Maina M W - Performance of Guyed Telecommunication Masts

Department of Civil, Construction and Environmental Engineering FINAL YEAR PROJECT TITLE A STUDY INTO THE PERFORMANCE OF GUYED MASTS AND SELF SUPPORTING LATTICE TOWERS AS USED IN TELECOMMUNICATION BY: MAINA MADRIN WANJIRU REG. NO. E25-0114/04 SUPERVISOR MR. P. U. MULU

This project is submitted in partial fulfillment of the award of Bsc. Civil, Construction and Environmental Engineering of the Jomo Kenyatta University of Agriculture and Technology. March, 2010

DECLARATION I, Madrin W. Maina hereby do declare that this is my original work and has not been presented elsewhere for the award of a degree or any other purpose whatsoever. Signed……………………………………………………………………………Date……………

Madrin Wanjiru Maina

CERTIFICATION I have read this report and approve it for examination Signed…………………………………………………………………………Date……………

Mr. P. U. Mulu

DEDICATION I dedicate this project to my dear parents and brothers who have believed in me and dared me to dream. Thank you for your support and prayer throughout my education. Daniela you taught me that patience pays. Stanley you have believed in me all through. Martin you set the path for me to follow. Lewis you provided the necessary challenge.

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ACKNOWLEDGEMENT I register my appreciation to God almighty. I thank my supervisor Mr. P. U. Mulu for his informed advice. Your effort at providing advice is greatly valued. I tender my deep gratitude to the staff of TKM Maestro LTD, ROM east Africa and Alan Dick Company and my colleagues who guided and assisted me throughout my studies and in accomplishing this research work. In addition, I would like to thank my family and friends who stood by my side throughout my studies. God bless you all.

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Table of Contents DECLARATION ................................................................................................................. i DEDICATION .................................................................................................................... ii ACKNOWLEDGEMENT ................................................................................................. iii ABSTRACT ..................................................................................................................... viii CHAPTER 1 ....................................................................................................................... 1 1.0 INTRODUCTION .................................................................................................... 1 1.1 BACKGROUND INFORMATION ......................................................................... 1 1.2 STUDY JUSTIFICATION ....................................................................................... 1 1.3 PROBLEM STATEMENT ....................................................................................... 2 1.4 RESEARCH OBJECTIVES ..................................................................................... 2 1.4.1 Overall objective ................................................................................................ 2 1.4.2 Specific Objective .............................................................................................. 2 1.5 RESEARCH HYPOTHESIS .................................................................................... 3 1.6 LIMITATIONS OF THE STUDY............................................................................ 3 CHAPTER 2 ....................................................................................................................... 4 2.0 LITERATURE REVIEW ......................................................................................... 4 2.1 Self supporting towers .............................................................................................. 4 2.3 Guy cables ................................................................................................................. 9 2.3.1 Phillystran .......................................................................................................... 9 2.3.2 Pultruded Fiber glass........................................................................................ 10 2.4 Preload in Guy cables ............................................................................................. 11 2.5 Termination of Guys ............................................................................................... 11 2.6 Bracing types .......................................................................................................... 11 2.7 Tower Wind Loading .............................................................................................. 13 2.8 Tower design criteria .............................................................................................. 13 2.9 Loading conditions.................................................................................................. 14 3.0 Tower Erection........................................................................................................ 16 3.1 Connections............................................................................................................. 17 3.2 Basic criteria in the design of connections ............................................................. 17 CHAPTER 3 ..................................................................................................................... 22 3.0 RESEARCH METHODOLOGY............................................................................ 22 3.1 STRUCTURAL ANALYSIS.................................................................................. 22 3.1.2 An overview of the Staad- Pro Analysis Software and modeling ................... 24

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3.1.3 Outline of the procedure for calculating the wind loads on Lattice Towers and Guyed Masts ............................................................................................................. 25 3.1.4 Solidity Ratio calculation ................................................................................. 26 3.2 Cost Benefit Analysis ............................................................................................. 27 CHAPTER 4 ..................................................................................................................... 29 4.0 DATA COLLECTION ........................................................................................... 29 i)

Types of tower and masts foundation. ...................................................................... 30 5.0 RESULTS ............................................................................................................... 34 5.1 Nodal Displacements .............................................................................................. 34 5.2 Support Reactions ................................................................................................... 35

CHAPTER 6 ..................................................................................................................... 38 6.0 DISCUSSION ......................................................................................................... 38 6.1 Structural analysis results ....................................................................................... 38 6.2 Load carrying capacity ............................................................................................ 39 6.3 Cost benefit analysis ............................................................................................... 40 CHAPTER 7 ..................................................................................................................... 41 7.0 CONCLUSION ....................................................................................................... 41 7.1 Recommendations ................................................................................................... 41 REFERENCES ................................................................................................................. 43 APPENDIX ....................................................................................................................... 44

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LIST OF FIGURES Figure 1: Self supporting 3 leg tubular tower……………………………………………………..5 Figure 2: 3 leg tubular Guyed Mast…………………………………………………………….....7 Figure 3: Typical cable configuration………………………………………………………….....10 Figure 4: Bracing types…..……………...…………………………………………………….12-13 Figure 5: Connections………………………………………………...…………………….....20-21 Figure 6: Structural simulation model for a 3 leg tubular Guyed Mast………….………….........23 Figure 7: Structural simulation model for a 3 leg tubular Self Supporting Tower.…………........24 Figure 8: Design Process Flow Chart……………………………………………………….........28 Figure 9: Tower foundation………..………………………………………………………..........30 Figure 10: Types of support structures………………………………………………………..31-33

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LIST OF TABLES Table 1: Summary of the nodal displacements for the guyed mast………………………………34 Table 2: Summary of the nodal displacements for the self supporting tower…………………….34 Table 3: Support reactions for the guyed mast…………………………………………………...35 Table 4: Support reaction for the self supporting tower…………………………………………36 Table 5: Wind loading for the 24 meter self supporting tower and guyed mast………………….37 Table 6: Tubular section prices…………………………………………………………………...37

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ABSTRACT The telecommunication industry is one of the fastest growing industries in Kenya. It is also one of the most vital industries since all sectors of the economy require good and dependable communication. Consequently the telecommunication industry is undergoing rapid expansion and the main focus is on towers and masts for supporting the antennas and the microwave dishes. Most of the costs incurred in this expansion are actually in the fabrication and erection of these towers and masts. Towers and masts are normally produced of tubular or angular profiles with either a square or triangular arrangement. In previous studies, structures of tubular profiles have been found to perform better structurally than those of angular profiles. 3 legged designs have also been found to reduce a structure’s weight significantly by almost 25%. Comparing towers of similar profiles, masts have a higher load carrying capacity due to the support offered by the guy cables. Depending on the size of the cable used, a mast can bear a load greater than that carried by a tower of larger profile. They also exert less pressure on the ground and therefore smaller foundation designs can be used for their support. However smaller concrete pads have to be used to act as bases where the guys are anchored. Erection of masts is also a bit complicated due to the process of prestressing guy cables in order to remove the slack in them. Masts also require more work during the maintenance process since the cables need retensioning to ensure that they support the tower appropriately otherwise it would fail. Some of the highest support structures in this industry are masts. Masts however require more space on the ground to set up and are therefore more suitable for less built up areas. This also leads to higher land acquisition fees. On the other hand towers are easier to erect and demand less work since the extra task of pretensioning guy cables is avoided. Though towers require a bigger foundation, no extra concrete pads are needed as in the case of masts. Towers also occupy less space on the ground and therefore less land acquisition fees. In order to make a choice between masts and towers, careful considerations have to be made between the various factors to identify the costs and come up with the most economical designs. This study gives an overview of these factors and offers reasonable comparisons between masts and towers of similar profiles.

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CHAPTER 1 1.0 INTRODUCTION 1.1 BACKGROUND INFORMATION Kenya’s Mobile Telecommunication Sector is under the overall management of the Communications Commission of Kenya, CCK. It has recently undergone tremendous growth which has seen the total number of national operators rise to four since the first national operator was licensed in 1997. The main Transmission Equipment used in mobile communication is the Microwave Dishes and Antennas which have to be hosted by Towers. With four national competitors in the market demand for infrastructure especially towers is quite high. In Kenya the most common types of towers are the self supporting towers, guyed masts and poles. The selection of each depends on the client’s preference, Engineer’s opinion, location and the equipment loading. Poles are used in cases where equipment loading is quite minimal while the guyed masts and self supporting towers are used in cases of heavy loading. With all the above mentioned host structures, high costs are incurred in design, fabrication transportation and erection. In Kenya, the telecommunication industry mainly focuses on guyed masts and self supporting towers for supporting their equipment. The focus is particularly on the total costs as well as production and erection time of the towers. Their structural strength and foundation parameters are also areas of concern. The production costs have gone up as a result of escalation of international steel prices. There are no standard chosen structures for a particular equipment and wind loading and a selection of any of the above is highly dependent on a variety of factors. In this research a critical structural analysis of both of the above mentioned structures coupled with a cost benefit analysis of the tower erection processes will be done to come up with a clearly defined method of knowing which structure is suitable in which area of application. 1.2 STUDY JUSTIFICATION Telecommunication equipment host structures vary according to clients’ preferences, the engineers’ opinion on the structures’ safety, the structure’s location and the basic requirement of transmission and receiving of signals. Mobile penetration in the Kenyan population is estimated at less than 50%, this figure is expected to grow significantly in the next few years and this growth will come with lots of demands on the

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transmission infrastructure. Theses demands can only be met if the telecommunication infrastructure is expanded rapidly. With the current global economic crisis there is need to come up with efficient yet structurally sound host structures for Telecommunication equipment. This will significantly reduce costs currently being incurred by Telecommunication companies in their roll out and expansion programs. 1.3 PROBLEM STATEMENT The cost of steel in the current global outlook is very high and therefore efficient design and construction models have to be adopted. The high cost of steel is due in part to major construction resulting from emerging economies of most third world countries. Currently in Kenya, there are four national mobile operators but none of them enjoys a national wide coverage. Coverage is an important aspect in the GSM (Global system for mobile communication) industry; consumers are very much influenced to join a particular network if it has substantial coverage all over the country. Therefore the Telecommunication companies have to hasten their rollout programs in order to capture a considerable share of the market. This can only be done if the concerned parties are armed with the correct information during their decision making process. Making informed decisions will lead to reduced costs both monetary and in terms of time. Also efficient ways of tower fabrication and erection should be adopted in order to hasten the Network Rollout process. 1.4 RESEARCH OBJECTIVES 1.4.1 Overall objective To draw suitable comparisons between guyed masts and self supporting towers which if carefully considered will assist in proper decision making and eventually result in an overall drop in fabrication and erection costs. 1.4.2 Specific Objective To draw comparisons of the safe carrying capacities of guyed masts and self supporting towers subjected to the same loading conditions. To draw comparisons on the quantity of concrete required for the foundations of guyed masts and self supporting towers. To draw comparisons on the costs of fabrication of guyed masts and self supporting towers.

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1.5 RESEARCH HYPOTHESIS Guyed masts have a higher load carrying capacity but structurally self supporting towers have a longer life. 1.6 LIMITATIONS OF THE STUDY Factors affecting decisions on choice of telecommunication host structures are many and variable and therefore some of them like type of design and construction material will be assumed to be constant. The cost of steel is very high and therefore some tests may not be done physically and will be simulated.

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CHAPTER 2 2.0 LITERATURE REVIEW Towers and masts are designed to support antenna for radio links, television broadcasts, radar and associated electronic equipment. These towers with three or four legs may be designed as either self supporting or as guyed structures particularly when they are very high and a large area is available to anchor the guys. These structures are designed for use with specific antenna systems which often greatly affect the shape of the towers themselves, to produce an installation which enables the best operating conditions of the equipment to be achieved. The increase in radiating power and transmission band has led to a greater sophistication of antenna structures and an increase of their heights. The towers must support not only the antennas on their external surface but also accommodate all the accessories, cables, access and facilities, which are often bulky. Self supporting towers have also been used sometimes to provide in their upper part large shelters to house and protect electronic equipment. In the last decade telecommunications have expanded rapidly so that many existing structures have become obsolete and unsuitable for upgrading to meet new demands. 2.1 Self supporting towers This refers to towers which stand on their own i.e. without added support. Self supporting towers are normally used to carry the directive array antenna of radio links and the array antennas and dipole antennas for FM-TV where the area available is restricted for example in towns or on mountainous locations. Theses towers can be as high as 200m, but beyond such heights costs become prohibitive. Such towers are normally of square or triangular cross section and behave as vertical cantilevers. Their vertical profile may be of constant section or resemble a truncated pyramid, with legs splayed to achieve a base width which is approximately 10% of the height. This width may be increased to provide greater rigidity to the structure or to decrease stresses transmitted to the foundations. The structure is normally of latticed construction and is supplied in knocked down form to be assembled on site by bolting. In design the lattice elements are considered to be hinged at their ends, despite the partial fixity afforded by the bolted joints. In designing the tower, external loading is considered to be applied to the nodes so that all of the elements are primarily subjected to axial loading. Nominally 6.8-8.9 bolts or their equivalent are used and each bolt is used with a plane washer to prevent unscrewing. Special washers may be specified to ensure satisfactory performance on zinc coated members.

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Figure1. Self supporting three leg tubular towers

Each face of a tower usually has the same configuration, comprising diagonal and bracing members. The sections used differ and are selected not only to resist the internal forces to which they are subjected but also to minimize the surface area exposed to the wind. In the wider lower sections of the tower the member lengths of the legs and the diagonals may be high and it may be necessary to use stiffening elements to reduce the effective lengths of primary elements. In addition, cross bracing must be provided in the horizontal planes to stiffen the structure and distribute eccentric and torsion loads.

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This cross bracing section may be an expensive and important feature of the lower sections of the high towers because it is often used to support working platforms. Towers are usually connected to their foundations by pre set anchor bolts. In the case of heavily loaded towers anchorage can be obtained by inserting the legs themselves during the pouring of the concrete foundations. Towers sometimes need to be a special shape to accommodate the installation of the antennas, in certain cases the requirements are for long sections of constant width in the upper part with the result that member sizes are chosen not to resist the imposed stresses but rather to provide the required rigidity. TV- FM towers with antennas composed mainly of dipole panels, normally have square cross sections with open members. The upper part of the sections has lengths varying from 15- 25m. The widths vary from 0.6 to 2.5m to conform to panel dimensions and their respective locations. The antenna characteristics and their radiation demand a high structural rigidity to limit the deflections caused by wind loading. Some specifications give a limit of 10 0 at the bottom of the uppermost antenna subjected to a wind speed of 90km/h which is considered to be the upper limit. Similar constraints of vertical and horizontal flexibility (sway and twist) are specified by microwave radio link towers. The horizontal loading generated by the wind action on the antennas is normally higher than that supplied by the same wind to the remainder of the structure.

2.2 Guyed Masts

Guyed radio masts have a wide application as they are the most economic and technically efficient, in terms of fabrication and erection of the various types of antenna support structures. Such masts are composed of a vertical shaft which is normally latticed and held at several levels by guys connected to anchorages in the surrounding area. The shaft may have either a square or rectangular cross section. A triangular section is the most frequently used and because of the clear economic advantages of the smaller number of guys (particularly if they are insulated) and the simple internal bracing. The legs of the tower may be made of tube angles or in severe environmental conditions of solid rods. The legs of the triangular sections shaft can also be made of hot rolled sections with the flanges partially bent to get an angle of 600 or of cold rolled profiles. The diagonals of majority of the designs use open sections but round sections (tubes, bars) are especially suitable for reducing the wind loading on the structure. The elements are usually bolted as in self supporting towers and supplied in fully knocked down form. Welded sections are used for small and light masts where transportation cost is not important. A section length can range from 3m on small masts to 6 -8m in taller ones. Most widths range from

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0.5 to 3m depending on the height and the number of guy levels. An important part of the design consists of optimizing the number of guys and the width of the mast.

Figure2: 3 Leg Tubular Guyed mast

The minimization of the number of guys particularly in the insulated ones leads to shafts with spans up to 10m. Due account must be taken of the guy dimensions and the cost of the adjusting and fixing details. The mast may be considered in design as a vertical beam on elastic supports which are provided by the guys. The guys are fixed at one end to frames at the required heights on the tower and at the other end to anchor blocks in the ground at computed distances from the tower base. The whole structure is therefore highly deformable and must resist the external loads mainly due to

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wind. The vertical loading is the structure weight, the weight of the antennas and accessories and the vertical components of the tension forces in the guys. Moments and shears in the shaft are generated by the wind directly loading the structure, the guys, the antennas and the accessories and by the secondary moments and support displacements. Although the guys have the function of providing transverse support for the mast, they are also used to vertically align the mast through the use of adjusting devices or turnbuckles. These also allow adjustment to be made to the guys’ pretension in the no wind condition. As the size and the weight of the guyed mast increases, the guys can reach a size which causes difficulties and problems with the fixing devices. With the advent of television broadcasting, the need has arisen for masts of greater height. Guyed masts of 400m or more are now being erected and heights in the future could even be greater. The guying of such tall masts presents complex problems and bridge strand ropes of large size are now used for this purpose. The higher strength of these strand ropes permits a reduction to be made of the diameter of the guys. This decrease in diameter reduces wind and ice loads which may be important in the overall design. The higher modulus of elasticity of bridge strand ropes also gives certain other advantages. Uniformity in tensioning and deflection is necessary for satisfactory operation of the guys. It is therefore important that the stretch of the guys be minimized: they must also present a high modulus of elasticity and approach a condition of true elasticity. This is achieved by prestressing which removes the inherent constructional looseness of the guy under sustained loading. Proof loading of the completed guy assembly tests of end attachments and permits accurate length measurements under load to be made. It is obvious that prestressing and proof loading are of use only on high guyed towers. The torsional rigidity of the shaft is secured through torque stabilizers (beams protruding from the shaft) normally located near the level of the antennas which require a limited torsional deviation; the number of guys at the stabilizer level must be doubled. Steel lattice towers are primarily produced of tubular or angular profiles. Traditionally lattice towers have been produced of angular profiles, circular tubes or solid round bars. In the very beginning, more than 100 years ago, the first steel lattice towers for telecommunication were produced of flat- sided profiles like the angular profiles since it was easy to produce and easy to assemble. However some 50 years ago the first lattice towers were produced of tubular profiles and solid round bars in order to reduce on wind load and save on material. Nowadays towers are in most cases produced of tubular profiles in the northern part of Europe. In the UK and America however the majority of the towers are produced of angular profiles. The choice of structure is controlled by the options according to the national codes, manufacturing process but also traditions and innovations within the design. These towers have varying advantages in terms of

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wind resistance, buckling capacity, ease of fabrication and overall erection cost. This calls for an intelligent choice, design and innovative solutions by the tower designer. Towers for telecommunication are designed to withstand the wind load on antennas, microwave dishes and grids, cables, ladders, safety brackets and rest platforms and on the structure itself. In some regions the towers are furthermore designed to withstand ice load and the combination of wind and ice load. Since the towers carry all antennas and microwaves links the stiffness criteria is set up for the towers in order to be able to use the network under sever weather conditions. The stiffness criteria are often the design driver of the towers, especially when they carry parabolas and the height of the tower is more than 40m Towers can also be classified as Greenfield or Rooftop .Greenfield towers are normally used on the ground with foundations built below the ground level. They are mostly used in the rural areas where land is not an issue. Rooftop towers on the other hand are placed on roofs used in towns where space is limited. An analysis of the building structure should always be done to determine whether the building can safely carry the additional loading without compromising on the structural integrity of the building. 2.3 Guy cables A guy may be described as a tension only member which provides horizontal support to the mast column at discrete levels. The lower end of the guy assembly is anchored to the ground and normally incorporates a means of adjusting the tension in the guy. The most commonly used guy wires are EHS (Extra High Strength) steel guy wires, Phillystran and Pultruded fiber glass. An EHS steel wire is made of a set of galvanized twisted steel strands and is the most commonly used guy cable. 2.3.1 Phillystran A non conducting guy wire material made out of Kevlar (aramid fiber) fiber core with a PVC jacket. Phillystran is both strong and light weighted. The purposes of the PVC jacket are: 1. To protect the cable from abrasion during installation. 2. To prevent moisture from wicking into the core. 3. Most importantly, to protect the core from UV (Ultra Violet) damage.

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2.3.2 Pultruded Fiber glass Pultruded fiber glass is much more elastic than steel. In order to have the same spring rate as steel guys and hence the same ability to stabilize a tower, the cross-sectional area of the fiberglass must be larger (4.83 times) that the steel. In the case of an EHS guy, the equivalent solid fiberglass rod diameter would be twice the EHS size you want to replace. The following are the various guy assemblies that are used:

Figure 3: Typical cable construction

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2.4 Preload in Guy cables It is recommended that the guy wires should have an initial tension (preload) of approximately 10- 15% of its ultimate breaking strength to stretch out the slack in them. The exact amount of preload depends on the type of guys used and how high the mast they are attached. When you add preload to a guy wire, you will straighten it out as it is not perfectly straight as it is. However, due to gravity, the wire will never be entirely straight and there will always be a concave curve even though it is pulled beyond the specified pre- load. If you add too large pre- load to your wire, you are reducing its ability to absorb additional load (from the tower moving) before it reaches its breaking strength. The larger the diameter of the guy, the higher spring rate which implies that it can better resist a change in length (i.e. movement of the tower) for the same loading force. However, a large diameter will make the guy wire heavier and hence; it requires more preload tension to pull out the slack. As a rule of thumb, if the guy is attached in the top of the tower (100%), the tension should be 8% of the tensile strength. For 80% of the tower’s height, 10% tension should be applied. If the anchor point is at 65% of the tower height, 15% tension can be applied as you loose a lot of wind load in this last type of installation. The breaking strength will improve the control of the flexibility and still not cut down on the cable strength. (Source: communication tower handout – Alberto Escudero pg 17) 2.5 Termination of Guys The guy wires need to be terminated in a way so that they can be securely attached to the guy anchors. For EHS dead ends are the most reliable and easiest way to terminate a guy. A concrete base is needed for each of the guy cables. An anchor is used to connect the guy cable to the concrete foundation. Anchors are very critical and incase one gets to be loose, the support of that particular guy cable is lost and consequently when wind blows, the whole mast can come down. An earth anchor is used in cases of small masts in combination with stable soils. In case of rocky soils, earth anchors can be drilled into rocks. It is essential that the guys can pivot as freely as possible at their attachments as any tendency to restrain the guys may result in fatigue damage. The guys will inevitably vibrate more or less due to wind on the mast. 2.6 Bracing types There are two types of bracing, primary and secondary bracing. Primary bracing are members other than legs carrying the shear force due to imposed loads on the structure. Secondary members are members used to reduce the effective length of the main legs and sometimes that of

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the bracing. They are normally considered unstressed and are only loaded due to deformation of the structure.

Five types of bracing and horizontal combinations are normally adopted in towers and are:

1. X-bracing without horizontals, call it xx-bracing (Fig a.) and is statically determinate for each panel. 2. XB –bracing. X bracing with horizontals is called XB –bracing (Fig b) and is statically indeterminate. The horizontals are the redundant members and carry only nominal forces. 3. K –bracing. K bracing (Fig c) is statically determinate and gives larger headroom. Therefore it is used next to the ground. 4. Y –bracing. Y bracing (Fig d) is statically determinate and provides better headroom space and could be used at ground or lower panels. 5. W –bracing. It is a kind of overlapping panels and statically determinate. It is suitable for small panels. 6. Arch –bracing. Arch bracing (Fig f) is the bracing in the lower most panels, K- bracing in the next panel and XB –bracing in the top panel In most of the transmission line towers, the lower most (or the ground) panel is usually K – braced and the remaining either XX or XB or W –braced. As the heights of the microwave or TV or radio towers are high, a combination of the braces is used. The lower panels are either arch or Y or K and the upper panels are usually X –braced and few panels with XB bracing.

a) XX- bracing

b) XB- bracing

c) K -bracing

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d)Y- bracing

e) W- bracing

f) Arch- bracing

Figure 4: Bracing Types

2.7 Tower Wind Loading Apart from the wind load on the antennas, dishes, cables and other ancillaries, the lattice structure itself contributes significantly to the wind resistance of the tower. The wind resistance of circular profiles used in both designs was analyzed and taken into account in the wind analysis section of this study. For circular profiles the wind resistance is furthermore dependent on the wind speed if the flow is supercritical or sub critical. Rooftop towers are designed similarly except that instead of a foundation a support beam grillage is used. A structural evaluation of the whole building in this case is necessary to determine whether the building can carry the additional loading. 2.8 Tower design criteria Specifications and standards give the minimum criteria for the design and construction of towers and antenna support structures. They are generally specifications for general steel structures but in some cases they are especially formulated for towers and masts. They include guidance such as theoretical or empirical procedures to predict the structural response to the environmental actions as well as the quantitative information concerning the physical parameters to be adopted in

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design. The bases of the proposed design approach must allow for a clear manner the reliability of the tower with reference to various requirements data to be assessed and must also allow the designer to incorporate the results given experiences and other relevant new knowledge. Standards and specifications may therefore be regarded as guides to the development of design criteria and a mixture of philosophy and technology. A limit state design method is based on the use of characteristic values of load, resistance and partial safety factors to be used in the detailed design. Such standards employ wind loading criteria based on a yearly probability, and are not intended to cover all the possible conditions on a single site. As the rule the survival resistance to exceptional conditions like hurricanes is not checked, more often the design requirement is the survival condition in very strong winds. In the presence of such severe loading, the structural design construction details and the materials will be selected in such a way that the structure will not be damaged in a measure disproportional to the cause. As a rule all the design specifications based on international standards need detailed references to the standards have been specially used for design of towers and include both the loading conditions and the rigidity criteria suitable for system operation; these criteria depend upon the characteristics of the used antennas and sometimes are fundamental in the construction due to special shapes imposed on the structure. 2.9 Loading conditions The main loads on a tower are; 1. The self weight of the structure and all attached devices 2. Wind loading 3. Live loading (weight imposed by the workers during construction and placing of equipment. 4. Wind and ice loading in temperate climates. The loads generated by the weights of the structure and accessories are clear and easily computed. The loads caused by wind depend on the tower shape its orientation if not circular or latticed the ratio between the effective and total area of a side face and the shape of the cross section of elements of the tower. The designer must use the best available information on wind speed and if necessary perform tests to obtain the data. The first step in the computation of wind loading is the assumption of a basic wind speed referred to the specific site of the tower. The required value of the design load can be calculated from wind frequency probability maps. The probability data must be considered very carefully particularly if the records were obtained over a short period, or if there is a

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possibility of long cyclic changes. In modern regulations the modifications are based on knowledge of high wind speeds, computed from static analysis of wind records in the official meteorological stations of each country. The recording is performed in open sites, where the degree of exposure is uniform. The wind speeds given in standard regulations refer to normal sites. Direct records of the true wind speed must be obtained for particular sites like promontories, mountains, gorges or where records and experience suggest that speeds indicated in the standards are not adequate. When towers are located in sites where their collapse would involve grave risks to human life or major economic damage, the storm frequency design must be higher than one in 100 years. The degree of safety against wind generated damage can therefore be based on the probable frequency of design wind speed, which may be identified by the following characteristics.

1. Basic wind speed 2. Gust factor 3.

Variation of speed and gust factor versus height.

The most significant factor which affects the resistance and hence all the wind loads is the wind speed itself. The wind speed increases with height above ground; considerable data exist on the influence of height and a number of empirical formulas have been proposed and are commonly used. Another characteristic factor affecting the wind speed is the gust wind speed. The gust factor is defined as the ratio between the gust speed and the maximum wind speed within a certain period. The basic wind speed is defined as the average speed at 10m height in 1 minute, and the design speed as the basic wind speed times the gust factor. The design frequency must not be higher than once in every 100 years and is obtained from wind records over as long as a period as possible with a minimum of 30- 40 years. The design wind loading consists of all of the horizontal forces applied by the wind to the structure. When the elements are connected to form a lattice structure the overall force applied by the wind to the elements acting like single pieces. The computation of the shape factor for the whole structure is theoretically very difficult, however it is possible to compute a simple formula if shape factors are determined as solidity ratios (exposed surface divided by the complete structure surface). When two trusses or elements are located one behind the other, the wind load on the truss or element leeward is reduced by the screening effect of the windward structure. The total load applied to the structure must include all the loads applied to pipes, cables, waveguides, ladders etc. located on either the faces, or internally. The screening effect for such objects can

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often be neglected and this component of wind loading is evaluated from their surfaces and shape factors. The above considerations apply to strong winds. In the case calm periods of regular or quiet winds they are rapid and succession of gusts and calm periods. This type of motion is characterized by vortices with diameters ranging from fractions of centimeters to some meters. When computing the hourly average, some of the irregularities of the original motion disappear, and the average flow is quite regular. It is possible to consider the total motion as an average flow with superimposed secondary movements originated by vortices; therefore oscillations due to vortices can be present even in the case of regular winds. Tall and slim structures can have, under these effects at certain speeds resonance induced which may become critical (self exciting phenomena). It is also possible that with a dynamic interaction between the structure and the forces generated by the structure accelerations and the irregular and repeated reactions of air masses on the structure, that the oscillations can become bigger as can the deformations. Their effects must be considered in the design of specific tower layouts. The ice and snow accumulation on the structure has two effects; 1. Increase weight. 2. Increase of surface exposed to the wind. Both the effects are important for towers with low section members. The first effect is easily computed: the increase in area is partially compensated for by the reduction in shape factor, generated by an increase of the solidity factor. It is also advisable to evaluate carefully, before stating the design conditions, the wind speed in the presence of ice because it is often less than the maximum speed envisaged. The solid ice located on the components of the antenna supports, or on the antennas themselves, is normally considered for calculation purposes as a muff surrounding the structural members and antennas with thickness ranging from 10 to 15mm; the value in special cases can reach 50mm. 3.0 Tower Erection Both self supporting towers and guyed masts are erected by similar methods, depending on the heights and weights involved. The structures are supplied in knocked down form, to be bolted on site, and therefore can be erected raising either single pieces, or sections of tower which have been preassembled on the ground. Guyed towers must be installed with special care, so as to maintain adequate stability during the interruption of installation. The lower end of the shaft will rest on the base hinge. Or on a group of auxiliary jacks which avoid the pre- positioned base insulator. The shaft is supported vertically by temporary guys. The crane can then be used, as in self supporting towers to erect the bottom

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sections of the guyed tower. The upper part of the tower is erected using a Jin pole, installed on the external side of a face, and complete sections are hoisted into position. When the guyed tower is very heavy, the Jin pole can be installed inside the tower, and the hoisted pieces may consist of faces or legs with connected diagonals. During the erection provisional guys may be installed at intermediate positions of the shaft, up to the level of the permanent guys. When the erection is completed, and the provisional guys and the base jacks are removed, the mast rests on its ball joint support. The verticality can then be adjusted by operating on the guys. The correct guy pretension must then be applied as called for by the design. Plumb: For guyed masts the maximum deviation from the true vertical shall be 1 part in every 400. For self supporting towers the maximum deviation from the true vertical shall be 1 part every 250. Linearity: for guyed masts the maximum deviation from a straight line between any two points shall not exceed 1 part in every 1000. 3.1 Connections The design of connections can be approached from a number of directions: the type of structure, the type of fastener, the type of loading and the designer’s special interest. The dominant concerns in the design of connections in buildings, bridges, and towers, such as offshore drilling platforms differ. Bolts, welds and devices such as cable sockets transmit forces in different ways. Static loads, dynamic loads, and the expected number of repetitions of either pose different problems. Structural engineers and fabricators have shared interests and responsibilities, but the focus on the former may be on obtaining a desired type of behavior and that of the latter on ensuring practicable fabrication and erection. 3.2 Basic criteria in the design of connections Fundamental to the design of any connection are interrelated criteria of strength, stiffness, ductility, predictability, practicability and cost. Strength Ultimate strength, either fracture or maximum inelastic resistance, is the most meaningful strength criterion for connections. Initial yielding is an inadequate measure. The common practice of pre- tensioning a high strength bolt causes some inelastic behavior. Stress concentration around holes, at the ends of welds, and at bends in connection components often results in local yielding under service loads. So, too, do residual stresses. Elongation or flexure caused by differential

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rotation between the members joined may also cause working load yielding of the joints. There is no harm in local yielding provided: 1. It does not cause loosening of the fasteners. 2. The resulting deformations are self- limiting. 3. It is not sufficiently progressive over the anticipated number of load applications to cause excessive deflection, instability, or fracture through alternating plasticity or fatigue. For most connections a reliable estimation of ultimate strength requires test evidence. At low loads stresses are not uniformly distributed among bolts of a group or along a weld, even in concentrically loaded connections. At higher loads there may be a change toward uniformity as inelastic redistribution takes place. The component plates or shapes of a connection are pulled, compressed, or flexed in complex, multi- directional ways. There may be considerable beneficial strain hardening at higher loads, the neglect of which would result in unrealistic estimates of resistance. Theory and analysis, particularly advanced methods of finite element analysis, provide the basis for understanding and interpreting observed behavior, and analytical procedures are essential for routine design. But it must be recognized that most such procedures are still largely empirical. Since the above is true with respect to ultimate strength, it follows that checks for the serviceability of connections must also be empirical or judgmental. Related to the estimation of strength is the complex question of a proper safety factor. Ideally, a safety factor should have a sound probabilistic basis. But many more data than presently exist are required before truly probabilistic design of most connections can become a reality.

Stiffness Connections are major determinants of both the stiffness and the strength of structural systems. It is obvious that a structure with pinned connections will deflect more than one consisting of the same members but rigid connections. The connection stiffness also affects the distribution of forces in the structure and thus the relative effectiveness of its members.

Ductility Connection ductility is necessary and desirable for several reasons. It is good practice to minimize stress concentration but it is impractical to eliminate it completely. Even in well proportioned connections there may be local stress peaks. Fasteners and joint components must be able to yield locally, or else premature fracture may result. It is also generally impractical to design connections to remain elastic in strong earthquakes. They must be ductile enough to

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undergo large plastic deformations without fracturing or becoming unstable, and it is desirable that they contribute to absorbing the energy of a major shock. In buildings designed on the basis of inelastic analysis, many sections of the frame must have large inelastic rotation capacity. Although it is not essential that connections participate in plastic hinge rotation, they are often in advantageous positions for doing so. When properly used, structural steel and its fastenings provide an extremely tough, ductile structural system. But without proper precautions, steel can be alarmingly brittle.

Predictability The response of structures and structural components that remain elastic under all loading conditions is, in theory, predictable. The formulation and solution of the proper elastic equations may be difficult in complicated elastic systems, but once accomplished; the results can be expected to be quite reliable. Properly proportioned rigid connections will normally approach the ideal of elastic behavior under service loads and loads substantially greater than the service loads. But this is not the case for semi- rigid connections. Most require some local, self – limiting inelastic deformation. Much of this may take place during erection or under the first few applications of the service loads, after which they can be expected to exhibit imperceptible inelastic response under normal fluctuations of gravity and lateral loads. But all classes of connections can be expected to become inelastic at higher loads, and this has a substantial influence on the calculation of the ultimate resistance of the system. Further, under strong earthquake loading, connections may be forced to deform inelastic through several cycles of fluctuating load. For these reasons it is important to have the means of predicting the monotonic and repeated load force-deformation characteristics of connections, particularly beam- to – column connections in building structures and any kind of connection for bridge structures, where fatigue and fracture also play a role. These characteristics must include the nonlinear effects of the inelastic deformation of the connection material itself, such things as bolt deformation and slip, and the deformation of the connected members within the joined region (the so- called ‘panel zone’ effect for example) It is expecting too much to believe that design engineers will attempt to calculate a response history for all semi rigid connections. For many structures of modest size and conventional construction that are amply redundant, it is not necessary to do so. But for structures in which knowledge of the true behavior of the system is essential to ensuring adequate performance under

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service loads and a reasonable, predictable margin of resistance against failure, such calculations are needed

Practicability and cost The cost of connections is a major item in the cost of a steel structure. Except for rare cases that justify special plant and equipment, they have to be made and assembled by conventional fabrication and erection techniques. A connection should therefore be as uncomplicated as possible to fabricate it, and still satisfy the relevant performance criteria. Rigid connections are normally more complex and costly than flexible ones for example. When they are used, it should be for sound reasons of overall system functionality or economy. The proper connection types should be determined in the early stages of system design. It can be costly or practically impossible to modify a design based on flawed connection concepts once it has reached the production stage. Related to this is the fact that even though the structural engineer may have used elaborate analysis and proportioning methods in designing major connections, the realities of practice are that changes may have to be made in the shop or in the field at short notice. Therefore, the essential performance criteria for any connection could be explainable in simple physical terms and understood by all parties having responsibility for the completed structure.

a) Tower leg bolted to base

b) Tubular tower leg connection

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c) Flange plate connection

Figure 5: Examples of connections

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CHAPTER 3 3.0 RESEARCH METHODOLOGY This study was conducted in two phases. The first phase entailed structural analysis of a guyed mast and a self supporting lattice tower on Staad- Pro analysis software, assessment of the design results which included: Tower nodal displacements, support reactions, wind loading on the individual tower elements, structure overall weight and the utilization ratios of all the structural elements. Secondly the performance of the structures structurally and economically followed. This phase of the study focused on the structures as a whole, the availability of their individual members in the market, tower fabrication and erection time. This research was done by use of questionnaires mailed to engineers working in various companies that specialize in the telecommunication infrastructure provision. The companies are namely: TKM Maestro limited. ROM Kenya limited. Alan Dick Company. Safaricom limited. The research on this two support structures was limited to use of tubular sections. This is because previous studies have shown that tubular sections perform better in the following ways: Deflection: The deflection of towers made of tubular sections from the vertical is lower than those of angular sections. This is attributed to the lesser surface area exposed to the wind loading thereby reducing the total loads of the structure. Buckling capacity: The elements that affect the buckling capacity of members are radius of inertia, buckling length, eccentricity and the buckling curve. The radius of inertia of a circular tube is larger than that of an angular profile. Therefore tubular sections have a higher buckling capacity and hence do not require heavy bracing. Bracing: Due to their lower buckling capacity, towers of angular sections require heavier bracing. The extra number of elements makes logistics more complicated and the erection time more consuming.

3.1 STRUCTURAL ANALYSIS In order to determine the effect of wind loading on the various types of steel sections two StaadPro analysis models were modeled and subjected to equal wind loading of 36m/s wind speed and

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location is a small town in Rift Valley (BS 6399: 1997 Part2: code of practice for wind loads ). The two models are namely: 24 meter Three leg Tubular Guyed Mast. 24 meter Three leg Tubular self supporting lattice Tower. The structural behaviors of these towers were then assessed from the Staad- Pro analysis output. Aspects looked at include: Deflection at tower top, nodal displacements individual member loads and Failure ratios. Rigidity is one of the requirements in tower design for telecommunication use. This is because wave transmission is from tower to tower and deflection at the tower top is undesirable as this would interfere with the signal. (Refer to the Staad- Pro analysis output attached).

Figure 6: Staad Pro structural simulation model for a 3 leg tubular guyed mast

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Figure 7: Staad Pro structural simulation model for a 3 leg tubular self supporting tower

3.1.2 An overview of the Staad- Pro Analysis Software and modeling STAAD- Pro is a general purpose program for performing the analysis and design of a wide variety of types of structures. The basic three activities which were carried out to achieve that goal i.e. a) Model generation b) The calculations to obtain the analytical results c) Result verification are all facilitated by tools contained in the program’s graphical environment. The Staad manual contains four sample tutorials which guide the user through those 3 activities. Staad- Pro is an integrated 3D analysis, design and draughting package. The system is used by over 3500 companies and is supported worldwide by offices in the USA, UK, France, Germany,

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Norway and India. Staad- Pro is applicable to almost every type of building structure, from simple portal frames and multi storey buildings to more complex structures such as bridges, offshore structures. Staad- Pro includes 2D, 3D elastic, P- Delta and dynamic analysis. There are extensive modeling features and structures can be analyzed using beams and finite elements. The user can choose from the graphical input or generator which includes a library of structure, edit a simple ASCII file or use AUTOCAD to create the structure geometry then import it into Staad- Pro. Staad- Pro can automatically optimize the weight of the structure, group members together and give a steel take off for estimating purposes. The design can be carried out using any type of section, including angles, channels, plate girders etc. The program includes 10 standard steel section tables including British and European sections and the user can also define personalized sections for analysis and design. The design codes implemented in Staad- Pro include British steel BS5950 the latest issue of Euro code 3, the French, German, Norwegian, Swedish, Australian, Canadian, and American steel and concrete codes. Other European design codes are in development. The package is multilingual. The user can change between French, English and German. Other languages are under development. As well as general building and structural applications StaadPro has found extensive use in the offshore industry and in the bridge design. Staad- Pro offers Bridge design to BS5400 for steel and composite bridges. The offshore packages include modules for wave loading fatigue and transportation analysis. Design options include the American code, API punching shear checks and the Norwegian NPD or NS3472 steel codes. Staad- Pro 2004 in which this project is done is the next generation of the STAAD product line, the most powerful structural engineering software in the world. With over 150,000 installations, 15,000 clients, design codes for 30 countries and NRC/NUPIC certification, Staad- Pro analysis software has a feature that allows the user to check the degree to which a given member in the structure is put to use. This degree is given in a range between 0 and 1. A value of 1 shows that the member is put to full use i.e. 100% utilization, values greater than this show that the member has been over utilized and will therefore fail. Therefore it is safe to keep the values between 0.5 and 0.9. At this range members will be utilized by between 50% and 90%. 3.1.3 Outline of the procedure for calculating the wind loads on Lattice Towers and Guyed Masts Lattice towers and Guyed Masts of square and equilateral sections constitute special cases for which it may be convenient to use overall force coefficient in the calculation of wind load. The

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wind load should, for convenience be calculated for the condition when the wind blows against one face. The wind loaf F acting in the direction of the wind should be taken as

Where: Ae effective area of the face q Is the dynamic pressure of the wind and, Cf is the overall force coefficient For square lattice towers the maximum load occurs when the wind blows onto a corner. It may be taken as the wind load for the face on wind. For triangular lattice towers the wind load may be assumed to be constant for any inclination of the wind to face. 3.1.4 Solidity Ratio calculation The solidity ratio ø is equal to the effective area of the frame normal to the wind direction divided by the area enclosed by the boundary of the frame normal to the wind direction. When single frames are composed of circular section members it is possible that the larger members will be in the supercritical flow regime (i.e. DVs > 6m2/s) and the smaller members will not (i.e. DVs