with standing a vertical load of not less than 1.5 kN. 14. Rest platforms The platform, which is provided with chequered plate or welded wire mesh along with suitable railing internally to enable a climbing person to take rest while climbing towards the top of tower is called ‘Rest Platform’. Generally rest platforms are provided for every 10m height. Rest platforms shall be provided for all towers of height exceeding 20m. 15. Working platforms or GSM/CDMA Antennae platforms The platforms which are provided at the level where GSM/CDMA antennae are provided externally around the tower with suitable railing are called Working platforms. The GSM/CDMA antennae can be mounted to this platform. The shape of this platform may be square, hexagonal or circular. Circular platform is preferably used as antennae can be fixed in any azimuth. The height of railing shall be minimum 1000mm with toe, knee and handrail protection.

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16. Mounts for Microwave antennae The structural elements, which are provided to connect microwave antenna to the tower members, are called Antenna mounting structures. This shall facilitate level and azimuth adjustment. Struts shall be provided for dishes of dia greater than or equal to 1.2m. 17. Mounts for GSM/CDMA The GSM/CDMA mount shall facilitate azimuth and orientation in vertical plane adjustment. 18. Template The temporary structure by means of which the anchor bolts or stubs in the tower foundation are set is called Template. Templates are very useful in setting stubs such that the distance between the stubs and their alignment and slope is in accordance with the design so as to permit assembling of superstructure without undue strain or distortion in any part of the structure. 19. Bolts, Nuts and Washers A bolt is a metal pin with a head formed at one end, shank threaded at the other end receive a nut. Bolts are important structural elements of the tower. The total strength and serviceability of the tower is dependent on the strength of bolts in transferring the load from one member to another member. Hence, proper care should be taken while designing, fabrication and at erections stages. Generally 16mm dia bolts are used for towers. Nuts are screwed to the bolt at one end with washers of sufficient thickness to avoid any threaded portion of the bolt being within the thickness or the parts bolted together. Both flat and spring washers are used for communication towers. The advantage of spring washers over flat washers is that, the former in addition to the developing the full bearing area of the bolt, also serve to lock the nuts. However, it is difficult to get correct quality for spring washers, and the spring washers are too brittle and consequently break when the nuts are fully tightened. Furthermore, the spring washers, unlike flat washers tend to cut into and destroy the galvanizing. 20. Stub The piece of bottom most leg member, which is embedded in to the foundation, is called Stub. The stub is embedded in to the foundation pit with help of stub setting template before concreting. The stub transfers the load coming from the tower to the foundation more effectively as compared to base plate and anchor bolt system. 21. Stitch plate The plates which are provided to hold two angles in position in a starred angle or back to back are called Stitch plates. 22. Base plate The plates, which are generally, square in plan to which bottom most leg is fixed to distribute load coming from the tower leg to the foundation pedestal is called Base plate. 23. Anchor bolts The bolts, which are generally in, L-shape in elevation provided to transfer the uplift and side thrust of the tower. One end of the anchor bolt is embedded in to concrete and the

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other end is fixed to the base plate. The portion, which is left above the concrete, shall be galvanized as per standards. 24. Stiffeners The stiffeners shall be provided on the top and/or bottom of the flange plates to stiffen the flange where it is inadequate in bending. 25. Flange plates The plates connected to tubes at both the ends, by which the two tubes are interconnected are called Flange plates. Generally these are circular in plan. 26. Pack plates The plates which are provided in a butt joint when it is required to join two members of different thickness are called Pack plates. 27. Bracket members The inclined member which is connected to the leg in outside to support working platform is called Bracket members. 28. Railing The structure which is provided all-round the platforms to avoid danger of free falling of working personal is called Railing. Generally it is of 1m height above the platform. 29. Chequered plate The plate which is provided as flooring for platform is called Chequered plate. The shape and size of chequered plate depends on the size and shape of cross section of the platform. 30. Welded wire mesh Welded wire mesh is alternative for chequered plate. Welded wire mesh is made up of round bars, which are welded together in both directions. As compared to chequered plate welded wire mesh is economical, but is not comfortable as compared to chequered plate. 31. Flats Flats are generally used for fixing welded wire mesh, for railing, for safety cage and for cable ladder. 32. Anchor plate The plate, which is provided at the bottom of anchor bolts to hold them in position, is called Anchor plate. 33. GSM/CDMA antenna The antennae for Radio signal transmission.

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34. Microwave antenna These antennae are either with Radome or without Radome. Dishes with Radome transfer very less load compared to without Radome. They are circular or parabolic in shape. 35. Aviation lamp The lamp, which is provided at the top of tower as per aviation requirements and standards, is called Aviation lamp. For towers of height more than 50m apart from the top, one more aviation lamp at 40m level is provided. 36. Lightening arrester The equipment provided at the top of tower to avoid danger due lightening is called Lightening arrester.

DESIGN OF COMMUNICATION TOWER

4.0 Introduction The following are the steps involved in design of communication tower : • Selection of configuration of tower • Computation of loads acting of the tower • Analysis of the tower for above loads • Design of tower members according to the codes of practice 4.1 Selection of Configuration of tower Selection of configuration of a tower involves fixing of top width, bottom width, number of panels and their heights, type of bracing system and slope of tower. The configuration of tower should be such that it should give minimum weight. After fixing the top width and bottom width, panel heights and their type of bracing systems should be selected. The legs are braced by the main bracings, both of these are often propped by additional secondary bracing to reduce effective buckling lengths. 4.2 Computation of loads coming on to the tower The different loads that are acting on tower are : • Wind load on tower members, antenna, ladder and platform members • Dead load of tower member, antennae and their mounts • Live load on antenna platforms and rest platforms • Seismic load • Miscellaneous loads like erection loads and temperature loads etc., Wind load is the most important of all and it often controls the design. The seismic load may not be critical as the mass of the structure is not heavy and it is more near to the ground. Dynamic effects due to wind become critical in slender and tall towers. In most cases of towers up to 100m height the dynamic effect may not control the design. Live loads on towers are negligible when compared with other loads. 4.2.1 Wind load on tower

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4.2.1.1 General Wind is air in motion relative to the surface of the earth. The primary cause of wind is traced to earths rotation and differences in terrestrial radiations. The wind generally blows horizontal to the ground at high wind speeds. Since vertical components of atmospheric motion are relatively small, the term ‘wind’ denotes almost exclusively the horizontal wind. The wind speeds are assessed with aid of ‘Anemometers’ or ‘Anemographs’ which are installed at heights generally varying from 10 to 30m above ground at meteorological observatories. 4.2.1.2 Basic wind speed The designer should select the basic wind speed depending on the location of the tower. The basic wind speeds for different locations in India are available in publication. 4.2.1.3 Design wind speed The basic wind speed (Vb) is modified to induce the effects of risk factor (k1), terrain and height (k2) and local topography (k3) to get the design wind speed Vd. Thus Vd = Vb k1 k2 k3 ; Where Vd = Design wind speed at any height z in m/sec. k1 = Probability factor (risk coefficient) k2 = Terrain height and structure size factor k3 = Topography factor Design wind speed up to 10m height from mean ground level shall be considered as constant. 4.2.1.4 Risk probability factor (k1) For the suggested life period of the structure taken in the design, the corresponding k1 factors for different class of structures for the purpose of design are given. 4.2.1.5 Terrain categories (k2) Selection of terrain categories shall be made with due regard to the effect of obstructions which constitute the ground surface roughness. The terrain category used in the design of a structure may vary depending on the direction of wind under consideration. Whenever sufficient meteorological information is available about the nature of wind direction, the orientation of any structure may be suitably planned. 4.2.1.6 Topography (k3) The basic wind speed Vb takes the account of the general level of site above sea level. This does not allow for local topographic features such as hills, valleys, cliffs, escarpments or ridges, which can significantly affect the wind speed in their vicinity. The effect of topography is to accelerate wind near the summits of hills or crests of cliffs, escarpments or ridges. The effect of topography will be significant at a site when the upwind slope is greater than 3 degrees and below that, the value of k3 may be taken to be equal to 1.0. The value of k3 varies between 1.0 and 1.36 for slopes greater than 3 degrees.

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4.2.2 Design wind pressure (Pd) The design wind pressure Pd at any height above mean ground level is obtained by the following relationship between wind pressure and wind velocity. Pd = 0.6 Vd

2 ; Where Pd = Design wind pressure in N/m2 at height z Vd = Design wind velocity in m/s at height z 4.2.3 Wind force on the structure The force on a structure or portion of it is given by F = Cf Ae Pd ; Where Cf = Force coefficient Ae = Effective projected area Pd = Pressure on the surface The major portion of the wind force on the tower is due to the wind acting on the frames and the antennas. 4.2.4 Wind force on single frame Force coefficients for a single frame having either a) Flat-sided members or b) All circular members in which all the members of the frame will have either:

i) D Vd less than 6 m2/sec or ii) D Vd greater then 6 m2/sec

These values are given . 4.2.5 Wind force on circular sections The wind force on any object is given by F = Cf Ae Pd ; Where Cf = Force coefficient Ae = Effective area of the object normal to the wind direction Pd = Design pressure of the wind For most shapes, the force coefficient remains approximately constant over the whole range of wind speeds likely to be encountered. However, for objects of circular cross section, it varies considerably. For a circular section, the force coefficient depends upon the way in which the wind flows around it and is dependent upon the velocity and kinematic viscosity of the wind and diameter of the section. The force coefficient is usually quoted against a non dimensional parameter, called the Reynolds number, which takes account of the velocity and viscosity of the medium(in this case the wind), and the member diameter. 4.2.6 Wind load on antennae Wind load on antennae shall be considered from Andrew’ Catalogue. In the Andrew’s catalogue the wind loads on antennas are given for 200kmph wind speed. The designer may calculate the antenna loads corresponding to design wind speed.

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For any other type of antenna which is not covered in Andrew’s catalogue such as GSM (Global System for Mobile) and CDMA, the wind load shall be calculated by multiplying exposed area of such antenna by design wind pressure. After arriving antenna loads drag coefficient / force coefficients shall be applied as per and force coefficients on all GSM/CDMA antennae. These antenna loads should be applied at appropriate levels specified by the user. 4.2.7 Wind load on ladder & platforms The wind load on ladder and accessories shall be considered while calculating wind loads on tower. Ladder exposed area per running meter is calculated and this has to be added to the exposed area of tower members for calculating the wind loads. Similarly exposed area of platforms is being added to exposed area of corresponding panels at that level. 4.2.8 Dead load of tower members Dead load of tower members shall be considered while designing the tower. The dead load includes weight of main members only. The additional weight of plates, cleats, ladder and platforms can be arrived by applying load factor for the self weight of main members. 4.2.9 Live load on antenna platforms and rest platforms Live load on antenna platforms and rest platforms shall be considered while designing the tower. Generally it shall be considered as 2 kN as per standards. 4.3 Analysis of tower In the analysis of towers, the greatest uncertainty is associated with the insufficient knowledge of wind loads. Owing to this a corresponding uncertainty is also introduced in understanding the dynamic and static response of tower structure, which is subjected to uncertain loads. Any improvement or refinement in the method of analysis cannot in anyway remove this uncertainty. Therefore a static linear three dimensional structural analysis is sufficient for most of the time but when the magnitude of the wind load is large enough to cause non linearity in structural behaviour of tower a non linear 3D analysis may become necessary. Dynamic analysis of self supporting lattice towers are rarely necessary unless there are special circumstances such as high masses at the top, uses as a viewing platform or circular or almost solid sections of mast which could be responsive to vortex shedding or galloping. For guyed towers the non-linear behaviour of guy is a primary influence and cannot be ignored. The choice of initial tension, for example, can have a great effect on the deflections (and dynamic behaviour). The effects of the axial loads in the mast on column stiffness can be significant. 4.3.1 Assumptions for analysis of communication tower The following assumptions are made while performing analysis of a communication tower. a. All members of a bolted type tower framework are pin-connected in such a manner

that the members carry axial loads only.

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b. The bolt slippages throughout the structures are such as to allow the use of the same modulus of elasticity for the entire structure, thus permitting the use of the principle of super-imposition for stress analysis.

c. Shear is distributed equally between the two members of a double web system, i.e., Warren system.

d. Shear is carried by the diagonal member under tension in a Pratt system with members designed for tension only, the other member being inactive.

e. Plan members at levels other that those at which external loads are applied or where the leg slope changes, are designated as redundant members.

f. Any face of the tower subjected to external loads lies in the same plane, so far as the analysis of the particular face is concerned.

g. The members on all the four faces of the tower share loads equally. h. Vertical loads placed symmetrically and the four legs share dead weight of the

structure equally. i. All the members placed horizontally or at an angle less the 15 degrees to the

horizontal will be checked independently for specified point load, causing bending stresses.

4.3.2 Method of analysis The tower structure is basically a statically indeterminate structure. For this type of structures Space analysis or 3D analysis will be used. 3D analysis cannot be handled manually but can be done with the help of appropriate powerful computer programme. 4.4 Design of communication tower After the loads acting on the tower are determined, and the analysis of tower is carried out, the sizes of the various members is fixed. Since the axial force is the only force for a truss element, the member has to be designed for either compression or tension. As the reversal of loads induce alternate nature of forces the members are to be designed for both compression and tension.

MANUFACTURING OF COMMUNICATION TOWER Towers are manufactured either with L angles or tubes. Various activities of Tower manufacturing process are: • Preparation of BOM • Raw material procurement • Inspection of raw material • Fabrication of Tower components • In line process inspection • Galvanizing • Number printing • Packing & dispatch 5.1 Preparation of BOM

After completing the tower design, a structural assembly drawing is prepared. This gives complete details of joints, member sizes, bolt gauge lines, sizes and lengths of bolts, washers, first and second slope dimensions, etc. From this drawing, a more detailed drawing is prepared for all the individual members. This is called a shop drawing or fabrication drawing. Since all parts of the tower are fabricated in accordance with the shop drawing, the latter should be drawn to a suitable scale, clearly indicating all the details required to facilitate correct and smooth fabrication.

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5.2 Raw Material Procurement An enquiry is sent to the steel manufacturers / suppliers who are already finalized through a vendor selection criteria. Raw material is then procured as per the sizes (which are optimized from the Bill of Material) required for a particular project. Also these materials are procured just before commencement of fabrication. 5.3 Inspection of Raw Material The raw material before, it is dispatched from the supplier, is inspected for the following: 5.3.1 Angles / Chambers

• Width of the flange • Thickness of the flange • Straightness • Chemical properties • Yield strength • Elongation strength

5.3.2 Plates

• Thickness of the plate • Straightness • Chemical properties • Yield strength • Elongation strength

The results are compared with the standard parameters as per relevant standards and if these are well within the specified tolerances, then the material is accepted.

5.4 Fabrication processes

During fabrication, various operations such as the following should receive particular attention:

a. Straightening of all steel sections and truing them up by pressure before and after fabrication.

b. Hot bending (angle sections up to 65 x 65x 6mm and plates not exceeding 6mm in thickness with bends not exceeding 5° could be cold bent).

c. Cropping, shearing and gas cutting: Cropping and shearing being permitted on sections up to 12mm thick and sections over 12mm thickness being sawn or gas cut.

d. Punching and drilling, punching being permissible on sections up to 12mm thick. In the case of bent members or plates, punching or drilling should be done after the bending operation is completed.

e. Marking by die-punch before galvanizing.

All steel sections, before and after any work is done on them, should be carefully leveled, straightened and made true. All workmanship and finish should ensure that all similar parts are interchangeable, and that, when assembled the adjacent surfaces are in close contact throughout.

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All dimensional tolerances on the rolled sections used for fabrication should be within the provisions , the fabrication tolerances should conform to: 5.4.1 Tolerances on distances a. On overall length of the member = ± 1.6mm. b. On gauge distance = + 0.5mm. c. Between consecutive holes = +0.5mm (cumulative). d. The edge distance should not vary by more than 1 mm. 5.4.2 Tolerances concerning bolt holes a. Allowable taper in punched hole = 0.8mm in diameter. b. Deviation in the bolt hole distances should be limited to + 1 mm.

GALVANISATION

Introduction The zinc coating (Galvanizing) is most commonly used for the protection of structural iron and steel instead of any other metallic coating because of the ease of its application and also because its residual corrosion products become basic in character and exerts a retarding influence on further resolution. Galvanizing gives a two-fold protection against corrosion. Firstly, the coating provides an impermeable barrier against corrosion between the steel and its environment, secondly, zinc due to its Electro-chemical properties, protects sacrificially any moderate sized areas of steel that may be exposed by local damage to the coating. 6.1 Base metal for galvanizing 6.1.1 Steel Mild steel is the most common material that is galvanized and the variations in the range of compositions used have little influence on the galvanization process. The steel, however, should contain minimum amount of segregation, slag inclusions, rolled-in mill scale, etc. 6.2 Preparation of the metal surface for galvanizing 6.2.1 Cleaning If an article is contaminated by oil, grease or paint, pretreatment in special solvents is necessary for their removal. Several proprietary reagents are available. Generally sodium hydroxide solution obtained by dissolving 10 to 15 kg of sodium hydroxide in 100 litres of water is used. When using sodium hydroxide solution, the temperature of the solution may be usually kept between 85 and 90°C and the immersion time varying from 1 to 20 minutes depending on the nature and degree of contamination. When using other proprietary degreasing agents, manufacturer recommendation should be followed. Immediately after degreasing, the work should be rinsed in hot water (60°C) followed, if possible, by a final rinse in cold running water. An ideal arrangement for rinsing is to

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provide an inlet and outlet on two opposite sides of the rinsing tank; the inlet should be at the bottom of the tank and water should overflow from the top. This way the rinse water is in a dynamic state thereby ensuring an efficient and a thorough rinsing operation. When lubricating materials have contaminated with the surface of the metal, it may be necessary to heat the part of bluing or scaling temperature in order to burn off the offending material. Since this is an expensive and difficult process, prior care should be taken to avoid such contamination. 6.2.3 Pickling Both hydrochloric acid and sulphuric acid solutions may be used for pickling. Hydrochloric acid is more safe and in general widely used. Hydrochloric acid solution Dilute technical grade acid conforming with an equal volume of water The actual concentration of hydrochloric acid solution and the time of immersion will depend on the nature of the work to be pickled. To obtain better results a suitable inhibitor should be used with hydrochloric acid. Agitation Mild agitation of the work in the pickling tank reduces the time of pickling. Raise or lower the work once or twice to change the acid layer in contact with the work. Air agitation is not recommended. 6.2.4 Rinsing After pickling, the article should be rinsed in running water. Two rinse tanks are preferable, the water cascading from one into the other, that is, cascading from the second tank into the first tank. 6.2.5 Fluxing The rinsed article, in the dry process, is dipped in a strong solution of zinc ammonium chloride (ZnCl2.3NH4C1), although ammonium chloride is also used to a certain extent. The actual concentration of the flux solution and its temperature depend on the work being undertaken and on individual circumstances. The working level is generally between 200 to 400g of zinc ammonium chloride per liter. Some wetting agent is usually added to the flux solution. The temperature may range from room temperature to 80°C. When dry galvanizing is adopted, the article shall be thoroughly dried after fluxing over a hot plate or in an air-oven. The temperature should be about 120°C and should not exceed 150°C as the flux decomposes above this temperature. In the wet process, a deep flux cover is used on the zinc bath and the work is immersed through the flux layer with or without fluxing. In this case drying is not considered essential. The article that has been prefluxed and dried should be galvanized without delay, as the flux coating picks up moisture from the air and also tends to oxidize. The recommended time limit for galvanizing is within an hour of fluxing.

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6.3 Hot dip process

This is the earliest type known and is very extensively used at present. In hot dip galvanizing, the process consists first of thoroughly cleansing the steel by pickling in acid and subsequently transferring it to a bath of molten zinc. The object of pickling is to remove from the surface of the work all mill and surface scale and other foreign matter that would prevent intimate contact between the two metals. The bath must be maintained at a temperature 450-460°C. The coating produced by hot dip galvanizing is not merely a layer of zinc spread over the surface but also a layer of new products firmly bonded to the surface. The layer consists of iron and zinc alloy, formed between the zinc surface layers and the steel base. The thickness of this layer depends on the length of time of immersion and the temperature of the molten zinc and can be controlled if flexibility of the coating is required. 6.4 General process of galvanization 6.4.1 Quality of zinc Zinc containing at least 98.5 percent purity should be used for the purpose of galvanizing. The molten metal in the galvanizing bath should contain not less than 98.5 percent by mass of zinc. 6.4.2 Aluminum Additions Aluminum may be added to the galvanizing bath in the dry process to the extent of about 0.005 percent (0.007 percent max) (0.05-0.07 g/kg of zinc) to reduce the rate of oxidation of the molten metal and brighten the appearance of the article. In the continuous strip galvanizing process, addition of Aluminium is made in the bath in the form of Zn-Al alloy to maintain Aluminium between 0.12 to 0.20 percent to control alloy layer thickness and thereby imparting better adherence. Lead is also added in the form of Zn-Pb alloy to provide spangle on the surface: 6.4.3 Bath temperature The control of bath temperature is essential if the quality of the product is to be consistent and zinc is to be used economically. Articles should be galvanized at the lowest possible temperature, which will allow the free drainage of zinc from the work piece during withdrawal. A low temperature reduces the formation of ash and dross, besides safeguarding the pot and conserving fuel. The bath temperature may vary from 440°C to 460°C and a working temperature of 450°C is commonly used. 6.4.4 Water quenching Where the article is withdrawn through a flux blanket, the quench water needs to be changed frequently to prevent the accumulation of corrosive salts. For this purpose tanks having overflow weir may be used with advantage. Light gauge articles should be spun quickly through the surface of water so that they retain sufficient heat after quenching to enable quick drying. Heavy articles retain sufficient heat for drying. 6.4.5 Post treatment The zinc coating on freshly galvanized surfaces when exposed to humid, poorly ventilated conditions during storage and/or transport react with moisture, carbon dioxide,

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oxygen, etc, in the atmosphere forming a mixture of salts which are white in colour. This is known as 'white rust' or 'wet storage stain'. Normally a post treatment like chromating is recommended. This is a temporary treatment and retards white rust attack. The chromating solution contains one percent sodium dichromate and half percent sulphuric acid solution the solution is kept at room temperature and its temperature should never be allowed to rise above 65°C. The galvanized articles are dipped into the chromating solution after the galvanizing and water quenching operations. In case of continuous strip galvanizing, the strip is sprayed with chromating solution, such as chromic acid and properly spread uniformly by means of squeezer rolls. Temperature of the chromic acid bath is maintained around 70°C to 75°C. 6.5 Testing and inspection 6.5.1 Freedom from defects The zinc coating should be adherent, smooth, reasonably bright, continuous and free from such imperfections as flux, ash and dross inclusions, bare and black spots, pimples, lumpiness and runs, rust stains, bulky white deposits and blisters. 6.5.2 Uniformity in thickness Galvanize articles shall be tested for uniformity in thickness of coating in accordance with tests. For quick approximate measurements of thickness, magnetic gauges may be used, but such instruments shall be suitably calibrated before use. 6.6 Adhesion tests 6.6.1 Pivoted hammer test for zinc coated fabricated products The adherence of the zinc coating on steel shall be determined by the pivoted hammer test. The hammer shall be made of normalized 0.3-0.4 percent carbon steel. The hammer blow shall be controlled by holding the pivoted base of the handle on a horizontal surface of the galvanized member and allowing the hammerhead to swing freely through an arc from vertical position to strike the horizontal surface. The test shall consist of two or more standard blows forming parallel impressions with 6mm spacing and a common axis. No part of an impression shall be closer than 12mm to the edge of the member. Removal or lifting of the coating in the area between the impressions shall constitute failure. An extruded ridge less than 2mm wide immediately adjacent to the impression shall be disregarded. The specimen is tested in several places throughout its length. 6.6.2 Knife test for zinc coated hardware and assembled steel products When the coating is cut or pried into, such as with a stout knife applied with considerable pressure in, manner tending to remove a portion of the coating, it is possible only to remove small particles of the coating and it is not possible to peel off any portion of the coating so as to expose the underlying iron or steel. 6.6.3 Storing, Packing and Handling Sufficient care is exercised while storing, packing and handling of galvanized products. While storing and transporting them, adequate ventilation is provided as otherwise 'white rust' or 'wet storage stain' may result when galvanized coatings react with humidity and atmospheric gases.