design of tower

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)

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

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.

(R.P. SINGH)

CONTENTS

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

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
4.1 GENERAL

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

STANDARDISATION OF TOWER DESIGN

6.1 INTRODUCTION

6.2 STANDARDISATION IN POWERGRID

CHAPTER-7

FORMAT OF TOWER ERECTION CHECKING

___________________________________________________________________________

CHAPTER
ONE
_________________________________________________________

TOWER CONFIGURATION

1.1 Purpose of transmission tower

Back to contents page

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


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 0OC 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 32OC (90OF) 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
Percent

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.

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

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
vertical lift force FLV.

1.5 Maximum & minimum temperature :-


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.

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.

(c) 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

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.

(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

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.

(iii) Arbitrary load to provide for the weight of a man with tools.

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.

--------------------------------------------------------------------------
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

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.

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

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.

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,

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.

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

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.

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.

Fig.2.8 shows two types of tower configuration for

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


Angle towers (Type B,C or D) with suitable

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.

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

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.


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.

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.

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

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

(Metric tones)
400 kV 220 kV 220 kV 132 kV 132 kV 66 kV 66 kV

Single Double Single Double Single Double Single

Circuit Circuit Circuit Circuit Circuit circuit Circuit
Span (m) 400 320 320 320 320 245 245
Conductor Moose Zebra Zebra Panther Panther Dog 6/4.72 Dog 6/4.72

54/3.53 mm 54/3.18 mm 54/3.18 30/3 mm 30/3 mm mm Al. + Al. +

al. + 3.53 Al + mm Al. + Al. + Al.+7/3 7/1.57 mm 7/1.57 mm

mm Steel 7/3.18 mm 7/3.18 7/3 mm mm Steel Steel Steel

Steel mm Steel Steel
Groundwire 7/4 mm 110 7/3.15 mm 7/3.15 7/3.15 7/3.15 7/2.5 mm 7/2.5 110

Kgf/mm2 110 mm 110 mm 110 mm 110 110 Kgf/mm2

quality Kgf/mm2 Kgf/mm2 Kgf/mm2 Kgf/mm2 Kgf/mm2 quality

quality quality quality quality 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 23.16 13.4 9.2 8.3 4.9 3.2 2.0

Tower
Weight of towers for 279 202 135 126 76 2 48

a 10-km line

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

weights.

--------------------------------------------------------------------------
CHAPTER
THREE
Chapter-3
--------------------------------------------------------------------------

TOWER FABRICATION
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.

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.

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)

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

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.

3.2.5 Most of the transmission line specifications do

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


At present, both flat and spring steel washers are

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

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

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.

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


Redundant sub-members usually require only one

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.

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

(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

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 :-


3.14.1 The river crossing towers and any other towers in

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

tower with aviation paints.

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

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

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

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.

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.

d. Fire protection equipments.

--------------------------------------------------------------------------
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.

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

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

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

rests on a strut of the tower immediately below the

leg joint. The gin pole then has to be properly

guyed into position.

The first face of the section is raised. To raise

the second face of this section it is necessary to

slide the foot of the gin on the strut to the

opposite of the tower. After the two opposite faces

are raised, the lacing on the other two sides is

bolted up. The last lift raises the top of the

towers. After the tower top is placed and all side

of the lacings have been bolted up, all the guys are

thrown off except one which is used to lower the gin

pole. Sometimes whole one face of the tower is

assembled on the ground, hoisted and supported in

position. The opposite face is similarly assembled

and hoisted and then bracing angles connecting these

two faces are fitted.

4.1.3 Ground assembly method


This method consists of assembling the tower on

ground, and erecting as a complete unit. The

complete tower is assembled in a horizontal position

on even ground, at some distance from tower footing.

The tower is assembled in a linewise position to

allow the cross-arms to be fitted. On sloping

ground, however elaborate packing of the low side is

essential before assembly commences. After the

assembly is complete the tower is picked up from the

ground with the help of a crane and carried to its

location and set on its foundation. For this method

of erection, a level piece of ground close to the

footing is chosen for the tower assembly. This

method is not useful when the towers are large and

heavy and the foundations are located in arable

land where building and erecting complete towers

would cause damage to large areas or in hilly

terrain where the assembly of complete tower on

slopping ground may not be possible and it may be

difficult to get crane into position to raise the

complete tower.

In India, this method is not generally adopted

because of prohibitive cost of mobile crane, and

non-availability of good approach roads to the

location.

4.1.4 Helicopter method


n the helicopter method, the transmission tower is

erected in sections. For example bottom section is

first lifted on to the stubs and then upper section

is lifted and bolted to the first section and the

process is repeated till the complete tower is

erected. Sometimes a complete assembled tower is

raised with the help of a helicopter. Helicopters

are also used forlifting completely assembled towers

with guys from the marshalling yards, where these

are fabricated and then transported one by one to

line location. The helicopter hovers over the line

location while the tower is securely guyed. The

ground crew men connect and tighten the tower guyed

and as soon as the tie lines are bolted tight, the

helicopter disengages and return to the marshalling

yards for another tower. This method is adopted

particularly when the approach is extremely

difficult.

4.2 Earthing


Once the geometry of the tower and the line

insulation level are fixed, the one factor which

affects the lightning performance of a line that can

be controlled during the construction phase of the

line, is the Tower-footing resistance.

Consequently, this should be measured during this

phase of the work and, if necessary, extra earthing

provided. The measured resistance alters if the soil

conditions change due to seasonal variations.

When the footing resistance exceeds a desired value

from the lightning protection point of view, the

towers are earthed generally with pipe type and, in

special cases, with counterpoise type earthing. In

the former case, a 25mm diameter galvanised iron

pipe, 3,050mm long, is used with 6.5mm diameter

holes drilled at 150mm apart to facilitate ingress

of moisture, and is surrounded by a layer of finely

broken coke of 25mm granular size and salt.

The earthing should be done in accordance with the

stipulations made in IS:3043-1972 and IS:5613 (Part

II/Section 1)-1976.

The general earthing arrangement is shown in Figures

4.9 and 4.10. Where the tower stands on rock,

efforts should be made to obtain a good ground by

carrying a length of galvanised steel tape from the

tower leg to the pipe driven in soil, at as short a

distance from the tower as possible. The connecting

tape is burried in a groove cut in the rock surface

and adequately protected from damage.

4.2.1 Measurement of Tower Footing Resistance

The megger can be used in two ways to measure the

resistivity of the soil, namely, the three point

method and the four-point method. The four-point

method is simpler and more accurate and is briefly

described below.

a) Soil Resistivity

Four similar electrodes are burried in the

earth to a depth B at equal distances A from

one another in a straight line. The megger

connections are shown in Figure 4.11. If the

crank of the instrument is then rotated at the

stipulated speed (usually 100 rpm), the

resistance R, as read on the scale, is the

resistance of the earth between the two

equipotential surfaces with which P1 and P2 are

in contact.

If the depth of the electrode in soil B (in

cm) is small in comparison with A, the

resistivity of the soil is given by the

following expression.

2x22 x AxR
P = -----------
7

Where

P = earth resistivity in ohms/cm3

A = spacing between the electrodes in cm, and

R = resistance in ohms as read on the megger.

For all practical purposes, A should be at least

twenty times that of B.

b) Tower Footing Resistance

For measuring tower footing resistance,

Terminal C-1 of megger shall be connected with

tower leg instead of electrode C-1. The value

of resistance read on the megger multiplied

with multiplying factor gives the tower footing

resistance in ohms.

4.3 Tack welding :-


All bolts/nuts below waist level in single circuit

tower or bottom cross arm in Double circuit tower,

shall be tackwelded to prevent theft of tower

members.

Two 10mm thick welding tacks should be done on each

bolt & nut in the diagonally opposite direction by

suitably selecting welding electrods of size 1.6mm

to 2.5mm equivalent to over cord-S, code AWS-E6013

(Advani-Oerlikon). After removing slag over tack

welding, zinc rich (90% zinc content) cold

galvanising paint equivalent to epilux-4 of Berger

Paint shall be applied on the welding.

4.4 Permissible tolerances in tower erection


As per IS;5613 (Part 3/Sec.2) :1989, the following

tolerances in tower erection are permitted:

4.4.1 No member of a tower shall be out of straightness

by more than one in 1000. Members failing the

requirements shall be straightened before erection

in a manner that shall not damage their properties

or the protective finish.

4.4.2 The tower shall not be out of vertical by more than

1 in 360 before stringing is carried out.

Annexure - E/1


POWER GRID CORPORATION OF INDIA LIMITED

(CONSTRUCTION MANAGEMENT)

LINE CONSTRUCTION

ERECTION ACTIVITY

Tools & plants reqd. for Tower erection gang

1. Ginpole/Derric Pole 75/100mm

dia. and of length 8.5-9m. 2nos.

2. Polypropylene rope 25mm dia. 700 m.

19mm dia.1000 m.

3. Single sheave pulley Closed type 8 nos.

4. Crow Bars(25mm dia and

1.8m length) 16 nos.

5. Spanners,(both Ring and Flat)

Hammers,Slings,(16mm dia.and

1m length), hooks (12mm dia)

D shackle,Tommy Bars. As per reqt.

6. Tents,Buckets,Water drums, camping,

cots, tables, chairs, and petromax

etc. As per reqt.

7. D Shackle 7.6 cm (3 in.) 6 nos.

8. Hexagonal box spanner with fixed

liver and end of the liver

pointed to use Reqd. sizes

hole bar. Each size 6 nos.

9. Safety equipments :

i. Safety helmets 40 nos.

ii. Safety belts 10 nos.

iii. Safety shoes 50 nos.

iv. Welding Goggles 2 nos.

v. First Aid Box 1 no.

Note : The quantity of safety equipments may be changed as

per manpower engaged.

Annexure - E/2


POWER GRID CORPORATION OF INDIA LIMITED

(CONSTRUCTION MANAGEMENT)

CONSTRUCTION ACTIVITY

MANPOWER REQUIREMENT

FOR

TOWER ERECTION GANG

One Engineer shall be earmarked exclusively for the work of

Tower Erection being carried out by different gangs.

Following manpower is required for each Tower Erection gang.

1) Supervisor 1 No.

2) Fitter 8 Nos.

3) Skilled workers 12 Nos.

4) Unskilled workers 20 Nos.

Note: Average output per gang per month will be approximately

80 MT. The man power may be regulated depending upon

requirements

--------------------------------------------------------------------------
CHAPTER
FIVE
--------------------------------------------------------------------------

GUIDELINES

GL-1 PRE-ERECTION CHECKS


NAME OF LINE___________________ LOCATION NO. _____________

NAME OF CONTRACTOR_____________ TYPE OF TOWER ____________

Before taking up tower erection works, following preparations

need to be made.

1.1 Foundation checks

1.1.1 Tower erection work shall be taken up only after

concreting is cured and set for 14 days as per

technical specifications. This is essential so that

concrete gains sufficient strength to withstand

various forces acting during and after tower

erection.

1.1.2 The stubs shall be set such that the distance

between the stubs and their alignment and slop is in

accordance with the approved drawings so as to

permit assembling of superstructures without undue

strain or distortion in any part of the structure.

To ensure above following checks are necessary

before tower erection.

(a) Level of all the four stubs shall be on one

horizontal plane in order to ensure correct

and smooth tower erection. The level of top of

the stubs shall be checked to ensure that these

are on one horizontal plane.

(b) Distance between the stubs shall be as per

approved drawing so that correct and smooth

tower erection is achieved. Hence distance

(diagonals) between the stubs are measured and

verified for its correctness.

1.1.3 Revetment/Benching wherever required shall be

completed so that there is no danger to foundation

during and after tower erection. However, if it is

felt that, non-completion of Revetment/Benching is

not going to harm foundation during and after tower

erection, the same may be programmed and executed on

later date.

1.2 Tower materials

1.2.1 It shall be ensured that approved structural

drawings and Bill of Material with latest revision

are available at site to facilitate tower erection.

Preferably one set of structural drawings properly

laminated and Bill of Material in Bound Book shall

be available at site with each gang.

1.2.2 All tower Members shall be available at site as per

approved Bill of Material and shall be serially

placed on ground in order of erection requirement.

1.2.3 It shall be checked that no tower Member, Nut/Bolt,

accessories are rusted, bent or damaged.

1.2.4 All required sizes of Bolts/Nuts, spring/packing

washers in required quantity are available at site.

1.2.5 If any defects in protective surface finish are

found in case of hot dip galvanised members, the

damage shall be repaired by applying two coats of

zinc-rich paint having atleast 90% zinc contents

conforming to I.S. code.

1.2.6 Members bent in transit shall be straightened such

that the protective surface finish is not damaged.

1.3 Tools & plants

1.3.1 All the tools and plants required for safe and

efficient tower erection shall be available at

site.A list of necessary tools and plants is given

at Annexure-E/1.

1.3.2 All the tools and plants shall be tested as per

approved safety norms and relevant test

certificates shall be available. In addition to

above, periodic testing of tools and plants shall

be carried out and its safe working capacity shall

be worked out.

1.4 Personal protective equipments

1.4.1 All the persons working on tower shall wear safety

helmet, safety belt and safety shoes,.Similarly all

the persons working on ground shall wear safety

helmet and safety shoes. List of personal protective

equipments is given at Annexure-E/1.

1.4.2 Safety equipments shall be tested as per safety

norms and necessary test certificate shall be

available. Also, a periodic check shall be carried

out to ensure requisite strength.

1.5 Manpower

1.5.1 Manpower engaged for the purpose of tower erection

shall be skilled and competent enough to ensure

safe, smooth and efficient tower erection activity.

1.5.2 A list of necessary manpower required for tower

erection is given at Annexure-E/2.

1.6 Miscellaneous

1.6.1 If there is any LT/HT power line near the vicinity

of tower erection, necessary shutdown of the power

line shall be obtained in writing from the concerned

Agency in order to avoid electrical hazards caused

by accidental touching of stay/Guy ropes with power

line.

1.6.2 In order to develop and maintain cordial relations

with field owners, it is desired that crop/tree

compensation of foundation is paid to the owners

before taking up tower erection works.

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