Tes p-122.05-pi-r0

PAGE NO. 2 OF 34TEP122.05PIR0/MAA
TRANSMISSION ENGINEERING STANDARD TES-P-122.05PI, Rev. 0
Date of Approval: May 7, 2007
TABLE OF CONTENTS
1.0 SCOPE
2.0 MATERIALS
2.1 Structural Steel
2.2 Bolts, Nuts and Washers
2.3 Galvanizing
2.4 Other Materials
3.0 CLASSIFICATION OF LOADS
3.1 Climatic Loads
3.2 Longitudinal and Torsional Loads
3.3 Construction and Maintenance Loads
4.0 APPLICATION OF WIND LOAD ON TOWERS
5.0 OVERLOAD CAPACITY FACTORS
6.0 TYPES OF TOWERS
7.0 BROKEN WIRE CONDITIONS
7.1 Single Circuit Towers
7.2 Double Circuit Towers
8.0 COMPUTATION OF LOADS
8.1 Types of Loads
8.2 Sag and Tension
8.3 Tabulation of Loads and Loading Trees
8.4 Transverse Loads
8.5 Vertical Loads
8.6 Longitudinal Loads
8.7 Other Loads

9.0 PERMISSIBLE STRESSES
9.1 Axial Stresses in Tension
9.2 Axial Stresses in Compression
9.3 Stresses in Bolts
10.0 EFFECTIVE SLENDERNESS RATIOS AND LIMITING CONDITIONS
10.1 Leg Members
10.2 Other Compression Members
10.3 Redundant Members
10.4 Joint Restraint
10.5 Limiting Values of Slenderness Ratios
11.0 MINIMUM THICKNESSES
11.1 Structural Members
11.2 Gusset Plates
12.0 CONNECTIONS, END & EDGE DISTANCES
12.1 Bolting
12.2 Framing
12.3 End & Edge Distances
12.4 Center-to-Center Bolt Hole Spacing
13.0 ADDITIONAL REQUIREMENTS
13.1 Single Circuit Strung Condition
13.2 Stub Angle Sizing
13.3 Considerations for Linemen Weight
APPENDIX 1
14.0 BIBLIOGRAPHY

1.0 SCOPE
1.1 This standard stipulates various design considerations to be adopted in the
design of self supporting, latticed steel, square based towers for use in the
overhead transmission line system of Saudi Electricity Company (SEC), Saudi
Arabia.
1.2 This standard includes classification of loads, loading conditions, combination of
loads, overload factors and permissible stresses.
1.3 This standard does not cover design considerations for guyed steel towers and
special towers for river crossings and other long span crossings.
2.0 MATERIALS
2.1 Structural Steel
The tower members including cross-arms shall be of structural steel made by
either open hearth, basic oxygen or electric furnace process and shall conform to
the provisions of latest revisions of the following standards or equivalent:
ASTM A36M 250 MPa, minimum yield stress
ASTM A572M (Grade 345) 345 MPa, minimum yield stress
2.2 Bolts, Nuts and Washers
Bolts, nuts and washers shall conform to ASTM A394, ASTM A563M and
ASTM F436M respectively or equivalent.
2.3 Galvanizing
Structural steel members, including stub angles, shall be hot-dip galvanized after
fabrication in accordance with the requirements of 01-TMSS-01 to a galvanizing
thickness as specified in 20-TMSS-01.
Bolts and other fasteners shall be galvanized in accordance with the
requirements of 01-TMSS-01 and 20-TMSS-01.
2.4 Other materials used in the construction of towers shall conform to 20-TMSS-
01.
3.0 CLASSIFICATION OF LOADS
Transmission lines are subjected to various types of loads during their lifetime. These
loads are broadly classified into three distinct categories:

- Climatic Loads: Loads imposed on towers and line components by the action of
wind and coincident temperature.
- Longitudinal and Torsional Loads: Loads imposed on towers due to breakage of
line components, sabotage, or cascade failure.
- Construction and Maintenance Loads: Loads imposed on towers during
construction and maintenance activities. Checking of strength of tower members
for these loads ensures safety of workmen during construction and maintenance
activities.
Towers shall be designed to withstand factored loads, including their simultaneous
application, arising out of above loading conditions, per details given below for each
category:
3.1 Climatic Loads
These are random loads imposed on towers, insulator strings, hardware,
conductors and ground wires due to the action of wind and do not act
continuously.
Loads due to climatic effects shall be considered for the following wind and
coincident temperature conditions and applied on towers and line components
(conductors, ground wires, insulators, spherical markers, warning lights, spacer
dampers, hardware etc.).
-Standard Wind: A wind pressure of 430 N/m2
at minus 1°C (-1°C)
-High Wind: A wind pressure of 1064 N/m2
at an every day temperarure
Notes:
i. The word “wire” shall mean conductor, ground wire, and OPGW
when used as “intact wire”, wire tension, wire loading, etc.
ii. The word “ground wire” shall mean OHGW and OPGW.
iii. Every day temperature for various SEC Operating Areas shall be as
in Table 05-1 below:
Table 05-1: Every Day Temperature
SEC Operating Area
Every Day
Temperature, o
C
Central 25
Eastern 27
Western 30
Southern 25 & 30

3.1.1 Standard Wind Loading
Under this condition of loading, the following shall be assumed:
a. All wires intact
b. Wind acting normal to the longitudinal face of tower
c. A wind pressure of 430 N/m2
acting on the projected area of
tower members of front face multiplied by the shape factor as
given in Clause 4.0
d. A wind pressure of 430 N/m2
acting on the projected area of
ground wires and conductors, on effective projected area of
insulator strings, spherical markers, warning lights, spacer
dampers, hardware, etc.
e. Wire (conductor, ground wire, OPGW) tensions corresponding to
430 N/m2
wind at minus 1°C (-1°C) temperature, final condition
3.1.2 High Wind Loading (Transverse)
a. All wires intact
acting on the projected areas of
tower members of front face multiplied by the shape factor as
given in Clause 4.0 for heights up to 10m above ground level. For
tower heights above 10m, the wind pressures shall be increased
per the procedure given in Clause 4.0 multiplied by the shape
factor.
Basic wind pressures and wind pressures inclusive of shape
factors for heights above 10m are given in Table 05-2 for
guidance.
acting on full projected area of
insulator strings, spherical markers, warning lights, spacer
dampers, hardware, etc.
e. Wire tensions corresponding to 1064 N/m2
wind at an every day
temperature, final condition

3.1.3 High Wind Loading (Longitudinal)
a. All wires intact
b. Wind acting normal to the transverse face of tower
c. Same as in Clause 3.1.2c except that the front face is transverse
face.
d. Wind load on conductors and ground wires due to longitudinal
wind shall be taken as nil.
Wind load on insulator strings, spherical markers, warning lights,
spacer dampers, hardware, etc., same as that for High Wind
Loading (Transverse) case Clause 3.1.2d, but in longitudinal
direction.
e. Same as in clause 3.1.2.e
3.1.4 High Wind Loading (Oblique, 45°wind)
a. All wires intact
b. Wind acting at 45 degree to the longitudinal face of tower
x 0.71 acting on projected areas of
tower members on transverse and longitudinal faces
simultaneously multiplied by the shape factor as given in Clause
4.0 for heights up to 10m above ground level
For tower heights above 10m, the wind pressures shall be
increased as per the procedure given in Clause 4.0 and 71 percent
of full wind pressure multiplied by the shape factor applied on
transverse as well as on longitudinal faces simultaneously
(1064 x Cos2
45º) acting on full
projected area of ground wires and conductors, on effective
projected area of insulator strings, spherical markers, warning
lights, spacer dampers, hardware, etc.
e. Same as in Clause 3.1.2.e

Notes:
i. For the purpose of calculating wind load on bundled
conductors, wind pressure shall be considered acting on
full projected area of each conductor in the bundle
ii. For the purpose of computing the wind load on insulator
strings, the effective projected area of the insulator string
shall be assumed as 50 percent of the projected area of the
cylinder with diameter equal to that of the insulator skirt.
For multi-strings, wind shall be considered acting on each
limb of multi-string.
iii. No reduction in wind span shall be considered for oblique
wind and for line deviation on angle towers.
3.2 Longitudinal and Torsional Loads
Designing a tower for longitudinal and torsional loads ensures adequate strength
of tower in longitudinal direction and provides margin for containment of failure
due to cascading effect. Longitudinal loads may be imposed on towers due to
failure of line components, conductors, ground wire or reduction/removal of one
side tension.
3.2.1 Broken Wire Loading
The number of conductors and/or ground wire to be considered broken
shall be as defined in clause 7.0 “Broken Wire Conditions” for
Tangent/Suspension, Angle (Tension), Anchor and Dead-End towers.
a. Conductor(s) and/or ground wire(s) broken as given in Clause 7.0
applied to the sum of the projected
areas of tower members of front face multiplied by shape factor
as given in Clause 4.0
acting on full projected areas of
insulator strings, spherical warning markers, warning lights,
spacers, hardware, etc.
e. Wire tensions corresponding to 430 N/m2
wind at minus 1°C (-
1°C), initial condition

3.2.2 Anti Cascading Loadings
Cascade failures may be caused by failure of items such as insulators,
hardware, joints, failure of major components such as towers,
foundations, conductors or from climatic overloads or from casual
events such as misdirected aircraft, sabotage etc. The security measures
adopted for containing cascade failures in the line are to provide anchor
towers at specific intervals of every 10 to 12 km, which shall be
designed for anti-cascade loads. These towers serve the purpose of
sectionalizing the line for containing the cascade failure.
Loads and application of loads specified in Clause 3.3.1 for Angle
(Tension), Anchor and Dead-End towers under “Stringing Loads” shall
meet the requirement for anti-cascading checks.
3.3 Construction and Maintenance Loads
3.3.1 Stringing Loads
Stringing loads are imposed on towers during stringing and pulling
operations. These loads shall be assumed to act at wires
(conductors/ground wires) attachment points in longitudinal direction,
normal to the transverse face of tower.
a. Under this condition of loading, the following shall be assumed:
i. Pulling load due to stringing equal to tension of
conductors/ground wires at minus 1°C (-1°C), no wind,
initial condition
ii. Transverse loads on account of wind on tower and line
component shall be taken as nil.
iii. Transverse loads due to line deviation shall be based on
wire tensions at minus 1°C (-1°C), no wind, initial
condition.
b. Application of stringing loads on Tangent/Suspension, Angle
(Tension), Anchor and Dead-End towers shall be as follows:
i. Tangent/Suspension Towers
- Stringing load at any one phase conductor
attachment point with remaining phase conductors
and ground wires intact
- Stringing load at any one ground wire attachment
point with other ground wire, if provided, intact

- Stringing of any one phase conductor and/or
ground wire when temporarily dead-ended
(anchored) at a distance such that the angle
between the traveler on the tower and anchor on
the ground is 15º to the horizontal.
ii. Angle (Tension), Anchor and Dead-End Towers
Stringing loads at all phase conductor and ground wire
attachment points simultaneously. All stringing loads shall be
considered acting in one direction only and all wires considered
intact.
3.3.2 Heavy Vertical Loads
This loading condition represents the pick up load during stringing. The
upper tie members and lower main chord members of cross-arms are
subjected to heavy stresses due to heavy vertical loads during
construction activities and are required to be adequately sized for the
safety of workmen.
Under this loading condition, loads shall be considered acting at any
one conductor or ground wire attachment point at a time, the following
shall be assumed:
a. A vertical load equal to twice the vertical load resulting from
intact maximum design weight span plus weight of insulator
strings and 1500 Newtons to account for the weight of linemen
and tools
b. Transverse loads on account of wind on tower and line
components shall be taken as nil.
c. Transverse loads due to line deviation shall be based on wire
tensions at minus 1°C (-1°C), no wind, initial condition.
d. Pulling loads due to stringing shall be equal to tension of
conductors/ground wires at minus 1°C (-1°C), no wind, initial
condition.
4.0 APPLICATION OF WIND LOAD ON TOWERS
The wind pressure, qf, in N/m² is given by:
qf = Kz q10 SF (Eq. 05-1)

Where:
Kz = Exposure coefficient, which increases with height above ground level
=
7/2
10
H
⎥⎦
⎤
⎢⎣
⎡
H = Height above ground level, in meters
q10 = Basic wind pressure at 10 meters above ground, in N/m2
= 0 613 10
2
. × V
V10 = Wind velocity at 10 meters above ground. For high wind
loading V10 = 150 km/hr = 41.67 m/sec
q10 = 0.613 x (41.672)2
= 1064 N/m2
SF = Shape Factor (Pressure Coefficient)
= 2.0 Applied to the sum of the projected areas of members on
the front face of tower having cylindrical surfaces
= 3.2 Applied to the sum of the projected areas of members on
the front face of tower having flat surfaces
Wind pressures for high wind conditions for heights above 10 meters are given below in
Table 05-2 for guidance:
Table 05-2: Wind Pressures for Heights above 10 meters
Height Above
Ground
(m)
Exposure
Coefficient
(Kz)
Basic Wind
Pressure
(N/m2
)
Wind Pressure on
Cylindrical Surface
(N/m2
)
Wind Pressure on
Flat Surface
(N/m2
)
10 1.000 1064 2128 3405
20 1.219 1297 2594 4150
30 1.369 1457 2914 4660
40 1.486 1581 3162 5060
50 1.584 1685 3370 5395
60 1.669 1776 3552 5685
70 1.744 1856 3712 5940
80 1.811 1927 3854 6170

5.0 OVERLOAD CAPACITY FACTORS
The overload capacity factors to be applied to various loads under different loading
conditions shall be as given in Table 05-3.
Table 05-3: Overload Capacity Factors
Types of Loads
Standard
Wind
Loading
High
Wind
Loading
Broken
Wire
Loading
Stringing
Loading
Heavy
Vertical
Loading
Wind Load 2.50
Transverse
Loads Wire Tension
Load at angle
1.65
1.10 1.50 1.25 1.25
Vertical Loads 1.50 1.10 1.50 1.25 1.25
In general 1.10
Longitudinal
Loads
At dead-ends 1.65
1.10 1.50 1.25 1.25
Oblique Loads - 1.10 - - 1.25

6.0 TYPES OF TOWERS
The selection of the most suitable types of towers for transmission lines depends on the
actual terrain of the line. The types of towers generally suitable for most of the SEC
transmission lines are given in Table 05-4.
Table 05-4: Types of Towers
Type of Tower Deviation Angle
0º to 3º for 69kV to 230kV Transmission LinesTangent Tower with
Suspension Strings 0º to 2º for 380 kV Transmission Lines
Small Angle Tower with
Tension Strings
2º to 10º for 69kV to 230kV Transmission Lines
10º to 30º for 69kV to 230kV Transmission LinesLight Angle Tower with
Tension Strings 2º to 10º for 380kV Transmission Lines
30º to 45º for 69kV to 230kV Transmission LinesMedium Angle Tower with
45º to 60º for 69kV to 230kV Transmission LinesLarge Angle Tower with
Heavy Angle Tower with
Tension Strings
60º to 90º for 69kV to 380kV Transmission Lines
Anchor Tower with Tension
Strings
0º to 3º for 69kV to 380kV Transmission Lines (for
sectionalizing the line to avoid cascade failures)
Dead-End/Terminal Tower
with Tension Strings
0º to 20º angle of entry/take-off for 69kV to 380kV
Transmission Lines
Transposition Tower with
Tension Strings
0º to 2º for 69kV to380 kV Transmission Lines
Notes: i. The angles of line deviation specified are for the design span (design
ruling span). The span may, however, be increased up to an optimum
limit by reducing the angle of line deviation, provided the required
ground and phase clearances are met.
ii. Dead-End towers shall be designed for the maximum as well as
minimum angle of entry/take-off angle as specified in the above table.
iii. Tangent towers may be designed for zero angle of deviation and may be
used up to the maximum deviation angle specified in the above table by
reducing the design wind span corresponding to equivalent transverse
loads due to line deviation angle.

7.0 BROKEN WIRE CONDITIONS
The following broken wire conditions shall be assumed for the design of towers:
7.1 Single Circuit Towers
The combination of broken wire cases, whichever is more stringent than others
for a particular member.
7.2 Double Circuit Towers
7.2.1. Tangent/Suspension Towers – Any one phase or ground wire broken,
whichever is more stringent for a particular member.
7.2.2 Angle (Tension) Towers – Any two phases broken on the same side
and same span or any one phase and one ground wire broken on the
same side and same span, whichever combination is more stringent for
a particular member.
7.2.3 Anchor and Dead-End Towers – All phases and ground wires intact on
one side and broken on the other side of towers.
Notes: i. Phase shall mean all the sub-conductors in the case of bundled
conductors.
ii. Broken conductor shall mean all sub-conductors of bundle
broken.
8.0 COMPUTATION OF LOADS
8.1 Types of Loads
Three types of loads act on transmission towers, namely transverse, vertical and
longitudinal. Towers shall be designed to withstand simultaneous action of these
loads multiplied by overload factors as in Clause 5.0, depending on design
condition.
8.2 Sag and Tension
Sag and tension values for conductors and ground wires shall be determined for
various loading conditions keeping in view the tension limiting conditions
specified in TES-P-122.03 for computing tower loads.
8.3 Tabulation of Loads and Loading Trees
Transverse, vertical and longitudinal loads for various loading conditions
specified in this standard shall be determined in accordance with the procedures

given in Clauses 8.4, 8.5 and 8.6 and tabulated or loading trees prepared before
proceeding with stress calculations and design.
Sample loading calculations for a tangent tower are given in Appendix-1.
8.4 Transverse Loads
8.4.1 Wind on Conductors and Ground Wires
Loads due to wind on wires (conductors/ground wires) Fwc, in
Newtons, applied at wire attachment points in the direction of
transverse axis of tower, shall be determined by the following
expression:
Fwc = Pd L D N (Eq. 05-2)
Where:
Pd = Design wind pressure in N/m²
L = Wind span, being sum of half the spans on both sides of
attachment point, in meters. Values of wind spans as
specified in the Scope of Work and Technical
Specifications (SOW/TS) or the relevant engineering
standard (as the case may be) shall be adopted.
D = Diameter of conductor/ground wire, in meters
N = Number of sub-conductors in a bundle
Note : Wind span for broken conductor is generally in the range
of sixty percent (60%) of intact wind span.
8.4.2 Wind on Insulator Strings
Wind load on insulator strings, Fwi, in Newtons, applied at attachment
points shall be determined from the following expression:
Fwi = Pd Ai (Eq. 05-3)
Where:
Pd = Design wind pressure in N/m2
Ai = Effective projected area of insulator strings in sq. m.
Effective projected area of the insulator strings shall be
assumed as fifty percent (50%) of the projected area of the

cylinder with a diameter equal to that of the insulator
skirt.
Note: In case of multi-strings including V-strings, wind pressure
on all strings of multi-string shall be considered without
any masking effect.
8.4.3 Line Deviation
Load due to line deviation, Fd, in Newtons, applied at attachment
points in the direction of transverse axis of tower, shall be determined
by the following expression:
Fd = 2 N T Sin θ/2 for intact spans (Eq. 05-4)
Fd = N T Sin θ/2 for broken spans (Eq. 05-5)
Where:
T = Tension of conductor/ground wire under a given loading
condition, in Newtons
θ = Angle of line deviation, in degrees
8.4.4 Wind on Tower
A latticed tower consists of panels of varying heights. These panels are
formed between the intersection of legs and bracings. It is conventional
to determine wind load on each panel and transfer the same to wire
attachment points as point loads. In addition to lumping the wind loads
on wire attachment points, it is sometimes desirable (in case of taller
towers) to lump wind loads at extension tops and at one or two points
in the tower body also. The practice of lumping tower wind loads at
wire attachment points reflects a practical approach and facilitates
application of tower wind loads during full scale testing of tower.
Tower wind may also be distributed throughout the height of tower, as
is the case when tower is designed on computer and wind loads are not
input as point loads at wire attachment points. This may reflect true
field conditions provided that all the tower members are included in
tower model during analysis. Further this may require redistribution of
wind loads at wire attachment points and at some panel points to
facilitate application of wind loads during full scale tower testing.
Wind load, Fwt, in Newtons, for wind normal to the latticed face of the
tower, on a panel height “H” assumed to be acting at center of gravity
is given by the expression:

Fwt = Pd Ae (Eq. 05-6)
Where:
Pd = Design wind pressure in N/m2
Ae = Net projected surface area of legs, bracings, cross-arms
and secondary members of latticed face of the panel, in
sq.m. Projections of bracing members of the adjacent
faces and of the plan and hip bracings may be neglected
while determining net projected area of members.
Note: For transverse wind, longitudinal face of tower is latticed
face and for longitudinal wind, transverse face of tower is
the latticed face. For diagonal (oblique, 45°) wind, wind
pressure for both transverse and longitudinal faces is 0.71
times the design wind pressure. Total transverse load at
each conductor/ground wire attachment points is the sum
of Fwc, Fwi and Fd. Transverse loads due to tower wind of
each panel, Fwt is distributed as explained above.
8.5 Vertical Loads
8.5.1 Weight of Conductors, Ground Wires and Hardware
Loads due to weight of wires, Vc, in Newtons, shall be determined as
below and applied at wire attachment points:
Vc = W1 Wt N + W2 (Eq. 05-7)
Where:
W1 = Unit weight of conductors and ground wires, in N/m
W2 = Weight of spherical warning markers, warning lights,
spacers and hardware, etc., in Newtons
Wt = Weight span, being the horizontal distance between the
lowest points of conductors/ground wires on the two spans
adjacent to the tower under consideration, in meters
The lowest point is defined as the point at which the tangent to the sag
curve or to the extended sag curve is horizontal. Values of weight spans
as specified in SOW/TS or relevant engineering standard (as the case
may be) shall be adopted.

In a transmission line, three types of weight spans may be encountered.
They are:
- Minimum weight span
- Maximum downward weight span and
- Maximum upward weight span
Tangent/Suspension towers shall be designed for both minimum and
maximum downward weight spans. Minimum weight span may
govern design of foundation under uplift and also some tower
members. Minimum weight span also governs the swing of suspension
insulator strings.
Tangent/Suspension tower shall never be used at locations, which are
subjected to uplift.
Angle (Tension), Anchor and Dead-End towers may be required to be
designed for upward weight span in addition to downward weight span.
Upward forces on a tower result due to higher elevation of wire
attachment points on adjacent towers.
8.5.2 Weight of Hardware and Insulator Strings
Load due to weight of hardware and insulator strings, VL,in Newtons,
shall be applied at wire attachment points. Weight of insulator string
shall be taken as the sum of weight of all the insulators and hardware in
a string/multi-string.
8.5.3 Weight of Linemen and Tools
Weight of Linemen and tools, VL , in Newtons, shall be applied at
ground wire and conductor attachment points for the specified loading
conditions.
8.5.4 Self-Weight of Tower
Self-weight of tower is the weight of all the structural members, plates,
nuts, bolts, washers, step bolts, ladders and all other accessories of
tower. The weight of insulator washing facilities such as wash
platforms, interconnecting walkways and handrails are to be included
where the insulators are at a height of forty (40) meters and above.
Self-weight of tower shall either be calculated on the basis of unit
weight of each member of tower and increased by about 10 to 12
percent to account for the weight of nuts, bolts, washers, cleats, plates
etc. and distributed at panel points or shall be generated and distributed
automatically when tower will be designed on computer. The computer

generated weight shall also be increased by a suitable percentage to
account for weight of above items plus weight of redundant members.
Total vertical load at each wire attachment point is the algebraic sum of
Vc, Vi, and VL
.
8.6 Longitudinal Loads
Longitudinal loads are caused by any of the following loading conditions:
8.6.1 Longitudinal and Oblique Winds Under High Wind condition
Longitudinal loads due to high wind in longitudinal and oblique
directions shall be calculated for insulator strings and towers as per
procedure in Clauses 8.4.2 and 8.4.4 and in accordance with
assumptions in Clauses 3.1.3.c and 3.1.4.c.
These loads shall be applied in longitudinal direction, normal to the
transverse face of tower for longitudinal wind and in transverse and
longitudinal directions for diagonal/oblique wind.
8.6.2 Broken Wire Condition
Unbalanced pulls shall be based on wire tensions determined according
to loading criteria specified in Clause 3.2.1.e and applied at wire
attachment points, normal to the transverse face of tower.
Number of conductors and ground wires to be considered broken shall
be as in Clause 7.0.
a. Tangent/Suspension Towers
Longitudinal loads for broken conductors get reduced due to
swing of suspension strings. The longitudinal load shall be
calculated using tension in the broken conductor as sixty percent
(60%) of the tension in the intact conductor.
Longitudinal loads for broken ground wires shall be without any
reduction in tension.
b. Angle (Tension), Anchor and Dead-End Towers
Longitudinal loads for broken conductors/ground wires shall be
the component of tension in longitudinal direction corresponding
to minimum design angle of deviation for angle (tension) towers.
Longitudinal loads for broken conductors/ground wires shall be
taken as nil for Anchor and Dead-End towers.

8.6.3 Stringing Condition
Stringing loads shall be assumed equal to tension of conductors/ground
wires at minus 1°C (-1°C), no wind, initial condition and applied in the
longitudinal direction:
- at any one conductor or ground wire attachment point at a time in
case of Tangent/Suspension towers.
- at all conductor and ground wire attachment points
simultaneously in case of Angle (Tension), Anchor and Dead-End
towers.
- Stringing of any one phase conductor and/or ground wire when
temporarily dead-ended (anchored) at a distance such that the
angle between the traveler on the tower and anchor on the ground
is 15º to the horizontal.
8.6.4 Intact Span Condition for Anchor and Dead-End Towers
Longitudinal loads due to one sided tensions for Anchor and Dead-End
towers shall be equal to full tension of conductors and ground wires
under worst loading condition. These loads shall be applied
simultaneously at all wire attachment points in one direction only.
8.7 Other Loads
Towers shall be designed to include the effects of transverse loads due to wind
on spherical markers and phase conductor warning lights etc. and their vertical
weights. These loads including the overload capacity factors shall be included in
the phase conductors and overhead ground wires loads and shall be considered
mandatory in the design of towers regardless of the actual project requirements.
9.0 PERMISSIBLE STRESSES
9.1 Axial Stresses in Tension
9.1.1 The design tensile stress on the net cross-sectional area An of
concentrically loaded tension member, shall not exceed minimum
guaranteed yield stress Fy of the material. In case the angle section is
connected by one leg only, the design tensile stress on the net sectional
area shall not exceed 0.9 Fy.
9.1.2 The net cross-sectional area, An, is the gross cross-sectional area Ag
(the sum of the products of the thickness and the gross width of each
element as measured normal to the axis of the member) minus the loss
due to holes or other openings at the section being investigated. If there

is a chain of holes in a diagonal or zigzag line, the net width of an
element shall be determined by deducting from the gross width the sum
of diameters of all the holes in the chain and adding for each gage
space in the chain the quantity s²/4g, where s = longitudinal spacing
(pitch) and g = transverse spacing (gage) of any two consecutive holes.
The critical net cross-sectional area, An, is obtained from the chain
which gives the least net width.
9.1.3 Plain angle sections bolted in both legs at both ends shall be considered
to be concentrically loaded. Leg members of latticed steel towers fall in
this category.
9.1.4 Plain angle sections used as bracing members in latticed towers are
connected by one leg only. The allowable tensile stress for such
members shall be limited to 0.9 Fy. If the legs are unequal and the short
leg is connected, the unconnected leg shall be considered to be of the
same size as the connected leg.
9.1.5 While developing structural drawings, care shall be taken such that the
centroid of the bolt pattern lies between the heel of the angle and the
center line of the connected leg. If this condition is not met, the
connection shall be checked for block shear in accordance with
equation (3.10-1) of ASCE Standard 10-97 “Design of Latticed Steel
Transmission Structures”.
9.2 Axial Stresses in Compression
9.2.1 The following provisions are applicable only to 90° angle sections.
9.2.2 The design compressive stresses in various members shall not exceed
the values given by the formulas in Clause 9.2.3.
9.2.3 The allowable unit stress, Fa, in MPa on the gross cross-sectional area
or on the reduced area where specified, of axially loaded compression
members shall be:
Fa =
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−
2
/
2
1
1
cC
rKL
Fy when KL/r ≤ Cc (Eq. 05-8)
Fa =
( )2
2
/ rKL
Eπ
when KL/r > Cc (Eq. 05-9)
Cc =
yF
E2
π (Eq. 05-10)

Where:
Fy = Minimum yield stress of steel, MPa
E = Modulus of elasticity of steel
= 2 x 105 MPa
K = Effective length factor
KL/r = Largest effective slenderness ratio of any unbraced
segment of a compression member
L = Unbraced length of the compression member, cm
r = Appropriate radius of gyration, cm
9.2.4 The formulas given in Clause 9.2.3 are applicable provided that the
largest value of w/t (width-thickness ratio) does not exceed the limiting
value given by
(w/t)Lim =
210
Fy
(Eq. 05-11)
Where:
w = Distance from edge of fillet of angle section to the
extreme fiber, mm.
t = Thickness of leg, mm
9.2.5 Where the width-thickness ratio exceeds the limit given in Clause
9.2.4, equations (05-8) and (05-10) given in Clause 9.2.3 are used by
substituting for Fy the value Fcr given by
Fy
tw
Fcr ⎥
⎦
⎤
⎢
⎣
⎡
−=
(w/t)
)/(677.0
677.1
Lim
when (w/t)Lim ≤ w/t ≤
378
Fy
(Eq. 05-12)
and
Fcr = 2
)w/t(
65550
when w/t >
378
Fy
(Eq. 05-13)
9.2.6 The maximum permissible value of w/t for any type of steel shall not
exceed 25.

9.3 Stresses in Bolts
9.3.1 Shear Stress: The shear strengths of bolts conforming to ASTM A394,
Type 0, are given in Table 05-5 and these shall not be exceeded. For
bolts in double shear the specified single shear values shall be
multiplied by 2.
Table 05-5: Shear Strengths of ASTM A394 Type 0 Bolts
Nominal Bolt
Size
(mm)
Single Shear Strength
Through Threads
(kN)
Single Shear Strength
Through Body
(kN)
16 50 60
20 80 95
22 100 120
24 120 140
Notes:
i. The above Table for ASTM A394, Type 0, bolts is based
on shear strength of 380 MPa (55,200 psi unit shear
strength) across the area at root of threads and 316 MPa
(45,880 psi unit shear strength) across the nominal area.
ii. For bolts conforming to other recognized International
Standards and sizes, the chemical composition shall
conform to or be better than ASTM A394, Type 0 and the
allowable shear stress Fv on the effective area shall be
0.62Fu where Fu is the specified minimum tensile strength
of the bolt material. The effective area is the gross cross-
sectional area of the bolt if threads are excluded from the
shear plane or the root area if the threads are in the shear
plane.
iii. It is recommended that shear strength values through
threads shall be used for design purposes to take care of
any possibility of threaded portion of bolt extending into
plane of shear during erection.
9.3.2 Bearing Stress
The maximum bearing stress, calculated as the force on a bolt divided
by the product of the bolt diameter and the thickness of the connected
part is limited between 1.2 to 1.5 times the specified minimum tensile
stress Fu of the connected part (member) or the bolt. A bearing stress
value of 1.25 times the minimum tensile stress Fu shall be adopted in
this standard to permit reduction in the end and edge distances.

For bolts conforming to ASTM A394, Type 0, the bearing stress on
bolt diameter for structural steels specified in this standard shall be as
follows:
ASTM A36M 500 MPa
ASTM A572M (Grade 345) 560 MPa
10.0 EFFECTIVE SLENDERNESS RATIOS AND LIMITING CONDITIONS
The effective slenderness ratio, KL/r, of compression and redundant members shall be
determined as follows:
10.1 Leg Members
For leg members bolted in both faces at connections,
r
L
r
KL
= 0 ≤
L
r
≤ 150 (Eq. 05-14)
10.2 Other Compression Members
For members with a concentric load at both ends of the unsupported panel,
KL
r
L
r
= 0 ≤
L
r
≤ 120 (Eq. 05-15)
For members with a concentric load at one end and normal framing eccentricity
at the other end of the unsupported panel,
KL
r
L
r
= +30 0 75. 0 ≤
L
r
≤ 120 (Eq. 05-16)
For members with normal framing eccentricities at both ends of the unsupported
panel,
KL
r
L
r
= +60 05. 0 ≤
L
r
≤ 120 (Eq. 05-17)
For members unrestrained against rotation at both ends of the unsupported panel,
KL
r
L
r
= 120 ≤
L
r
≤ 200 (Eq. 05-18)
For members partially restrained against rotation at one end of the unsupported
panel,

KL
r
L
r
= +28 6 0 762. . 120 ≤
L
r
≤ 225 (Eq. 05-19)
For members partially restrained against rotation at both ends of the unsupported
panel,
KL
r
L
r
= +46 2 0 615. . 120 ≤
L
r
≤ 250 (Eq. 05-20)
10.3 Redundant Members
KL
r
L
r
= 0 ≤
L
r
≤ 120 (Eq. 05-21)
If members are unrestrained against rotation at both ends of the unsupported
panel,
KL
r
L
r
= 120 ≤
L
r
≤ 250 (Eq. 05-22)
If members are partially restrained against rotation at one end of the unsupported
panel,
KL
r
L
r
= +28 6 0 762. . 120 ≤
L
r
≤ 290 (Eq. 05-23)
If members are partially restrained against rotation at both ends of the
unsupported panel,
KL
r
L
r
= +46 2 0 615. . 120 ≤
L
r
≤ 330 (Eq. 05-24)
10.4 Joint Restraint
A single bolt connection at either the end of a member or a point of intermediate
support shall not be considered as furnishing restraint against rotation. A
multiple bolt connection, detailed to minimize eccentricity, shall be considered
to offer partial restraint if the connection is to a member capable of resisting
rotation of the joint. A multiple bolt connection to an angle or angle chord
member, detailed to minimize eccentricity, shall not be considered to offer
partial restraint if the connection is made only on a gusset plate without also
being framed to the restraining member.
To justify using the values of KL/r in equations 05-19, 05-20, 05-23 and 05-24,
the following evaluation is suggested:
- The restrained member must be connected to the restraining member
with at least two bolts.

- The restraining member must have a stiffness factor I/L in the stress
plane (I = Moment of inertia and L = Length) that equals or exceeds the
sum of the stiffness factors in the stress plane of the restrained members
that are connected to it; and
- Angle members connected solely to a gusset plate should not be
considered to have end restraint against rotation. An angle member with
an end connection to both a gusset plate and the restraining angle
member should have adequate bolts in the restraining angle member to
provide end restraint against rotation.
Angle members connected by one leg shall have the holes located as close to the
outstanding leg as practical. Except for some of the smaller angles, normal
framing eccentricity implies that the centroid of the bolt pattern is located
between the heel of the angle and the centerline of the connected leg.
In calculating the slenderness ratios of the members, the length L shall be the
distance between the intersection of the center of gravity lines at each end of the
member.
10.5 Limiting Values of Slenderness Ratios
10.5.1 The limiting values of effective slenderness ratio, KL/r, for
compression members shall be as follows:
Leg members, ground wire peak members, and
main members of cross-arms in compression 150
Other members carrying computed stresses 200
Redundant members/secondary members
carrying nominal stresses 250
10.5.2 Slenderness ratio, L/r, of a member carrying
axial tension only shall not exceed 375
Other tension members 500

11.0 MINIMUM THICKNESSES
11.1 The minimum thickness of structural members shall be as given in Table 05-6:
Table 05-6: Minimum Thickness of Members
Type of Members Minimum Thickness, mm
Leg members, ground wire peak members and
main members of cross-arms in compression
6
Other stress carrying members 5
Redundant members* 4
Stub Angle 8
Note: *The redundant members shall be designed for 2.5% of the load in the
supported member.
11.2 Gusset plates
These shall be designed to resist the shear, direct and flexural stresses acting on
the weakest or critical sections. Minimum thickness of gusset plate shall be 2mm
more than the lattice connected to it only in case when the lattice is directly
connected on gusset plate outside the leg member. In no case the thickness of
gusset plate shall be less than 5mm.
12.0 CONNECTIONS, END & EDGE DISTANCES
Bolted connections for transmission towers are normally designed as bearing type
connections. It is assumed that bolts connecting one member to another carry the load in
the connection equally.
The end and edge distances specified in this standard are based on bearing stress value
of 1.25 times the minimum specified tensile stress of weaker of the material of bolt or
connected part and these do not include any allowance for fabrication and rolling
tolerances.
12.1 Bolting
12.1.1 Minimum Diameter of Bolts
The diameter of bolts shall not be less than 16mm.
12.1.2 Preferred Sizes of Bolts
Bolts used for erection of transmission line towers shall preferably be
of diameter 16mm and 20mm. Preferably one size of connection bolts
and nuts shall be used for Tangent/Suspension type towers. However,

for Angle (Tension), Anchor and Dead-End type towers, two sizes of
connection bolts may be used. All bolts shall be provided with hot-dip
galvanized steel locknuts.
The length of bolts shall be such that the threaded portion does not lie
in the plane of contact of members. The projected portion of the bolt
beyond the locknut shall be between 3 to 8mm but not less than three
(3) effective threads.
12.1.3 Gross and Net Area of Bolts
For the purpose of calculating the shear stress the gross area of bolt is
taken as the nominal area of bolt only when it is ensured that the
threaded portion of bolt will not extend into the plane of contact of
connected members. In this standard the shear area of bolts has been
taken as the area at the root of threads (see Clause 9.3.1 and Table 05-
5). The bolt area for bearing shall be taken, as d x t where d is the
nominal diameter of the bolt, and t is the thickness of the thinner of the
parts joined.
The net area of a bolt in tension shall be taken as the area at the root of
the thread.
12.1.4 Holes for Bolting
The diameter of the hole drilled/punched shall be 1.5mm more than the
nominal diameter of the bolt.
12.2 Framing
The angle between any two members common to a joint of a trussed frame shall
preferably be greater than 20° and never less than 15° due to uncertainty of stress
distribution between two closely spaced members.
12.3 End & Edge Distances
The end distance is the distance from the center of a hole to the end of the
member, whether this end is perpendicular or inclined to the line of force.
The edge distance is the distance from the center of a hole to the rolled or
sheared edge. It is a perpendicular distance between the nearest gage line of
holes to the rolled or sheared edge running parallel to the gage line.
12.3.1 Stressed members
The required end distance is a function of the load being transferred in
the bolt, the tensile strength and thickness of the connected part.

For stressed members the end and edge distances are given in Table 05-
7. These are minimum and shall not be underrun by fabrication &
rolling tolerances.
Table 05-7: End and Edge Distances for Stressed Members
Bolt diameter (mm)
Description
16 20 22 24
Thickness limitation for punched
holes for ASTM A36M Steel (mm)
16 20 22 24
End distance (sheared or
mechanically guided flame cut
end)
24 30 33 36
Edge distance (Rolled Edge) 21 26 28 32
Edge distance (sheared or
mechanically guided flame cut
edge)
23 28 30 34
The values of end and edge distances given in Table 05-7 are
applicable for all types of steels specified in this standard except that
punching thickness limitations are applicable to ASTM A36M steel.
For high strength steels (ASTM A572M, Grade 345) the thickness of
material for punched holes shall be the thickness specified in Table 05-
7 minus 1.5mm for maintaining the same end and edge distances.
Where the thickness of the angle section is more than the limiting value
of thickness given in Table 05-7 and holes are punched, the end and
edge distances will be governed by the following equations.
End distance (mm) = t + d/2 when t ≥ d (Eq.05-25)
Edge distance (mm) = 0.85[t + d/2]when t ≥ d (Eq. 05-26)
(Rolled edge)
Edge distance (mm) = 0.85[t + d/2] + 2.0 when t ≥ d (Eq. 05-27)
(sheared or mechanically guided flame cut)
Where:
d = Nominal diameter of bolt, mm
t = Thickness of connected leg, mm
The above provisions shall not apply if holes are drilled. Values of end
and edge distances shown in the Table 05-7 shall also be applicable for
member thicknesses in excess of those shown in Table 05-7, for drilled
holes.

12.3.2 Redundant Members.
The end and edge distances of redundant members shall not be less
than the values given in Table 05-8.
Table 05-8: End and Edge Distances for Redundant Members
Bolt diameter (mm)
Description
16 20 22 24
End distance (sheared or
mechanically guided flame cut end)
20 24 27 30
Edge distance (Rolled Edge) 17 21 23 26
Edge distance (sheared or
mechanically guided flame cut edge)
19 23 25 28
12.4 Center-to-Center Bolt Hole Spacing
The center-to-center bolt hole spacing is fixed to meet the following
requirements:
12.4.1 Strength requirement, given by:
Smin=
tF
2.1
U
Ρ
+ 0.6d (Eq.
05-28)
Where:
Smin= Minimum center-to-center spacing between two holes in
the line
of transmitted force, mm
P = Load transmitted through the bolt, Newtons
UF = Minimum tensile strength of steel, MPa.
t = Thickness of connected part (leg), mm.
d = Diameter of bolt, mm.
12.4.2 Installation requirement, given by:
Sinst = Width across flats of nut + 10mm.

The minimum bolt spacing for various bolt diameters, considering
above requirements and bolt bearing stresses specified in this standard
are given in Table 05-9.
Table 05-9: Minimum Center-to-Center Bolt hole Spacing
Bolt diameter (mm)
Description
16 20 22 24
Minimum Spacing (mm) 35 45 50 55
13.0 ADDITIONAL REQUIREMENTS
13.1 Single Circuit Strung Condition
For double circuit towers the design shall be suitable both for single circuit and
double circuit strung conditions.
13.2 Stub Angle Sizing
The unbraced portion of the stub between the last bracing connection and top of
concrete is subjected to combined axial and bending stresses. The stub angle
shall be suitable to withstand combined stresses.
The stub angle and the shear connectors shall be designed in accordance with the
ASCE Standard 10-97 “Design of Latticed Steel Transmission Structures” for
the maximum groundline reactions with overload capacity factors. Bonding
between the stub angle and concrete shall be ignored.
The thickness of the stub angle shall be 2mm more than the thickness of the leg
member to which it is connected.
13.3 Considerations for Linemen Weight
Horizontal or near horizontal tower members shall be capable of supporting a
vertical load of 1100 Newtons (vertical weight of linemen and tools). This load
shall be applied independently of all other loads without permanent distortion of
the members.

APPENDIX - 1
(Sheet 1 of 2)
Project Title:
__________
Code No.
________
Tower Type
_________
Drawing No.
__________
Page No.
_______
Design Span (m)
Line Angle (Deg):
Wind Span (m)
Normal:
Broken:
Max. Weight Span (m)
Normal:
Broken:
Min/Uplift Weight Span (m)
Normal:
Broken:
LOADING CONDITION (Tick as Applicable)
Standard
Wind
High Wind
Transverse
High Wind
Longitudinal
High Wind
Oblique
Broken Wire Stringing Heavy
Vertical
TRANSVERSE LOADS OLF Newtons
OHGW/OPGW
Wind Load on OHG Wire.....................................................................
Wind Load on SM, VD, HS*................................................................
Angle Pull............................................................................................
TOTAL
x.......
x.......
x.......
=
=
=
=
Conductor
Wind Load on Conductor.....................................................................
Wind Load on Insulators......................................................................
Wind Load on SP, VD, WL*...............................................................
Angle Pull...........................................................................................
TOTAL
x ......
x ......
x ......
x.......
=
=
=
=
=
VERTICAL LOADS MAXIMUM
OHGW/OPGW
Weight of OHG Wire............................................................................
Weight of SM, VD, HS*.......................................................................
Weight of Lineman and tools.................................................................
TOTAL
x......
x......
x......
=
=
=
=
Conductor
Weight of conductor.............................................................................
Weight of SP, VD, WL*....................................................................
Weight of Insulators..............................................................................
Weight of Lineman & tools.................................................................
TOTAL
x......
x......
x......
x......
=
=
=
=
=
* SM = Spherical Markers VD = Vibration Dampers
HS = Hardware Sets SP = Spacer Dampers
WL = Warning Lights
NOTE: Loads due to wind on tower shall be calculated and applied in transverse and longitudinal directions as the
case may be.
Continued on sheet 2

APPENDIX - 1
(Sheet 2 of 2)
Project Title:
___________
Code No.
_________
Tower Type
__________
Drawing No.
__________
Page No.
_______
Design Span (m)
Line Angle (Deg):
Wind Span (m)
Normal:
Broken:
Max. Weight Span (m)
Normal:
Broken:
Min./Uplift Weight Span (m)
Normal:
Broken:
LOADING CONDITION (Tick as Applicable)
Standard
Wind
High Wind
Transverse
High Wind
Longitudinal
High Wind
Oblique
Broken Wire Stringing Heavy
Vertical
VERTICAL LOADS MINIMUM OLF Newtons
OHWG/OPGW
Weight of OHG Wire...........................................................................
Weight of SM & VD*..........................................................................
Weight of Lineman and tools................................................................
TOTAL
x......
x......
x........
=
=
=
=
Conductor
Weight of conductor.............................................................................
Weight of SP, VD, WL*.......................................................................
Weight of Insulators..............................................................................
Weight of Lineman & tools....................................................................
TOTAL
x......
x......
x......
x........
=
=
=
=
=
* SM = Spherical Markers
VD = Vibration Dampers
HS = Hardware Sets
SP = Spacer Dampers
WL = Warning Lights
LONGITUDINAL LOADS (WIRE PULL, WIND ON INSULATORS**) Newtons
OHGW/OPGW ..................................................................................
Conductor ..................................................................................
(**Applicable for longitudinal and diagonal winds only)
=
=
NOTE: Loads due to wind on tower shall be calculated and applied in transverse and longitudinal directions as the case
may be.

14.0 BIBLIOGRAPHY
1. ASCE Manual No. 74 “Guidelines for Electrical Transmission Line Structural
Loading”, American Society for Civil Engineers, First Edition (1991).
2. ASCE Standard 10-97 “Design of Latticed Steel Transmission Structures”,
American Society for Civil Engineers, Second Edition (2002).
3. IEC 60826: "Design Criteria of Overhead Transmission Lines", Third edition
2003.
4. Holland H. Farr, "Transmission Line Design Manual", United States Department
of the Interior.
5. Rural Electrification Administration (U.S. Department of Agriculture), "Design
Manual for High Voltage Transmission Lines".

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