Guided by;D Sreehari Rao,

Assistant Professor, Dept. of Civil Engg.

ANALYSIS AND DESIGN OF TELECOMMUNICATION

TOWER

By;G Sireesha ( 11202012),

P Hari Krishna (11202024),S Sri Hari (11202030).

INTRODUCTION

In the present era the technology in communications has developed to a very large extent. The communication industries have seen a tremendous increase in last few years which have resulted in installation of large number of towers to increase the coverage area and network consistency. In wireless communication network these towers play a significant role hence failure of such structure in a disaster is a major concern. Therefore utmost importance should be given in considering all possible extreme conditions for designing these towers. In most of the studies, the researches have considered the effect of wind only on the four legged self-supporting towers. In this dissertation, a four legged lattice tower is analyzed and designed along with foundation details.

BASED ON STRUCTURAL ACTION

SELF SUPPORTING TOWERS

The towers that are supported on ground or on buildings are

called as self-supporting towers. Though the weight of

these towers is more they require less base area and are suitable in many situations. Most of the TV, MW, Power transmission, and flood light towers are self-supporting

towers.

GUYED TOWERSGuyed towers are normally

guyed in three directions. 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 other

restrictions to mount dish antennae on these towers and require large anchor blocks to

hold the ropes.

MONOPOLE

It is single self-supporting pole, and is generally placed

over roofs of high raised buildings, when number of antennae required is less or

height of tower required is less than 9m.

TYPES OF COMMUNICATION TOWERS

Self Supporting Tower Guyed Tower Monopole Tower

Based on the type of material sections :Based on the sections used for fabrication, towers are classified into angular and hybrid

towers (with tubular and angle bracings). Lattice towers are usually made of bolted angles. 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.

Based on cross section of tower:Towers can be classified, based on their cross section, into square, rectangular, triangular,

delta, hexagonal and polygonal towers. Triangular Lattice Towers have less weight but offer less stiffness in torsion. With the increase in number of faces, it is observed that weight of tower increases. The increase is 10% and 20% for square and hexagonal cross sections respectively.

Based on the number of segments:The towers are classified based on the number of segments as Three slope tower; Two

slope tower; Single slope tower; Straight tower.

OBJECTIVES The objective of this project is to design a Telecommunication tower, along with foundation details, and to analyze it, below mentioned basic parameters are considered :• Base width.• Height of tower.• Soil Bearing Capacity.• Configuration of Tower.

Following research has to be carried out for meeting the above objectives:• Soil exploration studies.• Terminology of communication tower and its components.• Different behaviors of towers.• Methodology for analysis and design of communication towers.

To meet these objectives the following work has to be done:

• Towers are configured with keeping in mind all field and structural constraints on AutoCAD 2015.• Loading format including reliability, security and safety pattern are to be evaluated. • Wind loading is calculated on the longitudinal face of the towers.• Now all the towers are modelled and analyzed as a three dimensional structure

using STADD.Pro V8i and STAAD(X) Tower V8i.• Finally tower members are designed as an angle sections.

DESIGN OF COMMUNICATION TOWERS

The following are the steps involved in design of communication tower:

• Selection of configuration of tower.• Computation of loads acting on tower.• Analysis of tower for appropriate loading conditions.• Design of tower members according to codes of practices.• Design of foundation according to codes of practices.

CONFIGURATION

A communication tower, like any other exposed structure, has a super structure shaped, dimensioned and designed to suit the external loads and self-weight

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 following are key parameters in configuration of tower.

• Width at bottom level = 4.00 m• Width at top level = 1.20 m• Overall Height = 30.00 m• No. of levels = 09 levels • Slope of outline of tower = 87º 8’ 15.34” (with horizontal)

ELEVATION OF LEVELS

LEVELS HEIGHT (M) BASE WIDTH (M) BRACING PATTERN OF FACE

0 0 4 K2 Brace Down1 6 3.3 Double K1 Brace Down2 10.5 2.775 Double K1 Brace Down3 14 2.367 Double K1 Brace Down4 17 2.017 XX Bracing5 19 1.783 XX Bracing6 21 1.55 XX Bracing7 22.5 1.375 XX Bracing8 24 1.2 XX Bracing9 30 1.2 XX Bracing

LOAD CONSIDERATIONS

In case of communication towers self-weight of tower is most important component of tower design. The tele communication steel tower is a pin-jointed light structure, It is still assumed that their behavior is similar to simple truss.

The percentage of openings in Tower structure will be more than 30%, so wind loads acting on the tower will be of less magnitude compared to chimneys, but the major cause of failures of telecommunication tower throughout the world though still remains to be high intensity winds (HIW). The major problem faced is the difficulty in estimating wind loads as they are based on a probabilistic approach. There has been several studies in telecommunication towers taking into consideration the wind as well as dynamic effect.

The loadings which are considered during this project are:1. Dead loads or Vertical loads

( i.e. self weight of tower members, Self weight of antennas, labour and equipment during construction and maintenance.)

2. Transverse loads (Wind load on exposed members of the tower and antenna.)

Wind load on tower:The wind load on tower can be calculated using the Indian standards IS: 875(Part 3)-1987[3] and BS: 8100 (Part 1)-1996[4].

Wind load on antennae:Wind load on antennae shall be considered from Andrew’s catalogue. In the Andrew’s catalogue the wind loads on antennas are given for 200kmph wind speed. The designer has to calculate the antenna loads corresponding to designwind speed.

DESIGN WIND PRESSURE•At Tirupati region, the design wind speed = 39 m/s

•Design speed at the site = Vz = K1K2K3Vb

Risk co-efficient = K1 for 100 years life = 1.08 K2 , terrain factor for 30 m and class B of Terrain category 3 = 1.03 Topography factor (K3) = 1+ Cs For the given plain topography K3 = 1 ( As C=0 )

• Vz = 1.08*1.03*1*39 = 55.62 m/s

• Design wind pressure = Pz = 0.6 Vz2 = 1.86 KN/m2

Similarly the design wind pressures for different levels are calculated and tabulated as follows:

Height (m) Design wind pressure (KN/m2 )

0 0

10 1.36

12 1.44

15 1.55

18 1.63

21 1.70

24 1.75

27 1.81

30 1.86

MODELLING AND ANALYSIS

The lattice tower model was analyzed in ANSYS as well as in STAAD. Pro V8i and STAAD(X) Tower V8i software package. The model was created using the coordinate data for the points and the element connectivity table and suitable cross sectional properties were assigned to the elements created. The boundary condition was stimulated in the model by fixing the three lowermost nodes of the modeled structure. The loads calculated above are applied at appropriate nodes and the stress parameters, deformation of the structure under the effect of the applied load is studied.

DESIGN OF MEMBERS

Suitable steel sections are initially assumed as members of the tower for analyzing the structure. Once the analysis is done members are finalized based on the stresses developing in them, following the codal provisions provided by Indian Standards.

• The maximum allowable stresses in the members are given in IS 802 (Part-1).• Limiting slenderness ratios for members are given in IS 802(Part-1).• Effective Length of compression members should be assumed as per IS

806(1968).

DESIGN OF FOUNDATION

DESIGN OF SLAB BASE

As per IS 800:2007,• Bearing strength of concrete = 0.6fck

• But for practical consideration bearing strength = 0.45fck

Area of plate required = • Where Pu = Factored load

• Load on each leg is = 400KN• Factored load on each leg = 600KN• Area of plate required = = 53333.33 mm2

Side of each base plate = 300 300 mm2

• Minimum thickness required (ts) = ()0.5

Where W = = = 6.66 N/mm2

a = 95 mm and b = 95 mm

ts = ()0.5

ts =25 mm

(As ts tf (truss angle thickness ts = 12mm), hence safe.)

Connect base plate to foundation concrete using 4 No’s 20mm diameter and 300mm long anchor bolts.

If weld is to be used for connecting column to base plate check the weld length of filler weeds.

DESIGN OF RAFT FOUNDATION

Initially assume footing size = 5m 5mUniform load on footing (W) = = = 32 KN/m2

Consider per meter width then load is = 32 KN/mMaximum bending moment at center of footing = 100 KNmBending moment required Mu = 0.138fckbd2

100106 = 0.138 25 1000 d2

d = 170.25 mm d = 200 mm.

Area of steel required:Mu = 0.87 fy Ast d (1-)100106 = 0.87415Ast200(1- )

Assume concrete grade = M20

Steel grade = Fe415

Ast required = 1596.36 mm2

Assume diameter of bars = 12 mmNo. of bars required = = 15 barsSpacing of bars = =250 mm

Design of concrete column for slab base:

Axial load on the column = 600KN.According to code axial load on column = 0.4fckAc + 0.67fyAst (As per IS 456:2000)600103 = 0.4fckAc + 0.67fyAst

600103 = 0.425Ac + 0.67fyAst

Assume 1 of steel of concrete area.600103 = 0.425Ac + 0.67Ac

Ac = 46946.6 mm2 = 216.67 216.68 mm2.

Ac = 220 220 mm2.

Hence provide 300 300 mm2 square column at 350mm from edge.Height of this column above the raft footing = 450 mm

Area of steel = 1% of column cross section = 0.01 300 300 = 900 mm2

Assume 20mm dia bars then No. of bars = = 3 bars

RESULT

A Telecommunication tower of 30m high is analyzed and designed.

1. The configuration of the tower is as follows:• Height of tower = 30m• Base width = 4m• Top width = 1.2m• Type of tower = Four legged lattice tower with two slopes.• Number of members = 564

2. Wind load is calculated using STADD.Pro V8i using IS: 875(Part 3)-1987[3].

The total wind load acting on the structure is 2719 Kg.

3. Design has been done according to IS: 802 using STADD.Pro and following results are obtained:• a. Total weight of steel required in superstructure = 9758 Kg.• b. Materials required in super structure: S. No Profile Length(m) Weight(Kg)

1. ISA 100x100x12 120.33 21302. ISA 80x80x10 170.63 20193. ISA 90x90x10 418.69 5609

Total = 9758

4. Raft foundation of 5m x 5m has been designed along with slab baseand column base to transfer the loads to raft. The details of foundation are:

a. Allowable Bearing Pressure = 250 KPab. Thickness of slab base = 25 mmc. Thickness of column base = 450 mmd. Thickness of Raft foundation = 22 mm

CONCLUSION

In the present era, technology is growing at a rapid phase which require adequate communication means like mobile phones, internet, radio communication etc. So there is need for proper communication systems including radio stations, Communication towers. If we could optimize the design of towers and use less resources, it will save a lot of money and resources. In olden days angle sections are used in making of truss in towers, currently tubular sections are preferred as they are more economical.

The wind load acting on the telecommunication towers will be comparatively less in magnitude as it is open structure with more openings, but failure of the towers is mainly due to High Intensity Winds and Earthquakes. So high factor of safety should be given to wind loads and seismic loads.