TL _ Vol. IV - Tower Erection

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FOR INTERNAL CIRCULATION ONLY

user’s manual

of

Construction

(part one)

Transmission Lines

Volume-4

Tower Erection

Construction Management

Power Grid Corporation of India Limited

(A Government of India Enterprise)

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DOCUMENT CODE NO. : CM/TL/TOWER ERECTION/96 JUNE, 1996

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FROM THE DESK

OF

DIRECTOR (PERSONNEL)

Four “M’s” viz. men, material, machine & money are vital to run an organization. However the key to success of the organization lies the way our employees structure and manage the construction, operation and maintenance activities of transmission system. Construction activitiy in transmission system is an important aspect and time, quality and cost are it’s critical parameters.

Experience, no doubt, is a great teacher and a valuable asset. However, the knowledge of underlined principles of sound working is also equally important. Preparation of these user’s manuals is the work of our experienced senior field staff and I find these to be very useful to our site personnel.

These manuals for transmission lines (Vol. 1 2 & 4) alongwith SFQP (Vol. 1) will be of immense help to our line staff to manage their resources in a more efficient and systematic way to achieve high quality and reduced time.

I find sincere efforts have gone into preparation of these manuals for which I congratulate Construction Management team and I am sure the authors will continue their efforts to bring out more and more such manuals.

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CHAPTER-I

TOWER CONFIGURATION

1.1 PURPOSE OF TRANSMISSION TOWER

1.2 FACTORS GOVERNING TOWER CONFIGURATION 1.3 TOWER HEIGHT

1.4 ROLE OF WIND PRESSURE

1.5 MAXIMUM & MI8NIMUM TEMPERATURE 1.6 LOADING OF TOWER

CHAPTER-2

TYPES OF TOWERS

2.1 CLASSIFICATION ACCORDING TO NUMBER OF CIRCUITS 2.2 CLASSIFICATION ACCORDING TO USE

2.3 400KV SINGLE CIRCUIT TOWERS 2.4 400KV DOUBLE CIRCUIT TOWERS 2.5 RIVER CROSSING TOWERS

2.6 RAILWAY CROSSING TOWERS 2.7 HIGH WAY CROSSING TOWERS 2.8 TRANSPOSITION TOWERS 2.9 MULTI CIRCUIT TOWERS 2.10 TOWER EXTENSIONS 2.11 LEG EXTENSIONS

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2.12 TRUNCATED TOWERS

2.13 WEIGHT OF DIFFERENT TYPES OF TOWERS

CHAPTER-3

TOWER FABRICATION

3.1 GENERAL

3.2 BOLTING

3.3 WASHERS

3.4 LAP AND BUTT JOINTS 3.5 GUSSET PLATES

3.6 BRACING TO LEG CONNECTIONS 3.7 CONNECTION TO REDUNDANT MEMBERS 3.8 CROSS-ARM CONNECTIONS

3.9 STEP-BOLTS AND LADDERS 3.10 ANTI-CLIMBING DEVICES 3.11 DANGER AND NUMBER PLATES 3.12 PHASE AND CIRCUIT PLATES 3.13 BIRD GUARD

3.14 AVIATION REQUIREMENT

3.15 PACKING, TRANSPORTATION AND STORAGE OF TOWER PARTS

CHAPTER-4

METHODS OF ERECTION

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4.1.1 BUILT UP METHOD 4.1.2 SECTION METHOD 4.1.3 GROUND ASSEMBLY 4.1.4 HELICOPTER METHOD 4.2 EARTHING 4.3 TRACK WELDING

4.4 PERMISSIBLE TOLERANCES IN TOWER ERECTION

ANNEXURE-E/1 - TOOLS & PLANTS REQUIRED FOR TOWER

ERECTION GANG

ANNEXURE-E/2 - MANPOWER REQUIREMENT FOR TOWER

ERECTION GANG

CHAPTER-5

GUIDE LINES FOR SUPERVISION

GL-1 PRE-ERECTION CHECKS

GL-2 CHECKS DURING TOWER ERECTION

GL-3 TIGHTENING AND PUNCHING

GL-4 FIXING OF TOWER ACCESSORIES

GL-5 EARTHING

GL-6 PRE-STRINGING TOWER CHECKS

CHAPTER-6

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6.1 INTRODUCTION

6.2 STANDARDISATION IN POWERGRID

CHAPTER-7

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Chapter-1

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___________________________________________________________________________ CHAPTER

ONE

_________________________________________________________

TOWER CONFIGURATION

1.1 Purpose of transmission tower

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The structures of overhead transmission lines, comprising essentially the supports and foundations, have the role of keeping the conductors at the necessary distance form one another and form earth, with the specified factor of safety to facilitate the flow of power through conductor form one point to another with reliability, security and safety.

1.2 Factors governing tower configuration

1.2.1 Depending upon the requirements of transmission system, various line configurations have to be considered ranging from single circuit horizontal to double circuit vertical structures with single or V-strings in all phase, as well as any combination of these.

1.2.2 The configuration of a transmission line tower depends on:

(a) The length of the insulator assembly.

(b) The minimum clearances to be maintained between conductors

and between conductor and tower.

(c) The location of ground wire or wires with respect to the

outermost conductor.

(d) The mid span clearance required from considerations of the

dynamic behavior of conductors and lightning protection of the line.

(e) The minimum clearance of the lower conductor above ground

level. 1.3 Tower height

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The factors governing the height of a tower are:

(a) Minimum permissible ground clearance (H1)

(b) Maximum sag (H2).

(c) Vertical spacing between conductors (H3).

(d) Vertical clearance between ground wire and top conductor (H4).

Thus the total height of the tower is given by H = H1 + H2 + H3 + H4

in the case of a double circuit tower with vertical configuration of conductors as shown in Fig. 1.1.

1.3.1 Minimum permissible ground clearance

From safety considerations, power conductors along the route of the transmission line should maintain clearances to ground in open country, national highway, rivers, railway tracks, tele-communication lines, other power lines etc. as laid down in the Indian Electricity Rule or standards or code of practice in vogue.

1.3.2 Maximum sag of Lowermost Conductor

The size and type of conductor, wind and climatic Conditions of the region and span length determine the conductor sag and tensions. Span length is fixed from economic considerations. The maximum sag for conductor span occurs at the maximum temperature and still wind conditions. This maximum value of sag is taken into consideration in fixing the overall height of the steel structures. In snow regions, the

maximum sag may occur even at 0O_{C with conductors loaded with ice in}

still wind conditions. While working out tension in arriving at the maximum sag, the following stipulations laid down, in I.E. Rules (1956) are to be satisfied.

(i) The minimum factor of safety for conductors shall be based on

their ultimate tensile strength.

(ii) The conductor tension at 32O_{C (90}O_{F) without external load shall}

not exceed the following percentages of the ultimate tensile strength of the conductor.

Initial unloaded tension . . 35

percent

Final Unloaded tension . . 25

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In accordance with this stipulation, the maximum working tension under stringent loading conditions shall not exceed 50 percent of the ultimate tensile strength or conductor. Sag-Tension computations made for final stringing of the conductors, therefore, must ensure that factor of safety of 2 and 4 are obtainable under maximum loading condition and every day loading condition, respectively.

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1.3.3 Spacing of conductors

The spacing of conductors is determined by considerations which are partly electrical and partly mechanical. The material and diameter of the conductors should also be considered when deciding the spacing, because a smaller conductor especially if made of aluminum, having a small weight in relation to the area presented to a cross wind, will swing synchronously (in phase) with the wind, but with long spans and small wires, there is always the possibility of the conductor swinging non-synchronously, and the size of the conductor and the maximum sag at the centre of span are factors which should be taken into account in determining distance apart at which they should be strung.

1.3.4 Vertical clearance between ground wire and top conductor.

This is governed by the angle of shielding i.e. the angle which the line joining the ground wire and the outermost conductor makes with the vertical, required for the interruption of direct lightning strokes at the ground and the minimum mid span clearance between the ground wire and the top power conductor. The shield angle varies from about 20 degrees 30 degrees, depending on the configuration of conductors and the number of ground wires (one or two) provided.

1.4 Role of wind pressure

The wind load constitutes an important and major component of the total loading on towers and so a basic understanding of the computation of wind pressures is useful.

In choosing the appropriate wind velocity for the purpose of determining the basic wind pressure, due consideration should be given to the degree of exposure appropriate to the location and also to the local meteorological data.

The country has been divided inot six wind zones of different wind speeds. The basic wind speeds for the six wind zones are:

Wind Zone Basic wind speed-m/s

1 33 2 39 3 44 4 47 5 50 6 55

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Fig. 1.2 shows basic wind speed map of India as applicable at 10m height above mean ground level for the six wind zones.

In case the line traverses on the border of different wind zones, the higher wind speed may be considered.

1.4.1 Variation of wind speed with height

At ground level, the wind intensity is lower and air flow is turbulent because of friction with the rough surfaces of the ground. After a certain height, the frictional influence of the ground becomes negligible and wind velocity increases with height.

1.4.2 Wind force on structure

The overall load exerted by wind pressure, on structures can be expressed by the resultant vector of all aerodynamic forces acting on the exposed surfaces. The direction of this resultant can be different from the direction of wind. The resultant force acting on the structure is divided into three components as shown in Figure 1.3.

These are :

(a) A horizontal component in the direction of wind called drag force

FD.

(b) A horizontal component normal to the direction of wind called

horizontal lift force FL H.

(c) A vertical component normal to the direction of wind called the

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1.5 Maximum & minimum temperature

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A knowledge of the maximum and the minimum temperature of the area traversed by transmission line is necessary for calculating sag and tensions of conductors and ground wires, thereby deciding the appropriate tower design. The maximum and minimum temperature normally vary for different localities under different diurnal and seasonal conditions.

The absolute maximum and minimum temperature which may be expected in different localities in the country are indicated in the map of India in Fig.1.4 and 1.5 respectively. The temperature indicated n these maps are the air temperatures in shade.

The absolute maximum temperature values are increased suitably to allow for the sun’s radiation, heating effect of current, etc. in the conductor. The tower may be designed to suit the conductor temperature of 75 degree C (max) for ACSR and 85 degree C (max) for aluminum alloy conductor. The maximum temperature of ground wore exposed to sun may be taken as 53 degree C.

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1.6 Loading of transmission line towers

1.6.1 As per revision o IS;802 regarding materials, loads and permissible stresses in transmission line owes, concept o reliability, security and safety have been introduced.

(a) Reliability

The Reliability that a transmission system performs a given task, under a set of conditions, during a specified time. Reliability is thus a measure of the success of a system in accomplishing task. The complement to reliability is the probability of failure or unreliability. In simple terms, the reliability may be defined as the probability that a given item will indeed survive a given service environment and loading for a prescribed period of item. (b)

Security:-The ability of a system to be protected from a major collapse such as cascading effect, if a failure is triggered in a given component. Security is a deterministic concept as opposed to reliability which is a probabilistic concept.

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Safety:-The ability of a system not to cause human injuries or loss of lives. It relates mainly to protection of workers during construction and maintenance operation. The safety of public and environment in general is covered by National regulations. 1.6.2 Nature of loads on Transmission Tower

Transmission lines are subjected to various loads during their life time. These are classified into three distinct categories, namely:

(a) Climatic

loads:-Which relates to reliability requirements.

(b) Failure containment

loads:-Which relates to security requirements.

(c) Construction & maintenance

loads:-Which relates to safety requirements. 1.6.3 Computation of various loads on towers

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The loads on of various loads on towers consist of three mutually perpendicular systems of loads acting vertical, normal to the direction of the line, and parallel to the direction of the line.

It has been found convenient in practice to standardise the method of listing and dealing with loads as under:

Transverse load Longitudinal load Vertical load Torsional shear Weight of structure

Each of the above loads is dealt with separately below:

(a) Transverse load due to wind on conductors and ground wire

The conductor and ground wire support point loads are made up of the following components:

(i) Wind on the bare (or ice-covered) conductor / ground

wire over the wind span and wind on insulator string.

(ii) Angular component of line tension due to an angle in the

line (Figure 1.7).

The wind span is the sum of the two half spans adjacent to the support under consideration. The governing direction of wind on conductors for an angle conditions is assumed to be parallel to the longitudinal axis of the cross-arms (Fig.1.8). Since the wind is blowing on reduced front, it could be argued that this reduced span should be used for the wind span. In practice, however, since the reduction in load would be relatively small, it is usual to employ the full span.

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(b) Transverse load due to line deviation

The load due to an angle of deviation in the line is computed by finding the resultant force produced by the conductor tensions (Fig. 1.7) in the two adjacent spans. It is clear from the figure that the total transverse load = 2T Sin Ø/2 where Ø is the angle of deviation and T is the conductor tension.

(c) Wind load on tower

In order to determine the wind load on tower, the tower is divided into different panels having a height ‘h’. These panels should normally be taken between the intersections of the legs and bracings.

1.6.3.2 Longitudinal load

(a) Longitudinal load acts on the tower in a direction parallel to the

line (Fig. 1.6B) and is caused by unequal conductor tensions acting on the tower. This unequal tension in the conductors may be due to deadending of the tower, broken conductors, unequal spans, etc. and its effect on the tower is to subject the tower to an overturning moment, torsion, or a combination of both. In the case of dead-end tower or a tower with tension strings with a

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broken wire, the full tension in the conductor will act as a longitudinal load, whereas in the case of a tower with suspensions strings, the tension in the conductor is reduced to a certain extent under broken-wire conditions as the string swings away from the broken span and this results in a reduced tension in the conductor and correspondingly a reduced longitudinal load on the tower.

(b) Torsional load:

The longitudinal pull caused by the broken wire condition imposes a torsional movement, T, on the tower which is equal to the product of unbalanced horizontal pull, P and its distance, from the centre of tower in addition to the direct pull being transferred as equivalent longitudinal shear, P as shown in Fig.1.9. The shear P and the torsional movement T = Pe gets transferred to tower members in the plane ABCD.

1.6.3.3 Vertical Load

Vertical load is applied to the ends of the cross-arms and on the found wire peak (Fig.1.6C) and consists of the following vertical downward components:

(i) Weight of bare or ice-covered conductor, as specified, over the

governing weight span.

(ii) Weight of insulators, hardware etc., covered with ice, if

applicable.

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1.6.3.4 Weight of structure

The weight of the structure like the wind on the structure, is an unknown quantity until the actual design is complete. However in the design of towers, an assumption has to be made regarding the dead weight of towers. The weight will no doubt depend on the bracing arrangement to be adopted, the strut formula used and the quality or qualities of steel used, whether the design is a composite one comprising both mild steel and high tensile steel or make use of mild steel only. However, as a rough approximation, it is possible to estimate the probable tower weight from knowledge of the positions of conductors and ground wire above ground level and the overturning moment.

Having arrived at an estimate of the total weight of the tower, the estimated tower weight is approximately distributed between the panels. Upon completion of the design and estimation of the tower weight, the assumed weight used in the load calculation should be reviewed Particular attention should be paid to the footing reactions, since an estimated weight which is too high will make the uplift footing reaction too low.

1.6.3.5 Various loads as mentioned above shall be computed for required reliability, security and safety.

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Chapter-2

Types of Towers

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CHAPTER

TWO

TYPES OF TOWERS

2.1 Classification according to number of circuits

The majority of high voltage double circuit transmission lines employ a vertical or nearly vertical configuration of conductors and single circuit transmission lines a triangular arrangement of conductor, single circuit lines, particularly at 400 KV and above, generally employ horizontal arrangement of conductors. The arrangement of conductor and ground wires in these configurations is given at Figure No. 2.1 to Figure No. 2.5.

The number of ground wires used on the line depends on the isoceraunic level (number of thunderstorm days/hours per year) of the area, importance of the line, and the angle of coverage desired.

Single circuit lines using horizontal

configuration generally employ two ground wires, due to the comparative width of the configuration; whereas lines using vertical and offset arrangements more often utilise one ground wire except on higher voltage lines of 400 KV and above, where it is usually found advantageous to string two ground wires, as the phase to phase

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spacing of conductors would require an excessively high positioning of ground wire to give adequate coverage. Details of different types of 400 KV single circuit and 400 KV double circuit towers are given at Clause No. 2.3 and 2.4.

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2.2. Classification according to use

Towers are classified according to their use independent of the number of conductors they support.

A tower has to withstand the loadings ranging from straight runs up to varying angles and dead ends. To simplify the designs and ensure an overall economy in first cost and maintenance, tower designs are generally confined to a few standard types as follows.

2.2.1 Tangent suspension tower

Suspension towers are used primarily on tangents but often are designed to withstand angles in the line up to two degrees or higher in addition to the wind, ice, and broken-conductor loads. If the transmission line traverses relatively flat, featureless terrain, 90 percent of the line may be composed of this type of tower. Thus the design of tangent tower provides the greatest opportunity for the structural engineer to minimise the total weight of steel required.

2.2.2 Angle towers

Angle towers, sometimes called semi-anchor towers, are used where the lines makes a horizontal angle

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greater than two degrees (Figure 2.6). As they must resist a transverse load from the components of the line tension induced by this angle, in addition to the usual wind, ice and broken conductor loads, they are necessarily heavier than suspension towers. Unless restricted by site conditions, or influenced by conductor tensions, angle towers should be located so that the axis of the cross-arms bisects the angle formed by the conductors. Theoretically, different line angles require

different towers, but for economy there is a limiting number of different towers which should be used. This number is a function of all the factors which make the total erected cost of a tower line. However, experience has shown that the following angle towers are generally suitable for most of the lines :

1. Light angle - 2 to 150_{line deviation}

2. Medium angle - 15 to 300_{line deviation}

3. Heavy angle - 30 to 600_{line deviation}

(and dead end)

While the angles of line deviation are for the normal span, the span may be increased up to an optimum limit by reducing the angle of line deviation and vice versa. IS:802 (Part I) - 1977 also recommends the above classification.

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The loadings on a tower in the case of a 60 degree angle condition and dead-end condition are almost the same. As the number of locations at which 60 degree angle towers and dead-end towers are required are comparatively few, it is economical to design the heavy angle towers both for the 60 degree angle condition and dead-end condition,

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whichever is more stringent for each individual structural member.

For each type of tower, the upper limit of the angle range is designed for the same basic span as the tangent tower, so that a decreased angle can be accommodated with an increased span or vice versa. It would be uneconomical to use 30 degree angle

towers in locations where angles higher than 2 degree and smaller than 30 degree are encountered. There are limitations to the use of 2 degree angle towers at higher angles with reduced spans and the use of 30 degree angle towers with smaller angles and increased spans. The introduction of a 15 degree tower would bring about sizable economics. Pilot suspension insulator string

- This shall be used if found necessary to restrict the jumper swings to design value at both middle and outer phases.

Unequal cross arms

- Another method to get over the difficulty of higher swing of Jumper is to have unequal cross arms.

2.3 400 kv single circuit towers

The bundled conductors are kept in horizontal configuration with a minimum clearance of 11 mtrs. phase to phase.

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The latticed parts are fully galvanised. Galvanised hexagonal round head bolts and nuts are used for fastening with necessary spring or plate washers.

Normally 4 types of single circuit towers are used as detailed below

a) "A" type towers :

These towers are used as tangent towers for straight run of the transmission line. These are called suspension or tangent towers. These towers can carry only vertical loads and are designed for carrying the weight of the conductor, insulators and other accessories. These towers are also designed for a deviation upto 2 degrees.

b)" B" type towers :

These towers can be used as sectionalising towers without angle and angle towers from 2 degrees up to 15 degrees deviation.

c) " C" type towers

These towers can be used for deviations ranging from 15 degrees up to 30 degrees. They are also being used as transposition towers without any angle.

d) "D" type towers :

These towers can be used as Dead End or anchor towers without any angle on the tower. Also these towers can be used for deviations ranging from 30

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degree - 60 degree.

These towers are usually provided as terminal towers near gantry with slack span on one side or as anchoring tower before major river crossing, power line crossing, railway crossings etc.

Fig. 2.8 shows two types of tower configuration for 400 KV single circuit towers.

A section of 400 kv single circuit towers is shown in Fig.2.9.

2.4 400 KV Double circuit towers

These towers are designed to carry two circuits consisting of 3 phases each, having bundled conductors. Here, the circuits are placed in a vertical configuration. A minimum phase to phase clearance of 8 mtrs. is maintained. A minimum clearance of 11 mtrs. is maintained from one circuit to another. Two earthwires are placed above each circuit in such a way to provide the required shielding angle.

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Like single circuit towers, these towers are also galvanised, lattice steel type structures designed to carry the tension and weight of the conductor alongwith the insulators, earthwire and its accessories.

Normally these towers are identified as P (D/C suspension towers), Q, R & S (D/C tension towers) or as DA, DB, DC and DD respectively.

As in the single circuit towers, DA/P towers are used as suspension towers from O degrees-2 degrees deviations. DB/Q,DC/R and DD/S towers are used as tension towers with angle of deviation from 2 degrees-15 degrees, 15 degrees-30 degrees and 30 degrees - 60 degrees respectively.

DB towers are also used as sectionalising towers without angle.

DC tower is also used as transposition tower without any angle.

The Double Circuit towers are used while crossing reserved forest, major river crossings, narrow corridors near switchyards etc. so as to make provision for future transmission lines since the approval from various authorities can be obtained at one time (for example, from forest, aviation authorities etc.) and to minimise expenditure in laying foundations in rivers.

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400 kv double circuit towers.

2.5 River-crossing tower

The height and weight of the towers vary considerably depending on the span, minimum clearance above water, ice and wind loads, number of `unbroken' conductors, etc. Usually the governing specification requires that towers employed for crossing of navigable water ways be designed for heavy loading conditions and utilise larger minimum size members than the remainder of the line. In addition to these structural requirements, it is often necessary to limit the height of tall crossing towers because of the hazard they present to aircraft.

Fig.2.10 shows a view of 400 kv double circuit River crossing tower.

2.6 Railway crossing tower

Angle or dead end towers (Type B,C or D) with suitable extensions and with double tension insulator strings are employed for railway crossing in conformity with the relevant specification of Railway Authorities.

2.7 High way crossing tower

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extension and with double tension strings are employed for high way crossing.angle towers are used for National High way crossing to make the crossing span as a single section so as to facilitate independent and prompt striginig.

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2.8 Transposition tower

2.8.1 Power transmission lines are transposed primarily to eliminate or reduce disturbances in the neighboring communication circuits produced by the geometric imbalance of power lines. An incidental effect of transposing power line section is the geometric balancing of such circuits between terminals which assumes balanced conditions at every point of the power transmission system. Improvements and developments in both the communications and power fields have, however, greatly reduced the need for transposition of high voltage lines at close intervals. In fact, in India, the central standing committee for coordination of power and telecommunication system has ruled that "the power supply authorities need not provide transposition on power lines for coordination with telecommunication lines".

2.8.2 However, when transposition are eliminated, there are the effects of geometric imbalance of the conductor arrangements on the power system itself, and the residual current to be considered. The imbalance of the three phase voltages due to asymmetry of conductor arrangement is not considered serious in view of the equalizing effect

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of the three phase transformer bank and synchronous machinery at various points on the system. The remaining consideration viz. residual currents due to the elimination of transposition, might be important from the point of view of relay settings to prevent causing undesirable tripping of ground current relays. Operating experience has shown that many disturbance on high voltage line occur on transposition towers and statistical records indicate that at least one of the four outages is physically associated with a transposition.

2.8.3 A good practice would be to adopt about 200 KM as the permissible length of the line without taking recourse to special transposition structures, transposition being confined to substation and switching station only, provided they are located at suitable intervals.

2.8.4 Tower type C under O degree deviation limit and with suitable modification shall be used for transposition for line maintaining all the required

clearances and shielding. Arrangement of

transposition is shown at Figure 2.7. A view of 400 kv single circuit transposition tower is also shown in Fig.2.11.

2.9 Multi circuit towers.

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To transmit bulk power at a economical rate, Multi circuit towers are used. It may be mentioned here that a double circuit line is cheaper than two independent single circuit lines and four circuit line cheaper than two double circuit lines. However, the capital outlays involved become heavy and it is not easy to visualise the manner in which the loads build up and the powerflow takes place in the longterm prospective. Further, reliability considerations become very important at extra high voltages. A balance has therefore to be struck between the two somewhat opposing considerations.

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2.10 Tower extensions

All towers are designed in such a way that they can be provided with standard tower extensions. Extensions are designed as +3, +6 +9 and + 25 in Mtrs. These extensions can be used alongwith standard towers to provide sufficient clearance over ground or while crossing power lines, Railway lines, highways, undulated, uneven ground etc. A view of 400 kv single circuit towers crossing anoth er 400 kv single circuit line is shown at Fig. 2.12

2.11 Leg extensions

Leg extensions are designed to provide extension to tower legs which are located at uneven ground where different legs of the tower are at different levels.

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Standard designs can be made for 1.5, 2.5 and 3.5 M leg extensions.

These leg extensions can be utilised where towers are located on hill slopes, undulated ground etc. By providing leg extensions, specially in hilly areas, heavy cost of benching/revetment can be avoided completely or reduced substantially.

2.12 Truncated towers (Tower reductions)

Similar to extension towers, truncated towers can also be used for getting the sufficient electrical clearance while crossing below the existing Extra High Voltage lines. For instance,a DD-6.9 Mtrs. truncated tower has been used in 220 KV RSEB S/Stn. at Heerapura (Jaipur). In this particular case 2 nos. of 400 KV S/C lines are already crossing over the 220 KV D/C Kota-Jaipur RSEB feeders with A+25 Mtrs. extension type of towers. While constructing another D/C 220 KV line from Anta to Jaipur which was also to be terminated in the same sub-stn. either to under cross these 400 KV S/C lines by using gantry system or to make use of the existing A+25 Mtrs. extension towers. But with the existing A+25 Mtrs extension tower, required clearance between the earth wire of the 220 KV line and hot Conductor of 400 KV lines were not within the permissible limit. So for getting the required

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electrical clearance either to remove the earthwire of 220 KV line or to use truncated tower. So to avoid the removal of earth wire a `DD' type truncated tower (-6.9 Mtrs.) has been used in order to cross these lines safely and with the required permissible electrical clearances.

The truncated tower is similar to normal tower except 6.9 Mtrs of bottom section of normal tower has been removed, the other section of the tower parts remain un-changed.

This is a ideal crossing in an area where one line has already crossed over the existing lines with Special extension tower and we have to accommodate another line in the existing crossing span.

2.13 Weight of different types of towers

The weight of various types of towers used on transmission lines, 66 KV to 400 KV, together with the spans and sizes of conductor and ground wire used in lines are given in Table 2.1. Assuming that 80 percent are tangent towers, 15 percent 300

towers and 5 percent 600_{towers and dead-end}

towers, and allowing 15 percent extra for extensions and stubs, the weights of towers for a 10 kms. line are also given in the Table 2.1.

Table 2.1 Weights of towers used on various voltage categories in India

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(Metric tones) 400 kV Single Circuit 220 kV Double Circuit 220 kV Single Circuit 132 kV Double Circuit 132 kV Single Circuit 66 kV Double circuit 66 kV Single Circuit Span (m) 400 320 320 320 320 245 245 Conductor Moose 54/3.53 mm al. + 3.53 mm Steel Zebra 54/3.18 mm Al + 7/3.18 mm Steel Zebra 54/3.18 mm Al. + 7/3.18 mm Steel Panther 30/3 mm Al. + 7/3 mm Steel Panther 30/3 mm Al.+7/3 mm Steel Dog 6/4.72 mm Al. + 7/1.57 mm Steel Dog 6/4.72 Al. + 7/1.57 mm Steel Groundwire 7/4 mm 110 Kgf/mm2 quality 7/3.15 mm 110 Kgf/mm2 quality 7/3.15 mm 110 Kgf/mm2 quality 7/3.15 mm 110 Kgf/mm2 quality 7/3.15 mm 110 Kgf/mm2 quality 7/2.5 mm 110 Kgf/mm2 quality 7/2.5 110 Kgf/mm2 quality Tangent Tower 7.7 4.5 3.0 2.8 1.7 1.2 0.8 30 Deg. Tower 15.8 9.3 6.2 5.9 3.5 2.3 1.5

60 Deg. And Dead-end

Tower

23.16 13.4 9.2 8.3 4.9 3.2 2.0

Weight of towers for

a 10-km line

279 202 135 126 76 2 48

Note: Recent designs have shown 10 to 20% reduction in weights.

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CHAPTER THREE ---TOWER FABRICATION 3.1 General

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.

Towers used are of bolted lattice type. In no case welding is allowed. All members, bolts, nuts and fittings are galvanised. Spring washers are electro galvanised.

Fabrication of towers are done in accordance with IS codes which is ensured by visit to the fabrication workshops and undertaking specified tests, in the presence of POWERGRID quality engineers. The following may be ensured during fabrication of the towers.

Chapter-3

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i) Butts, splices should be used and thickness of inside cleat should not be less than that of heavier member connected. Lap splices are used to connect unequal sizes.

ii) While designing, joints are to be made so that eccentricity is avoided.

iii) Filler should be avoided as far as practicable.

iv) The dia of hole = dia of bolt + 1.5 mm

v) Drain holes are to be provided where pockets of depression are likely to hold water.

vi) All similar parts should be interchangeable to facilitate repairs.

vii) There should be no rough edges.

viii) Punched holes should be square with plates and must have their walls parallel.

ix) It should be checked that all burrs left by drilling or punching should be removed completely. Drilling or reaming to enlarge defective holes is not allowed.

3.2 Bolting

3.2.1 The minimum diameter of bolts used for the erection of transmission line towers is 12 mm. Other sizes commonly used are 16 mm and 20 mm.

3.2.2 The length of the bolt should be such that the threaded portion does not lie in the plane of contact of members.

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Figure 3.1 shows the wrong uses and the correct uses of bolt threads.

3.2.3 Table 3.1 gives the minimum cover to free edge and bolt spacing as per IS:802 (Part II)-1978 Code of Practice for Use of Structural Steel in Overhead Transmission line Towers. The bolts used with minimum angle sizes restrict the edge distances as given in Table 3.2 for the bolt sizes of 12 mm, 16 mm and 20 mm used on 40 x6 mm, 45x6 mm and 60x 8 mm angle sizes respectively.

Table 3.1 Spacing of bolts and edge distances

(mm)

---Bolt Hole Bolt spacing Edge distance(min)

Dia dia min. Hole Hole

centre centre to rolled to edge sheared edge ---12 13.5 32 16 20 16 17.5 40 20 23 20 21.5 48 25 28 ---(See next page)

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Table 3.2 Maximum edge distance possible with minimum angle size (mm)

---Size of bolted Maximum edge

Bolt dia. leg of angle distance that

section and its can be

thickness actually obtained 12 40x6 17 16 45x6 18 20 60x8 25

3.2.4 The bolts may be specified to have Whitworth or other approved standard threads to take the full depth of the nut, with the threading done far enough to permit firm gripping of the members but no farther, and with the threaded portion of each bolt projecting through the nut by at least one thread. It may also be specified that the nuts should fit hand-tight to the bolts, and that there should be no appreciable fillet at the point where the shank of the bolt connects to the head. Emphasis should be laid on achieving and maintaining proper clamp load control in threaded fastners. If a threaded fastener is torqued too high, there is a danger of failure on installation by stripping the threads or breaking the bolt or making the fasteners yield excessively. If the bolt is torqued too low, a low preload will be induced in the fastener assembly, possibly inviting fatigue or vibration failure. For every bolt system, there

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is an optimum preload objective which is obtained by proper torquing of the bolt and nut combination. The three techniques for obtaining the required

pretension are the calibration wrench method, the turn-of-the-nut method and the direct tension indication method.

The calibrated wrench method includes the use of manual torque wrenches and power wrenches adjusted to stall at a specified torque value. Variations in bolt tension, produced by a given torque, have been found to be plus minus 10 percent.

The turn-of-the-nut method has been developed where the pretensioning force in the bolt is obtained by specified rotation of the nut from an initially snug tight position by an impact wrench or the full effort of a man using an ordinary wrench. This method is found to be reliable, cheapest and preferred.

The third and the most recent method for establishing bolt tension is by direct tension indicator. There are patented load indicating washers, where correct bolt tension could be assessed by observing the deformation. Upon tightening the bolt, the washers are flattened and the gap is reduced. The bolt tension is determined by measuring the remaining gap.

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not specify the maximum permissible group length of bolts. It is a good practice to ensure that no bolt connects aggregate thickness more than three times the diameter of the bolt. Further more, the grip strength developed by a bolt depends not only upon the thickness of the members but also on the number of members to be connected. This is due to the fact that the surface of the members may not be perfectly smooth and plain and, therefore, if the number of members to be connected is too many, the full grip strength would not be developed. In the tower construction, the need for connecting more than three members by a single bolt rarely arises, it would be reasonable to limit the number of the members to be connected by a single bolt to three. The limitation regarding the thickness of the members and the number of members to be connected is necessary not only from the point of view of developing maximum grip strength but also from the point of view of reducing the bending stresses on the bolt to a minimum.

3.2.6 The threaded portion of the bolt should protrude not less than 3 mm and not more than 8 mm over the nut after it is fully tightened.

3.3 Washers

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being used in the construction of transmission line towers in India. The advantage of spring washers over flat washers is that the former, in addition to developing the full bearing area of the bolt, also serve to lock the nuts. The disadvantages, however, are that it is extremely difficult to get the correct quality of steel for spring washers, and also that they 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 galvanising.

When spring washers are used, their thicknesses should be as recommended in IS:802 (Part II)-1978 and given in Table. 3.3

Table 3.3: Thicknesses of spring washers

(mm)

---Bolt dia. Thickness of spring washer

12 2.5 16 3.5 20 4.0

With regard to the locking arrangement, the general practice is to lock the nuts by centre punching of the bolts or punching the threads. In special cases such as tall river-crossing towers which are subjected to unusual vibrations, the bolts are secured from slacking back by the use of

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lock nuts, by spring washers, or by cross-cutting of the thread.

A minimum thickness of 3mm for washers is generally specified.

In our transmission lines, we are using spring washers under all nuts of tower. These spring washers are electro-galvanised.

3.4 Lap and butt joint

(figure 3.2 and 3.3)

Lap splices are normally preferred for leg members as these joints are generally simpler and more economical compared to the heavier butt joints which are employed only if structural requirements warrant their use.

In lap splices, the back(heel) of the inside angle should be ground to clear the fillet of the outside angle.

3.5 Gusset plates

In the case of suspension towers, the stresses in the web system are usually small enough to keep the use of gusset plates to the minimum. On heavier structures, however, the web stresses may be very large and it may not be possible to accommodate the number of bolts required for the leg connection in the space available on the members, thus

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necessitating the use of gusset plates. Plates may also be 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 leg in order to limit the bending stresses induced in the main members due to eccentricity in the joints. To satisfy this condition, it may sometimes become necessary to use gusset plates.

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3.6 Bracing to leg connections

Typical connections of diagonals and struts to a leg member are shown in Figure 3.4.

The number of bolts required in these simple connections is derived directly from the member load and the capacity per bolt either in shear or bearing. Diagonal members which are clipped or coped for clearance purposes must be checked for capacity of the reduced net section. Note that gusset plates are not used at leg connections, but eccentricity is kept to a minimum by maintaining a clearance of 9.5mm to 16mm between members.

If the leg does not provide enough gauge lines to accommodate the required bolts in a diagonal connection, a gusset plate as shown in Figure 3.5 may be employed. The thickness of gusset plate must be sufficient to develop the required load per bolt.

Typical gusset plate connection at waist lines on the normal face for a wasp-waist tower is shown in Figure 3.6.

3.7 Connection of redundant members

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bolt connection to transfer their nominal loads. Thus, gusset plates can easily be avoided if clipping and coping are used to advantage. Typical connections, shown in Figures 3.7, 3.8 and 3.9 indicate the methods of clipping or turning members in or out to keep the number of bolts to a minimum. Figure 3.7 illustrates the use of a small plate rather than connecting five members on one bolt, as it has been found that erection of more than four thicknesses per bolt is particularly awkward.

3.8 Cross-arm connections

The cross-arm to leg connection (Figure 3.10) must be considered as one of the most important joints on a tower since all loads originating from the conductors are transferred through the cross-arms to the tower shaft by means of these bolts. Because of its importance, a minimum of two bolts is often specified for this connection.

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An example of a hanger-to-arm-angle connection on `Vee' cross-arm is shown in Figure 3.11, Both vertical and horizontal eccentricities may become excessive if the detail of this joint is not carefully worked out. Suspension towers are provided with holes at the ends of the cross-arms, as shown in Figure 3.10, for U-Bolts which receive the insulator string clamps. Strain towers, however, must be supplied with strain plates (Figure 3.12) which are not only capable of resisting the full line tension, but also shock and fatigue loads as well as wear.

3.9 step bolts and ladders

The step bolts usually adopted are of 16mm diameter and 175mm length. They are spaced 450mm apart and extend from about 3.5 metres above the 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 withstanding a vertical load of not less than 1.5 KN. Step bolts are provided from 3.5 m to 30 m height of the superstructure. For special structures, where the height of the superstructure exceeds 50 metres, ladders along with protection rings are provided

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(in continuation of the step bolts on the longitudinal face of the tower) from 30 metres above ground level to the top of the special structure. A platform, using 6mm thick chequered plates, along with a suitable railing for access from step bolts to the ladder and from the ladder to each cross-arm, and the ground wire support is also provided.

3.10 Anti-climbing devices

All towers are provided with anti-climbing devices at about 3.5 metres above ground level. The details of anti-climbing devices are shown in Figure 3.13.

3.11 Danger and number plates

Provision is made on the transverse face of the tower for fixing the danger and number plates while

developing the fabrication drawing. These

accessories are generally fixed at about 4.5mm above the ground level. Fig. 3.18 and Fig.3.16 show the details of danger and number plates respectively.

The letters, figures and the conventional skull and bones of the danger plates should conform to IS:2551-1982 Specification for Danger Notice Plates and they are to be painted in signal red on the

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3.12 Phase and circuit plates

Each tension tower shall be provided with a set of phase plates. The transposition towers should have the provisions of fixing phase plates on both the transverse faces. The details of phase plate are given in Fig. 3.15.

All the double circuit towers shall be provided with circuit plate fixed near the legs. The details of circuit plates are indicated in Fig.3.17.

These plates shall also be fixed at about 4.5m above ground level.

3.13 Bird guard

Perching of Birds on tower cross arms results in spoiling of insulator discs of suspension strings which leads to tripping of line. To overcome this problem, bird guards are fixed over suspension insulator string. The details are given at Figure No. 3.14.

Bird guards shall be used for type-I string only.

3.14 Aviation requirements

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the vicinity of an airport shall be painted and the crossing span shall be provided with markers to caution the low flying air craft.

3.14.2 The full length of the towers shall be painted

over the galvanised surface in contrasting bands of orange or red and white. The bands shall be horizontal. Fig.2.10 shows the river crossing

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3.15 Packing, transportation and storage of tower parts.

3.15.1 Packing :

a) Angle section shall be wire bundled. Cleat

angles, gusset plates, brackets, fillet plates, hangers and similar loose pieces shall be bolted together to multiples or securely wired together through holes.

b) Bolts, nuts, washers and other attachments

shall be packed in double gunny bags accurately tagged in accordance with the contents.

c) The packing shall be properly done to avoid losses/damages during transit. Each bundle or package shall be appropriately marked.

3.15.2 Transportation.

The transport of steel towers from the works to

the nearest railway station presents no special difficulty. The towers are delivered in trucks having one or two towers per truck according to the weight involved. A station having a loading bay is highly desirable, as this greatly facilitates handling. The lorries can be backed against the bay and the ease of handling will then offset any slight increase in haulage

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costs from a station less well equipped. The parts of each tower should be kept separate so that they can be delivered from the bay direct to the tower site. Tower sets are made up in sections, since it is impracticable for the corner angles to be in one length. Each section is carefully marked at the works. In each section there are generally one or more panels and these are marked to facilitate erection. The tower sets should be carefully checked when unloaded from the trucks and then placed in a suitable position on the bay where they can be picked up easily as a complete unit. If the steelwork is delivered in bundles, the checking is even more important and there are two meth-ods of doing this. Some Engineers prefer laying the steelwork out in members while others prefer it laid out in towers and in our

opinion the latter method has many

advantages. Shortages are easily spotted and scheduled and the tower can be loaded and taken to its particular position. All bolts, washers, nuts and small parts should be in bags and labelled with the number of the tower they are intended for. A word of warning re-garding the handling of the long corner angles should be clearly displayed. These must be carefully

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transported or they may get bent and it is a very difficult job to straighten them without damaging the galvanising. All material transport shall be undertaken in vehicles suitable for the purpose and free from the effects of any chemical substances. Tower members shall be loaded and transported in such a manner that these are not bent in transit and sharp-bent members are not opened up or damaged.

3.15.3 Storage.

A. The selection of location of a

construction store is important as the movement of construction materials is time consuming process and it requires detailed planning and Managerial attention. The selection of location is generally based on the following criteria.

a. Close proximity to rail heads, National

Highways.

b. Proximity to urbanisation and towns.

c. Availability of water and electrical

power.

d. Distance from the proposed line and

approach.

e. Type of land. (The store should not be located on marshy or wet lands. Also, the

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low lying and water stagnant areas)

f. Availability of land in sufficient area.

g. Communication facilities.

h. Availability of labour for the work in the

stores.

B. Once land is selected, it is better to identify the space for towers, insulators, conductors, hardware and the tools & plants of erection contractor. The selection of place for each type of material should be very judicious and in such a way that inward or outward movement of one item should not be through the stacking of the materials of other item. Proper board markings and pointers may be kept for each item for easy identification.

C. Tower parts should not be kept directly on the ground and it should be placed above stones of proper size or sleepers to avoid contact with mud.

D. It is always preferable to stack the tower

parts in a neat and systematic fashion in tower wise order. On request of erection gang, store-keeper should be able to provide him one full set of tower without any difficulty and delay.

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E. The following points may be ensured in the stores.

a. Complete fencing of the store yard. b. 24 hours vigilant security.

c. Proper lighting.

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CHAPTER FOUR ---METHODS OF ERECTION 4.1 GENERAL

There are four main methods of erection of steel transmission towers which are described as below

i. Built-up method or Piecemeal method.

ii. Section method

iii. Ground assembly method.

iv. Helicopter method

4.1.1 Built up method

This method is most commonly used in this country for the erection of 66 KV, 132 KV, 220 KV and 400 KV Transmission Line Towers due to the following advantages.

i. Tower materials can be supplied to site in knocked down condition which facilitate easier and cheaper transportation.

ii. It does not require any heavy machinery such as cranes etc.

iii. Tower erection activity can be done in any kind of terrain and mostly through out the year.

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iv. Availability of workmen at cheap rates.

This method consists of erecting the towers,

member by member. The tower members are kept on ground serially according to erection sequence to avoid search or time loss. The erection progresses from the bottom upwards, the four main corner leg members of the first section of the tower are first erected and guyed off. Sometimes more than continuous leg sections of each corner leg are bolted together at the ground and erected.

The cross braces of the first section which are

already assembled on the ground are raised one by one as a unit and bolted to the already erected corner leg angles. First section of the tower thus built and horizontal struts (bet members) if any, are bolted in position. For assembling the second section of the towers, two gin poles are placed one each on the top of the diagonally opposite corner legs. These two poles are used for raising parts of second section. The leg members and braces of this section are then hoisted and assembled. The gin poles are then shifted to the corner leg members on the top of second section to raise the parts of third section of the tower in position for assembly. The gin pole is thus

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moved up as the tower grows. This process is continued till the complete tower is erected. Cross-arm members are assembled on the ground and raised up and fixed to the main body of the tower. For heavier towers, a small boom is rigged on one of the tower legs for hoisting purposes. The members/sections Are hoisted either manually or by winch machines operated

from the ground. For smaller base

towers/vertical configuration towers, one gin pole is used instead of two gin poles. In order to maintain speed and efficiency, a small assembly party goes ahead of the main erection gang and its purpose is to sort out the tower members, keeping the members in correct position on the ground and assembling the panels on the ground which can be erected as a complete unit.

Sketches indicating different steps of erection by built up method are shown at Figure 4.1 to Figure 4.7.

List of Tools and Plants and Manpower for Tower Erection is given at Annexure E/1 and E/2.

Guying arrangement - Guying arrangements are to be done at waiste level/bottom cross-arm level as well as in the girder level/top cross-arm level depending on SC/DC towers and it is to be

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installed at 450_{from vertical. The deadments}

for guying arrangements is to be properly made. A sample of deadments drawing is enclosed at Figure 4.8 for reference. Guying should be steel wire or polypropylene rope depending upon requirements. Nominal tension is to be given in guying wire/rope for holding the tower in position.

4.1.2 Section method

In the section method, major sections of the tower are assembled on the ground and the same are erected as units. Either a mobile crane or a gin pole is used. The gin pole used is approximately 10 m long and is held in place by means of guys by the side of the tower to be erected. The two opposite sides of the lower section of the tower are assembled on the ground. Each assembled side is then lifted clear of the ground with the gin or derrick and is lowered into position on bolts to stubs or anchor bolts. One side is held in place with props while the other side is being erected.

The two opposite sides are then laced together with cross members diagonals; and the assembled section is lined up, made square with the line, and levelled. After completing the first section, gin pole is set on the top of the first section. The gin