Introduction to power system power point ch-4&5.pdf
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1. 1
Towers and masts
1.1 Introduction
A tower or mast is a structure which has relatively small cross section and
have large ratio between the height and the maximum width. A tower is also
known as a pylon. A tower is a single cantilever freely standing, self-supporting
structure fixed as its base. A mast is a pin-connected structure to its foundation and
braced with guys and other elements. The communication towers, radio and
television towers, power transmission towers,etc.., are the examples of structure
belonging to the tower family. The power transmission towers are used to support
transmission cables over long distances. The tall transmission tower should have
necessary clearance, when the transmission cables have maximum sag. The towers
may be built up with three or more legs. But generally the towers are constructed
with four legs ,spaced suitably. The transmissions line towers are self flexible type,
self supporting wide baseband guyed type. The self supporting types of towers are
rigid in both transverse and the longitudinal direction. The flexible types of towers
are not rigid in longitudinal direction, i.e. in the direction along the transmission
cables. When the direction of power line is straight then the line towers are used.
When the direction of power line is changed then the angle towers are provided.
Electric power is today playing an increasingly important role in the life of
the community and development of various sectors of economy. Developing
countries like India are therefore giving a high priority to power development
program. In fact the economy is becoming increasingly dependent on electricity as
it is the basic input for any activities in the society.
Soon after independence, the economic importance of electricity was
recognized and legislation was enacted for creation of the requisite organization
2. 2
and legislation was enacted for the creation of the requisite organizational base
central authority at the national level and electricity board at state level for
implementing a planned development program framed under the successive plans,
beginning with the first five year plan which was launched in 1950-51.
While in absolute terms the level of electricity energy consumption in India
is far below the developed countries, the per capita consumption has increased
from 14.6 KWh in 1950 to 191 KWh in 1986. The installed generating capacity
and the transmission and distribution networks to carry power from the generating
stations to the local centers and from there on to the ultimate consumers have
increased manifold.
As a background to the development of power transmission networks in
India and the importance of proposal to review briefly the energy scene and the
growth of the power supply industry and give a broad perspective into the future.
1.2 Types of tower
Towers may support single, double or multiple circuits, along with
supporting the ground wire. Transmission line towers are designed as
Self supporting towers
Flexible towers
The self supporting towers are subjected to two types of loads.
a) Wind loads acting transversely
b) Longitudinal horizontal loads occurring when part or all of the conductors
snap in one span, forcing the structure to resist the unbalanced cable pulls
from the unbroken span.
3. 3
These towers are therefore rigid in both directions. Whereas the flexible towers are
rigid in only transverse direction (to resist wind loads) and their longitudinal
stability is provided by the cables. Due to this reason self-supporting towers are
preferred, but then it is essential to provide self-supporting towers at some suitable
intervals to prevent the simultaneous collapse of the whole power line.
Towers may be of two types based on its alignment
1. Line towers or tangent towers
2. Angle towers
Towers on straight line portion of the line are known as line towers. Angle
towers are provided at angles in the lines and designed to resist the angular
component of the cable pulls. These are placed in such a way that the axis of the
cross-arms bisects the angle between the deviated transmission lines.
IS : 802 (part F)-1977 code of practice for the use of structural steel in over
head transmission line towers recommends the following four types of towers
a) Tangent tower : 00 to 20 line deviation
b) Small angle tower : 100 line deviation with suspension insulators
c) Medium angle tower : 300 line tower
d) Large angle 600 and end tower
Power conductors are supported by one or more strings of insulators hanging
vertically from the cross arms. The conductors or wires hang between the towers,
and are in tension. The spacing of tangent towers depends upon the terrain.
Tangent towers are spaced from 200 to 400 m apart for lines with voltage of 220-
300 kV and from 400 to 600 m for lines of high voltage because of high voltage
carried by the conductors, there should be a clear vertical distance of 6 to 10 m
4. 4
between the ground level and suspended conductors. Due to this reason the height
of the tower ranges from 20 to 40 m depending upon the spacing of towers. The
weight of a single suspension tower for 220-500 kV may range between 40 to 80
KN. Where from terrain considerations, it is a advantageous to have to have
tangent towers with 00 line deviation, the towers may be designed accordingly. The
angle of line deviation specified above is for normal span. The span may, however
be increased up to an optimum limit by reducing the angle of line deviation.
1.3 Loads on transmission line towers
The transmission line towers are subjected to the following loads:
1. Vertical loads:
weight of tower structure
weight of insulator strings and fittings
weight o power conductors
weight of ground wire
weight of ice coatings( if any)
weight of maintenance crew (line man) with tools (1.5 KN)
2. Transverse or horizontal loads (T)
wind load on conductors
wind load on ground wire
wind load on insulator string
wind load on tower structure
transverse components of tensions in conductors and earth wire
3. Longitudinal loads (P)
Unbalanced pull due to a broken conductor
Unbalanced pull due to a broken ground wire
5. 5
Seismic load on wires
Load due to temperature variation
4. Tension (M)
Due to earth wire broken
Due to conductor broken
The design is based upon two conditions: Normal wire condition and broken wire
condition
As per IS:802 (Part 1), the following broken wire condition may be assumed in the
design.
TABLE-1: Types Of Towers
(a) single circuit towers
(i) tangent tower (2o)
(ii) small angle tower (10o)
(iii) medium angle tower (30o)
(iv) large angle tower (up to 60o) and
Dead end tower.
Any one power-conductorbroken or
one ground- wire broken, whichever is
more stringent for a particular
member
(b) Double circuit towers
(i) tangent tower (2o)
(ii) small angle tower (10o)
(iii) medium angle tower (30o)
Any one power-conductorbroken
or one ground- wire broken; whichever
is more stringent for a particular
member.
Any two of the power conductors
broken on same circuit and on the
same span, or any one of the power
conductors and one on ground wire
broken on the same span; whichever
combination constitutes the most
stringent condition for a particular
member.
Three power conductors brokenon
the same circuit and on the same span
6. 6
(iv) large angle tower (up to 60o) and
Dead end tower:
or any two of the power conductors
and any one ground wire broken on the
same span; whichever combination
constitutes the most stringent condition
for a particular member.
(c) cross-arms : In all tower designs, the power-
conductorsupports and earth-wire
supports shall be designed for the
broken-wire supports shall be designed
For the broken-wire condition also.
1.4 Design span length: The following is used in for various types of span:
1. Normal span: It is center to center distance between towers.
2. Wind span : The wind span is the sum of the two half spans adjacent to the
support under consideration.
3. Weight span: The weight span is the horizontal distance between the lowest
points of the conductor, ontwo spans adjacent to the tower. The lowest point
is defined as the pint at which tangent to the sag curve or to the sag curve
produced, is horizontal.
1.5 Determination of tower height
The factors governing the height of the tower are:
7. 7
1. Minimum permissible ground clearance (h1)
2. Maximum sag (h2)
3. Vertical spacing between conductors (h3)
4. Vertical clearance between ground wire and top conductor (h4)
Thus the total height of the tower is given by
H = h1+ h2+ h3+ h4
8. 8
Minimum permissible ground clearance
From safely considerations, power conductors along the route of
transmission line should maintain requisite clearances to ground in open country,
national highways, rivers, etc.,
Rule 77(4) of the Indian electricity rules, 1956 stipulates the following
clearances above ground of the lowest point of the conductor.
For extra high voltage lines, the clearances above ground shall not be less
than 5.182 m plus 0.305 m for every 33,000 volts or part thereof by which the
voltage of line exceeds 33,000 volts.
Thus the value for 132 KV line is 6.10 m
Spacing of conductors
Considerable differences are found in the conductor spacing adopted in
different countries and on different transmission systems in the same country.
The spacing of conductors is determined by considerations, which are 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 out of the vertical plane farther than a conductor of larger cross section.
Usually conductors will swing synchronously with the wind, but with long spans
and small wires, there is always the possibility of the conductors swinging non-
synchronously, and the size of the conductor and the maximum sag at the center of
the span are the factors which should be taken into the account in determining the
distance apart at which they should be strung.
9. 9
According to the IS: 802(part-I) 1977 the loading conditions and the
permissible stress and the clearances requirements are given
Minimum height of the conductor above ground level = 6.7 m
Minimum spacing between power conductors = 3.5 m
There are many other empirical formulae in use, deduced from spicing which have
successfully operated in practice while research continues on the minimum
spacing, which could be employed.
Vertical clearances 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 stroke at the ground and the
minimum mid span clearances between the ground wire and the top most
conductor. The shield angle varies from about 250 to 300, depending upon the
configuration of the conductors and the number of ground wires provided.
According to the Indian standards the height of the top most power
conductor shall be half of the horizontal spacing of power conductor.
Sag (dip) and tension in uniformly loaded conductors
In order to determine the total height of the tower above the ground level, in
addition to the clearance required the maximum sag in the conductor may be found
by knowing the tension in the conductor at the corresponding temperature.
The tension in the conductor T2 at the maximum atmospheric temperature, t2
may be determined by the following equation
10. 10
T2
2(T2-T1) + ((W1
2L2) / (24T1
2)EA) + (t2-t) α EA) = ((W2
2EA) / 24)
The maximum sag in the conductor at the corresponding temperature can be
determined using the following relation.
D = (W2L2) / (8PH)
PH =horizontal component of tension in the conductor
D = max sag of the conductor
E= Young’s modulus of elasticity of the conductor material
A= Area of cross-section of conductor
L= Span of conductor
W = uniformly distributed load
11. 11
CHAPTER-2
2.1 DETERMINATION OF THE DIMENSIONS OF TOWER
Single circuit three phase
Volt of 132 KV
Conductor
30mm ACSR (Aluminum cable steel raft conductor consisting of 54
strands of 3mm dia of Aluminum and T strands of 3mm dia steel)
Unit weight of conductor = 16.75 KN/m
Permissible axial tension = 35.60 KN
Young’s modulus of elasticity = 0.842×105 N/mm2
Coefficient of expansion = 0.00001992/ 0c
Shape factor for conductor =0.67
Ground wire
10mm dia galvanized steel wire has been used
Permissible axial tension = 25.4 KN
Clearance requirements
Vertical height of the conductor above ground level = 6.7m(minimum)
Vertical spacing between power conductors = 3.5m(min)
Horizontal spacing between power conductors = 6.25m(min)
Height of ground wire above top most power conductor should be half of the
horizontal spacing of power conductors
Others
Variation of temperature 50c to 600c
Uniform intensity of wind 1.5 KN/m2
Weight of span of tower wind span = 250m
12. 12
2.2 GEOMENTRY OF THE TOWER
Total height of tower is decked in view the clearance requirements and
minimum requirements for power conduction
Clearance requirement
Vertical height of conductor above ground = 7m
Vertical spacing between power conductors=4m
Height of ground wire above top most power conductor
Horizontal spacing between power conductors = 7m
Height of ground wire =(7/2)= 3.5m
Total height = 14.5m
Maximum sag for power conductor
Below lowest wire support
D= (WL3) / (8pH)
Both the supports of wire are considered at one level. The span of tower 250 m
Weight of conductor at maximum temperature and no wind W2 = 16.76 N/ m
Horizontal compression of tension pulls
T2
2(T2-T1) + ((W1
2L2) / (24T1
2)EA) + (t2-t) α EA) = ((W2
2EA) / 24)
T1 = Permissible tension in the conductor = 35.06 KN
T2 = Tension in the cable at mid span
13. 13
A = Effective cross sectional area (It is calculated on the basis of net area of each
strand)
A = (54 + 7) (π/4) x 32 = 431.8 mm2
(t2-t1) = Temperature variation = 550 C
E = 0.824 x 105 N/mm2
Since wind gusts are not likely to cover complete span and the swinging of
conductor continues, the intensity of wind is developed to 75 percent.
WL, wind load = 70% of wind intensity x shape factor x dia of the
conductor
= (0.75 x 1.5) x (0.67 x 0.03)
= 0.0225 KN/m
Weight of the conductor at min temperature with wind, W1 = √(W2
2 + W1
2)
=√ (16.762+ 0.02252)
=0.02876 KN/ m
Maximum sag of the conductor
D = (W2L2) / (8PH)
W= uniformly distributed load of intensity
L= linear meter of projected conducted
D= 0.01676x 250 / (8 x 13.546) = 9.66
14. 14
Height of the tower
Clearance required = 14.5 m
Maximum sag of conductor = 9.66 m
Total = 24.16 m
Height of the tower may be fixed as 25 m
Width of base of the tower
From stability requirement the width of the tower at the base is kept as 1/14
of height of the tower = 6.25 m
Width of tower may at base may be adopted as 6.5 m
Length of members
Length of members as projected in the plane of the paper.
DD1 = 2 x ((6.5/2) – (2/2)) x (3/20) + 2 =2.675
CC1 = 2 x ((6.5/2) – (2/2)) x (6/20) + 2 = 3.350
BB1 = 2 x ((6.5/2) – (2/2)) x (13/2 0) + 2= 4.925
AA1 = 2 x ((6.5/2) – (2/2)) = 6.5
EE1= FF1= GG1 = 2 m
ED =D1E1 =C1D1
Pythagoras theorem
√(32+2.33752)= 3.8031
16. 16
Table 2 : Length of the members in tower
Horizontal Vertical Diagonal
CG1 = 2
FF1 = 2
EE1 = 2
DD1 = 2.675
CC1 = 3.35
BB 1 = 4.925
AA1 = 6.5
ED = 3.01 = E1D1
Dc = 3.01 = C1D1
CB = 3.01 = B1C1
BA = 3.01 = A1B1
GH = 1.414 = G1H
DE1 = 1.414 = D1E
CD1 = 1.414 = C1D
BC1 = 1.414 = B1C
AB1 = 1.414 = A1B
2.3 FORCES ACTING ON TOWER
Lateral forces due to wind
The lateral forces due to wind acting at every panel joint is found as a
product of intensity of wind and the exposed area of members of the tower
intensity of the projected area of the windward face in addition with 50% of that of
leeward face. In order to estimate the projected area, the sizes of member are taken
as
Column sizes for complete length ISA 150 x 150 x 10mm
Horizontal bottom panel joints ISA 90 x 90 x 8mm
Other horizontal and diagonals secondary members of nominal size 65 x 65
x 8mm
The members sizes are compared with those found after the design.
17. 17
Panel joint B (exposed area for windward and leeward)
Column = 2((BA + BC)/2) x width of member x 50% of area for leeward face
= 2((7.51 + 7.151)/2) x 0.150 x 1.5
= 3.21795 m2
Horizontal = 1 x (BB2 x 0.090 x 1.5)
= 1 x 4.925 x 0.090 x 1.5
= 0.665m2
Diagonal = 2 x ((BA1 + BC1)/ 2) x 0.065 x 1.5
= 2 x ((9.035 + 8.131)/ 2) x 0.065 x 1.5
= 1.674m2
Secondary members
6.5 – 4.925 = 1.575
1.575/ 2 = 0.7875
0.7875 / 7 = X / 3.5
X = 0.39375
4.925 + 2(0.39375) = 5.7125m
Secondary members = (1 x 5.7125 x 0.065 x 1.5)
= 0.5570m2
Total area at panel B
18. 18
Column = 3.21795m2
Horizontal = 0.665m2
Secondary = 0.5570m2
Total = 4.43995m2 or 4.44m2
Lateral force due to wind = 4.44 x 1.5 = 6.65KN
Panel Joint C
Column = 2 x ((CB + CD) / 2) x 0.150 x 1.5
= 2 x ((7.151 + 3.01)/ 2) x 0.150 x 1.5
= 2.286m2
Horizontal = 1 x 3.35 x 0.090 x 1.5
= 0.45225m2
Diagonal = 2((CB1 + CD1)/ 2) x 0.065 x 1.5
= 2 x ((8.131 + 4.25148)/ 2) x 0.065 x 1.5
= 1.21m2
Secondary members
4.925 – 3.35 = 1.575 / 2 = 0.7875
0.7875 / 7 = X / 3.5
X = 0.39375
3.35 + 2 (0.39375) = 401375m
19. 19
Secondary members = (1 x 401375 x 0.065 x 1.5)
= 0.40340m2
Total = 6.52KN
Panel Joint D
Column = 2 x ((DC + DE) / 2) x 0.150 x 1.5
= 2 x ((3.01 + 3.01) / 2) x 0.150 x 1.5
= 1.355m2
Horizontal =1 x 2.68 x 0.090 x 1.5
= 0.3611m2
Diagonal = 2((DE1 + DC1)/ 2) x 0.065 x 1.5
= 2 x ((3.803 + 4.25148)/ 2) x 0.065 x 1.5
= 0.3926m2
Total = (1.355 + 0.361 + 0.3926) = (2.1083 x 1.5) = 3.162KN
Panel joint E
Column = 2 x ((ED + 1)/ 2) x 0.150 x 1.5
= 2 x ((3.01 + 1)/ 2) x 0.150 x 1.5
= 0.90225m2
Horizontal = 1 x 2 x 0.090 x 1.5
= 0.27m2
20. 20
Diagonal = 2 x ((3.803 + 2.23) / 2) x 0.065 x 1.5
= 0.5883m2
Area of cross arm = 0.75
Total = (0.90225 + 0.27 + 0.5833 + 0.75) = (2.51047 x 1.5) = 3.765KN
Lateral load due to wind acting on the conductor
IS:802 (Part 1) 1967 recommends that projected area of conductor is
that as 0.667 times its diameter and intensity of wind is directed by % in order to
account the swinging effect of the conductors.
250 x (0.067 x 0.03) x (0.667 x 1.5) x 0.8 = 4.0041m
Lateral load due to direction of conductor from the tangent line
Three adjacent towers 1, 2, 3 are shown in figure. Each conductor
deviate from the tangent line = 1.5. The maximum permissible tension. T in the
cable is 35.60KN. The lateral load acting as the conductor at the level of
conductor.
2T sinᶿ = 35.60 x Sin 1.50
= 2 x 35.60 x 0.026126
= 1.8638KN
Total lateral load at the point joint E
3.7657 + 4 +1.838 = 9.625KN
Panel Joint F
Column = 2 x ((2 + 2)/ 2) x 1.5 x 0.150 = 0.9m2
21. 21
Diagonal = 2 x ((2.23 + 2.23)/ 2) x 1.5 x 0.065
= 0.434
Horizontal = 1 x 2 x 1.5 x 0.090 = 0.27m2
Total = 0.9 + 0.434 + 0.27 + 0.75 = 2.354m2
Lateral load due to wind = 2.354 x 1.5 = 3.531KN
Lateral wind acting on conductor same as E and diameter conductor = 1.86KN
(3.531 + 4 + 1.86) = 9.391 KN = 9.5 KN
22. 22
Panel joint G
Total lateral load at the panel joint G is equal to that at panel joint F = 9.5KN
Panel joint H
Cross arm and bracing = 0.80m2
Load = 0.80 x 1.5 = 1.2KN
Lateral load due to wind acting on the conductor
= 250 x (0.667 x 0.010) x (0.667 x 1.5) x 0.8
= 1.334
Lateral load due to deviation of ground from the tangent line
2 x 25.40 x Sin 1.5 = 1.3297 KN
1.2 + 1.334 + 1.3297 = 3.86 say 4 KN
Dead load acting on the tower
Self-weight of the tower is found from Ryle’s formula
W = 0.04 H M (1/2)
W = self-weight of tower in KN
H = height of tower in m
M = total moment due to lateral forces about the base in KN-m
M = PH x 25 + Po x 24 + PF x 22 + PE x 20 + PD x 19 + PC x 14 + PB x 7
M = 4 x 25 + 9.5 x 24 + 9.5 x 22 + 9.6295 x 20 + 3.162 x 19 + 6.5 x 14 + 6.665 x 7
23. 23
M = 927.218 KN m
Therefore W = 0.04 x (927.218) (1/2) x 25
W = 30.45 KN
Trial weight of tower
The sizes of various members have been assumed to determine the
lateral forces due to wind. The trial weight of the tower may be found by
measuring the lengths and by multiplying by their respective unit weights(which
may be noted from ISI Handbook No: 1)
Column:
ISA 150 x 150 x 10mm
= 4(AB + BC + CD + DE + (E-G)) x 0.228
= 4(7.151 + 7.151 + 3.01 + 4) x 0.228
= 22.1816
ISA 65 x 65 x 8 @ 0.077 KN / m
= 4 x (2 x AB1 + 2 x BC1 + 2 x CD1 + 2 x DE1 + 8 x 2.23) x 0.077
= 4 x (2 x 9.035 + 2 x 8.131 + 2 x 4.25148 + 2 x 3.8031 + 8 x 2.23) x 0.077
= 21.14KN
Horizontal
ISA 90 x 90 x 8mm @0.108KN / m
= 4 x (4.925) x 0.108 =2.1276 KN
24. 24
ISA 65 x 65 x 8mm @ 0.077 KN / m
= 4 x (3.35 + 2.675 + (5 x 2)) x 0.077 = 4.9357 KN
Secondary member
ISA 65 x 65 x 8mm
= 4 x (5.7125 + 4.1375) x 0.077 = 3.0338 KN
Cross area = 5.5252 KN
Total estimated weight of tower =58.74 = 60 KN
Weight of 3 power conductors = 3 x250 x 0.01676 = 12.57 KN
Weight of ground wire = 1 x 250 x 0.006 = 1.5 KN
Weight of the lineman with tools are recommended in IS802 =1.50 KN
Total dead load = 15.57 + 58.94 = 74.51 say 75 KN
Various factors acting on the tower under topmost power conductor in broken
conditions are as follows
1. Lateral load due to wind at panel joints B, C, D, E, F and H remains and tired
whereas the lateral load at the panel joint G is as under
(a) Due to wind(unchanged) = 1.5 x 2.354 = 3.531
2. Lateral load due to deviation of the ground wire from the tangent line
(unchanged)
27 Sinᶲ = 35.60 x Sin 1.5 x 2
= 1.8638 KN
25. 25
3.Due to wind on the conductor (broken load) for 60% span
= (0.60 x 250) x (0.667 x 0.03) x (0.667 x 1.5 x 0.8)
= 2.4 KN
Total = 3.531 + 1.8638 + 2.4 = 7.794 KN
The broken power conductors cause longitudinal tensile face on the towers
= 60% of working tension
= 0.60 x 35.60 = 21.36 KN
The broken power conductor also causes torsion forces as shown
2 x F1 x a = T x b
F1 = (66.75 / 4) = 16.68 KN
Dead load:
As one conductor is broken its 40% weight is reduced from that above
calculated
(0.40 x 250 x 0.01676) = 1.6089
Total dead load = 75
Various forces acting on the tower underground wire in broken are given below.
Lateral load due to wind at the panel joints B, C, D, E, F and G remains unchanged
areas the lateral load at the panel joint h as under:
Due to wind (unchanged) = 1.20 KN
Due to deviation of ground wire from the tangent line = 1.39
26. 26
Due to wind on the ground wire (B.C) for 60%
= (0.60 x 250) x (0.667 x 0.010) x (0.667 x 1.5 x 0.8)
= 1.334 x 0.60 = 0.8004
= 0.8004 + 1.392 + 1.2 = 3.392 KN
Broken ground wire causes s longitudinal tensile force on the tower 60% of
working tension
Dead load:
As the ground wire is broken, its 40% weight is reduced from that calculated
in the condition
0.40 x 250 x 0.006 = 0.6 KN
Total dead load = 75-0.6 = 74.40 KN
The lateral force acting on, one face of the tower at various panel joints.
Stresses:
Stresses in the various members of the tower under the normal operating
conditions of the conductor are as below
The transmission line tower is highly intermediate. The stresses in the
various members may be found by approximate method. The tower is reduced to a
determinate plane frame as shown by neglecting the horizontal and secondary
members.
The horizontal reaction at the foot of each column is equal to half of the
sum if total lateral load acting in one face of tower.
27. 27
= 12.235 (24.27 / 2)
The maximum BM about the base of tower
MI = 0.50 x 927.218 = 463.609 KN m
The spacing between the columns is 6.5
Vertical reaction:
= 463.6209 / 6.5
= 71.324 KN
The axial forces in column and diagonal members of the bottom panel are found by
resolving the forces horizontally and vertically at A.
28. 28
CHAPTER-3
STRUCTURAL STEEL CONNECTIONS
3.1 GENERAL
Connections are vital for good behavior of the tower under-load.
Connections, if not designed with care, are a source of weakness not only in their
structural action but also because they may be initiators of corrosion. The design of
main members is made based on theories, which generally have been fully
understood, developed and refined. On the other hand, the connections are not
given the attention they merit. The behavior of connections is often so complex
that theoretical consideration must be rigid to avoid fluctuating stresses, which
may cause fatigue failure; be such that there is the least possible weakening of the
parts joined; and be easily installed, inspected and maintained.
In transmission line towers, wherever possible, triangulated frames,
which are assumed to be having pin-joined connections, are made such that the
center of the resistance of the connections and the line of action of the load
coincide. This is to avoid any eccentricity moment on the concentration is present,
the members and the connections are designed to provide adequate resistance
against the induced secondary moments.
In general, the governing ruled for the design of connections may be
stated as follows:
1. The product of number of bolts of rivets used and their strength must
not be less then force they are required to resist. In the case of the
welded connection, the product of tits length and its strength per unit
run must be larger than the force required to resist.
29. 29
2. The moment resistance of a group must not be less than the moment
they are required to resist.
3. The plate or section used to form the connection must be capable of
resisting safely the forces and the moments to which they safely the
forces and moments to which they are subjected.
3.2 TYPES OF CONNECTIONS
The connections are classified as
Riveted connections
Bolted connections
Welded connections
A brief description of these connections is now given.
Riveted connections
Riveting is a method of connecting a joint by inserting a ductile metal
pin into holes and forming a head at each end to prevent the joint from coming
apart. The rivets may be either field or shop rivet. For transmission tower work,
field rivets are commonly used. A hand-operated pneumatic riveting hammer
drives these. Shop rivets are made using a riveting press.
Bolted connections
In bolted connections, three types of bolts are usually adopted. They are
black colts, turned barrel bolts and high strength friction grip bolts.
30. 30
The term ‘black bolts’ is used for unfinished common or rough bolts.
Black bolts used where slip and vibration do not matter. They are usually made of
mild steel.
Turned barrels bolts are specially made from black round bars and
turned down to the exact diameter. The flat face of the nut and the head on the
inner side are usually machined. Washers also should be machined on both the
faces for these bolts. Holes for such bolts are either reamed or drilled.
Them ‘high strength friction grip’ bolts of high tensile steel, used in
conjunction with high tensile steel nuts and hardened steel tension in order that the
clamping force this provided will transfer loads in the connected members by
friction between the parts and not shear in, or bearing on, the bolts or connected
parts.
Welded connections
Sound and efficient welded connection can be only by proper technology
of welding and proper structural design of weld. Welds are basically of two types
Butt welds
Fillet welds
The parts of a butt weld are shown in figure. The edge preparation required before
welding and the difference between square and V grooves are also shown in figure.
The change in thickness from one plate to another should be gradual, as shown.
For best results (for having high resistance against fatigue), the reinforcement
should be ground off flush. The standard fillet weld has equal leg lengths and a flat
31. 31
convex face or concave face. The influence of this on size is also shown in the
figure. Plug and slot welds may be considered as special cases of fillet welds.
3.3 Design connections
Hand driven rivets are used
Normal diameter of the rivets=22 mm
Gross diameter of the rivets = 22+1.5= 23.5 mm
Permissible shear strength, τvf = 80 N/mm2
Single shear strength = πd2/4 x 23.52 x 80
= (πd2/4) x 552.5 x 80
=34699 N
Joint 1
Member 1-38 = 194070/34699 = 6 nos.
Member 1-42 = 297400/34699 = 1 no.
Joint 2
Member 2-39 = 135410/34699 = 4 nos.
Member 2-42 = 19970/34699 = 1 no
Joint 5
Member 5-6 = 19760/34699 = 6 nos.
Member 5-38 = 195880/34699 = 1 no
32. 32
Member 5-42 = 21340/34699 = 1 no
Member 5-46 = 157840/34699 = 5 no
Member 5-50 = 33780/34699 = 1 no
Joint 6
Member 6-5 = 19760/34699 = 1 no.
Member 6-37=133630/34699 = 4 no.
Member 6-42=31750/34699 = 1 no
Member 6-47= 101050/34699=3 nos.
Member 6-50= 22470/34699=1 no
Joint 9
Member 9-10= 16050/34699= 1 no.
Member 9-13=136740/34699=4 nos.
Member 9-46=159640/34699=5 nos.
Member 9-50=26940/34699=1 no.
Member 9-54=21510/34699=1 no.
Similarly the no of bolts for other joints are determined..
33. 33
CHAPTER-4
FOUNDATION
4.1 TOWER FOUNDATION
The tower foundation cost approximately 10 to 30 % of the overall cost of tower,
or 5 to 15 % of the cost of transmission lines, depending on the type of the
foundation, the loading on the tower and the type of the soil.
Foundation design is often one of the last steps in a long planning process that
precedes the actual construction of the transmission line. Some of the steps of this
process is carried out in parallel with others. Possible line routes are identified by
considering number of factors, including the placement of substations, local
climatic conditions, and environment features and underlying geological
characteristics. After the tower type have been selected, the final tower is
completed and tested. Only during actual tower site selection can individual
foundation designs be finalized, taking in to account the variety of difficult and
often conflicting requirements created by the rest of the process.
Experience shows that the collapse of a tower is often initiated by foundation
failures. Thus, while an inadequate foundation may load to collapse of the tower,
an over design may prove very uneconomical. It is a good practice to check the
towers for permissible deflection at the top . Since differential foundation
settlement also causes tower deflection at the top, and if the total deflection at the
top of the tower is to be restricted, the permissible deflection has to be carefully
apportioned between the structure deflection and that caused by the differential
foundation settlement. The design of the safe and economical foundation is based
34. 34
on soil properties, knowledge of soil structure interaction and settlement analysis
of tower foundation.
4.2 Load, safety factors and settlements
4.2.1 Load
The loads on the foundation are determined from an analysis of the tower. The
foundation is called upon to resist the following type of forces.
Uplift
Down thrust
Lateral load
Over turning moment
The basic vertical forces are derived from the deadweight of the tower and the
conductors. The wind contributes to the horizontal forces on the tower, producing
not only horizontal force on the tower, producing not only horizontal shear force
(lateral load) on the foundation, but also an uplift or the compression forces are of
primarily concern in tower foundation design. In the case of the heavy angle and
terminal structures, however, one pair of legs will be permanently subjected to
compression and the other pair to uplift, due to the permanent heavy loads imposed
by the deviation of the line. In the case, it is the general practice to design all the
four footings to withstand both types of loading. However, in two legged H-frame
towers, overturning moment on the foundation becomes the predominant criterion.
35. 35
4.2.2 Safety factors
The foundation is generally designed for factors of safety, which are 10% in excess
of those, adopted for towers. Accordingly, the overload factors assumed in the
designs are 2.2 under normal condition and 1.65 under broken condition. However,
IS:802-1977(part III), relating to transmission line tower foundation, does not
make any distinction with regard to factors of safety as twin towers and
foundation.
4.2.3 Tower deflections due to foundation settlement
IS: 802-1977 does not specify any limitation regarding tower deflections.
However, in accordance with the practice followed in countries like the USSAR, it
is worthwhile limiting the tower deflection to about H/140. For transmission line
structures, the lateral foundation movements which are caused by wind action or
broken-wire condition will not cause any significant change in the forces of tower
members. Even if there is a differential lateral deflection of the tower foundation
for individual legs, the tower, being very flexible, can safely tolerate this
deflection. The tower deflection at the top, which is measured relative to the
foundation and is normally resisted to H/140, is not affected by the deflection of
tower legs.
It is possible to estimate the contribution of foundation settlement to the total tower
deflection, which is due to both structural deflections!!!!!!! And deflection!!!!
Caused by the relative foundation settlement. It follows that the permissible
deflection due to foundation settlement! Should be less than H/140.
36. 36
4.3 Lateral deflection of the foundation
The lateral; load on foundation is resisted by soil pressure on the sides of the
foundation. The top of the foundation therefore undergoes a deflection relative to
the base of the footing or the tip of the pile. On this deflection depends the
magnitude of the bending moment in the shaft of the footing of the pile. This
limited to 12mm in accordance with IS: 802-1977 relating to the pile of the
foundation.
4.4 classification and properties of soils
The design of the tower depends upon the nature of loading and the type of soil
that supports the foundation. The soil is broadly classified as
1. Sandy soil (loose, medium and dense)
2. Clayey soil (soft, medium and stiff)
3. Clayey sand(sandy clays, silt clays and silt sand)
4. Rock(soft, medium and hard )
The following laboratory tests are usually conducted from the soil samples
collected.
1. Visual examinations and other identification tests
2. Determination of in-situ density
3. Determination of strength parameters namely cohesion C and angle of
internal friction ɸ, settlement characteristics such as rate of settlement,
compression index C, etc.,
4. Determination of elastic properties- modulus of compressibility (k),
coefficient of lateral sub grade reaction (η)
37. 37
Among the field tests the standard penetration test (SPT) and the static cone
penetration (SCPT) are extensively adopted. In the standard penetration test (SPT),
a 64 kg weight is dropped 76 cm to drive a sampling spoon into the ground. The
number of blows required to push the spoon to a given depth is correlated with a
number of soil properties. The advantage of SPT is that it is relatively quick,
simpler and inexpensive; but it is also subjected to a many kind of errors. Also
correlations of SPT measurements with those of soil stress and other parameters
are not particularly reliable.
In the SCPT, a shaft with a conical tip is slowly pushed into the ground while
electrical transducers measure both tip pressure and side friction. The SPT
generally gives more accurate measurements. It is also a faster method to identify
soil problems.
The following soil properties are used in the design of different types of
foundations.
Density ϒ
Relative density Dr
Angle of internal friction for sandy soils ɸ
Unconfined compressive strength Cu and cohesion C for clay soils
Modulus of compressibility Es
Coefficient of lateral sub-grade modulus(η for sand and K for clay)
Poisson’s ratio v
Compressive strength of rocks σ
38. 38
TABLE-3 Relation between N,ϒ,Dr, and ɸ for sandy soil
Description SPT value (N) Density (ϒ)
gm/cc
Relative
density Dr
Angle of
internal
friction ɸ
Very loose 0-4 1.1-1.6 0-15 <28
Loose 4-10 1.45-1.85 15-35 28-30
Medium 10-30 1.75-2.1 35-65 30-36
Dense 30-50 1.75-2.25 65-85 36-41
Very dense >50 2.1-2.4 85-100 >41
TABLE-4 RelationbetweenN value, and unconfined compressive strength Cu
and cohesion C for clays
consistency SPT value N Unconfined
compressive
strength Cu
kg/cm2
Cohesion C
Kg/cm2
Reduction
factor for side
friction α of
bored pile
Soft 0-4 0-0.5 0-0.25 0.7
Medium 4-8 0.5-1.0 0.25-0.5 0.5
Stiff 8-15 1.0-2.0 0.5-1.0 0.4
Very stiff 15-30 2.0-4.0 1.0-2.0 0.3
Hard >30 >4.0 >2.0 0.3
39. 39
Notes:
1. For non-cohesive soils the values of safe bearing capacity are to be
reduced by 50 percent if the water table is above or near the base of
footing.
2. The values of safe bearing capacity do not take into account the effect
of shape and size of footing, cohesive C, angle of internal friction ɸ,
effect of eccentricity, the SPT value N, etc. Hence, the values are to
be considered as average and approximate.
3. For other types of soils such as black cotton and peat, soil
investigation have to be necessarily carried out for determining the
safe bearing capacity level, the highest flood, estimated depth of
scour, etc.
4. The maximum and mean velocity of water current.
4.5 Classification and types of foundations
4.5.1 Classification
The foundation are classified as shallow or deep based on Df/B ratio
Where
Df =depth of foundation, and
B=breath of foundation.
40. 40
If Df/<1, then the foundation is considered shallow and if Df /B>1, it is considered
as deep foundation. Piles are classified as deep foundations. Even though footings
may have greater depth than breath in some circumstances, they are treated as
shallow foundation for the analysis of bearing capacity. This approximation leads
to a conservative estimate of the factor of safety and is, therefore, adopted for
convenience and ease in calculations.
4.5.2 Types
Concrete foundations are predominantly used for transmission line towers, Spread
foundations, which use pad and stem design or grillages, are commonly employed
when the site allows only shallow excavations. Grillage are used in remote areas
where difficult terrain preclude easy access for concreting.
The types of foundation generally adopted for transmissions line towers are as
follows
1. Straight drilled shaft
2. Belled drilled shaft (auger type with undercut)
3. Pad and stem type without undercut
4. Pad and stem type with undercut
5. Pad screw anchor type
6. Under-reamed pile type
7. Grillage
8. Raft foundation
9. Rock anchors
41. 41
Straight-drilled shaft
Angering a cylindrical hole and filling it with reinforced concrete construct this.
The diameter varies from 0.5 to 2m and the shaft depth varies from 3 to 15m. the
skin friction between the ground and the shaft is an important contribution for
resisting uplift in such foundations. This type is extensively used for tower
foundation in the USA and is likely to gain acceptance for wide use in India
Belled drilled shaft auger type with undercut
In this case the pit excavated by means of some mechanical equipment, very little
disturbance of the adjustment soil takes place, so that the undisturbed material
resists the uplift loads. This avoids the use of forms, except for the portion above
the ground level. This type of foundation has been found to develop an uplift load
2 to 3 times that of an identical footing without undercut.
42. 42
Pad and system type without undercut
These footings are generally provided for cohesive soils, such as incremented sand
or gravel, which will not stand in vertical excavation lines and are, therefore not
undercut in the pad. The practice, usually followed in India at the present, is to
provide pad type footings without undercut.
Pad and system type with undercut
These footings are in form cohesive soils, which stand upon on vertical excavation
lines and are undercut on the pad. The excavation does not require the use of any
mechanical equipment. Experience has shown that this type of footing develops
resistance to uplift to the extent of 2 to 3 times that given by the footing without
undercut.
43. 43
Pad screw anchor type
The screw footings are a hybrid design, not yet in wide use. It combines the
advantage of the pad as well as the drilled shaft type. It generally used in situations
where large uplifts force are to be resisted and the soil bearing capacity is low.
Under-reamed pile type
Augured footings with more than one bulb are used to increase the uplift capacity.
The load carrying capacity of these footings for both downward and uplift forces
we generally established by tests. In the absence of tests the safe load on this type
of pile is assumed IS:2911(part 3)-1980 relating to pile foundations recommends
the use of values of safe loads on piles, 3.5 m long and under-reamed to two and
half times the shaft diameter in clayey black cotton and medium dense sandy soils,
when such field test results are not available.
Grillage foundation
Earth grillage has found wide application in the USA, Canada and some
continental countries. The chief objection to earth grillage is that the steel may be
44. 44
easily attacked by corrosive constituents of the soil, and that the periodical
excavation necessary for the purpose of maintenance would loosen the soil and
consequently lessen the anchorage until the earth consolidates again. However, the
Canadian experience shows that, when the grillage is employed in medium dry
sand, clay or sandy clay soils, no special precaution is necessary to protect the
buried steel work apart from using galvanized steel for all the buried subterraneous
members and generally limiting the minimum thickness of steel to above 6mm. In
isolated cases, where excess moisture exists or chemically active soil is
encountered, the tower footings members are coated with some other form of
asphalt or completely encased in concrete. The steel is treated with one coat of
bituminous paint, and a topcoat of asphalt given every year at ground level and
0.6m below ground level.
In India, the Tata Hydro-Electric Co. has used grillage foundation with satisfactory
performance.
Raft foundation
This type of foundation is used in special circumstance such as river-crossing
towers and towers on embankments. Apart from reducing the ground pressure
considering, the raft at the bottom makes the foundation substantially rigid to
minimize differential settlement.
45. 45
Rock anchors
Rock anchors are suited is the areas with rock out-crop. Based on the amount of
uplift, the anchor may be a single bar or a group of bars welded to the tower leg.
Where solid rock is encountered, the vertical bars are below the stub angle, which
form the cage for the footing, may be drilled and grouted to a depth of about fifty
times the diameter into the rock.
46. 46
4.6 DESIGN OF FOUNDATION
1. Check whether foundation is shallow or deep
D=1.5 m;
b=0.3 m;
e=0.3 m; ultimate S=2.22 tons
d/b =5.0; e/b=1.0; S=ultimate lateral resistance
S/ (cb2) = 12.0;
Hence, S=12x1.5 (0.3)2=1.62
47. 47
1.62 < 2.22 (ultimate lateral resistance in BWC)
The foundation to be classified as shallow since the lateral resistance
capacity is less than that required in the problem.
2. Check for uplift
For the first approximation, neglect reduction of volume occupied by
concrete
∅=30o
vol. of pyramid = D/3[A1+A2+√A1A2]
=2.3/3[1.52+4.142+√2.25𝑥17.14]
= 19.74 m3
Vol. of concrete =0.744 m3
Total WL = (19.74-0.74) 1.5+0.74x 2.3
= 28.5 +1.71 =30.2 tons
Fs on BWC= 30.2/19.4
=1.56 >1.5 (satisfactory)
3. Check for stability against overturning under BWC
Point of rotation= B/6=1500/6
=250 mm from toe
Wt. of concrete foundation = 1.71 tons as calculated from volume)
Wt. of soil acting at heel = W/2
=28.5/2
48. 48
=14.25 tons = W2
M =750-250 mm
Uplift = 19.4 (BWC)
S =1.48 (given)
Condition =W3(5B/6)> u x m +s (D +e) - WfB/3
=14.25(5x1.5/6)>19.4x0.5+1.48(2.4) -1.71x1.5/3
=17.81>9.7+3.55-0.86
= 17.81>12.39
Hence it is safe by the condition.
49. 49
CONCLUSION
For this project work, we all are united and worked in a planned manner.
We did it in a mostproper way in each and every stage, which lead us to complete
this project in a most successful manner. This project has given us quality and
teamwork. The information and the implements, which we have gained from this
project, will help us in the later part of our career.
The design of steel transmission tower has been completely done. All the
necessary drawings have been provided. The design is strictly done in accordance
with code provisions.
This project contains valuable information for planners, practising
engineers and students associated with electrical power transmission. The book
should serve the needs of professional engineers and advanced students alike. It
will undoubtedly enhance the appreciation and the evaluation of knowledge in
transmission power line designing.
50. 50
REFERENCES
1. Ram Chandra, “Design of steel structures-vol II” seventh edition 1991.
2. P.C. Varghese, “Foundation engineering”
3. Dr.B.C Punmia, Ashok kumar jain, Arun kumar jain, “Design of steel
structures” Lakshmi publications, July 2005.
4. IS : 802 Part I-1977 (second revision)-code of practice for use of structural
steel in overhead transmission line towers.
