Breaking the Kubernetes Kill Chain: Host Path Mount
Types of-tower
1. 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)
3. 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)
4. 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
5. 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
6. 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
7. 6.1 INTRODUCTION
6.2 STANDARDISATION IN POWERGRID
CHAPTER-7
FORMAT OF TOWER ERECTION CHECKING
9. ___________________________________________________________________________
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
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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
10. Back to contents page
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
11. 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.
12.
13. 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
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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
14. 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.
15.
16.
17. 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.
18.
19.
20. 1.6 Loading of transmission line towers
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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
21. 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.
22.
23.
24. (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
25. 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.
26.
27. 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.
29. --------------------------------------------------------------------------
CHAPTER
TWO
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TYPES OF TOWERS
2.1 Classification according to number of circuits
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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
30. 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.
31.
32.
33.
34.
35. 2.2. Classification according to use
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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
36. 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.
37. 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,
38. 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
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The bundled conductors are kept in horizontal
configuration with a minimum clearance of 11 mtrs.
phase to phase.
39. 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
40. 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
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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.
41.
42.
43. 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
44. 400 kv double circuit towers.
2.5 River-crossing tower
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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
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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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Angle towers (Type B,C or D) with suitable
45. 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.
46. 2.8 Transposition tower
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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
47. 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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48. 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.
49.
50.
51.
52. 2.10 Tower extensions
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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
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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.
53.
54. 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)
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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
55. 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
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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
56. (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.
57. --------------------------------------------------------------------------
CHAPTER
THREE
Chapter-3
--------------------------------------------------------------------------
TOWER FABRICATION
Tower Fabrication
3.1 General
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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.
58. 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
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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.
59. 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)
60.
61. 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
62. 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
63. 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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At present, both flat and spring steel washers are
64. 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
65. 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)
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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
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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
66. 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.
67.
68. 3.6 Bracing to leg connections
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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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Redundant sub-members usually require only one
69. 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
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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.
70.
71.
72. 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
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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
73. (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
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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
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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
75. 3.12 Phase and circuit plates
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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
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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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3.14.1 The river crossing towers and any other towers in
76. 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.
77.
78.
79.
80.
81. 3.15 Packing, transportation and storage of tower parts.
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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
82. 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
83. 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
84. 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.
85. 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.
87. --------------------------------------------------------------------------
CHAPTER
FOUR
--------------------------------------------------------------------------
METHODS OF ERECTION
4.1 GENERAL
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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
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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.
88. 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
89. 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
90. 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
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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
91. 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.
92.
93.
94.
95.
96.
97.
98.
99.
100. 4.1.3 Ground assembly method
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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
101. location.
4.1.4 Helicopter method
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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
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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
102. 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
103. 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.
104. 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.
105.
106.
107.
108. 4.3 Tack welding :-
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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
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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.
109.
110. Annexure - E/1
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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
111. 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.
112. Annexure - E/2
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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
114. --------------------------------------------------------------------------
CHAPTER
FIVE
--------------------------------------------------------------------------
GUIDELINES
GL-1 PRE-ERECTION CHECKS
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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
115. 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.
116. 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
117. 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.