Transmission Line Manual

TRANSMISSION LINE MANUAL Publication No. 268

Central Board of Irrigation and Power Malcha Marg, Chanakyapuri, New Delhi - 110 021

CBI&P Panel of Experts on Transmission Lines Editor

'.J. Varma

Chairman D

I\n Ahh I\A,~Ii~

CENTRAL BOARD OF IRRIGATION AND POWER Esrablished 1\127

OBJECTIVES • To render expertise in the fields of water resources and energy; • To promote research and professional excellence; • To provide research linkages to Indian engineers, researchers and managers with their cOWlterparts in other countries andintemational organisations; • To establish database of technical and technological developments, and provide information services; • Teclmological forecasting.

(

ACfIVlTIES

1. AdvancemtDt of Knowledge iUld Tecbnologital Forecasting $. 1;;~' ~ ~ .:'-:~: :" ..':":" ~~ .'1'.: ~ '. : .: ,• .,': ,. ~.: ~t . ~ ~ ~ ::: ci6rlecttbri'~dc6rnPiiati'on ~f data and pooling ofteclmical knowledge and experiences. ' : .•. '.';" ~ i M'i";~"::"~ ~p~p~o.Iro.f ~baSesm water and power sectors in electronic format at the national level for easy access. ~::r.' ,n:; ··.r ",.: lllntrOductioo and' implementation of Internet, Intranet. E-Business and E-Commerce for infrastructure ,·il·~.t..~; ,.. ..,' f.I,~i~ities ... >: ; ':- .. ' .... :o'i..~; :.. / • :,,... 0','- Introduction of paperless office, flow charting and documentation management. !~'.{ f' 1\ .:*C pissentinaticilof.iri£ojrnation - Library and infonnation services. o~., i r ,: * Organismg nati~l and ~onal seminars, symposia, conferences, workshops, roundtables, etc, ;~(t::"'~ ~., ,* . RecOghizmgOUtstanding'contiibutions of engineers and managers by presenting them various CSIP awards.

t'

~

..

,"

'." ..... ,

'fttI, ~;,t:i

'~'~''':. ~ .

i

M

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e ~

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EXHIBIT 1.10 Sector·wise Utilisation of Funds for Power ~tl

(Figurt's Rs. crorf's) SI. Period No.

.

Total Funds utilised for Power

Sector wise Utilis.ation Generation

Transmission & Distribution

Others

Amount

%

Amount

%

Amount

%'

23

~

95

21

1. 1st F.Y. Plan (1951·56)

260

105

40

132

2. 2nd F.Y. Plan (1956-61)

460

250

54

115

51 . 25

3. 3rd F.Y. Plan (1961-66)

1252

777

62

301

24

174

14

4.

Annual F.Y. Plan (1966-69)

1223

676

55

291

24

256

21

5.

4th F.Y. Plan (1969-74)

2931

1505

51

768

26

658

23

6. 5th F.Y. Plan (1974·79)

7541

4467

59

2016

27

1058

14

7.

Annual Plan (1979-80)

2473

1429

58

720

29

324

13

8.

6th F.Y. Plan (1980-85)

18913

12116

64

4706

25

2091

11

9.

7th F.Y. Plan (1985-90)

38169

24528

64

9847

26

3794

HI

10. Annual Plan (1990-91)

10470

7003

67

2375

23

1092

10

11. Annual Plan (1991·92)

13904

10373

75

2661

19

870

6

12. 8th F.Y. Plan (192·97) Outlay

79730

49196

62

22432

28

8102

10

"

15

Transmission Line Manual Chapter 2

Tower Types and Shapes

CONTENTS Page 2.1

Scope

2.2

Types of Towers

2.2.2

Self-S~pporting

Towers

2.2.3 Conventional Guyed Tower

1

2.2.4 Chainette Guyed Tower

7

2.3

Tower Shapes

7

2.4

Tower Designation

7

2.4.2 Suspension Towers

8

2.4.3 Tension Towers

8

2.4.4 .Transposition Towers

8

2.4.5 Special Tower

CHAPTER 2

Page 1

TOWER TYPES AND SHAPES

1

1 1

7 7

7 8 8

8 8

2.1

SCOPE

2.1.1 The tower of various shapes had heen used in the past without considering detrimental influence on the environment. With conservation environmentalists attracting the highest attention and the public becoming more and more conscious of the detrimental effects of transmission line towers on the environment and occupation of land, transmission line tower designers have been endeavouring to develop towers with sllch shapes which blend with the environment. Other factors responsible for changes in shapes of towers are the need for the use of higher transmission voltages, limitation of right-of-way availability, audible noise level, radio and T.V. interference, electrostatic field aspects, etc. The types and shapes of Transmission Line Towers used in India and in other countries are discussed in this chapter. 2.2

TYPES OF TOWERS

2.2.1 The types of towers based on their constructional features, which are in use on the power transmission line are ~ven helow :

steel conforming to IS: 8500 is not readily available in the country, steel conforming to BS 4360 Gd 50B/ASTM A 572IJ1SNDE or any other InternationallNationai standards can be used. Some of the countries such as ; apan, USSR, Austria, Canada, France, etc., have explored use of other material such as steel formed angle sections, tubular sections, aluminium sections, etc., for fabrication of towers. In the case of heavy angle and long span crossing towers, some of the countries namely Russia, Norway, France, etc. are using single phase self-supporting towers. Selfsupporting towers usually have square/rectangular base and four separate footings. HoweveN'wer voltage narrow-based towers having combined monoblock footings may be used depending upon overall economy. Self-supporting towers as compared to guyed towers have higher steel consumption. Self supporting to\Ve~~~sed}~r compactline design. Compact tower may comprise fabricated steel body, cage and groundwire peak, fitted with insulated cross-arms. Qmwa~tion ~s also achi~ye~..E1..~angement of phases, ~ll~ing V insul_ator strings, etc. Compact towers iUiVereduced dimensions and require sm3iier right-of-way and are suitable for use in congested areas and for upgrading the voltage of the existing Transmission Lines ~Iso. Self-supporting towers are shown in Figures 1 & 2.

I.

Self-Supporting Towers

2.

Conventional Guyed Towers

2.2.3

~.

Chainette Guyed Towers

2.2.3.1 These towers comprise portal structures fabricated in "Y' and "V' shapes and have been use~ in some of the countries for EHV transmission lines upto 735 kV. The guys may be internal or external. The guyed tower including guy anchors occupy much larger land as compared to self-supporting towers and as such this type of construction· finds application in long unoccupied, waste land, bush tracts in Canada, Sweden, Brazil, USSR etc.

These are discussed in the subsequent paragraphs. 112.2

Self·Supporting Towers

Self-supporting broad based/narrow based latticed steel towers are used in India and other countries. This type \oftower has been in use in India from the beginning of this century for EHV transmission lines. Self-supporting towers e covered under Indian Standard (IS : 802) and other' ational and International Standards. These are fabricated, sing tested quality mild steel structurals or a combination f tested quality mild steel and High tensile steel structurals onforming to IS:2062 and IS:8500 respectively. As H.T.

Conventional Guyed Tower

2.2.3.2 Compact guyed towers are used on compact lines. The phases are arranged in such. a way that the phases are not interspersed by grounded metal parts of Tower. The phases can be placed in different configurations and are insulated from the supports. The conventional guyed towers

1

N

. ~ •

...!.

• .,a:.

ZONTAUWAS" OWER

••

~

.'. .!..

•

.,a.

.a ..

-' .

J)()UBLE CIRCUIT TOWER

SINGLE CIRCUIT TOWER

B-DELT AlCA T HEAD TOWER

...

C-VERTlCAUBARREL TOWER

'.

PHASE-I

I'HASE-2

E-TRIPLE POLE STRUCTURE FIGURE I : SELFISUPPORTING

TOWERS

PHASE-J

~ ~

""...,

~

~

"""" $::l

5.

INSUI,ATED

'FABRICATED TOWER BODY

COMPACT TOWER

MULTICIRCUIT TOWERS

FIGURE 2

4

...

and compact guyed towers are shown in Figure 3.

(ii)

Horizontal/Wasp Waist Type

2.2.4

(iii)

Delta/Cat Head

(iv)

H-Structure Type

Chainette Guyed Tower

Chainette guyed tower is also known as cross rope suspension tower, and consists of two masts each of which is supported by two guys and a cross rope which is connected to the tops of two masts and supports the insulator 'strings and conductor bundles in horizontal formation .. For angle towers, the practice is to use three separate narrow based masts each for carrying one set of hundle conductors or ~lse self-supporting towers. Each . narrow based mast is supported with the help of two main guys. Typical chainette guyed towers for suspension and angle location are shown in Figure 4.

In India, tower shapes at (i) and (ii) are used for single circuit line whereas tower shape at (i) has been used for double circuit and multi-circuit lines. In other countries al the above shapes have been used. Tower shape at (i) is structurally more stable and ideally suitable for multi-circuit lines. whereas tower shape at (ii) offer better performance from the consideration of audible noise, radio and television interference i.. electrostatic potential gradient at ground level and at the edge of the right-of-way. These towers shapes are shown in Figures 1 & 2.

2.2.5

Guyed towers will be~overed in a separate I~anual

2.4

TOWER DESIGNATION

2.3

TOWER SHAPES

2.4.1

Broadly, towers are designated as under:

Tower shapes in use are as follows:

(i)

Suspension Tower

Verticallbarrel Type

(ii)

Tension Tower

,

(i)

DOUBLE TENSION i

SUSPENSION :INSULATOR STRING

FIGURE 6 : ARRANGEMENT OF INSPAN TRANSPOSITION

....

Tower Types and SI

(iii)

Transposition Tower

(iv)

Special Tower

2.4.2

SUspension Towers

These towers are used on the lines for straight run or for small angle of deviation upto 2° or 5°. Conductor on sUspelisioh towers may be sUpported by means of I-Strings, V·Strings, or a combination of I & V Strings. 2.4.3

Tension Towers

Tension towers also known as angle towers are used at locations where the angle of deviation exceeds that

permissible on suspension towers and/or where the towers are subject to upliti loads. These towers are further classified as 2°/SO-15°, 15°.30°, 30°·600IDead end towers and are used according to the angle of deviation of line. One of the classes of angle towers .depending on the site conditions is illso designated as Section Tower. The section tower is introduced in the line after 15 suspension towers to avoid Cil.

:...

,

"""'

::- :E m en

-

~

r·

::r.

cf}~Anchor shackles

AS( .f,

Yoke plate

A/Ball eye

( I C·_·j

clevis

(

A

Socket eye horn holder typ e

I

I

I r·'.

I

.r\..

l,.oisc insulators.

I I

shackles

l

F

.

I I rcing horn

~YOke ~r--~=

1

~

I

~

I

I

I

r

I

:r""\

i

.,

I

~DiSC I

r"\

plate clamp

Double suspension string

Ball clevis

I I

l\ _ L r: .. i~J

,

Compression plan end clamp

Single tension string

Typical Insulator String Arrangement Double tension string

Figure

3. Typical Insulator String Arrangement for 220 kV AC Transmission Line

-+

::T

~ en en " en

-------------~-

C?)-Ball hook

Single susp ension string

~

s· _

...J..

insulators

3300 min.

255

~

o

U-clevis

Arcing suspensIon 200

....

eyeJl1 ~ e

,

.~)

."

+-+ -:- '-I

I

--

/1

)1

r

1-----

I

_ .--v'

_/ , ' :

c::>

l=C

l-rI :"--\.".J.--~ . :=-J,I'. I ,..... ' .--.... ":'l!.

0

.0--

-------.

g, -.0

I

I

0

--......"

I

\

I

\~j _. ---.o.- ___ _ _ _

--~--

, 3335!92

135

130

160

Of

~ ...,

3850!102

~~:::::~::~:::::::£::A£~:.~. ~ J~'~,~ . a~.1 '"~,£:'i2:iLg: J:l :'~ :.L:) :J i:J t£;'l~b J~a ":l ( SI. l. Figure

Pl·,."

'.;:;

4. Single Suspension Insulatior String for 400kV AC Transmission Lines

p 1.44.@)%. .t.SL4. .=c..

,=>~-=c"'

L

. .2 :L'L::,.

la:.III:l:,.

__

:..:.

~t."4"¥"'1". . ~"i "d; .· .1 !19!1 LI!£MI ~.

IJ£!lJ§!lJI!I,.t!.c 12111.1.1L.::._.....II.J.£P.azlll .

(j') (I)

~

-<

,!!O.J.",J!!!.

..• yll!13111.;'M, ..

,...,

"

o ..., o

,.£).

Horn holder ball eye

I I

E E

,.,., "",.,., ,.,.,

..

Ln

-.1'

I~

[orona control ring

E E 0 0 0-

,.,.,

E E

E E 0

::J

.~

I:

QJ

0 0,.,.,

E

E

E .§

.§

~

:::J

C

.§ .§ x I:

u

-.1'

rr,.,.,

000

:::J

E

E E

:::J

i

I I I

III

I:

C

ro

t-

QJ

.... 0

ro .&:. .....

3

:n -

:

!I °1 >£>,

,

:

I.f'l

r-

I.f'l

850 mm

Notes J

L I

J I J

n

1. Spring washers electro galvanised 2. Other ferrous parts not dip galvanised 1 12% tolerance on length of hardware"

4. Nominal spacing tolerance !(O.03xspacing+OJlmm for insulator discs only

I

Figure 5: Typical Arrangement of Single Suspension String for 400 kY Lines with Twin Bun died Conductor I

n

Tower Geometry

12

----------------------B

190

A

450 520

iii u

c:

IV

.... QJ

.... 0

e e VI N

..e

::J

e

·~I el e! 0 VI GO ~

I

I

~

Twi ted. shackle

I II I

5.5 dia 2.5· thick

L ISuspension .

I camp

Coroni! con trol ring

Note: 1. Spr·ing washers electro 9alvilnised

2. Others ferrous parts not dip galvanised 3. ~2% tolerance on length of hardwilre 4. Nominal spacing tolerilnce !'O.03xspilcing+0.31 mm for insulator discs only

Figure. 6: Typical Arrangement of Double Suspension String \for 400 kV Lines with Twin Bundled Conductor!

-L-

~~ ~'J t=.

:-=:

.-=-:

-

-=-=

------

~ ... -¥ ~ '_ r

~~

L.·

_U

_Ii

_2

jildbLtl_

9

c> c> 1.1">

1. Anchor shackle 2. Turnlu:kle

3. Anchor shackle 4. H. H. b-all eye 5. Ardng horn 6. SO CK e t clevis 7. Yoke plate 8. [orQna control ring 9. 0 e'il d end assembly

4604 min. 4980 max. 100 ,100

-I I

24x14 5= 3480 !96

Min. 35 /..

I

3082

-I

r-.-i

I

II/I

•

.,...

c> c::>

c::>

c::> .,...

c::> LI"I ~

600

600 Min. 4676-5057 ma-x.

Figure 7 : Single Tension Insulator String for 400 kV AC Transmission Lines

c::> LI"I ~

..4

~

2-2.5 mm thickness

mm minimum

5392 mm mOiximum" - - - ' .. - - . - - - . - - _ . --/-ll...:.l-J..

co

=

Lr\

---

B

I

~

~

'>, ;:5~

5457 mm m

.~

-.3 Ln

·'~~O "+/ /,9 i?o

,

'-

I

r

x2 =1860

VI

wtz

>

,.,t,

~t,

--

Xt,

T

1 4 - - - 1 - - - - - LM

------1

wb ~--~----LB---------~~

Figure 20: Electrical Clearance Diagram-Suspension Tower (Annexure-I: Analytical Calculations)

32

Towe; Geometry

-

Tal

--

,

\1

II \1

I I

>

~~

---xtl

.-,-

lB

Z

I101 mt

I 101m

K

L.

~l

I I I I \

I I r

t

wb

1 4 - - - - - + - - - L B - - - - -.. ...,1

r

Ia I\ 1

TABLE 3.3 Typical Details of the Insulator Strings Adopted in India on Transmission Lines at 66 kV to 800 kV AC and ± 500 kV HVDC

Line Voltage (kV)

Suspension String

Tension String

Type

No. of Discs

Length (mm)

Types

No. of discs

Length (mm)

5 2x5

965 1255

6 2x6

1070 1575

9 2x9

1630 1915

10 2x10

1820 2175

14 2x14

2340 2640

SIT DIT SIT DIT SIT

15

400

SIS DIS SIS DIS SIS DIS SIS

23

± 500 DC

VIS DIS

66 132 220

OIT

2x~5

2915 3345

3850

DIT

2x23

5450

2x38

7120

QuadlT

4x38

8450

2x40 2x35 4x35 1x40 2x40 2x29 2x31

7000 7550 7800 7000 7250 See Fig.14 & 15

QuadlT

4x35

9800

QuadlT

2x31

See Fig. 16

800 POWER GRID

V(A Towers) V(S&C Towers) SIS (Pilot D&E Towers) V (Pilot D&E Towers) V

UPSES

Note: (i) (ii) (iii)

Size of discs for insulator strings upto and including 220 kV Voltages is 255x145 mm. Size of discs for suspension and tension strings for 400 kv voltage is 280x145 mml255x145 mm and 280x170 mm respectively. Size of discs for 800 kV system of POWERGRID are 255x145 mm of 120 KN discs for DIS and SIS (Pilot D&E towers) and V (Pilot for D&E towers) and 280x170 mm 01210 KN for V (A, S & C towers) and quad tension string. In case of UPSES, the size of disc is 320x195 mm of 300 KN both for suspension and tension strings.

TABLE 3.4 Typical Swing Angles and Electrical Clearances for Suspension Insulator Strings adopted . in India on Transmission Lines at 66 kV to 800 AC and ± 500 kV HVDC SI. No.

Line Voltage (kV)

Assumed Value of Swing of Suspension String from Vertical (degrees)

Minimum Clearances Specified (mm)

1.

66

15 0 30 0 45 0 60 0

915 760 610 610

34

Tower Geometry (Table 3.4 Contd.) 2.

132

15° 30° 45° 60°

1530 1370 1220 1070

3.

220

15° 30° 45°

2130 1830 1675

22° 44°

3050 1860

4.

400 I-String

(

C \

)

c

5.

800 I-String

V-String Power Grid UPSEB

Power Grid 20° 25° 41° 55°/64° 105° to 115° V=90°

c

5600 4400

y

1300

5100/5600 5000/5500

TABLE 3.5 Typical Swing Angles and Electrical Clearances for Tension String (Single/Double) Jumper adopted in India on Transmission Lines at 66 kV to 800 kV and ±500 kV HVDC

SI. No.

line Voltage (kV)

Assumed Value of Swing of Jumper from Vertical (Degrees)

Minimum Clearances Specified (mm)

1.

66

10° 20° 30°

915 610 610

2.

132

10° 20° 30°

1530 1070 1070

3.

220

10° 20°

2130 1675

4.

400

20° 40°

3050 1860

5.

± 800

Power Grid 15°/20° 25°/30° 40°/45°

5600 4400 5000

" I

.

• '.1

w

..vV,

ANNEXURE·I

Analytical Calculation for Electrical Clearances on Transmission Lines (Refer Figures 20 and 21)

1.0 NOTATIONS

=

H

=

S

B

=

C

=

ocococ= T'

y~.

M'

B

Y2 =

W1

=

W!1,W I2 = W'1'W:? = ~.Y

=

Z

=

h"hm,hb

=

M = LT,LM,L B =

4>

D

=

=

Height of hanger Overall length of suspension insulator string upto the lower tip of corona control ring. Swing angles of suspension insulator string Specified electrical clearances to be maintained at swing angles corresponding to 91 & 92 respectively. Flange width of the nearest projecting angle sections connected to main and tie angle members. Distance of centre of gravity of main angle section Angle between main and inclined tie members of top, middle and bottom cross-arms. Vertical distance from underneath the cross-arm to nearest tip of corona control ring from centre line of tower corresponding to 91 & 92, Vertical distance from underneath the cross-arm to the farthest tip of corona control ring from centre line of tower corresponding to 91 &92, Horizontal distance from centre line of tower to nearest tip of corona control ring corresponding to 01 & 92, Horizontal distance from centre line of tower to the farthest tip of corona control ring corresponding to 91 &92, Half width of tower body at top cross arm level Half width of tower body at level corresponding to ~'1' ~12 Half width of tower body at level corresponding to XI1 , X!2' . Slopes of legs Height of Corona control ring Length of top, middle, bottom cross arm from centre line of tower body. Spacing between the conductors of bundle or jumpers. Height of top, middle and bottom cross arms Angle of deviation of line Jumper depth

36

Tower Geometry

2.0 ELECTRICAL CLEARANCE ON SUSPENSION STRINGS 2.1

Underneath the Cross-arm

At Angle of Swing

Electrical clearance Ayailable

6,

K,

=Y, -

(B + C)

=H +(8-M) Cos 6, - N. Sin 9, -

(B+C) X,

2

62

K2

= Y2 -

(B + C)

=H +(S'- M) Cos 9

2-

N Sin 92 - (B+C) X2 2

2.2

Electrical Clearance from Tower Body

Horizontal Clearance = (X\1 - WI') Cos B ~ X,

9, Horizontal Clearance

=

WI + Y, tan B

XI' =

~-

S. Sin 6, -

NCos'9, -

(B+C)

2

Y1

=

W12

=

WI + Y2 tan ~

X12

=

~

H + S. Cos 9, + NSin 9, - (B+C) 2 (XI2 - W12) Cos ~ ~ X2

-

S. Sin 62 -

NCos 92 -

(B+C)

2 H + S. Cos 92 + NSin 92 - (B+C) 2 Electrical Clearance from Lower Cross-ARM Tie (Inclined) Member Y2

2.3

-

W\1 =

tan oem (Lower X-arm) =

=

hm

Perpendicular distance to Tie member from the line point' is shortest. If oem < 9,. then clearance is required to be computed at swing angle of string corresponding to oem If oem > 9, and less than 92 , then the clearance is minimum when angle of swing is 91 Distance from lower tip of corona control ring to lower cross-arm tip

=P

p =(Lm - ~ ) + S. Sin 9, - N Cos 8,

Clearance available

=rJ=-lH+5 COs 9, + ~ Sin 9,) - Ptan u,,1 Cos 0., • (B+C) ~ X" i =[V - Y, . p tan aml Cos Urn· (B+C) ;e~1

IV

Similar check shall be made for 8

n

vi

3.0 ELECTRICAL CLEARANCES ON TENSION STRINGS 3.1

Electrical Clearance with Reference to Underneath of Cross-arm

Angle of Swing

Electrical Clearance Clearance =t + 0 Cos 91 Clearance =t + 0 Cos 82 -

3.2

Z Sin 91 - (B+C) ~ X1 2 Z Sin 92 - (B+C) ~ X2 2

Clearance from Tower Body·

SWING ANGLE 9, Shift deviation. Xt1

Wt1

, (

=

Projected length of Tension Insulator String upto Jumper connection for angle of

=

Cross-arm Length -

=

L, -

(Shift + 0 Sin 91 + Z Cos 91) 2 (S. Sin ~ + 0 Sin 9, + Z Cos 9,) 2 2

=

Clearance available from tower body =(Xt - Wt) Cos ~ (B+C) ~ X1 3.3

Clearance from Low Cross-Arm Tie (Inclined) Member

tance m = - - - Lm -Wmt AG

=V -

KH

= (Lm -

KG

=KH + Z Cos 9,

Y ; BH

=AG -

Z Sin 91

Lt ) + Shift + 0 Sin 91 -

AI

=AG-GI =AG -

BJ

= BH -

JH

=BH -

KG tan

oc

Z Cos 91 2

m

KH . tan

Clearance available from middle X-arm

oc m

=AE =AI Cos

oc

m- (B+C)

~

X1

· Save power for national productivity ~. MAHARASHTRA STAlE ElECTRICITY BOARD

Transmission Line Manual Chapter 4

Electrical Clearances

CONTENTS Page 4.1 Introduction

1

4.2 Minimum Ground Clearance

1

4.3 Minimum Clearance above RiverslLakes 4.4 Environmental Criteria for 800 kV Line

2

4.5 Air Clearance - General Consideration

2

4.6 Clearance and Swing Angles on Transmission Lines in India

2

4.7 Conductor Metal Air Clearances

3

4.8 Air Clearance-Analysis by CIGRE

4

4.9 Phase-to-Phase Air Clearances

5

4.10 Clearance between Conductor & Groundwire

6

4.11 Effect of Span Length on Clearance

7

4.12 Clearance at Power Line Crossings

7

4.13 Recommendation

8

ANNEXURES Annexure I

- Spacing between Conductor

11

Annexure II - Swing Angle for 800 kV Anpara - Unnao Line for Insulator Strings and Jumper

12

1

APPENDIX - Investigation Studies on Clearance and Swing Angles for Indian Power System

16

4

a

r 11

Chapter 4

ELECTRICAL CLEARANCES 4.1

Introduction

The design of a transmission line tower is distinctly classified into mechanical design and electrical 'design. The parameters which affect the design of a tower are di.§cus~d in Chapter-V, whereas loadings and mechanical design of a tower are discussed in Chapters 6 & 7 'of the Manual. In this chapter, the aspects leading to electrical design of· a tower are, therefore discussed. The electrical deslgn·oftower, infact, involves fixation of external insulation against different electrical phenomena. The extemallnsulation comprises self restoring air and solid insulation in the form of insulator strings consisting of disc insulators, mtg rod insulators etc. The electrical insulation of a tower is a function of steady state operating voltage of the syst&m and various events that occur in the system (energisation, re-energisation;-fault occurrence. and its clearance, lightning strokes etc.), For system upto and including "3b9kV voltage rating, the tower insulation is determined from the power frequency voltage and lightning impulse requirement where.as for system above300 kV rating, the power frequency and switching impulse voltages are the governing criteria.· The other factors which affect the electrical insulation are climatic conditions - altitude, relative humidity, pollution, etc. The various factors and statutory regulations which affect the electrical design of a t.ower are discussed as hereunder.

4.2 7 8

Minimum Ground Clearance

The minimum clearance above ground as per sub rule 4 of Rule 77 of I.E.Rules 1956 (latest revision) for AC system and for ± 500 kV HVDC system as adopted in India are as under: Vrltage (kV)

Nominal Highest (System)

Minimum ground clearance (mm)

66 72

132 145

220 245

400 420

--

--

800

±500

5500

6100,

7000

8800

12400

12500

To the above clearance, an additional clearance of 150 mm is added to provide for uneven ground profile and possible sagging error. , )

4.3

Minimum Clearance above Rivers/Lakes

In case of accessible frozen r'iversnakes, the minimum clearance abOve frozen riversnakes should be equal to the minimum ground clearance given in 4.2 above. '. , The minimum clearance of Power Conductor over the highest flood level in case -of ·non shall be as foliows:

navig~ble

rivers

System Voltage (kV)

Minimum clearance above higttesUloodJeveL(mm)*

72 145 245 420 800 ±500

3650 4300 5100 6400 9:400 6750

..

·(The maximum height of an obJect over the highest flood level of non-navigable rlverlll;consldClred:al·3000mm)

1

For navigable rivers, clearances are fixed in relation to the tallest mast in consultation with the concerned navigationaVport authorities.

4.4

Environmental Criteria for 800 kV Line

The Standing EHV committee of CEA (Working Group 9: Interference) have laid down the iollowing environmental criteria for 800 kV lines: Radio Interference should not exceed 50 dB for 80% of time duration during the year. For Television Interference, the minimum signal to noise ratio should be 30 dB. Audible noise should be less than 55 dB (A). Electrost~tic field at 2 m above ground below the outer most phase should be equal to or less than 10 kV/m and equal to or less than 2 kV/m at the edge of right of way. To comply with the above environmental requirements minimum ground clearance of about 15000 mm has been adopted in India for 800 kV lines.

4.5

Air Clearances - General Consideration

The air clearances applicable to transmission lines are categorised as minimum ground clearance, phase to grounded metal clearance, phase to phase clearance, clearance between power conductor and groundwire, clearance between pOwer lines crossing each other, power lines crossing telecommunication lines, railway tracks, roads etc. The phase to grounded metal clearances is a function of power frequency voltage and lightning impulse vottage in case of the transmission lines of voltage rating upto and including 245 kV and power frequency vottage and switching impulse voltage for lines above 245 kV voltage rating. The power frequency voltage is expressed in terms of service voltage or service voltage modified by events such as faults, sudden change of loads, ferranti effect, linear resonance,ferroresonance, open conductor, induced resonance from coupled circuits, etc. A line is subjected to lightning impulses due to shielding failure (direct stroke to power conductor), back flashover from tower to power conductors, vottage induction from nearby objects etc. The switching impulse voltage originates from line energisation, line reclosing, fault occurrence and clearing, switching off capacitive current (restriking effect) including line dropping and capacitor bank switching, switching of inductive currents (current chopping effect) including transformer magnetising currents and reactor switching, special switching operations including series capacitors, resonant ferro resonant circuits and secondary switching.

The air gap clearances tor phase to phase lightning impulse withstand voltages are the same as those for phase to ground lightning impuls~ withstand voltages.

4.6

Clearances and Swing Angles on Transmission Lines In India

Conductor metal clearances generally adopted in the country for transmission lines 66 kV and above are given as under:

"".1 ""VIII

.. VI'U.~:"V

(kV) 72AC

145 AC

245 AC

420 AC

Qlllijll:l

;;'Utiptml:iIOn

Swing from vertical (degree) Nil 15 30 45 60 Nil 15 30 45 60 Nil 15 30 45 60 Nil 22 44

800AC ±500 DC

Jumper

InSUIalor ~tnng Minimum clearance (mm) 915 915 760 610 610 1530 1530 1370 1220 1070 2130 1980 1830 1675

--

Nil 10 20 30

1530 1530 1070 1070

Nil 10 20

2130 2130 1675

--

---

--

--

Nil 3050 3050 20 1860 40 Discussed in the Appendix 40 3750

I

NiI*

--

--

--



I

1600

·V-Strings have been adopted. Notes: (i) Electrical clearance for suspension towers should be based on !single suspension strings. For road crossings, tension towers should be adopted. (ii) The details of insulator string adopted in the country for transmission lines 66 kV and above voltage are given in Chapter SP. 4.7

Conductor Metal Air Clearances

4.7.1

System VoHage

The air clearances for AC system given in document 11 (secretariat 48) of IEC referred in CiGRE document "Tower Top Geometry - WIG 22.06" issued in June "1995 and for DC system on the basis of values adopted by Power Grid for their ± 500 kV HVDC Rihand-Dadri line are given below: System VoHage (kV)

"72 145 245 420

800

Air Clearance (mm) 4.7.2

190 390 650

1200 1560

±500

1150

LIghtning and Switching Over-voltage

The air clearances corresponding to lightning impulse and switching over-voltages for AC system as per IEC 71-2 (1996) and for DC system as adopted by Power Grid for their ± 500 HVDe Rlhand-Dadri line are given as under.

"

System VoltagQ

Impulse withstand VoHage (kVp)

, (kV)

I

Air Clearances (mm)

Ufghtning Impluse Level

"Ligtilning': Switching \ '

.

--.

.

-

_. -

1.

2.

72AC

325 '

145AC

550 650

245AC

950 1050

,.

t I Switching Ir~plu~ Leval,'

.- - - . - ..•..•..• - - I

,

Conductor Structure

Rod Structure

Conductor Structure

Rod structure

3.

4.

5.

6.

7.

----

---

630

--

--

--

1100 1300

--

--

--

1700 1900

1900 2100

----

---

--

--

'"

420AC

1300 1425

950 1050

2400 2600

2600 2850

2200 2600

2900 3400

800AC

,1950 2100

1425 1550

3800 3900

3900 4200

4200 4900

5600 6400

±500DC

1800 ..

1000

--

--

3750

4.8

,

Air Clearance· Analysis by CIGRE

4.8.1 As a sequel to adoption of structural design based on reliability concept, CIGRE SC-22,WG06 had taken up study on tower top geometery to ascertain the swing angles of the insulator strings, air clearances, etc. for the meteorological data used for determining the structural strength. The WG based on CIGRE Publication 72 had interalia worked out air clearances corresponding to lightning and switching surges understill air,condition/small swing angle in Document "Tower Top Geometry" - June 1995 as given below. Minimum Phaseto-Earth Air Clearance (mm) ,

Nominal VoHage uR (kV)

Highest Voltage for Equipment urn (kV)

Lightning Impulse Withstand Voltage (kV)

Switching Impulse Withstand Voltage (kV)

1.

2.

3.

4.

5.

110

123

450 550

940 1130

230

245

850 950 ' 1050

----

..

---

1760 ,1970 2180

400

420

1175 1300 1425

850 950 1050

2430 . 2800 3250

500

525

1300 1425 1550

950 1050 1175

2800, 3250 3900

Values recommended for adoption are given separately.

- -- - ---- -----." .--,....... "'..-

I

1

~--

. . r-·

CIGRE Doc of June 95 adopted in ·other ·'countries .are,given Tables . A.;1 ;and i"~2. ,4i8:3

1 J J

hi

. Ie 9

The 'correlation between wind.pressure (speed) ',and'maximum ~angle :of iSWi~ Jdf.uspenslon 'strings(bdth 'r&V):adopted in.other.countries:indicates1h1lnhese;pressure·s.famtn1th8~:dJOO/o \to 70%,ofUHimate wind:pressure. :Further, '1hese'WfAd:pressures:corre'$pol'Itf'JtolllHrll'geeus :characterisedbyretum ;period :df:2·to '5 'years .againstfJOwer!frequency'Ndlt.;mmaaad1~ ;arnHightnlng/switchingsurges i!'l case of V;:strings. :If?.aJr~he·lretJuced 'a:ngle'lSf'sWI,,~~~, 'occasionally.a characteristic ,wind 'speed 'Is ':speclfied }'.COrresJi)OnCfing 'to' ;ndt1ffiI~'(over 'voRages incase 'of I,.suspenslon 'or pllot:suspension·:string.

)s

~

la ~ ~(

.

,

, I

:4:9

1Phase-lO-;Phase tArrClearances

4:9.1

'.Phase-to~phase·vertical.andhorizontal·.separation'betwsenipower:Conductors:df1thesame~

or'different circuits onthe same tower will be;estabIiShed1lY1:Onductormetal:t!teamrrae8l1scuased tn Paras 4:7& 4:8. However minimum clearances 'betweenphases ,as 'given ;inEiTiH21(t99B) are 'reproduced 'below: 4:9.1.1 Ughtning 'Impulse .,

'Standard .lightning Impulse withstand voltage (kVp)

Mlnlmum~Alr

Clearance'(mrtI)

Rod Structure

Conductor:Structme

325

630

-

450

900

550

1100

--

650

1300

.-

750

1500

.-

850

1700

16'00

950

1900

1700

1050

2100

19'00

1175

2350

22'0t)

1300

2600

:24.00

·1425

2850

.2600

1550

3100

1675

3350·

:311;'0'0

1800

3600

:3300

1950

3900

.:3600

2100

4200

3'9'00

,

~2900

,

I.

~

i

4.12.2 Power Lines Crossing Communication Lines The minimum clearance to' be maintained between a power line and a communication line, as per "Code 'of Practice for Protection of Telecommunication Lines of Crossings with Overhead Power Lines" should be ,', as follows: ",":'

,.

Nominal

66

132

220

400

Highest

72

145

245

420

800

2440

2750

3050

4480

7900

Voltage (kV) '~I .j

"(1'

Minimum clearance between power conductor crossing telecommunication line (mm)

4.12.3 Power Line Crossing Railway Tracks The minimum vertical clearance between the lowest conductor of a power line crossing the railway track as per "Regulations for Power Line Crossings of Railway Tracks· 1987" shall be as follows:

,,"

The minimum vertical clearance above rail track as al~o highest working point of the jtb when crane is deployed and the lowest point of any conductor of crossing including ground wire under condition of maximum sag.isgiven as under: . Voltage (kV) Nominal

Minimum Ciearance (mm) Highest

66 132 220 400

72 145

245 420 800

Above Rail Track

Over Crane

14,100 14,600 15,400 17,900 22,000

2,000 2,500 3,500 6,000 9,50Q

4.12.4 Power Lines Running Along or Across the Roads The minimum clearanct:: above ground for 66 kV and above voltage power lines running along or across • the road shall be 6,1 m as per Rules 77 of I.E, Rules 1956 provided the requirement stipulated in Sub·Rule . (4) of Rule 77 of IE Rules 1956 is met. . ~

As per electrostatic field effect of EHV transmission lines, the minimum clearance for line passing over the' road shall be corresponding to field gradient of 10 kV/m, It should not permit a short circuit current more than 5 rnA through an individual when touching a vehicle standing below the line. .

4.13

Recommendation

4.13.1 Air clearances and swing angles for various system voltage ratings are recommended as under:

~ ,

.

,

System voltage (kV)

Swing from vertical (degree)

145 AC

Jumper

Single SuspenSion Insulator String Minimum clearance (mm)

Swing from vertical (degree)

15 30 45 60

915 760 610 610

10 20 30

Nil

1530

Nil

1530

245 AC

30 45 60

1370 1220 1070

20 30

1070 1070

Nil

2130 1980 1830 1675

Nil 10

2130 2130 1675

3050 3050 1860

Nil

5600/5100 4400 1300

Nil

15 30 45 60 400 AC

Nil

22 44

20

20 40

3050 3050 1860

800 AC Zones 1&

Nil

II

22 45

Zones III &IV

Nil

27 55

Zones V & VI

Nil 30

60

15 30

5600/5100 4400 1300

20 40

5600/5100

Nil

4400 ' 1300

22 45

Nil

5100 4400 1300 5100 4400 1300 5100 4400 1300

4.13.2 The spacing between conductors for,long spans shall be established from the following formulae: Vertical Clearance (,-1)

0.75

Vf.,s +--i~-

+

V

T5U

Horizontal Clearance (m)

Where

= = =

Sag at 75" C Length of Insulator String in metres. Line Vo~age in kV

60 01 .~ ~

-i VI I //0

1----- ---- ---50

en L..

~

L

40

E

/

~I

Nominal Voltage : 500 k V "2Conductor : ACSR 410mm x4 Insulator Strings: 320mm x 26pc s. double strain

:

I

Depth of Jumper: 5,000 mm

I I

Catenary Angle :

I

.

::J ..."

.... 30

o

!

/1

L

QI

g' 20

~

~-----

10

l/

o

o

-Ii

V

10

20

30

40

OCI

+oc2: 5°

Without reinforcement

-

With reinforcement wire And reinforcement spacer

----_.-.-

50

Mean wind speed during 10 minutes [m/sec] Figure I : Swing characteristics of jumper conductor based on test carried out in Japan.

F:LXSin~12

r-=;

o o

0

0

0

Where.

(a) Suspension Insulator Strings

L. Length of insulator strings Line deviation angle (b) Jumper (wi thout pilot Suspension Insulator

e.

C:. r:nn c. \

Germany

Austria

Belgium

O. 75Jf + 1 k + ~m (Vertical)

O.62Jf+l k

+~m (Horizontal)

France

U.S.A.

O. 75V+3.26~ inch

Poland

Sweden

6.5~+O.7vcm

Czechoslovakia

Canada In which f = 1+40 =

1k

•

La

VR

.=

"5

=

V ~

= =

25+V+7~cm .

Max. Sag Sag at 40° C Length of Insulator String (assumed as 4 m) VoHage in kV Actual span in m limited to 450 m Reference span in m (50 m) Reference Voltage in kV (5 kV) Sag at 15° C

.. ." • ,

'

i

I I

rf~,

11.'.-

,

«! •• l t:

, c: fi .~

'1 fc'

t:

,; ~.: .. ',

'-:;

f: C'

!

tJi> .1

G

• • •.

•• •• •

'•.

Transmission Line Manual Chapter 5

Design Parameters

i..

CONTENTS Page

5.0 Abstract 5.1 Transmission Voltage 5.2 Number of Circuits 5.3 Climatic Conditions and Ecological Consideration 5.4 Environmental . .

5.5 Conductor 5.6 Earth Wire 5.7 Insulator Strings

2 2 11 11

12 13

17

-

5.8 Span

,, .~

s

')

COl T~I I

I

Design Parameters 5.0

ABSTRACT

The design of transmission line towers is entirely dependent on the selection of correct data/parameters. A good tower designer should accumulate all necessary design parameters before starting the design work. This chapter describes the design parameters required for developing a transmission line tower design. These design parameters should be correct and authentic in nature to ensure· reliability of transmission line under given conditions.

5.1

Transmission Voltage

This is very important parameter. All the electrical parameters such as air gap clearance, phase to phase clearance, ground clearance etc. are fully dependent on the voltage level. The power is transmitted either through A.C. System (alternating current) or through D.C. system (Direct Current) depending upon the requirement of power system of a particular region or country as a whole. In India the following transmission voltages have been standardised for transmitting the power :

A.C. System (i). (ii) (iii) (iv) (v) (vi)

66KV 110KV 132KV 220KV 400KV 800KV

D.C. System (i) .+/-500KV

For indigenous development of HVDC technology, Govt. of India had approved HVDe Research and Development proposal in Nov. 1981 and action plan in Nov. 1982 for taking the R&D project on an actual line to enhance the power level. APSEB and MPEB had offered 220KV DIC Lower Sileru (A.P) Barsoor (M.P) line for the experimental project. The HVDC Steering Committee in Oct. 1983 approved National HVDe project (NHVDC) to be taken in 3 stages. Stage I

1OOMW, + 100KV monopole

Stage II

200MW, + 200KV monopole

Stage III

400MW, + 200KV bipole

The National HVDC Stage I was approved by Government in Oct. 1984 for establishing a 100MW, ± 100 KV HVDC, 6 pulse monopole link between Lower Sileru and Barsoor by converting one circuit of 220KV D/C Lower Sileru-Barsoor line. The Stage I has been commissioned in Oct. 1991 and is in operation. The Stage II for uprating Stage I to 200 MWj +200KV, 12 pulse monopole has been approved by the Govt. in Sept. 1993 and scheduled to be commissioned by the end of 1997.

2

Design Parameters

5.2

Number of Circuits

The transmission line can be classified into three categories depending on the number of circuits. Each circuit consists of three phases. However, each phase may further consist of single, twin or multiple bundle of conductors. The three classifications based on the number of circuits are :(a)

SINGLE CIRCUIT

(b)

DOUBLE CIRCUIT

(c)

MULTI CIRCUITS (i)

Single Circuit : The transmission line which carries only one circuit.

(ii)

Double Circuit : The transmission line which carries two circuits.

(iii)

Multi Circuit: The transmission line which carries more than two circuits.

However, single circuit and double circuit transmission lines are popular throughout the world. Some of the utilities of the world have constructed multi circuit transmission lines also to avoid Right of Way problems in Urban areas but the number of such lines are very less as the multiple circuit lines are not advisable from the maintenance & reliability point of view. Some of the utilities of the world have constructed multivoltage lines which have more than two circuit of different voltage levels. Wherever Right of Way constraints are foreseen, multiple circuit and multivoltage lines are preferable. 5.3

Climatic Conditions

The reliability of a transmission system is largely dependent on the accuracy of the parameters related to climatic conditions considered for design. The design of tower will vary with variation in climatic conditions. The following are the main climatic parameters which play vital role in developing design of transmission line towers :1.

Wind

2.

Temperature

3.

Isokeraunic level

4.

Seismic Intensity

5.

Ice formation.

5.3.1 Wind

~:

5.3.1.1 The Wind speed have been worked out for 50 years return period based on the -tlPto-date wind data of 43 dynes pressure tube (DPA) anemograph stations and study of other related works available on the. subject since 1964. The basic wind speed data have been published by Bureau of Indian Standards in IS : 875-1988 in active cooperation with In9ian Meteorological Department as shown in Figure 1. This map represents basic wind speed based on peak gust velocity averaged over a short time interval of about 3 seconds and corresponds to 10m height above mean ground level in terrain Category-2 for 50 yrs. return period. Based on the wind speed map the entire country has been divided into six wind zones ... :........... v , .. :.. ,.1

"'''' ........,.1

,,* r::.r::.m/" ....,.

~n~ min win~ C!noorl nf ':l':lm/c:.~('

R~c:.i('

winn c:.nAAn fnr thA

TABLE I Wind Zone

Basic Wind Speed (m/sec)

1 2

33 39

3

44

4 5

47 50

6

55

,

NOTE : In case the line tranverses on the border of wInd zones, the hIgher wInd speed may be considered.

5.3.1.2 Reference Wind Speed VR

It is extreme value of wind speed over an average period of 10 minutes duration and is to be calculated from basic wind speed 'vb' by the following relationships:VR = vb/ko Where : Ko is a factor to convert 3 seconds peak gust speed into average speed of wind during 10 minutes period at a level of 10 meters above ground. Ko is to be taken as 1.375. 5.3.1.3 Design Wind Speed, Vd

Reference wind speed obtained in 5.3.1.2 shall be modified to include the fo.llowing effects to get the design wind speed : (i)

Risk Coefficient, K,

(ii)

Terrain Roughness coefficient, K2

It is expressed as follows :Vd = VR' K,. ~

5.3.1.4 Risk Coefficient K1

Table 2 gives the values of Risk Coefficient K, for different wind zones for three Reliability Levels. TABLE 2 Risk Coefficient K1 for Different Reliability Levels and Wind Zones Reliability Level

1

Coefficient K, for wind zones : 5 4 2 3

1(50 yrs. return period)

1.00

1.00

1.00

1.00

1.00

1.00

2(150 yrs. return period)

1.08

1.10

1.11

1.12

1.13

1.14

3(500 yrs. return period)

1.17

1.22

1.25

1.27

1.28

1.30

6

Design Parameters

4

5.3.1.5 Terrain Roughness Coefficient, K2 Table 3 gives the values of coefficient ~ for the three categories of terrain roughness corresponding to an average 10 minutes wind speed. TABLE 3 Terrain Roughness Coefficient Terrain Category Coefficient

~.

~

1

2

3

1.08

1.00

0.85

5.3.1.6 Terrain Categories (a)

Category 1 - Coastal areas, deserts and large streches of water.

(b)

Category 2 - Normal cross country lines with very few obstacles.

(c)

Category 3 - Urban built up areas or forest areas.

NOTE: For lines encountering hills/ridges, value of K2 will be taken as next higher value. 5.3.1.7Design Wind Pressure Pd The design wind pressure on towers, conductors and insulators shall be obtained by the following relationship :Pd

= 0.6 Vd2

Where Pd = design wind pressure in N/m 2 and Vd = Design wind speed in m/s. Design wind pressure Pd for all the three Reliability levels and pertaining to six wind zones and the three terrain categories have been worked out and given in· Table 4. TABLE 4 Design Wind Pressure Pd, in N/m2 (corresponding to wind velocity at 10m height) Reliability Level

Terrain Category

1

1 2 3

403 346 250

563 483 349

717 614 444

818 701 506

925 793 573

1120 960 694

2

1 2 3

470 403 291

681 584 422

883 757 547

1030 879 635

1180 1010 732

1460 1250 901

1120 960

1320 1130

1520 1300

1890 1620

1

Wind pressure Pd for wind zones 4 5 2 3

6

)

3

1 2

552 473

838 718

(A)

Wind Load on Tower

In order to determine the wind load on tower, the tower is divided into different panels having a height 'h'. These panels should normally be taken between the intersections of the legs and bracings. For a lattice tower, the wind load Fwt in Newtons, for wind normal to a face of tower, on a panel height 'h' applied at the centre of gravity of this panel is : Fwt = Pd. Cdt • Ae. GT Pd = Design wind pressure, in N/M2 Cdt = Drag Coefficient pertaining to wind blowing against any face of the tower. Value of ~dt for the different solidity ratios are given in Table 5. Ae = Total net surface area of the legs and bracings of the panel projected normally on face in m2. (The projections of the bracing elements of the adjacent faces and of the plan-and-hip bracing bars may be neglected while dete-rmining the projected surface of a face). GT = Gust Response Factor, perpendicular to the ground roughness and depends on the height above ground. Values of GT for the three terrain categories are given in Table 6. TABLE 5 Drag Coefficient, Ccit For Towers

. Solidity Ratio'"

Drag Coefficient, Cdt

Upto 0.05 0.1 0.2 0.3 0.4 0.5 and above

)'3.6 3.4 2.9 2.5 2.2 2.0

Note : Intermediate values may be linearly Interpolated.

"'Solidity ratio is equal to the effective area (projected area of all the individual elements) of a frame normal to the wind direction divided by the area enclosed by the boundary of the frame normal to the wind direction. TABLE 6 Gust Response Factor for Towers (GT) and for Insulators GI)

Height above ground m Upt010 20 30 40 50 60 70 80

Values of GT and GI fo(Aerrain Categories 1

1.70 1.85 1.96 2.07 2.13 2.20 2.26 2.31

Note : Intermediate values may be Interpolated.

fl 1.92 2.20 2.30 2.40 2.48 2.55 2.62 2.69

3

2.55 2.82 2.98 3.12 3.24 3.34 3.46 3.58

6

Design Parameters

(B)

Wind Load on Conductor and Groundwire

The load due to wind on each conductor and ground wire, Fwc in Newtons applied at supporting point normal to the line shall be determined by the following expression : Fwc = Pd. L. d. Gc. Cdc Where: Pd = Design wind pressure in N/m2; L

= Wind span, being sum

of half the span on either side of supporting point, in

metres. d

= Diameter of conductor/groundwire, in metres.

Gc = Gust Response Factor which takes into account the turbulance of the wind and the dynamic response of the Conductor. Values of Gc are given in Table 7 for the three terrain categories and the average height of the conductor above the ground. Cdc

= Drag coefficient which

is 1.0 for conductor and 1.2 for Groundwire.

Note : The average height of conductor/groundwire shall be taken upto clamping point on tower less two third the conductor/groundwire sag at minimum temperature and no wind.

The total effect of wind on bundle conductors shall be taken equal to the sum of the wind load on sub-conductors without accounting for a possible· masking effect of one of the subconductors on another. TABLE 7 Values of Gust Response Factor Gc. for Conductor/G-Wires

Terrain Category

Height Values of Gc for conductor of span, in m above Upto: ground, m 200 300Y\) 400 700 500 600

800

&above

1. Upto

10 20 40 60 80

1.70 1.90 2.10 2.24 ·2.35

1.65 1.60 1.871-1'5'1.83 2.04 :],·ov2.00 2.18 2.12 2.25 2.18

1.56 1.79 1.95 2.07 2.13

1.53 1.75 1.90 2.02 2.10

1.50 1.70 1.85 1.96 2.06

1.47 1.66 1.80 1.90 2.03

~.

10

1.78 1.73 2.041-:(\0.\1.95

1.69 1.88

60 80

1.83 2.12 2.34 ·2.55 2.69

1.60 1.80 2.05 2.20 2.32

1.55 1.80 2.02 2.17 2.28

10 20 40 60 80

2.05 2.44 2.76 2.97 3.19

1.77 2.06 2.38 2.56 2.73

1.73 2.03 2.34 2.52 2.69

3.

!~

2.46 2.56

2.37 l.! \'lJ2.28 2.48 2.41

1.65 1.84 2.08 2.23 2.36

1.98 2.35 2.67 2.87 3.04

1.93 2.25 2.58 2.77 2.93

1.83 2.10 2.42 2.60 2.78

./

2~2Z2·1~{2.20:L'l~2.13

1.88 2.15 2.49 2.67 2.85

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un

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Wind load on insulator strings 'Fwi' shall be determined from the attachment point to the centre line of the conductor in case of suspension tower and upto the end of clamp in case of tension tower, in the direction of the wind as follows: Fwi = 1.2 . Pd . Ai . Gi Where: Pd

= Design Wind pressure in

Ai

= 50 Per cent of the area of Insulator string projected on a plane parallel to the

N/m2

longitudinal axis of the string (1/2 x diameter x length). NOTE : Length of Insulator shall be co'hsidered as follows : Suspension Insulator!

from the centre point of conductor to the connection point of Insulator to the tower. Tension Insulator:

End of tension clamp to the connection point of insulator to the tower. Gi =

Gust Response Factor, perpendicular to the ground roughness and depends on the height above ground. Values of Gi for the three terrain categories are given in Table 6.

In case of multiple strings no masking effect shall be considered.

5.3.2 Temperature

To evolve design of tower, three temperatures i.e. Max. temperature, min. temperature and everyday temperature are very important. Tower height as well as sag and tension calculation of conductor and earthwire varies with the change in above three temperatures. The temperature range varies for different parts of India under different sea.sonal conditions. The absolute max. and min. temperatures which may be expected in different localities in country are indicated on the map of India in Fig 2 and Fig 3 respective.ly. The temperature indicated in these maps are the air temperature in shade. The max. conductor temperatures may be obtained after allowing increase in temperature due to solar radi.ation and heating effect due to current etc. over the absolute max. temperature given in Fig 2. After giving due thought to several aspects such as flow of excess power in emergency during summer time etc. the following three designs temperatures have been fixed : (a) (b) (c) (d)

(e)

Max. temperature of ACSR conductor = 75 deg.c Max. temperature of AAAC conductor = 85 deg.c Max. temperature of earthwire = 53 deg.c. Min. temperature (ice free zone) =- 5 deg C to +10 deg. c (depends on location of the trans. line of however Goc widely used in the country) a Everyday Temperature 3). C (for most parts of the country).

For region with colder climates (-5 deg.c or below) the respective Utility will decide the everyday temperature. 5.3.3 Lightning Consideration for Tower Design

As the overhead transmission lines pass through open country, they are' subjected to the effects of lightning. The faults initiated by lightning can be of following three types :

Design Parameters,

8

IS 802 ( Part 1I5ec 1 ) : 1995 .t'.

•

tI'

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-

MAP OF INDIA '~t

'\...,'

TEMPERATURE ISOPLETHSOC

i

(SRINAGAR

BASED ON DATA UP TO 11158 SURI!llED BY

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ORTHOMORPHC

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,~./2 TM = Load in Newtons Tl = Tension in Newton of conductor/groundwire at everyday temperature and nil wind.

_.

(6.5.2)(6.~.~~ .. (~.5.4j

$

= Angle of deviation of the ~~e..

Where "Fwc" and "Fwi" and "Fwd" are to be applied 6.7.2 Brokenwire Condition - Suspension, Tension and on all conductor/Groundwire points. But "Fwt" wind on Dead End Towers tower is to be applied on the tower at ground wire peak and 6.7.2.1 Transverse loads due to wind action on tower cross ann levels. For 400 kV and above, "Fwt" will also " structure, conductors, ground wire insulators shall be taken be" applied at any convenient level between Bottom Cross Arm and ground-level. In case of Normal tower with as nil. extension of any voltage rating one more level at the top of 6.7.2.2 Transverse load due to mechanical tension of extension panel shall be considered conductor o~ ground wire at everyday temperature and nil wind -on account of line deviation shall be considered as 6.6 TRANSVERSE LOADS (TS) - SECURITY follows:CONDITION TM =Tl x sin $12 6.6.1 SuspenSion Towers

6.6.1.1 Transverse loads due to wind action on tower structure, conductors, ground wires and insulators shall be taken as nil.

where TM Tl

.,

6.6.1.2 Transverse loads due to line deviation shall be based on component of mechanical tension of conductors and ground wires corresponding to everyday temperature and nil wind condition. For broken wire the component shall be corresponding to 50% of mechanical tension of conductor and 100% of mechanical tension of ground wire at everyday temperature and nil wind. 6.6.2 Tension and Dead End Towers

6.6.2.1 Transverse loads due to wind action on tower structure, conductors, ground wires and insulators shall be computed as per clause~.l. 60% wind span shall be considered for broken wire and 100% for intact wire.

$

c v c

5 o

6

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0:

6. O. 'I

L\. cr .1

de

=

Load in Newtons = 50% of tension in Newtons of conductor and 100% of tension of grollndwire at everyday temperature and nil wind "" ." for suspension tower and 100% for angle and dead end towers for both conductor and ground wire. = Angle of peviation of the tower.

Cc uU:

6.1

6.8

VERTICAL LOADS (VR) CONDITION

RELIABILITY

6.8.1 Loads due to weight of each conductor and groundwire based on appropriate weight span, weight of Insulator strings and accessories. " 6.8.2 Self weight of structure upto point under

..h -".a 15(

u.l

Loadings

f I

5

calculated corresponding to minimum design weight span plus weight of insulator strings & accessories only shall be taken.

Longitudinal loads. which might be caused on tension towers by adjacent spans of unequal lengths shall be neglected.

6.9

6.11.2 Dead End Towers

VERTICAL LOADS CONDmON

(VS)

-

SECURITY

6.9.1 Loads due to weight of each cond~ctor or groundwire based on appropriate weight span, weight of insulator strings and accessories taking broken wire condition where the load due to weight of broken conductor/groundwire shall be considered as 60% of weight span. (For intact wire the vertical load shall be considered as given in clause No. 6.8) 6.9.2 Self weight of structure upto point under- consideration of tower panel.

6.10

VERTICAL LOADS DURING CONSTRUCTION AND MAINTENANCE (VM) SAFETY CONDITION

6.10.1 Same as Clause 6.9.1 multiplied by overload factor of 2.0 6.10.2 Same as Clause 6.9.2 6.10.3 Load of 1500N shall be considered acting at each cross-arm tip as a provision of w~ight of line man with tools. f I

6.10.4 Load of 3500N at cross arm tip to be considered for cross-arm design upto 220 kV and 5000 N for 400 kV and higher voltages. 6.10.5 The cross arms of tension towers shalf also be designed for the following construction loads: Tension Tower

Vertical

Lifting point distance

with

Load, N

min. from the tip of cross-arm (mm)

;>

Twin bundle

10,000

600

Conductor Multibundle conductor

2u,000

1,000

6.10.6 All bracings and redundant members of the, tower which are horizontal or inclined upto 15 deg. from horizontal shall be designed to withstand an ultimate vertical load of 1500N considered as acting at centre, independent of all other loads.

.

6.11 LONGITUDINAL LOADS (LR) - RELIABILITY CONDITION 6.11.1 Suspension and Tension Towers ,,,.j jr.·

6.11.1.1

Longitudinal loads for Suspension and Tension be taken as nil.

tower~ ~hal1

6.11.2.1 Longitudinal loads for Dead End Towers shall be considered corresponding to mechanical tension of conduc~ors and groundwires for loading criteria defin.ed in Clause 6.4. 6.12

LONGITUDINAL LOADS (LS) - SECURITY CONDITION

6.12.1 Suspension Towers The longitudinal load corresponding to 50 per cent of· the mechanical tension of conductor and 100% of mechanical tension of ground wire shall be considered under everyday temperature and No wind pressure for broken wire only. 6.12.2 Tension Towers 6.12.2.1 Horizontal loads in longitudinal direction due to mechanical tensi"n of conductors and groundwire shall be taken for loading criteria specified in Clause 6.4 for broken wire(s). For intact wires these loads shall be considered as nil. 6.12.3 Dead End Towers Horizontal loads in longitudinal direction due to mechanical tension of conductors and groundwire shall be taken for loading criteria specified in Clause 6.4 for intact wires, however for broken wires these shall be taken as nil. 6.13

LONGITUDINAL LOADS DURING CONSTRUCTION AND MAINTENANCE (LM) SAFETY CONDITION

6.13.1 Normal Condition Towers

Sm'pensi.on and Tell$ion

These loads shall be taken as nil.

6.13.2 Normal Condition - Dead End Towers 6.13.2.1 These loads for Dead End Towers shall considered as corresponding to mechanical te.nsion conductor/groundwire at every day temperature and wind. Longitudinal loads due to unequal spans may neglected .

be of nil be

6.13.3 Broken Wire Condition 6.13.3.1 Longitudinal loads during construction simulating brokenwire condition will be based on Stringing of One Earthwire or One Complete Phase of sub-conductors at one time.

6

Loadings

6.13.3.2 Broken Wire Condition fo~ Suspension Tower Longitudinal loads during Stringing on Suspension Tower should be nominally imposed only by the passing restriction imposed during pushing of the running block through the Sheave. It will apply only on one complete phase of Sub-conductors or One Earthwire. It will be taken as 10,000 N on one Sub-conductor or 5,000 N on one Earthwire.

6.13.3.3 Broken Wire Condition for Tension and Dead End Towers Angle Towers used as dead en,d during stringing simulating broken wire condition shflll be capable of resisting longitudinal loads resulting from load ·equal to twice the sagging tension (sagging tension is 50 per cent of the tension at every day temperature and no wind) for one earthwire or one complete phase of sub- conductors which is in the process of Stringing. At other earthwire or conductor attachrllent points for which stringing has been completed, loads equal to 1.5 times tbe sagging tension will be considered. However, the structure will be strengthened 'by installing temporary guys to neutralise the unbalanced longitudinal tension. These guys shall be anchored as far away as possible to minimise vertical load. 6.14

I

LOADING COMBINATIONS UNDER RELIABILITY, SECURITY AND SAFETY CONDITIQNS

2. Vertical Load as per Clause 6.10 3. Longitudinal Load as per Clause 6.13.3 and 6.13.4 6.15 ANTI·CASCADING CHECKS All angle towers shall be checked for the following anti-cascading conditions with all conductors arid OW intact only on one side of the tower. 6.15.1 Transverse Loads These load shall be taken under no wind condition. 6.15.2 Vertical Loads These loads shall be the weight of conductorl groundwire intact only on one side of tower, weight of insulator strings and accessories. 6.15.3 Longitudinal Loads 6.15.3.1 These loads shall be the pull of conductorl ground wire at everyday temperature and no wind applied simultaneously at all points on one side with zero degree line deviation. 6.16 BROKEN WIRE CONDITION 6.16.1 Single Circuit Tower Anyone phase or ground wire broken, whichever is more stringent for a particular member. 6.16.2 Double, Triple and Quadruple Circuit Towers

6.14.1 Reliability Condition (Normal Condition)

6.16.2.1 Suspension Towers

6.14.1.1 Transverse Loads as per Clause 6.5

Anyone phase or groundwire broken whichever is more stringent for a particular member.

,

1 6.14.1.2 Vertical Loads as per Clause 6.8

t 6.14.1.3 Longitudinal Loads as per Clause 6.11. 16.14.2 Security Condition (Broken Wire Condition)

16.14.2.1

Transverse Loads as per Clause 6.6

\

,I

i\ 6.14.2.2 Vertical Loads as per Clause 6.9 !

6.14.2.3 Longitudinal Loads as per Clause 6.12. 6.14.3 Safety Condition (Construction and Maintenance) : 6.14.3.1 Normal Conditions 1. Transverse Loads as per Clause 6.7.1 2. Vertical Loads as per Clause 6.10. 3. Longitudinal Loads as per Clause 6.13.1 and 6.13.2 ~; 6.14.3.2 Brokenwire Condition

6.16.2.2 Small and Medium Angle Towers

Any two phases broken on the same side and same span or anyone phase and one ground wire broken on the same side and.same span whichever combination is more stringent for a particular member. 6.16.3 Large Angle/Dead End Towers Any three phases broken on the same side and same span or any two phases and one ground wire broken on the same side and same span whichever combination is more stringent for a particular member. 6.17 BROKEN LIMB CONDITION INSULATOR STRING

FOR

'V'

6.17.1 For 'V' Insulator strings, in normal condition one limb broken case shall be considered. In such a case the! transverse and vertical loads shall be transferred to outer limb

Transmission Line Manual Chapter 7

Design of Tower Members

\

,

CONTENTS Page 1

7.1

GENERAL 7.1.1 Technical Parameters 7.2 STRESS-ANAL"'(SIS 7.2.1 List of Assumptions 7.2.2 Graphical Diagram method 2 7.2:3 Analytical Method 2 7.2.4 Computer-Aided Analysis. 2 7.2.4.1 Plane - Truss method or, 2-Dimensional analysis ') 7.2.4.2 Space - Truss method or, 3-Dimensional analysis 2 7.2.5 Comparison of various methods of stren analysis 3 7.2.6 Combination of Forces to determine maximum stress in each member 3 (i.e., Leg-Member, Bracing-Transverse and Longitudinal, X-arm and G.W. Peak) . 7.3 MEMBER SELECTION 4 7.4 SELECTION OF MATERIAL 4 7.4.1 Use of hot rolled angle steel sections 4 7.4.2 Minimum flange width 4 7.4.3 Minimum thickness of members 4 7.4.4 Grades of steel 4 7.5 SLENDERNESS RATIO LIMITATIONS (LlR) 4 7.6 COMPUTATION OF LIR FOR DIFFERENT BRACING SYSTEMS 4 7.7 PERMISSIBLE STRESSES IN TOWER MEMBERS 5 7.7.1 Curve-l to curve-6 5 7.7.2 Reduction due to bIt Ratio 5 7.8 SELECTION OF MEMBER 5 7.8.1 Selection of Members in Compression 5 7.8.2 Selection of Members in Tension 5 7.8.3 Redundant Members 6 7.9 Bolts and Nuts. 6 Annexures I II III IV V VI

W VIII IX X XI XII XIII XIV

Conductor Details Earthwire Design Loads Graphical Diagram Method Analytical Method Computer Aided Analysis ~mb3DAn~~

Output Giving Summary of Critical Stresses Chemical Composition and Mechanical Properties of Mild Steel Chemical Composition and Mechanical Properties of High Tensile Steel Section List Equal Section Commonly Used For Towers & As Per IS:808 (Part - V) 1989 Llr Consideration for Bracing System in a Transmission Tower . Permissible Axial Stress in Compression Reference Table for Maximum Permissible Length of Redundant Members

7 8 9 II 13 21 M 28 32 33 34 36 37 43

CHAPTER 7 DESIGN OF TOWER MEMBERS

7.1 GENERAL 7.1.1 Technical Parameters Design data for transmission line Towers are discussed in chapters 2 to 6.

7.2. SlRESS ANALYSIS The exact stress analysis of transmission tower requires calculation of the total forces in each member of the tower under action of combination of loads externally applied, plus the dead weight of structlle. The design of structure must be practical so that it is done as a production assignment. Basically the stress analysis of any tower requires application of the laws of statics. As. tower is a space frame the solution becomes complex. if all extemalloads are applied ~imultaneously. Different categories of loads are taken separately for calculation of stress in each member. stresses so calculated. for different types of loads are superimposed to arrive at overall stress in the member.

7.2.1 List of Assumptions (a) All members of a bolted type tower frame work are pin-connected in such a manner that the members carry oxialloads only. (b) The bolt slippages throughout the structures are such as to allow the use of the same modulus of elasticity for the entire structure. thus permitting the use of the principle of super-imposition for stress analysis. (c) Shear is distributed equally between the two members of a double web system. i.e .. warren system. (d) Shear is carried by the diagonal member under tension in a Pratt system with members designed for tension only. the other member being Inactive. (e) Torsional shears applied at crossarm level for square tower are resisted by all the four tower faces equally. (f) Plan members at levels other than those at which external loads are applied or where the leg slope changes. are designated as redundant members. (g) Any face of the tower subjected to external loads lies in the same plane. so far as the analysis . of the particular face is concerned. except earth wire cross-arm and peak. (h) Transverse loads are shared by the members on the transverse faces of the tower equally. Similarly. the longitudinal loads are shared equally by the two longitudinal faces. (i) Vertical loads placed symmetrically and dead weight of the structure are shared equally by the four legs. Vertical load at cross-arm panel will be shared by web member. in some cases.

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Figure 2(b):

R.c.c.

Spread Type Foundation (Step) with 1S'Omm Working Clearance

The R.C.C. spread type footings can be suitably designed for variety of soil conditions. R.C.C. footings. in some situations may be higher in cost although structurally these are the best.

9

When loads on foundations are heavy and/or soil is poor. the pyramid type foundations may not be feasible from techno-economical considerations and under such situations. R.C.C. spread type footings are technically superior and also economical. R.C.C. spread footing with bottom step/slab when cast in contact with Inner surface of excavated soil will offer higher uplift resistance as compared to the footing having 150 mm side clearance as shown in Figure 2(c).

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Figure 2(eJ: R.Le. Spread Type Foundation (Step) Cast Directly Contact with the Soil & without 1S0mm WorkinQ Clearance

(c)

Block Type

This type of foundation Is shown in Figures 3 & 5 (a). It consists of a chimney and block of concrete. This type of foundation is usually provided where soft rock and hard rock strata are encountered at the tower location. In this type of foundation, concrete is poured in direct contact with the inner surfaces of the excavated rock so that concrete develops bond with rock. The uplift resistance in this type of footing Is provided by the bond between concrete and rock. The thickness and size of the block Is decided based on uplift capacity of foundation and bearing area required. It is advisable to have footing having a minimum depth of about 1.5 m below ground level and check this foundation for the failure of bond between rock and concrete. The values of ultimate bond stress between the rock and the concrete to be considered for various types of rocks are given in Annexure-iV for guidance. However, the actual bond stress between rock and concrete can be decided by tests. Block type foundations are being provided by some power utilities for soft and hard rock strata. However, under cut type of foundations for soft rock and rock anchor type of foundations for hard rock are sometimes preferred by some power utilities because of their soundness even though thes.e may be more costly in comparison with Block type foundations. (d)

Under-Cut Type

These type of foundations are shown In Figures 4 (a),(b) & (c). These are constructed by making under-cut in soil/rock at foundation level. this type of foundation Is very useful in normal dry cohesive soil, hard murrum, fissured/soft rock, solis mixed with clinker, where soli is not collapsible type i.e.. it can stand by itself. A footing with an under-cut generally develops higher uplift resistance as compared to that of an identical footing without under-cut. this is due to the anchorage in undisturbed virgin soil. The size of under-cut shall not be less than 150 mm. At the descretion of power utility and based on the cohesiveness of the normal dry soil, the owner may permit undercut type of foundation for normal dry cohesive soil. (e)

Grouted Rock and Rock Anchor Type

Typical Grouted Rock and Rock Anchor type,footing is shown in Figure 5(b). This type of footing is suitable when the rock is very hard. It consists of two parts viz. block of small depth followed by anchor bars embeded in the Grouted Anchor Holes. The top part of the bsu is embeded in the concrete of the shallow block. The depth of embedment. diameter and number of anchor bars will depend upon the uplift force on the footing. The diameter shall not be less than 12 mm. The grouting hole shall normally be 20 mm more than the diameter of the bar. .. The determination of whether a rock formation is suitable for installation of rock anchors is an engineering judgement based on rock quality. Since, the bearing capacity of rock is usually much greater, care must be exercised in designing for uplift. The rock surfaces may be roughened, grooved. or shaped to increase the uplift capacity. The uplift resistance will be determined by considering the bond between reinforcement bar and grout/concrete. However. an independent check for uplift resistance should be carried out by conSidering the bond between rock & concrete block which in turn will determine the min. depth of concrete block to be provided In hard rock. Anchor strength can be substantially increased by provision of mechanical anchorages. such as use of eye- bolt. fox bolt or thread~d rods as anchoring bars or use of keying rods in case of stub angle anchoring. The effective anchoring strength should preferably be determined by testing. "

11

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Fig u re 3: Block Foundation ( Friction Tun .. \

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ELEVATION

PLAN 'A- A'

Figure 4 (a ) : Pyramid Type Foundation (with under-cut)

13

ELEVATION

PLAN A-A

Figure 4 (b): R. C. C. Spread Type Foundation (Under Cut Type)

r

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150 100 50 C.l.

X : Min depth as per bond requirement ELEVATION

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Figure 4 (c) : Block Foundation (Unit Cut Type)

15

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Figure 5 (a): Hard Rock Foundation (Block Type)

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Figure 5(b) : Rock Anchor Type Foundation

17

Open cast Rock foundation is not recommended in Hard Rock. However, where rock anchor 1ype foundation Is not practicable, open cast rock type foundation may be adopted as a special case. (f)

Augur Type/Under Reamed Pile Type

Typical types of foundations are shown In Figure 6(a). The cast-In-situ reinforced concrete augured footings have been extensively used in some westem countries like USA. Canada and many countries in our continent. The primary benefits derived from this type of foundations are the. saving In time and man-power. Usually a truck mounted power augur is utilised to drill a circular hole of required diameter, the lower portion of this may be belled, If required, to a larger diameter to Increase the uplift resistance of the footing. Holes can be driven upto one metre in diameter and six metre deep. Since, the excavated hole has to stand for some time before reinforcing bars and cage can be placed In position and concrete poured, all kinds of solis are not suitable for augured footing. Usually, stiff clays and dense sands ar~. capable of being drilled and standing up suffiCiently long for concreting works and Installation of stub angle or anchor bolts, whereas loose granular materials may give trouble during construction of these footings, Bentonite slurry or similar material is used to stabilise the drilled hole. In soft soils, a steel casing can also be lowered Into the hole as the excavation proceeds, to hold the hole open. Uplift resistance of augured footing without bell Is provided by the friction along the surface of the shaft alone and hence it's capacity to resist uplift is limited. Augured footing can be constructed according to the requirement, vertical or battered and with or without expanded base. (g)

Under-Reamed Pile Type

The under-reamed piles are more or leSs similar to augured footings except that they have under reaming above bottom of shaft. These can be generally constructed with hand augur. The bore Is drilled vertically or at a batter with the augur, having an arrangement of cutting flanges (edges) to be opened by the lever. This arrangement makes it possible to make under-reams at various level of bores as shown in Figure 6(b). The advantage of this foundation is foster construction. The load carrying capacity of these footings, both for downward and uplift forces should be established by tests. The safe loads allowed on under-reamed piles of length 3.50 m and under reamed to 2.5 times the shaft diameter in clayey, black cotton and medium dense sandy solis may be taken from IS: 4091 for guidance. These types of foundation are useful in case of expansive type of block cotton soils. (h)

Steel Grillage Type

These types of foundation are shown in Figures 7(a)&(b). These are made of structural steel sections. Steel grillages can be of various designs. Generally, it consists of a layer of steel beams as pad for the bearing area. The footing reaction Is transmitted to the pad by means of heavier joists or channels resting cross-ways on the bearing beams. For smaller towers, the horizontal shears at foundation from the component of force In the diagonal members is transferred to the adjoining soil by shear plates of adequate size proyided at the point where the bottom most diagonal bracings Intersect the main leg/stub usually about a metre below the ground surface as shown in Figure 7 (a). In case of heavy towers like angle or dead end, the lateral·force is taken up by addition of suitable bracing members shown in Figure 7 (b) which trdnsfer the shear down to the grillage beams.

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19

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Figure 6(b): Augur Type Foundation (Unde,r Reamed Pi Ie Type)

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Figure 7(a): Steel Grillage Type Foundation

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PLAN 'A_A'

Figure 7 ( b) : Steel Grillage Type Foundation

The grillage Is designed to resist the down thrust and uplift. The angle of earth frustum Is developed from the bottom of the footing. In this type of foundations, there is no solid slab as compared to concrete foundations. However, if the distance between the grillage members is not greater than the width of members, the gross area of grillage can be utilised in calculating bearing pressure. If the distance between members is large, only the net area of grillage can be taken into account for calculating the bearing pressure on the soil. The placement and compaction of the backfill is very critical to the actual load carrying capacity of this type of foundations. As a precaution against corrosion, a coot of bituminous paint is usually applied to the footing. When backfill Is well compacted to eliminate air pockets, the lower portion of the footing may not suffer any appreCiable corrosion of steel. Weathering steel or galvanised steel can also reduce the chances of corrosion, but none of these can prevent corrosion when the soil at the tower location is unfavourable and chemically aggressive. When doubt arises, It may be necessary to test the soil and sub-soil water samples to ascertain their corrosiveness before using a steel grillage footing. Grillage footings require much more steel than a comparable concrete footing, but erection cost is small in comparison to that of the concrete footing resuMing in often economical and always quicker construction. Other advantages include their simplicity in construction procurement of complete foundation with tower parts from the manufacturer of towers and elimination of concrete work at site. These foundations are also very helpful in restoring the collapsed transmission lines because of quicker construction. The disadvantage of this type of foundation is that these foundations have to be designed before any soil borings are made and may have to be enlarged and require a concrete base if actual soli conditions are not as good as those assumed in the original design. These types of foundation are generally provided in case of firm soils and are usually adopted for locations where concreting is not possible and head loading is difficult. This type of foundation is not . popular in our country. (i)

Steel Plated Type

A typical pressed steel plate foundation is shown in Figure 8. This arrangement is similar to the steel grill foundation shown in Figure 8 except that the base grillage has been replaced by a pressed steel plate. This type of foundation Is usually adopted for locations where concreting work is not possible and head loading is difficult. This type of foundation is suitable only In case of good, cohesive and firm soil. The size of plate is decided based on uplift capacity required and also based on footing area necessary from bearing capacity consideration. The net horizontal force at the level where bottom most diagonal bracing Is attached to the stub is resisted by the passive pressure of -the soil. The advantage of this type of foundation Is It's simpliCity. However, one has to be careful in excavation at the bottom .. The plate must rest firmly In contact with the surrounding soli. The disadvantage of this type of foundation is possibility of corrosion of steel and large settlement because of loose sand under the plate. This type of foundation is not popular in our country. G)

Pile Type

A typical pile type foundation is shown In Figure 9. This type of foundation is usually adopted when soil is very weak and has very poor bearing capacity or foundation has to be located In filled-up soli or sea mud to a large depth or where tower location falls within river bed and creek bed which are likely to get scourea during floods.

23

ti.L.

G.l.

c o :;: IV

> >

IV

X

""

r-

A

A

ELEVATION

PLAN A-A

Figure 8: Steel ,Plate Type Foundation

r A

HFL Water flow

HFL

..

SECTION

PLAN 'A-A'

Figure 9: Pile Type Foundation 25

The pile foundations are designed based on the data of soil exploration at the tower location. The 1mPOrtant parometers for design of pUe foundation are the type of soli, angle of Intemal friction, cohesion and unit weight of soil at various depths along the shaft of pile, maximum discharge of the river, maximum velocity of water, high flood level, scour depth etc. Pile foundation usually costs more and may be adopted only after detailed examination of the site condition and soil data. The downward vertical load on the foundation is carried by the plies through skin friction or by point bearing or both: while the uplift is resisted by the dead weight of the concrete In piles and pile caps and frictional resistance between pile and soU surrounding the pile. For carrying heavy lateral loads, battered piles may be advantageously used. Piles are of different types such as driven pre-cast piles, cast-In-sltu concrete bored piles and cast-in-situ concrete driven piles. Concrete driven piles whether pre-cast or cast-in-sltu, require heavy machinery for their construction and as such may not be possible to use for transmission line foundations because of remoteness of the sites and smail volume of work~ Mostly, cast-In-sltu concrete bored piles are provided In transmission line proJects since, they do not require heavy machinery for their construction. Load carrying capacitY of different types of piles should normaily be established by load tests. When It Is not possible to carry out load tests, the capacity of pile can be determined by static formula as given In IS: 2911 using soil properties obtained from soil investigation of tower location where pile foundation is proposed to be provided. (k)

Well Type

A typical well type of foundation for transmission line tower is shown in Figure 10. This type of foundation Is usually provided where tower location falls within the course of major river having larger discharge, heavy floods during monsoon and large scouring of river bed during floods. The cast-In-sltu weils of R.C.C. or brick masonary are sunk by continuous excavation from within the wells. The basic parameters required for the design of well are soil properties like angle of internal friction, cohesion, and density at various levels along the depth of well, maximum flood discharge, maximum velocity of water, the scour depth, etc. The well has to be taken below the estimated scour level to a sufficient depth for obtaining desired load carrying capacity of the well. Kentel edge may have to be used during sinking of the well for penetrating the hard strata and also to prevent it's tilting during sinking operation. The top of the wells is normally kept above the high flood level. After the well has been sunk to It's design depth, the well Is filled up with sand and suitable well cap Is constructed on the top of the well to accommodate the tower and it's anchor bolts/stubs. The filled up well acts as solid pier. Well type foundations are very costly and require more time for their construction and may be adopted only after detailed examination of the site condition and soil data. 10.8

REVETMENT ON FOUNDATION

The revetment on foundation is usually required when the tower is to be founded on a slope of hill or in deserts where there is possibility of soil flying away during dust storm. The typical details of revetment for hilly location are shown in Figure 11. The bench cutting is first done to level the siope. The foundation is cast with shorter and longer stubs If it is not possible to fully level the slope. Revetment is necessary to prevent erosion of soil due to water flow from uphill and also to ensure proper anchorage against uplift. . 10.9

SOIL RESISTANCES FOR DESIGNING FOUNDATION

As discussed in para 10.2, the foundations of Transmission line towers are subjected to three types of . loads viz. the downward thrust (compression), the uplift (tension) and the side thrust (horizontal shear)..

Tower leg

t Water flow

Scouring action

U

Sand filling

(utting edge Bottom (one. plug ELEVATION

PLAN A-A

Figure 10: Well Type Foundation 27

A

Tower body

lope of..~~...

.....

\... 1IP'-

Tower foundation

ELEVATION

Figure 11: Rivetment on Foundation The soil resistances available for transferring the above forces to earth are described below: (a)

Uplift Resistance

The soil surrounding a tower foundation has to resist a considerable amount of upward force (tension). In fact, In the case of self-supporting towers, the available uplift resistance of the soli becomes the most decisive factor for selection of the type of footing for a particular location. It is generally considered that the resistance to uplift is provided by the shear strength of the surrounding soli and the weight of the foundation. Various empirical relationships linking ultimate up-lift capacity of foundation to the physical properties of soil like angle of Internal friction (t/» and cohesion (C) as well as to the dimensions and depth of the footing have been proposed on the basis of experimental results. However, the angle of earth frustum Is considered for calculating the uplift resistance of soil. Typical values of angle of earth frustum are given In Annexure -I for guidance. The angle of earth frustum is taken as 2/3 of angle of Internal friction (t/» or the value given In Annexure I I. _ _ _ .. aJl ..... : _ ...... _

... _ .

11 __ ,1. __ .1.L _

.L _ _ _ _ L

-

_·1'

••

The uplift resistance is estimated by co uti th . of cone whose sides make an angle ;e formula for calculating volume covered under Inverted frustum of a cone is given in Annexure-V.

~~) ;~ ~:~i~~~~~~~~~~~~~~t!~v:::~s~~

It shoUI?, how~v~r, be noted that effective uplift resistance, apart from being a function of the properties of sOil like angle of intemal friction (¢J) and cohesion (C) is greatly affected by the degree of co,:,pactlon and the ground water table. When the back fill Is less consolidated with non-cohesive matena!, the effective uplift resistance will be greatly reduced. In case of foundation under water table, the buoyant weights of concrete and back fill are only considered to be effective. The uplift resistance of footing with undercut projections within undisturbed soils in firm non-cohesive soils and fissured/soft rock shall generally be larger than that of conventional footings. (b)

Lateral Soil Resistance

In foundation design of towers. the side thrusts (horizontal shears) on the foundation are considered to be resisted by the passive earth pressure mobilized in the adjoining soils due to rotation of the footing. Passive pressure/resistance of soil is calculated based on Rankine's formula for frictional soils and unconfined compressive strength for cohesive soils. (c)

Bearing Capacity

The downward compressive loads acting on the foundation including moments du~ to horizontal shears and/or eccentricities, wherever existing. are transferred from the foundation to earth through be.drtng capacity of the soil. The limit bearing capacity of soil is the maximum downward intensity of load which the soil can resist without shear failure or excessive settlement.

10.10

DESIGN PROCEDURE FOR FOUNDATION

The design of any foundation consists of following two parts :

(1)

'\ I

Stability Analysis

Stability analysis aims at removing the possibility of failure of foundation by tilting, overtuming, uprooting, and sliding due to load intensity imposed on soli by foundation being In excess of the ultimate capacity of the soil. The most important aspect of the foundation design is the necessary check for the stability of foundation under various loads imposed on it by the tower which it supports. The foundation shOUld remain stable under all the possible combinations of loadings, to which It Is likely to be subjected under the most stringent conditions. The stability of foundation should be checked for the following aspects: (a)

Check for Bearing Capacity

The total downward load at the base of footing consists of compression per leg derived from the tower design, buoyant weight of concrete below ground level (i.e .. difference in the weight of concrete and soli) and weight of concrete above ground level. ' While calculating over weight of concrete for checking bearing capacity of soil, the pOSition of water table should be considered at critical location i.e .. which would give maximum over weight of concrete. In case of foundation with· chimney battered along the slope of leg, the centre line of chimney may not coincide with the C.G. of the base slabs/ pyramid I block. Under such situation, oxlal load in the chimney can be resolved into vertical and horizontal components at the top of base

29

slab/pyramid/block. The additional moments due to the above horizontal loads should be considered while checking the bearing capacity of soli. Further, even In cases where full horizontal shear Is balanced by the passive pressure of soli. the horizontal shears would cause moment at the base of footing as the line of action of side thrusts (horizontal shears) and resultant of passive pressure of soil are not In the same line. It may be noted that passive pressure of soil is reactive force from the soil for balancing the external horizontal forces and as such mobilized passive pressure In soil adjoining the footing can not be more than the external horizontal shear. Thus. the maximum soil pressure below the base of the foundation (Toe pressure) will depend upon the vertical thrust (compression load) on the footing and the moments at the base level due to the horizontal shears and other eccentric loadings. Under the action of down thrust and moments, the soli pressure below the footing will not be uniform and the maximum toe pressure 'P' on the soli can be determined from the equation:

W

MT

ML

P=-+ - + BxB ZT ZL

Where. 'W' is the total vertical down thrust including over weight of the footing; 'B' is dimension of the footing base; MT & ML are. moments at the base of footing about transverse and longitudinal axes of footing; and ZT & ZL are the section modulii of footing which are equal to (1/6) B3 for a sqlJare footing. The above equation is not valid when minimum pressure under the footing becomes negative. The maximum pressure on the soil so obtained should not exceed the limit bearing capacity of the soil. (b)

Check for Uplift Resistance

In the case of spread foundations, the re.sistance to uplift is considered to be provided by the buoyant weight of the foundation ,and the weight of the soli volume contained In the inverted frustum of cone on the base of the footing with sides making an angle equal to the angle of earth frustum applicable for a particular type of the soil. Referring to Figure 13. the ultimate resistance to uplift is given by : UP :: Ws + Wf where 'Ws' is the weight of soil in the frustum of cone; (The method of calculation of Ws is given in Annexure-V). 'Wf' is the buoyant weight/overload of the foundation (Refer Figures 13 & 14). Depending upon the type of foundation i.e .• whether dry or wet or partially submerged or fully submerged, the weights 'Ws' and 'Wf' should be calculated taking Into accounUhe location of ground water table. Under-cut type of foundation offers greater resistance to uplift than an Identical footing without under-cut. This Is for the simple reason that the angle of earth frustum originates from the toe of the under-cut and there Is perfect bond between concrete and the soli surrounding It and there Is no n~d to depend on the behaviour of backfilled earth. Substantial additional uplift resistance Is developed, due. to use of under-cut type of foundation. However. to reflect advantage of additional uplift resistance In the design the density of soil for under-cut foundation has been increased as given in Annexure -I.

In cases wnere HU:;IUIII VI C:UIItI I-IYIUIIIIU \:7 '-\:7- __ ,-_ .... -,--_ frustum Is assumed truncated by a vertical plane passing through the centre line of the tower base. VI

IY't'V ..........} " ' ......

Check for Side Thrust

(c)

In towers with inclined stub angles and having diagonal bracing at the lowest panel point. the net shearing force of the footing is equal to the horizontal component of the force in the diagonal bracing whereas in towers with vertical footings. the total horizontal load on the tower is divided equally between the number of legs. The shear force causes bending stresses in the unsupported length of the stub angle as well as in the chimney and tends to overtum the foundation. When acted upon by a lateral load. the chimney will act as a cantilever beam free at the top and fixed at the base anq supported by 'the soil along it's height. Analysis of such foundations and design of the chimney for bending moments combined with down thrust/uplift Is very important. Stability of a footing under a lateral load depends on the amount of passive pressure mobilized in the adjoining soil as well as the structural strength of the footing in transmitting the load to the soil (Refer Figure 12),

S I

r-----7,

,,, I

,I

IIIIi , I

I

.c.

~! ,I , I

I I,

I t

I

: 1I

/-1I ,I

I

I

I

I

,I

I

I

I

/

I I

,, I

• ,I 'I I 'I I

I

,, I

I I I I I

3.0Cu~

passive Pressure ,in , Non Cohesi~ S~II .

Passi~ Pres.surO'in jr

Cohe~~~~_~,~l~~.. _', .__.~

Cu =Undrained Cohes!,o_~ ~r Soil Kp =Coemcient of passive earth pressure K =1• Sin • . p l-.Sin. 1 = Unit Wt. of Soil Ikg.lm 3)

Figure - 12 31

S"

•

I

\

\

I

\

I

\

I

\

\ \

I I

I

\

I

\

.&;

I

\

I

\

m

I

\

r\~

I

I

t

A

I-

B

A

I-

-I

I

ELEVATION

PLAN

Figure. 13

s

~

\

I

\

B3

I

\

I

\

/

\ \ \ \ \

I

/ / /

-.

\

\~ \~f

A

2

/

.&;

/ I

/

m

/ I

\

. ~.

B

-I PLAN 'A-A'

ELEVATION

Figure.14

(d)

Check for Over-Turning

Stability of the foundation against overturning under the combined action of uplift and horizontal shears may be checked by the following criteria as shown in Figure 14 :

0) (II) Oii)

The foundation over-turns at the toe. The weight of the footing acts at the centre of the base; and Mainly that part of the earth cone which stands over the heel causes the stabilising moment. However, for design purpose, this may be taken equal to half the weight of the cone of earth acting on the base, It is assumed to act through the tip of the heel.

For stability of foundation against overturning, stabilising moment should be more than overtuming moment. (e)

Check for Sliding

In the foundations of transmission towers, the horizontal shear is comparatively small and possibility of sliding is generally negligible. However, resistance to sliding Is evaluated assuming that passive earth pressure conditions are developed on a vertical projections above the toe of foundation. The friction between bottom of the footing and soil also resist the sliding of footing and can be considered in the stability of foundation against sliding. The coefficient of friction between concrete and soil can be considered between 0.2 to 0.3. However, the frictional force is directly proportional to vertical downward load and as slJch may not exist under uplift condition. For cohesive soil the following formula can be applied for calculating the passive pr~ssure to resist sliding: P =2 C tane + rh Ta~29 Where C

e

h

y. (2)

= = = =.

Cohesion (2t/m2 min.) 45° + 1/2 of angle of earth frustum Height of foundation Unit Wt.of soil

Structural Design of Foundation

Structural design of concrete foundation comprises the design of chimney and the design of base slab/pyramid/block. The structurql design of different elements of concrete foundation is discussed In the following paras: (a)

Structural Design of Chimney

The chimney should be deSigned for maximum bending moments, due to side thrust In both transverse and longitudinal direction combined with direct pull (Tension) / direct down thrust (Compression). Usually, combined uplift and bending will determine the requirement of longitudinal reinforcement In the chimney. When stub angle is embedded in the chimney to Its full depth and anchored to the bottom slab/pyramid/block, the chimney is designed. Considering passive resistance of soil leaving 500 milimetres from ground level. This is applicable for all soils - cohesive, non-coheslve and mixture of cohesive and non-cohesive soils. In hilly areas and for fissured rock, passive resistance of solis will not be considered. Stub angle will not be considered to provide any reinforcement. In certain cases, when stub is embedded in the chimney for the required development length alone and same is not taken upto the bottom of foundation or leg of the tower is fixed at the top of the chimney /pedestal by anchor bolts, chimney should be designed by providing 'reinforcement to

33

withstand combined stresses due to direct tension (tension)/down thrust (compression) and bending moments, due to side thrust in both transverse and longitudinal direction. The structural design of chimney for the above cases should comply with the procedures given in IS: 456-1978 and SP: 16 using limit state method of design except as specifically provided In this document.

CASE-I:

WHEN SlUB ANGLE IS ANCHORED IN BASE SLAB/PYRAMID/BLOCK

When the stub is anchored in base slab/pyramid/block reinforcement shall be provided in chimney for structural safety on the sides of the chimney at the periphery. From the equilibrium of internal and external forces on the chimney section and using stress and strains of concrete and steel as per IS:456, the following equations as given In SP: 16 are applicable. n

Pu =O.36k+!; (Di/lOO) (Fsi-Fci) + (pS/100) (Fss-Fcs)/Fck ... (1) 2 FckB3 1-11 m~ I m = modular ratio . . --I

IK = M

i

. =O.36k(O.5-0.416k) FckB3 2 i-

I

- - ; c cbc cs:t. + ~ ~ _c st ____

+1:

=permissible bending com.press stress in =permissible len~!~ str~_s.~n!~eel_________.=--~.

(pi/100) (Fsi-Fci)/Fck) (YijD)

... (,,)

i-1

D:-Total deptll of stub:

Where Asi pi .Fci Fsi

= = = =

Vi

=

n Fss Fcs Fck

= = = =

Cross-sectional area of reinforcement In ith row 100 Asi/B3 2 Stress in concrete at the level of ith row of reinforcement Stress in the ith row of reinforcement., Compression being positive and tension being negative distance from the centroid of the section to the i'h row of reinforcement; positive towards the highly compressed edge and negative towards the least compressed edge Number of rows of reinforcement Stress in stubs Stress in concrete Characteristic compressive strength of concrete

CASE-jl: WHEN SlUB IS PROVIDED IN CHIMNEY ONLY FOR ITS DEVELOPMENT LENGTH· When stub is provided in chimney only for it's development length, chimney has to be designed for and reinforcement provided for combined stresses due to direct pull(tension)lThrust (compression) and bending moments. The requirement of longitudinal reinforcement should be calculated in accordance with IS: 456 and SP: 16 as an independent concrete column. In this case, from the equilibrium of internal and external forces on the chimney section and using stress and strains-of concrete and steel as per IS:456, the following equations as given in SP: 16 are applicable. n

Pu 2 =O.36k(O.5-0.416k) FckB3

+L

(pi/100) (Fsi-FCi)/Fck)

...

(~)

i-1

In each of the above cases, for a given axial force, compression or tension, and for area of

n

E

Mu 2 =0.36 k (0.5 -0.416 K) + (pi/100) (Fsi-Fci) / Fck) (Yi/D) FckB3 i"'l

...

(~.

reinforcement. the depth of neutral axis Xu=kB3 can be calculated from equation (1) or (3) using stress strain relationship for concrete and steel as given In IS: 456-1978. After finding out the value of 'k' the bending'capacity of the chimney section can be worked out using equation (2) or (4). 'The bending capacity of the chimney section should be more than the maximum moment caused In the chimney by side thrust (horizontal shear). Chimney is subjected to biaxial moments i.e., both longitudinal and transverse. The structural adequacy of the chimney in combined stresses due to axial force (tension/compression) and bending should be checked from the following equation: \

< 1,0 \,

Where, MT and ML are the moments about transverse and longitudinal axis of the chimney: Mut and Mul are the respective moment of Resistance with axial loads of Pu about transverse and longitudinal axes of chimney which would be equal In case of square chimney with uniform distribution of reinforcement on all four faces:

_nPu/Puz is an exponent whose value would be 1.0 when axial force is tensile and depends on the value of when axial force is compressive where: Puz = 0.45 Fck Ac + 0.75 Fy As + 0.75 Fys Ass In the above equation, Ac is the area of concrete; As is the area of reinforcement steel; Ass is the cross sectional area of stub, to be taken as zero; Fy is the yield stress of reinforcement steel: and Fys is the yield stress of stub steel, to be taken as zero. Pu/Puz

n

0.2 0.8

2.0

1.0

For intermediate values. linear interpolation may be done. The solution of equations (3) & (4) for case-2 Is given In SP-16 in the form of graphs for various grades of concrete and steel and these can be readily used. IMPORTANT CODAL STIPULATIONS FOR STRUCTURAL DETAILING OF CHIMNEY

While designing the chimney. the important codal provisions as given below should be followed: (a)

In any chimney that has a larger cross-sectional area than that required to support the load. the minimum percentage of steel shall be based on the area of concrete required to resist the

35

direct stress and not on the actual area. (b)

The minimum number of longitudinal bars provided in a column shall be four in square chimney and six in a circular chimney.

(c)

The bars shall not be less than 12 mm in diameter.

(d)

In case of a chimney in which the longitudinal reinforcement is not required in strength calculations, nominal longitudinal reinforcement not less than 0.15% of the cross sectional area shall be provided.

(e)

The spacing of stirrups/ lateral ties shall be not more than the least of the following distances: (1) (2) (3)

The least lateral dimension of the chimney Sixteen times the smallest diameter of the longitudinal reinforcement bar to be tied Forty-eight times the diameter of the transverse stirrups/lateral ties.

(f)

The diameter of the polygonal links or lateral ties shall be not less than one-fourth of the diameter of the largest longitudinal bar, and in no case less than 6 mm.

(g)

Structural Design of Base Slab

The base slab in R.C.C. Spread foundations could be single stepped or multi stepped. The design of concrete foundations shall be done as per limit state method of design given in IS : 456 - 1978.

IMPORTANT CODAL STIPULATIONS FOR R.C.C. FOUNDATIONS The important provisions applicable for concrete foundations which are necessary and should be considered in the design are explained below: (a)

Footings shall be designed to sustain the applied loads, moments and forces and the induced reactions and to ensure that any settlement which may occur shall be as nearly uniform possible, and the bearing capacity of the soil is not exceeded.

(b)

Thickness at the edge of footing in reinforced concrete footings shall be not less than 15 cm (5 cm lean concrete plus 10 cm structural concrete). In case of plain concrete footing, thickness at the edge shall not be less than 5 cm).

(c)

Bending Moment (i)

The bending moment at any section shall be determined by passing through the section of a vertical plane which extends completely across the footing, and computing the moment of the forces acting over the entire area of the footing on the side of the said plane.

(ii)

The greatest bending moment to be used in the design of an isolated concrete footing which supports a column/pedestal shall be the moment computed in the manner prescribed in c(i) above at sections located as follows : (1) (2)

(d)

At the face of the chimney; At sections where width/thickness of the footing changes.

Shear and Bond

The shear strength of footing Is govemed by the more severe of the following two conditions:

(e)

(1)

The footing acting essentially as a wide beam. with a potential diagonal crack extending in a place across the entire width; the critical section for this condition shall be assumed as a vertical section located from the face of the chimney at a distance equal to the effective depth of the footing in case of footings on soils;

(2)

Two-way action of the footing. wHh potential diagonal cracking along the surface of truncated cone or pyramid around the concentrated load;

Critical Section

The critical section for checking the development length in a footing shall be assumed at the same planes as those described for bending moment In para (c) above and also at all other vertical plahes where abrupt changes of section occur. STRUCTURAL DESIGN OF BASE StAB SHALL BE DONE AS PER THE PROVISION OF E-1 OF APPENDIX-E-'OF IS: 456-1978.

When a plain concrete pyramid and chimney type footing Is provided and pyramid slopes out from the chimney at an angle less than 45° from vertical. the pyramid Is not required to be checked for bending stresses. Thus. in such cases. the footing Is designed to restrict the spread of concrete pyramid of slab block to 45° with respect to vertical. 10.11

CONCRETE TECHNOLOGY FOR TOWER FOUNDATION DESIGNS

While designing the various types of concrete footings. it is better to know about certain aspects. of co~crete technology which are given below: (a)

Properties of Concrete

The grade of the structural concrete used for tower foundations should not be leaner than M-15 (1 :2:4) having a 28-day cube strength of not less than 15 N/mm2 and concrete shall confolm to IS: 456. For special foundations like pile foundations. richer concrete of grade of M 20 (1: 1.5:3) having a 28-day cube strength of not less than 20 N/mrrt should be used. M-15 grade concrete shall have the nominal strength of not less than 15 N/mm2 at the end of 28 days as ascertained form the cube te.st. Such strength at the end of 7 days shall not be less than 10 N/mm2. The density of the concrete will be 2300 kg/m 3 for plain concrete and 2400 kg/m 3 for R.C.C. Oth.er· properties of concrete are given In IS: 456. (b)

Properties of Steel

The high yield stress cold deformed reinforcement bars used in the R.C.C. shall conform to IS: 1786-1979 and shall have yield stress of not less than 415 N/mm2. When mild steel reinforcement bars are used in R.C.C.. they shall conform to IS: 432 (part - I) and shall have yield stress of not less 26 N/mm2 for bars of size upto 20 mm diameter and 24 N/mm2 for bars above 20 mm diameter. 10.12

PULL-OUT TESTS ON TOWER FOUNDATION

The pull-out tests conducted on foundations help In determining the behaviour of the soil while resisting . the up-lift forces. The feed back from this pull-out test results. In a particular type of soli. can be conveniently used In the

37

designs of foundations. The procedure of pull-out tests, equipments and results are discussed In detaU below: (0)

Selection of Site

Trial pits of size 1.Ox1.0x3.0(d) metre are mode and the strata of the soil Is observed. It is ascertained that the strata available at the location Is one In which we are Interested (I.e., a particular type of soil or combination of soils is available). Soil samples are takEm from and around the'slte and subjected to various tests, particularly relating to the density of soli, bearing capacity of soli, cohesion and angle of intemal friction etc. (b)

Design of Foundation for Pull-Out Test

Design of foundations for pull-out test is carried out with a different view point os compared to the design of actual foundations for tower. this Is due to the fact that the pull-out tests are conducted to measure the pull-out resistance of the solis and therefore all the other ports of the foundation viz concrete, reinforcement and the pull-out bars should be strong so that these do not fall before the soli/rock fails. Based on the actual tower foundation loadings (down thrust, uplift and side thrust) and the soil parameters obtained from the tests, a foundation design is developed. The design has a central rod running from the bottom of the footing upto a height of about 1.5 m to 2.0 m above ground, depending on the jacking requirements. The central rod is surrounded by a cage of reinforcement bars. A typical design developed for the pull-out test is shown in Figure 15.

30_O -r--_ _

G.l.

±_ =-1

Pull-out bar

300

GoL.

(2.4-0.5)m therfore the soil pressure will only be mobilised in 1.9m depth. Resisting soil force F = 518.86

X

1.92= 1873.09 kg

Moment due to side thrust at the base of the footing = 4983x(2.95+0.225) - 1873.09x(O.55+ 1.9/3) = 13605.2 kg m V 7.0

Check for Bearing Capacity

165598/1.036+ 11523

2x(165598/1.036)xO.192570xO.6

+ --------------

NC= 5.19 2

1/6x5.19]

16538.86 1818.85 + --------- + --------1/6 X 5.19] 1/6x5.19] = 6362' + 1585.3 710+ 78 = 8736 kg/m2 < 13675 kg/m2

Hence O.K.

154376/1.036 + 11523 BWC=

2 x (154376/1.036)xO.192570x(0.6) + -------------1/6x5.19J

5.19 2 24082.70

13605.2

+------ +-----1/6 x 5.19]

1/6x5.19]

= 9056 Kg/m2 < 13675 Kg/m2 B.O

· '!

Hence O.K

Design of Chimney A)

Compression with bending

Area of steel in compression ASC = 24x n/4 x(2.0)2 = 75.40 cm 2 . percentage of steel p/fck = 1.785/15 =0.119

, I.

= p = ASClB3 2 X 100 :B3=65 cm =1.785

59

Normal Condition Puc

=165598 Kgs =1624516 N

Puc

1624516 - - - - - - = 0.256

--= fck.bd

15x650x650

.x d' =50(20/2) =60 therefore d'/d =0.10

d =650

As per chart 44 of 5p.16 For the values of Puc/fckbd Mux1/fckbd 2 =0:65 -+ Mux1

= 0.256 & p/fck = 0.119

=0.165x15x650x650 2 = 679.7 X 10 N-mm 6

= 679.7 KN-m

Also Muy1 = 679.7 KN-m From the calculation shown in $ 6.0 Moment at the root of the chimney Mux =5907x(2,4+0.225) - 1873.09x(1.9/3) =14320.21 kg m =140.5 kN m Muy =825x(2.4+0.225) - 825x(1.261/3) =1818.88 kg m =17.84 kN m Ref: Clause 38.6 of 15-456-1978 PUZ

PUC

=0,45xfckxAC+0.75 fy ASC =0,45x15x(650)2+0.75x415 x{24x1t/4x20 =5198650.2 N =5198.65 KN =165598 Kgs =1624.5 KN

PUC

. 1624.5

--= PUZ

=0.3125 5198.65

for PUC/PUZ

f ::~::)

2)

=0.3125; ocn =1.1875

r

+

::~::) r ~

= 0.154+0.013 = 0.167 < 1.0 Hence O.K.

___ 17.84 11.1875

'r-_ _ _ ] 1.1875+ 140.50

+ 679.7

679.70

)..

BROKEN WIRE CONDITION

PUC =154376 kgs =1514.4 KN PUC/fckbd =1514.4x1000/15x650x650 =0.239 p/fck =0.119 .As per chart 44 of SP16 MUX1/fckbd 2 =0.167 MUXI =0.167x15x650x65Q2 = 687.90 x 10' N-mn =687.90 KN-m Also MUY1 =MUXl =687.90 KN-m From the calculation shown in $ 6.0 Moment at the root of the chimney Mux

= 8283x(2.4+0.225) - 1873.09x(1.9/3)

=20557.21 kg m =201.67 kN m =4983x(2.4+0.225) - 1873.09x(1.9/3) =11894.71 kg m

Muy

= 116.69 kN m

=" 5198.65 KN

PUZ

PUC/PUZ

(MUX)

=1514.4/5198.65 =0.2913; ocn =1.152

l"n

[ (MUX1)

J •

1.152

201.67 [ 687.90

=0.243+0.129 =0.373 < 1.0 Hence OK B)

Tension with Bending NORMAL CONDITION

PUt

=140917 Kgs

=1382396 N

PUtlfckbd =1382396/15x650x650 =(-)0.22 p =1.785 p/fck =0.119 d'/d =0.10 From Chart 79 of SP 16 61

J1.152 .

116.69 ] [ 687.90

Muxl/ fck bd2 =0.085 Muxl :; 350.15 kN m Muxl Muyl 350.15 kN m

=

=

Mux =140.5 kN m Muy = 17.85 kN m As per c1. 38.6 of 15-456-1978

[::~::) r

+

[

:~::)

r:

=1.0 for tension with bending

ocn

(MUX) ]

140.5 350.15

(MUY) ]

+

[ (MUX1)

=[

1.0

[ (MUY1)

I

+

[

17.85 350.15

I

=0.452 < 1.0 Hence O.K. BROKEN WIRE CONDITION

=130185 Kgs

PUt

=1277.1 kN PUt/fckbd

= 1277115/15x650x650 = (.)0.202

p = 1.785 p/fck =0.119 d'/d =0.10 From Chart 79 of 5P 16 Mux1/ fck bd 2 =0.09 Mux1 370.75 kN m Mux1 Muy1 = 370.75 kN m

=

Mux =201.67 kN m Muy =116.7 kN m As per c1. 38.6 of 15-456-1978 ocn (MUX)

+

[

ocn

(MUY)

< 1.0 [ (MUX1)

an

(MUY1)

=1.0 for tension with bending

[ (MUX) (MUX1) -

1

::~::) ]

+

[

+

[:::::: ]

1

-

[201.67] 370.75

= 0.858 < 1.0 Hence O.K.

9.0

Design of Base Slab Design Bearing Pressure = (PIA) + (P.ex/Z) +MAX{ST moment, SL moment}!Z = 6362 + 1585.3/2 + 710 = 7865 kglm2 ~ 0.07715 N/mm 2 d, = Eff. depth at Section XX = 550-50-16-8 = 476 mm d2= Eff. depth at Section YY = 350-50-16-8 =276 mm

a)

COMPRESSION REINFORCEMENT (i) Bending Moment at Section X-X

I

'

Bearing Pressure = 7865 kglm2 = 0.07715 N/mm 2 MUX1 = 0.07715x (8-83)2/8 x 5190 = 0.07715 x (5190-650)2/8 x 5190 1031708030 N-mm = 1031.6 kN m

=

MU, LIM = 0.36 Xu, Max/d (1-0.42 Xu, max/d) bd 2fck As per C1. 37.1 f of IS - 456 for Fe 415 grade steel Xumax/d = 0.48 Mu, LIM = 0.36x0.48 (1-0.42xO.48)x1740 x (476)2x15 815.8 kN m < 1031.7 kN m '-,-

=

Mux1/bd1

= 1031.7 x 10' I (1740x476 2) =2.618> 2.06

Hence section to be designed as doubly reinforced section. d'/d (50+16+8) 1476 0.15

=

=

63

From table 49 of SP 16 Pt 0.8956, Pc 0.192 Hence Ast (1740x476xO.8956)/l 00 7418 mml Provide 37 bars of 16mm dia. Ast provided =7437 mm1> 7418 mml Asc =(1740x476xO.192)/100 =1590.2 mml Provide 8 bars of ·16 mm dia. This is the minimum reinforcement to be provided at section

=

(ij)

= = =

x-x for uplift.

Bending Moment at Section Y_ Y

Muy1·

=0.07715 x(5190-1740)1 x 5190/8 =595.73 kN m

.

Muy1/bd1

= 595.73 x 10'1 (4690x2762) =1.67 < 2.06 Hence section to be designed as singly reinforced section. From table 1 of SP 16 Pt 0.546 Hence Ast (4690x276xO.546)/l 00 =7068 mm2Provide 37 bars of 16mm dia. Ast provided 7437 mm2> 7067 mm2

=

(bJ

= =

UPLIFT REINFORCEMENT

=1409171 (5.19 -0.65 =5314.9 Kglm2

Bearing Pressure P2

2

2)

=0.052139 N/mm 2

(j)

Bending Moment at Section X-X

MUX2

=0.052139 x (5190-650)2/8 x 1000

=134333520 N-mm/M

MUX2 = 0.87 x 415 x Ast x 476 (1 - Ast x 415/1 000~15) Ast 820.81 mm21M-width = 8.21 eM2 1M-width Ast reqd. =8.21x1.74 14.29 eM2 Provide 8 bars of 16 mm • Ast Provided 16.08 em2> 14.29 cm 2 Hence depth provided at Section X-X is ok.

C7

= -~

=

=

(ij)

Bending Moment at Section Y_ Y

= = =

=

MUY2 0.052139 x (5190-1740)2 I 8 x 1000 77573055 N-mm/M MUY2 =0.87 x 415 x Ast x 276 (1 - Ast x 415/1000x276x15) Ast 850.9 mm 2/M-width a.51 eM2 1M-width Ast reqd. =: 8.51x4.69 39.91 eM2 Provide 22 bars of 16 mm • Ast Provided 44.22 em 2> 39.91 eM2 Hence depth provided at Section Y-Y is ok.

=

=

;

,

c).

CHECK FOR ONE WAY SHEAR

At Section X-X Design bearing Pressure p 7 0.07715 N/mm 2

Shear force =VI =

B-B 1

xP

-d1

2

=0.07715x[(5190-650) /2-476] x1000 =138407 N/M width

=138407/476x1 000 =0.291 N/mm2

Shear Stress

% of Steel (p) =(Ast/bd)x1 00 = ((74.37x100) / (5190x476) xl00 = 0.301 As per table 13 of IS-456-1978 Allowable Shear Stress 0.3806 N/mm2>0.291 N/mm2 Hence O.K.

=

At Sec-Y-Y p = 0.07715 N/mml

=

Shear force:: V2

B-B 2 - d2

xp

2

':: 0.07715x [6190-1740) 12-276 )x1000 :: 111790 N in ShearStle$:: 111790J276x1000 :: 0.4050 NAn 2

=

Ast/bdxl00 74.37xl00/ (5190x276) xl00 = 0.5192 . Allowable Shear Stress = 0.468 N/mm 2>0.405N/mm 2 Hence OK d).

CHECK FOR 7WO WAY SHEAR

At Section X-X

=

p 0.07715 N/mm 2 Shearforce ='V2 [B2-(B J+D1)2] x p =0.07715x[51902-(650+476)2] 1980304 N Shear Stress 1980304/4x476[650+476] 0.924 N/mm2

=

= =

65

Allowable Shear stress = 0.25 x (15) 1/2 . = 0.968N/mm2 > 0.924 N/mm2 Hence OK

At Sec-y-y p = 0.07715 N/mm2 5hearforce = V2 [B2-(B 3+01)2] x p = 0.07715x[519()2(1740+276)2] = 1764563 N Shear Stress = 1764563/4x276[1740+276] = 0.793 N/mm2 Allowable Shear stress = 0.25 x ..J15 = 0.968N/mm2 > 0.793 Hence OK e)

CHECK AGAINST UPROOTING OF STUB:

Design Uplift = 140917 Kgs. Stub section = 200x200x16 Stub depth below GL = 2800 mm UltLoad resisted by stub in slab due to Bond Us = [Ox{Xx2.0+(X-Ts)x2.0}-Npx{X+(X-Ts)}xklxs Where X = flange width of stub. o = Depth of stub in slab. s = Ultimate permissible bond stress between stub & concrete Ts = Thickness of stub section. Np = No. of cleat pair (pair consist of outer and inner cleat) k = Flange width of cleat section. Us = [40x{20x2+(20-1.6)x2.0}-3x{20+(20-1.6)x11]x10 = 18048 Kg. Ultimate permissible bearing stress in concrete = 68.84 kg/cm2 Use outer cleat = 3 nos. 11 Ox11 Ox8 - 440 mrn long yse inner cleat = 3 nos. 11 Ox11 Ox8 - 250 mm long . provide 4 nos. of"16 dia. bolts per cleat pair of 5.6 grade

Load resisted by cleat in bearing Uc Where b Lo U Ct

= bx(Lo+U)xNpx(k-Ct) = Ultimate Bearing pressure in concrete = Length of Outer cleat = Length of Inner cleat = Thicness of cleat section. --':;-1

Uc=68.84~~9+25)x3x(11-0.8)

= 136923 Kg

(j)

:

{

Ultimate shear strength of bolts Ub

= total no. of boltsx2.0x2.01 x3160 (considering M-16 bolt gradeS.6 & double shear for cleat connected in pair) = (4x3)x2.0x2.01 x3160 = 152438 Kg (ii)

Ultimate bearing strength of bolt in stub or cleat = Total nos. of boltsx1.6x(Ts or2xCt)x5200 take Ts or2xCt which ever is less = (4x3)x1.6x1.6x5200 = 159744 Kg (iii) Effective strength of stub and cleat = Us+ .Least of the strength of case [ (i), (ii), (iii) ] = 18048+136923 = 154971 Kg which is more than UIt.Uplift=140917kg (Hence safe) f)

CHECK FOR BOND: Design bearing pressure = 0.07715 N/mm2 (5190-2650) Maxm. Shear force =

- 476 [

1

x5190xO.07715

= 718333 N As per Appendix - E of Is - 456 - 1978 Xu/d = 0.87 fy Ast/0.36 fck bd 0.87x415x7437

=-------0.36x15x5190x476 = 0.2013

J = 1-Xu/d xl/3

=1-0.2013/3 =0.933

Bond Stress =718333/0.933x476x37x 1t x16 = 0.87/N/mm2 < 1.6 N/mm 2 Hence OK.

10.0

Check for Sliding

/

F1 =1/2x1.5x6480xO.65 =3159 F2 =1/2x (23'9'5+3831) x 0.9XO.65 =1821 F3 = (0.2/2) (38J2+4151)x1.74 = 1389 F4 =(0.25/2) (45-50+4151) (4.69+5.19)/2 =5373 F5 = (0.1/2)(4550+4710) x 5.19 = 2403

=14145 67

.I

.

' 1.0 F.O.S. in BWC =14145/8283 =1.71 > 1.0 Hence OK.

n.o

Check for Overturning Resultant Side Thrust

= {59072+825 2)1/2 =5964 kg (ii) Under BWC = (8283 2+4983 2)112 = 9666 kg (j) Under NC

Total Overturning Moment Under NC -

(j)

= (140917/1.036)x(5.19/2 -5.19/6) + 5964x(2.9S+0.225) - 5338x(5.19/2 -5.19/6) = 245016 kg m (ij) Under BWC = (130185/1.036)x(5.19/2 - 5.19/6) + 9666x(2.95+0.225) - 5338x(5.19/2) -5.19/6} = 238849 kg m Total Resisting Moment = 1/2 x(68.327xl440 + 44.311 x940) x (S/6 xS.19) ~ 302843 kg m

"

Factor of Safety Under NC =302843/245016 =1.236 > 1.0 Under BWC = 302843/238849 = 1.268> 1.0 Hence O.K. 12.0

Quantities Per Tower

: 42.06 m3 + 5.39 m3 (M15) (Ml0) Excavation Volume: 361.68 M3 Reinforcement : 4962 Kgs.

Concrete Volume

13.0

Reinforcement Detail

13.1

BAR BENDING SCHEDULE

Sketch

Length

Bar~

(mm)

(mm)

t

No. of Bars ~::(.

5090

n

,

\.~

t·.,

Unit wt. wt.llength wtlTower (kg/m)

(kgs)

(kgs)

.

5090

16

76

1'58

611'21

2444'84

2690

16

16

1'58

68'00

272:00

5352

16

44

1'58

372'07 1488'28

3350

20

20

2· 47

165'49

661'96

2307

6

13

0·22

6·60

26'39

Totol

4893'47

1640

100

L

425

100 4590

~

100

100

3000

~ 550

0

550

4894 kgs

69

13.2

REINFORCEMENT SKETCH

_~,-r-_~e.L.

Bar Mkd' 0 ' (4) bars of 20

650---'

o o 1.0 F.O.S (BWC) = 169213/130185 = 1.300> 1.0

6.0

Check for Bearing Capacity

165598/1.036 + 7069

165598/1.036 x 0.192570 x 0.6 x2

+

NC=

1/6x4.69 3 14676.0

+

=10701

1910

+ 1/6x4.691

1/6x4.691

K&,m2 < 62500 K&,m2 154376/1.036+7069

154376/1.036 xO.192570xO.6x2

BWC=-------------- + 1/6/6x 4.69 3 22220.0

+

.1/6x4.693

=11075 K&,m2 7.0

11743

+ 1/6x4.693

< 62500 K&,m2

Design of Chimney

.

Basic design calculations are similar to those given in Wet Type foundation. B.O.

Design of Base Slab

Basic design calculations are similar to those given in Wet Type Foundations. 9.0

Check for Sliding

Basic design philosophy is similar to that given in wet type foundation. J0.0

Check for Overturning

Basic design philosophy is similar to that given in wet type foundation.

11.0

Quantities Per Tower

35.07 (M15) + 4.40 (M1 0) 233.71 ml 4150 Kgs.

Concrete Volume (Ml) Excavation Volume (NEAT) Reinforcement 12.0

12.1 12.2

Reinforcement Details Similar to those given in wet type foundation. REINFORCEMENT SKETCH:

Similar to that given in wet type foundation. BAR BENDING SCHEDULE Similar to that given in wet type foundation.

ILLUSTRATION-VII SUBMERGED FISSURED ROCK TYPE FOUNDATION

8v C\J

o o o

o

oN

r()

~

N

o o o 10

Lean concrete (I : 3: 6) • • • • • Do.

•

6>

•• ,

2080

..

••••

• •••

All dimensions are in mm.

6090 6590 "

Sketch-8: Submerged Fissured Rock Type Foundation" "ALL DIMENSIONS ARE IN MM" 1.0

Volume

of Concrete (Cu.m)

2

6.59 x 0.05 6.59 2 x 0.10 0.25/3 [6.59 2 + 6.09 2 + 6.59 x 6.09) 2.08 2 x 0.2 0.66 2 x 2.625 TOTAL

= 2.171

= 4.343 =10.054

= 0.865 = 1.143 =18.577 79

2.0

Overlo~d Que

tp CqlJcrete (Kg.) COMP 235 8501

2

(0.66 x 0.225) x 2400 = (1 a,577 -.098) x (1400-940) (18.577 -0.098-2.171) x (140~940)

= =

UPLIFT 235

7502 8736

3.0

Dry $q;1 volu~ : Nil

4.0

Wet Soil volume: (Cu.m)

=

=

128.113 18.877 0.754

=

147.744

6.59 2 x 2.95 2 x 6.59 x 0.503 x 2.85 1t /3x (0.503) 2x2.85 = TOTAL

5.0

Check for Uplift

5.1

RESISTANCE AGAINST UPLIFT:

7737

=147.744 x 940 + 7737 =146616 Kgs.

F.O.S (NC) =146616/140917 =1.040> 1.0 F.O.S (BWC) = 146616/130185 =1.130> 1.0

6.0

Check for Bearing Capacity

NC

=

165598/1.036 x 0.192570xO.6x2

165598/1.036+8736

+ 1/6x6.59 3 16902.0

1787

+-----

+ 1/6x6.59 J

1/6x6.59J

154376/1.036+8736

154376/1.036xO.192570xO.6x2

+

BWC= 6.59 2

1/6x6.59 J

24445.0

13968

+

+ 1/6x6:59

J

= 5160 Kg/m2 < 62500 Kglm2

1/6x6.593

7.0

Design of Chimney Basic design calculations are similar to those given in wet type foundation.

B.O

Design of Base Slab Basic calculations are similar given to those in Wet Type Foundation.

9.0

Check for Sliding Basic design philosophy is similar to that given In wet type foundation.

10.0

Check for Overturning Basic design philosophy is similar to that given in wet type foundation.

11.0

Quantities Per Tower Concrete Volume Excavation Volume Reinforcement

65.62 (M15) + 8.69 (MlO) Ml 478.25\,,\~

7750 Kgs.

12.0

Reinforcement Details Similar to those given in wet type foundation.

12. 1

REINFORCEMENT SKETCH: Similar to that given in wet type foundation.

12.2

BAR BENDING SCHEDULE: Similar to that given in wet type foundation.

ILLUSTRATION: VIII DRY TYPE FOUNDATION

10

r-..

-

0

0 0

en

(\J

10

en

(\J

10

~

o

10

~_---.JL..-.-

All dimensions __

L _ _ _ _ _ _ _ _ _ _ _ _ _ _----!. ore in mm

~

4070

.\

Sketch 9: Dry Type (PCC) Foundation /I

ALL DIMENSIONS ARE IN MM" 81

1.0

. Volume of Concrete (Cu.m)

4.07 2X 0.05 1.725/3 .[4.072+ 0.622 + 4.07 x 0.62] 0.622 x (1.175 + 0.225)

0.828 =11.197 = 0.538

=12.563

TOTAL

l.O

~

Overload Due to Concrete (Kg.)

COMP/UPlIFT

=

0.622 x 0.225 x 2300 02.563-0.0865) (2300-1440)

199

= 10730 10929 -

3.0

(. I

I

'Jl

Dry Soil Volume (Cu.m)

4.07 2x 2.95 4.07X1.674X2X2.9

I -) .

= 48.867 = 39.516 = 8.510

1tI3x(1.674)2x2.9

TOTAL

\ '3. . .~

Wet Soil volume: Nil

S.O 5.1

Check for Uplift RESISTANCE AGAINST UPLIFT:

'< .--

"

Ij

-

--

."

= 96.893

4.0

~

, ,

..

..::

t

=96.893·x 1440 + 10929 =150455 Kgs. F.O.S. (NC) F.O.S (BWC) 6.0

=150455/140917

=150455/130185

=1.068> 1.0

=1.156> 1.0

Check for Bearing Capacity

NC

=

165598/1.036+ 10929

. 2x165598/1.036xO.192570 x1.775

+ 4.07 2

1/6x4.071

17475.0

+----

1340

+

1/6x4.07J

= 21708 Kglm2 < 27350 Kglm2

1/6x4.07J

154376/1.036+10929 BWC=---------------

2x154376/1.036XO.192570x1.775 +

1/6x4.073

25020.0

14541

+------

+ 1/6 X4.07 3

1/6 X4.07 3

= 22272 Kwm2 < 27350 Kglm2

7.0

Design of Chimney

Basic design calculations are similar to that given in wet type foundation. B.O

Check for Sliding

Basic design philosophy is similar to that given in wet type foundation. 9.0

Check for Overturning

Basic design philosophy is similar to that given in wet type foundation. 10.0

Quantities Per Tower

Concrete Volume Excavation Volume Reinforcement

: 50.252 m3 : 225.34 m3

ILLUSTRATION·IX HARD ROCK TYPE FOUNDATION 1.0

Volume of Concrete

0.65 2 x 0.225 1.65 2x 1.250

= 0.095

TOTAL

= 3.498

= 3.403

,; "

2.0

Overload of Concrete

0.095 x 2300 3.403 x 860

= 219 = 2927

6:'1>00-\44 D)

TOTAL

= 3145

83

.

c.L. III

~

G.L.

650 Sq, ......

Rock level

o

III N

....

o

o ....

o

III

.... N

o

o

III III N N +1

....

•

-~

I-~

0 III

• ¢= dia

22.54>-i

-4'

o

•

• of grout bar

•

e

-t-

III +1

o o

....'"

0 III

12 bars of

-4'

20'mm'7

• 0 III

-4'

III N

....

•

III N +1

125 ±2S

•

•

1.50

1.50

• 450

1600!50 U __ ..I

0 __ 1.

~_

.. _...a_,,: __

125 ±25 ~

3.0

Bea,ing CaP.1City 165598 + 3145

NC

= - - - - - - - = 70237 Kglm2 < 1,25,000 Kglm2 1.552

BWC

4.0

154376 + 3145 - - - - - - - = 65565 Kglm2 < 1,25,000 Kglm2 1.552

=

theelc for Uplift DESIGN UPLIFT NET UPLIFT

= 140917 Kgs. = 140917-1.55xl.55x1.25x2300 =134010 Kgs.

UPLIFT RESISTED BY 12 NOS. 20, ANCHOR BARS: 12 X 1t X 2.0 x 115 x 16 = 138733 > 134010

5.0

-

Check Against Uprooting of Stub DESIGN UPLIFT = 140917 KG NO. CLEATS PROVIDED = 3 NOS. 11 Ox11 Ox8 (Outer & Inner) NOS. OF BOLTS = 12 NOS. O~ 16MM DIA. Ult. resistence of stub in Bond = Us =[115x (20x2+{20-1.6) x2.0) -3x {20 ... (20-1.6)} xl1]x 10 = 75648 Kg. LEAST RESISTENCE OFFERED BY CLEATS IN BEARING/BOLT: = 136923 Kg. . (REFER CHECK FOR UPROOTING OF STUB CAL.) RESISTENCE AGAINST UPLIFT: =75648+136923 = 21i571 > 140917

6. 0

Bond Between Rock and Concrete = 160 x 120 x 4x4

=307200 > 134190

NOTE 1: 1. 2. ·3. 4. 5.

Minimum depth of slab should not be less than 1000 mm. ., . Stub to be cut, Holes to be drilled .and cold-zinc rich paint/galvanising to be appliro at site. Grout holest to be 20 mm bigger than dia of grout bar. Cement sand mix 1:1 Ratio to be used for grouting through grouting pump. Entire concrete block (slab) should be embedded in hard rock irrespective of level of hard rock encountered.

85

ILLUSTRATION -X DRY SANDY SOIL (WITH CLAY CONTENT 5-10%)

C.L. G.L.

225

100 ~

I

~ I-

n....-.....J

•

r--240V"'~-~.

50

ITsin_

4150.I

4650

wI-- 1"""In ,

All dimensions ore In mm

Sketch X : Dry Sandy Soil (with Cloy Content 5-10%)

nALL DIMENSIONS IN SKETCH ARE IN MM'I

1.0

Volume of Concrete (Cu.m)

4.65 2 x 0.05 4.65 2 x 0.100 0.25/3 x (4.65 2 + 4.15 2 + 4.65 x 4 .15) 2.9 2 x 0.2 0.65 2 x 2.625

= 1.081 = 2.162 =4.845 = 1.682 = 1.109 10.879

2.0

Overload of Concrete (Kg.)

•

0.65 2 x 0.225 x 2400 0.65 2 x 2.4 x (2400-1440) 2.92 x 0.2 x (2400-1440) 4.845 x (2400-1440) 4.65 2 x 0.1 x 2400

3.0

= = = = =

Dry Soil Volume (Cu.m) 4.65 2 x 2.85 2 x 4.65 X 2.85 2 x TAN20 PI/3 x TAN2 20 X 2.85 1

= 61.62

=27.494 = 3.211 92.325

COMP 228 973 1615 4651 5189

UPLIFT 228 973 1615 4651 5189

12656

12656

.'

.,

4.0

Total Resistance Against Uplift

= 92.325 x 1440 + 12656 = 1.45604 KG F.O.S (NC) = 145604/140917 = 1.033 > 1:0 F.O.S (BWC) = 145604/130185 = 1.118 < 1.0 5.0

Check for Bearing Capacity

165598/1.036 + 1.036 + 12656

2x (165598/1.036) xO.192570 x 0.6

NC=

+

4.65 2

1/6 X 4.65 3

14676

1910

+

+ 1/6 X 4.65 3

1/6x 4.65 3

=11172 KG/M2 < 25000 kg/m2 154376/1.036 + 12656 BWC=

+

4.65 2

1/6x 4.65 3

22220

+

2x (154376/1.036f x 0.192570 xO.6

11743

+ 1/6 X 4.65 3

1/6x4.653

= 11558 KG/M2 < 25000 KG/M2 7.0

Design of Chimney

Basic design calculations are similar to those given in wet type foundation B.O

Design

of Base Slab

Basic design calculations are similar to those given in wet type foundation 9.0

Check for Sliding

Basic design philosophy are similar to that given in wet type foundation 10.0

Check for Overturning

Basic design philosophy is similar to that given in wet type foundation 11.0

Quantities Per Tower

Concrete Volume (CU.M) Excavation Volume (CUM.M) Reinforcement (KG) 12.0

: 39.192 (M15) + 4.324 (M1 0) : 294.03 : 2740

Reinforcement Details

Similar to those given in wet type foundation. 13.0

Reinforcement Details

Similar to those given in wet type foundation. 14.0

Bar Bending Schedule

Similar to that given in wet type foundation.

87

• • I

ILLUSTRATION -XI PARTIALL Y BLACK COTTON SOIL TYPE FOUNDATION

I

0 0

10

0 0

~

N.

0 0 0

0 0

N

0 0

-

10

I I I I ",

0

10 N

0

52

I1----\4

,-

2 00 4000

..,

.

-t

4500

0

JAilIn mmd,m.'::,o.. "•

C

I I I

•I I

Sketch XI: Partially Black Cotton Soil Type Foundation

I I

IIALL DIMENSIONS IN SKETCH ARE IN MMII

I

1.0

J

Volume of Concrete (Cu.m)

4.502 x 0.05 4.502 x 0.100 0.25/3 (4.50 2 + 4.002 + 4.50 x 4.00) 2.8 2 x 0.2 0.65 2 x (2.4-1.5) 0.65 2 x 1.5 0.65 2 x 0.225

I

=1.013

~

= 2.025

=4.521

= 1.568

=0.380 = 0.634

=0.095

10.240 2.0

Overload of Concrete (Kg.)

0.095x2400 0.0.634 x (2400-1440) 0.38 x (2400-1440) 1.568x (2400-1440) 4.521 x (2400-1440) 2.025 x 2400

= = = = =

=

COMP 228 609 365 1505 4340 4860 11907

UPLIFT 228 609 365 1505 4340 4860 11907

,

-

Al A2 V

3.1

-

= 4.5 2+ 4x4.5xl.35 X TAN30 + 1f"(1.35x TAN30)2 . = 36.188 = 4.5 2+ 4 X 4.5 (1.35 TAN 30 + 1.5 TANO) +1f(1.35 X TAN 30 +1.5 TANO)2 = 36.188 = 1.5/3 06.188+36.188 + (36.188x 36.188)1/2 = 54.2822

Volume o( Normal Soil (Cu.m)

4.5 2 X 1.35 2 X 4.5 X 1.35 2 x TAN 30 1f/3 (1.35 1 x TAN2 30)

= 27.338 = 9.470 = 0.8588 = 37.6668

4.0

Total Resistance Against Uplift

= 54.2822 x 1440 + 37.6668 x 1440 + 11907 = 144313 KG F.O.S. (NC) = 144313/140917 = 1.024> 1.0 F.O.S (BWC) = 144313/130185 = 1.108> 1.0

5.0

Check (or Bearing Capacity

2x (165598/1.036) x 0.192570 x 0.6

165598/1.036+ 11907 NC=------------------ + 4.50 2 17450

+ ---------

1/6 X 4.503

1551

+

1/6 X 4.50 1

1/6x4.501

=12165 KG/M2 < 25000 KG/M2 154376/1.036+ 11907 Bwe=

2X(154376/1.036)XO.192570XO.6 +

4.502

1/6X4.503

24993

+ 116 X 4.503 =

7.0

14516

+ 1/6 X 4.503

12815 KG/M2 < 25000 KGfM 2

Design o( Chimney

Basic design calculations are similar to those given in wet type foundation B.O

Design o( Base Slab

Basic design calculations are similar to those given in wet typ~ ioundation 89

9.0

Check for Sliding

Basic design philosophy is similar to that given in wet type foundation 10.0

Check for Overturning

Basic design philosophy is similar to that given" in wet type foundation 11.0

12.0

Quantities Per Tower Concrete Volume (CU.M.) Excavation Volume (CU.M) Reinforcement (KG)

36.908 (M1S) + 4.052 (MlO) 243.03 2600

'Reinforcement Details

Similar to those given in wet type foundation. 13.0

Reinforcement Sketch Similar to that given in wet type foundation.

14.0

Bar Bending Schedule

Similar to that given in wet type foundation.

";':

Transmission Line· Manual '. ;:

Chapter 11

Construction of Transmission Lines

I I I

CONTENTS Scope 11.1 Survey 11.2 Manpower, Tools and Plants and Transport Facilitie~ 11.3 Environmental Consideration 11.4 Statutory Regulation for Crossing of Roads, Power Lines, Telecommunication Lines, Railway Tracks, etc. 11.5 Survey IN '"=I M E7HcJi}S 11.6 Foundations 11.7 Erection of Super Structure and Fixing of Tower Accessories 11.8 Earthing 11.9 Stringing of Conductors 11.10 Hot-Line Stringing of E.H.V. Lines 11.11 Protection of Tower Footings 11.12 Testing and COmmissioning 11.13 References Annexures

Page 1

I

• J

1

t

3

I

4 4 10

16 17 19 24 26 26 26 27-54

j

• •

, -

,J ~ J J

CHAPTER-XI

CONSTRUCTION OF TRANSMISSION LINES A. SCOPE

9.

This chapter will cover the environmental consideration, Survey, Excavation, Stub-setting and Concreting, Erection of Towers, Stringing of Conductor for the Construction of: Transmission Lines.

11.1 SURVEY (i) Reconnaissance Survey (ii) Alignment Survey (iii) Detailed Survey It would also cover soil investigation of representative sites along the route of the line to establish the distribution of foundations in different types of soils. 11.1.1 Erection of Transmission Line Erection of transmition line cov~rs Check Survey, Excavation, Setting of Stubs, Casting of Foundations. Erection of Towers, Stringing of Conductors and Groundwire, Final Checking and Commissioning. 11.2 MANPOWER, TOOLS AND PLANTS AND TRANSPORT of ACILITIES 11.2.1 Survey Average output per month per gang consisting of about 10 persons will be: (i) Alignment Survey I5km or (ii) Detailed Survey 20km or 20km (iii) Check Survey Wherever topographical survey is to be carried out the output will be less and will depend on the quantum of work.

11.2.1.1 Tools required/or Survey Gang 1. Theodolite with stand 2. Dumpy level with stand 3. Ranging rod

,

INa 1 No 5 Nos 2 Nos

4.

Levelling staff

5.

Engineers chain 30m

INo

20m

1 No

30m

INa

15 m

1 No

6.

Steel Tape

7.

Survey umbrella

8.

Chain pins

1 No 30 Nos

10.

11.2.1.2

Spades, Knives and axes for clearing the bushes and trees Tents, buckets, water drums, camping cots, tables, chairs, and petromax etc

Asper requirement As per requirement

Transport required/or Survey Gang Jeep with trailor

INo

11.2.2 Excavation Stub-setting and Concreting Average output per gang consisting of about 85 persons per month will be Excavation 400-500 m3 Normal soil 60 m3 Soft rock + 180 m3 Normal soil 150 m3 Soft rock °

Output of Hand rock will depend on situation Stub-setting & Concreting

60-70 m3

11.22.1 Tools and Plants required/or Excavation, Stubsetting and Concreting Gang L Stub-setting Templates As. per requirement 2. Stub-setting Jacks -do3. Form boxes/Chimneys -do4. Mixer machine - Diesel engine driven I No - Hand driven 2 Nos 5. Needle vibrator INa 6. Dewatering pump 2 Nos 7. Air compressor for drilling holes in rock INa Asper 8. High carbon drilling rods for drilling holes in rock requirement 9.•Exploder I No INa 10. Water tanker trailor 11. Theodolite with stand INa 12. Ranging rod 3 Nos 13. Dumpy level with stand. INa 14. Levelling staff INa 15. Survey umbrella INa 6 Nos 16. Concrete cube mould Asper 17. Wooden shuttering & poles requirement

2

. _... _._-_._---------

2 Nos dia and of length - 8.5-9 m 700m Polypropylene rope -25 mmdia 1000m -19mmdia 8 Nos Single sheave pulley - closed type 4 Nos - Open type 16Nos Crow bars (25 mm dia and 1.8 m length) Spanners (both ring and flat) hammers, Asper slings (16 mm dia and 1 m length) requirement hooks, (12 mm dia) 'D: shackle, tommy-bars

18. Mixing sheets 19. MeasUring box 20. Metal screen - 40 mm mesh -20mm mesh - 12.5 mm mesh . 2l. Sand Screen - 4.75 mm mesh 22.. Empty barrel (200 litres capacity) 23. SteeVAlwninium/Wooden ladder (3.5 m length) 24. 30 m metallic tape 25. 30 m steel tape 26. Engineers' spirit level I 27. Steel piano wire/thread

12 Nos 6 Nos 1 No 1 No 1 No 1 No 6 Nos

28. 29. 30. 31.

Crow bar Pikaxe Spade Shovel

20 Nos 12 Nos 25 Nos 8 Nos

112.3.2

32. 33. 34. 35. 36.

Gamelas Buckets Iron r~mer (4.5 kg) Masonry trowel Manila rope - (38 mm dia) -(12 mm dia)

30 Nos 12 Nos

11.2.4 Stringing of Conductor Average output per gang consisting of about 200 persons per month will be Tension Stringing method - Machine stringing -15km (i) for 400 kV Single Circuit -8km (ii) for 400 kV Double Circuit

37. Pocking rod (16 mm dia) - 3 m length

5 Nos 1 No 1 No

5 Nos 6 Nos 150 m 30 m 2 Nos

/

2. 3.

4. 5.

6.

Tents, buckets, water drums, camping cots, tables, chairs and petromax etc.

50 m

39. Hammer, Tommy bar, plumb bob, (0.45 kg) . Hook, (12 mm dia) spanners (bQth ring As per and flat) etc. requirement 40. Tents, buckets, water drums, camping As per cots, tables and chairs, petromax etc. requirement

1.

. 3.

2 Nos

- 1.5 m length 2 Nos 38. Blasting materials, binding wire Asper requirement

1122.2

2.

Transport required/or Stub-setting & Concreting Gang Truck 1 No .(For transportation of metal and sand from source, cement, reinforcement steel and other materials from site stores) Tractor with trailor 1 No Motor Cycle 1 No

11.2.3 Erection or Tower by Built up Method Average output per gang consisting of about 50 persons per month will be - 80 mt

1.

2. 3.

As per requirement

Transport required/or Tower Erection Gang Truck 1/2 No 1No Tractor with Tailor 1 No Motor Cycle

-Skm for ± 500 kV HVDC Multi-Circuit Requirement of manpower and average output per gang for carrying out various types of transmission lines by manual method is furnished hereunder (iii)

Manpower Average Output (Nos) per month (kIn)

Sl No

Description of line

l. 2.

-P6 kV Single CiItuit

75

·6i6 kV Double Circuit

3. 4.

132 kV Single Circuit 132 kV Double Circuit

75 100 100

5. 6.

220 kV Single CiItuit

125

220 kV Double Circuit

125

30 .15

7.

400 kV Single Circuit

225

IS

8.

400 kV Double Circuit

225

8

30 15 30 15

112.4.1 Tools and Plants required/or Stringing Gang/or Tension/Manual Stringing l. TSE sets (Tensionar & Puller of 8/10t capacity) 1 Set" 2. Running block for conductor lOONos ~- Runninl!: block for earthwire 60 Nos

5. Pilot wire each of 800 m length 10 Nos 6. Pilot wire joint 12 Nos 7. Ground roller for Tension/Manual Stringing 30/1 00 Nos 8. Wire mesh pulling grip (one end open) of required dia for conductor 6 Nos 9. Wire mesh pulling grip (one end open) of required dia for earthwire 2 Nos. 10. Wire mesh pulling grip (double end open) of required size for conductor 4 Nos 11. Articulated joint - Heavy duty (20 t) 10 Nos - Medium duty (10 t) 10 Nos - Light duty (5 t) 5 Nos 12. Drum mounting jack for conductor drum of lOt capacity 4 Sets 13. Tum table (5 t capacity) 2 Nos 14. Anchor plate (1.5 m x 1.0 x 8 mm) with 15 Nos. Anchor pins (45 mm dia and 850 mm long) 10 Sets 15. Hydraulic compressor machine - 100 t capacity with die sets 8 Nos 16. Travelling ground 12 Sets 17. Dynamometer -10 t 4 Nos - 2t 2 Nos 18. Pilot wire reel stand 4 Nos 19. Four sheave pulley with 12 mm dia 300 m length wire .rope 6 Sets 20. Four sheave pulley with 9 mm dia and 300 m length wire'rope 2 Sets 21. Four sheave pulley with 12 mm dia and 150 m length wire rope 4 Sets 22. Equiliser pulley (lOt capaci ty) 16 Nos 4 Sets 23. Conductor lifting tackle 4 Nos 24. Winch - motorisedlmanual - 10 t Capacity 25. Comealong clamp for conductor (bolted type/automatic) 50/20 Nos 26. Comealong clamp for earthwire (bolted type/automatic) 15/10 Nos 27. Tirfor (5 t capacity) 6 Nos 28. Aerial (chair for conductor) 6 Nos 4 Nos 29. Aerial trolly 16 Nos 30. Tum buckle - lOt 6 Nos - 3t ... 31. Tension/Sag plate (for tensioning purpose) 6 Nos 8 Nos 32. Sag board 4 Nos 33. Marking roller 2 Nos 34. Mismatch roller 6 Nos 35. Joint protector

s s s J'"

:t

36. Walkie talkie set 4 Nos 37. Theodolite with stand 1 No 38. Thermometer 3 Nos 39. Survey umbrella I Nos 40. Hydraulic wire cutter 2 Nos 41. Binocular 3 Nos 42. Flag (red & green) 30 Nos 43. Crow bar (1.8 m length) 10 Nos 44. Nail pullar 6 Nos 45. Wire rope -(19 mm dia) 1000 m -(16 mm dia) 150 m -(14 mm dia) 900m 46. Polypropylene rope - (25 mm dia) 500 m - (19 mm dia) 500 m 47. 'D' - Shackle - 190 mm long 40 Nos -150 mm long 125 Nos - 100 mm long 125 Nos 48. Bulldog clamp - 100 mm long 35 Nos 49. Hammers, spanners, (both flat and ring) round files, flat files screw drivers, cutting pliers, steel and metallic tapes, hacksaw frame and blades, deadmenlS, scafolding, slings etc. Asper requirement 50. Tents, buckets, water drums, ~ping cots, As per table, chair, petromax etc. requirement

112.42 Transport required/or Stringing Tension stringing

Manual stringing

1. Truck

4 Nos

4 Nos

2. 75 h.p. Tractor

2 Nos

1 No

3. 35 h.p./45 h.p. Tractor 5 Nos

6 Nos

and trailors 4. Jeep

2 Nos

2 Nos

5. Motor Cycle

1 No

INo

11.3

ENVIRONMENTAL CONSIDERAnON The route of transmission line should be aligned in such a way as JO minimise damages to crops and cutting of trees. Special care should be taken to avoid routing of transmission line through lands particularly in Reserved/Protected forests. Even ifline length increases, efforts should be made to keep the line of forests. If forest land cannot be avoided, standard extensions should be provided minimise cutting of trees by ensuring adequate ground clearances. The line also should be kept away from villages, bulk storage oil tanks, oil .. pipe lines, airports, petrol pumps, cluster of hutments, buildings containing inflammable materials such as explosives, cotton godowns, factories, aerodromes Helipads etc.

•

4

11.3.1. I~portant requirement for Choice of Route The transmission line connects two points which may be two power stations, power station and another sub-station or two sub-stations. The line route has to be shortest connecting the two points. However, it is important that due weightage be given while selecting the route to the accessibility of the line for construction as well as for maintenance or its total life span. By sljght deviation increasing the route length marginally, the line should be sited in areas which are not inaccessible. It should be possible to transport the materials and tools quickly in case of breakdowns. Wherever roads are existing the line should be approachable from such roads. It should avoid as far as possible waterlogged areas or areas prone to flooding for long periods. The transmission line route should avoid inhabited areas leaving sufficient margi~for growth of villages. It should avoid as far as possible the areas where intensive cultivation is done. As far as possible crossing of orchards and gardens should be avoided. The additional costs to be incurred in crop compensation during construction and delay in attending to break downs during operation and maintenance should be carefully weighed against increase in the route length as also increase in angle towers. It should be possible for the men patrolling the line to be able to reach every location, careful inspection of the towers, insulators and the accessories without any obstruction from the land owners. With intensive irrigation in certain areas it may be cheaper to have slight deviation, rather than having litigation delaying the project apart from the cost to be incurred in making payment for compensation. Heavily wooded areas should be avoided. Prior consultations should be held with the concerned Departments. With these general remarks the various considerations for the choice of route and the construction of the line are discussed in detail in the following paras. 11.4 STATUTORY REGULATION FOR CROSSING OF ROADS, POWER LINES, TELECOMMUNICATION LINES,RAILWAY TRACKS ETC 11.4.1 Road Crossing On all major road crossings, including National Highways, the towers shall be fitted with double suspension or tension insulator strings depending on the type of towers used. 11.4.2 Power Line Crossing Where a line is to cross over another line of the same voltage or lower voltage, suspension/tension towers with stan{lard extensions shall be used. Wherever the line to be constructed is crossing another important line for which shutdown is difficult, susPension towers with required extensions in combination with dead end towers shall be used. 11.4.3 Telecommunication Line Crossing The angie of crossing shallbe~ as near 90 degrees as

angle of crossing is below 60 degrees, the matter shall be referred to the authority incharge of the telecommunication system. Also in the crossing span, power line support shall be as ncar the telecommunication line as possible to obtain increased vertical clearance between the wires. The crossiug shall be in accordance with the code of practice for crossing between power and telecommunication lines. 11.4.4 Railway Crossing For Railway Crossing, to~ers shall be Angle/dead,end type and railway crossing construction shall conform to the regulations for Electrical Line Crossings with Railway Tracks ,issued by the Ministry of Rail ways from time to time. 11.4.5 River Crossing In case of major river crossing, towers shall be of suspension type using double suspension strings and the anchor towers on either side of the main river crossing shall be dead end type. Clearance required by the navigation Authority shall be provided in case of navigable rivers. For non-navigable rivers, clearance shall be reckoned with respect to highest flood level (HFL). 11.4.6 Other Provisions 11.4.6.1 The transmission linein the vicinity of Aerodrome shall meet the requirement laid down by the Director General, Civil Aviation, Government of India. 11.4.6.2 Requisite vertical and horizontal clearance to adjacent structures shall be maintained as per I.E. Rules. 11.4.6.3 The electrical clearance required for different kinds of crossing are given in Annexure-' A'. SURVEY \ N C) M GTHa..6SThe survey of high voltage transmission lines must be carried out accurately and expeditiously. A mistake in the field or subsequent office work may cause unnecessary expendit~e and inconvenience. It is, therefore, essential that every care should be taken in seuing out; levelling and plotting the profile of the route. The care and fore-thought given at the first stage of surveying goes a long way in achieving economy and successful successive operational stages. The survey of the transmission line till now is being carried out in India by conventional methods using only the Topa sheets and instruments like vernier theodolite, dumpy level, engineers' chains or measuring tapes, for selecting the route and further field works. However, in advancede 'used at locations where sub-soil water table is met between 6.75 metre to 1.50 metre below the ground level. 11.6.4.2.4 FullySubmerged To be used at locations where sub-soil water table is within 0.75 metre below the ground level. 11.6.4.2.5 Black Colton To be used at locations when soil is clayey type, not necessarily black in colour, which shrinks when dry, swells when wet, resulting in differential movement extending to a maximum depth of about 3.5 metres below ground level. 11.6.4.2.6 Fissured Rock To be used at locations where decomposed or fissured rock, hard gravel, kankar, limestone, laterite or any other soil of similar nature is met. Under cut type foundation is to be used for fissured rock locations. Rock anchor type foundation can also be used for fissured rock location where the under cut is not feasible. In case of fissured rock locations where water table is met at 1.5 metre or more below ground level submerged fissured rock foundations shall be adopted. When the water table in such location is met within 1.5 metre from ground level, fully Submerged Fissured Rock type foundations shall be adopted.

;~

...

11.6.4.2.7 Hard Rock The locations where chiselling, drilling and blasting is required for excavation, hard rock type foundations are to be used. For these locations rock anchoring is to be provided to resist uplift forces. 11.6.4.2.8 In addition to the above, depending on the site conditions other types of foundations may also be developed for: 1. Intermediate conditions under the above classifications to effect more economy or 2. For locations where special foundations (well type or piles) are necessitated. While classifying foundations of Wet, Partially Submerged, Fully Submerged foundations mentioned above, the worst conditions should be considered and not necessarily the conditions prevailing at the time of inspection. For instance. there are areas where sub-soil water rises when canal water letout in the fields raising sub-soil wa~r to a considerable degree. Simifarly the effect of monsoon or when the nearby reservoirs are full should also be considered and not the conditions prevailing in open season or summer when work is carried out normally. 11.6.5 Stub-setting The stubs are set in such a manner thai the distance between the stubs and their alignment and slope are as per

design so as to perm it assem bling of the superstructure without undue strain or distortion in any part of the structure. There are three methods by which this is generally accomplished. (i) Use of a combined Stub-setting Template for all the four stubs of the tower. (ii) Use of Individual Leg Template for each stub. (iii) Use as a Template the lower tower section or extension, where Stub-setting Template is not available. The first method is the most commonly used. The Stubsetting Template is composed of a light rigid framework which holds the stubs at the correct alignment and slOpe. The Stubsetting Template is generally of adjustable type which can suit the standard tower as well as towers with standard extensions: The Template is centred and levelled by sighting through transit The anchors or slubs are bolted to this Template, one at each comer of the Template, and are held in their proper position until the concrete is poured and has hardened. The procedure for setting stubs at.site is given in Annexure- 'J'. The second method is adopted for casting the foundation 'locations having individual leg extensions or locations having broad base for which use of a single Template for setting all the four stubs is unwieldy. The Individual Leg Template comprises a steel channel or joist having a length more than the size of the pit. by about 2 to 3 metres. A chamfered cleat is welded in the centre of the channeVjoist to provide the slope to the stub. The stub is bolted to the cleat of the Template for which holes as required for the slope of the stub are provided. The Individual Leg Templates are initially seton each pit approximately to the' required position w.r.t. the centre point of the tower and after that stubs are bolted to the cleat The stubs are then brought to proper position w.r.t. the centre of the tower with the help a Theodolite, Dempty level and a measuring tape, before fixing form boxes and pouring concrete. This type of Templates are very useful for casting the foundations of individual leg extensions in which the foundation pits are staggered and use of either a normal Stub-setting Template or the first section of the tower is not feasible. The foundation layout of unequal leg extensions is shown in Annexure- 'K' In the third method, lower section of the tower or extension is used for setting stub. In this method two opposite sides of the lower section of the tower are assembled horizontally on the ground, and the stubs are bolted to the same with correct slope and alignment. Each assembled side is then lifted clear of the ground with a gin pole and is lowered into the four pits excavated at four comers of the tower to their proper size and depth. The assembly is IifWEI in such a manner that stubs are not damaged. One side is held in place with props while the other side is being erected. The two opposite sides are then laced together with cross members and diagonals. Then the aSsembled section is lined up. made square with line and levelled. The propereJevation an~.levelling are done with a transit. When the

I Construction of Transmission Lines

J

The fOnTI work for slabs and pyramids shall be made symmetrical about the bases of the chimney to ensure interchangeable faces.

".J'

14 lining and levelling has been done, the bolts are tightened up to make the frame as rigid as is reasonably possible. Thereafter the fonn boxes for foundations are built and the concrete is poured. For heavy towers use of Stub-setting Template is recommended.

11.6.6 Concreting

11.6.6.1 Type For reasons of economy and progress it is nonnal practice to use coarse and fme aggregates available along the line route and/of nearest locations to the route. Ordinary plain or reinforced cement concrete given in IS: 456-1978 shall be used in overhead line foundations.

11.6.6.2 Mixes For main foundation, M 15 or 1:2:4 mix cement concrete shall be used. For lean concrete sub-bases or pads, M 10 or 1:3:6 mix cement concrete may be used. The properties of concrete and mix proportions shall be as given in IS :456-1978. It shall be pennissible to proportionate the concrete as follows. 11.6.6.2.1 Prepare a wooden measuring box of 35litres capacity (that is equal to 1 bag of 50 kg. of cement) with inside dimensions of 30cm x 30cm x 39cm alternatively a cylinder of 34cm diameter and 39cm height. The mix quantities according to the measuring box shall be as follows:

MIO Cement 1 bag 2 boxes Sand 3 boxes Metal 4 boxes 6 boxes Water 1 boxes less 3 litres 1box less 1 litre 11.6.6.2.2 Measurement of water may be made with separate water tight drwns of the above size or with 1 or 2 litre mugs. 11.6.6.3 One bag of cement is taken to contain 50 kg or 35 litres of ordinary portland cement MIS 1 bag

11.6.7 Form Work 11.6.7.1 General The fonn work shall confonn to the shape, lines and dimensions as shown on the foundation design drawings, and be so constructed as to be rigid during the placing and compacting of concrete, and shall be sufficiently tight to prevent loss of liquid from concrete. It shall be of light design, easily removable without distortions and shall be of steel or suitable materials. The inner surface coming in contact with concrete shall be smooth and free from projections. Window on one face shall be provided for pyramid fonns to facilitate concreting in the lower parts which shall be flxed after concrete in the bottom

11.6.7.2 Clearing and Treatment of Forms All rubbish, particularly chippings, shaving and sawdust and traces of concrete, if any, shall be removed from the interior of the fOnTIS before the concrete is placed. The surface in contact with the concrete shall be wetted and sprayed with fine sand or treated with an approved composition such as black or waste oil etc., before use, every time. 11.6.7.3 Stripping Time Under fair weather conditions (generally where average daily temperature is 20 degree or above) and where ordinary cement is used, fOnTIS may be stripped after 24 hours of the placing of concrete. In dull weather such as rainy periods and very cold temperature, the fOnTIS shall be removed after 48 hours of the placing of concrete. 11.6.7.4 Procedure when Removing Form Work All fOnTI work shall be removed without much shock or vibration as otherwise it would damage the concrete or the fonns. 11.6.8 Mixing 11.6.8.1 Concrete shall preferably be mixed in a mechanical mixer, but hand mixing shall be pennissible. 11.6.8.2 When hand mixing is adopted, it shall be carried out on impervious platfonns such as iron plain sheets properly overlapped and placed upon level ground. The coarse aggregate shall first be evenly spread out in required quantity over the sheets. The flne aggregate shall be evenly spread out over coarse aggregate next. The aggregates shall then be thoroughly mixed together and levelled. The required amount of cement shall now be spread evenl y over the mixed aggregates and wet mixing shall start from one end with required amount of water using shovels. The whole lot shall not be wetted; instead mixing shall proceed progressivel y. If the aggregates are wet or washed, cement shall not be spread out, but shall be put in progressively. 11.6.8.3 For mixing in mechanical mixers, the same order of placing ingredients in the loader drum shall be adopted, that is coarse aggregate shall be put in fIrst followed by sand, cement and water. 11.6.8.4 Mixing shall be continued until there is a unifonn distribution of materials and the mass is unifonn in colour and consistency but in no case shall mixing be done for less than 2 minutes. 11.6.8.5 If the aggregates are wet, the amount of water shall be reduced suitably. 1UiJj

Tr.llmmortation

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dation. In places where it is not possible, concrete may be mixed at the nearest convenient place. The concrete shall be handled from the place of mixing to the place of final deposit as rapidly as practicable by methods which shall prevent the segregation or loss of any of the ingredients. If segregation does occur during transport, the concrete shall be remixed before being placed. 11.6.9.2 During hot or cold weatherw concrete shall be transported in'deep containers. The deep containers, on account of their lower ratio of surface area to mass, reduce the rate ofloss of water by evaporation during hot weather and loss of heat during cold weather. 11.6.10 Placing and Compacting 11.6.10.1 The concrete shall be placed and compacted before seuing commences and should not be subsequently disturbed. The placing should be such that no segregation takes place. 11.6.10.2 Concrete shall be thoroughly compacted during the placing operation, and thoroughly worked around the rein. forcement, if any, around embedded fixtures and into comers of form work by means of 16mm diameter poking bars pointed at the ends. As a guide for compacting, the poking bars may be worked 100 times in an area of 200mm square for 300mm depth. Over compacting causes the liquid to flow out upward causing segregation and should be avoided. 1l.6.10.3 If, after the form work has been re!lloved, the concrete surface is found to have defects, all the damaged surfaces shall be repaired with mortar application composed of cement and sand in the same proportion as the cement and sand in the concrete mix. Such repairs shall be carried out well before the foundation pits are back filled. 1l.6.10.4 For precautions to be taken on concrete work in extreme weather and under water, the provisions of ~S : 456 : 1978 shall apply. 11.6.11 Reinforcement All reinforcement shall be properly placed according to foundation design, drawing with a minimum concrete cover of 50mm. The bars shall, however, be placed clear of stubs and cleats where fouling. For binding, iron wire of not less than 0.9mm shall be employed, and the bars may, be bound at alternate crossing points. The work shall conform to IS : 25021963 wherever applicable. In case of the foundation having steel reinforcement in pyramid or base slab, atleast 50mm thick pad of lean concrete of 1:3:6 nominal mix shall be provided to avoid the possibility of reinforcement rod being exposed due to unevenness of the bottom of the excavated pit. 11.6.12 Sizes of Aggregates The coarse aggregate (stone/metal) to be used shall be 40mm nominal size for slab/pyramid concrete and 20mm nominal size for chimney concrete conforming to IS: 3831979. These sizes are applicable to ordinary plain cement

concrete. For R.C.C. works the aggregate shall preferably be of 20mrn, ,nominal size. The fine aggregate (sand) shall be of preferably Zone I Grade to IS : 383-1979 which is the coarse variety with maximum particle size of 4.75mm. 11.6.13 Levelling Sub-base To take care of the unevenness at the bouom of the excavated pit it is necessary to provide a levelling sub-base not less than 1:3:6 proportion and 50mm thickness. 11.6.14 Back Filling, Following opening of form work and removal of shoring and shuuerings back filling shall be started after 24 hours of casting or repairs, if any, to the foundation concrete. Back fill ing shall normall ybe done with the excavated soil, unless it· consists of large boulders/stones, in which case the boulders shall be broken to a maximum size of 80mm. The back ruling materials should be clean and free from organIc or other foreign materials. The earth shall be deposited in maximum 300mm layers, levelled and wetted and tamped properly before another layer is deposited. Care shall be taken that the back ruling is started from the foundation ends of the pits towards the outer ends. After pits have been back filled to full depth, the stub-setting template may be removed. The back filling and grading shall be carried out to an elevation of about 75mm above the finished ground level to drain out water. After back filling 50mm high earthen embankment (bund) will be made along the sides of excavated pits and sufficient water will be poured in the back filled earth for atleast 24 hours. 11.6.15 Curing The concrete after setting for 24 hours shall be cured by keeping the concrete wet continuously for a period of io days after laying. The pit may be back filled with selected earth sprinkled with necessary amount ofwater and well consolidated in layers not exceeding 300mm. after a minimum period of 24 hours and thereafter both the back ruled ~ and exposed chimney top shall be kept wet for the remainder of the prescn'bed time of 10 days. The uncovered concrete chimney above the back filled earth shall be kept wet by providing empty cement bags dipped in water fully wrapped around the concre.te chimney for curing and ensuring that the bags be kept wet by the frequent pouring of water on them. 11.6.16 Tolerance The tolerances for various items connected to the found8tion works of transmission line are as under.

11.6.16.1 Stub-setting (Tower Footing) \~: 11.6.16.1.1 All the stub angles for tower legs shall be set accurately to the grade and alignment shown on the drawings. The difference in elevation between identical parts any two stub angles shall not exceed 1/1000 of the horizontal disl30ce

of

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Construction o/Transmission Lines

16 between the stubs,allowance being made for difference, if any, in the lengths of legs and extensions. The actual elevation of any stub angle shall not differ from the computed elevation by more than 1/100 of foundation depth. Stub angles shall be located horizontally so that each is within 6mm of its correct position, and the batter of the stub angles shall not differ from the correct t>1uer by more than either 1/100 of exposed stub length, or by the amount of playas offered by the clearance between bolts and holes of the stub-setting template. To ensure greater accuracy, the hole clearance shall not be greater than 1.5mm o~ the punched side of the Template members. 11.6.16.1.2 If the actual elevation of stubs is beyond 6cm as found after casting the foundation and on the plus side (that is, if the foundation is raised) equivalent depth of earthwork will be provided over the top of the foundation as per design requirements with particular reference to such location. By design requirements is meant the earth required lO resist uplift forces. 11.6.16.1.3 The following tolerances shall be applicable in case of position of foundations erected with reference to the tower positions spotted on Survey Charts: Type oCTower GutoC Aligrunent

From Centre Line of Route

From Transverse Centre line

Suspension

0.5 degree

25mm

±250mm

Tension

05 de!,'fee

25mm

±25mm

(Set at bi-section of deviation angle)

11.6.16.2 Concrete and Form Dimensions The maximum tolerance on the dimensions shall be ±10 mm. AlllOlerances shall not be on the negative side. 11.7 ERECTION OF SUPER STRUCTURE AND FIXING OF TOWER ACCESSORIES The towers shall be erected on the foundations not less than 10 days after concreting or till such time that the concrete has acquired sufficient strength. The towers are erected as per the erection drawings furnished by the manufacturers to facilitate erection. For the convenience of assembling the lOwer parts during erection operations, each member is marked in the factory lO correspond with a number shown in the erection drawing. Any damage to the steel and injuring of galvanising shall be avoided. No member shall be subjected to any undue over stress, during erection. 11.7.1 Method of Erection There are four main methods of erection of steel transmission lOwers which are described as below: (i)

Buil~~up method or Piecemeal method.

(iU

Section method

(iii)

Ground assembly method.

11.7.1.1 Built Up Metlwd This method is most common1y used in this country for the erection of 66 kV, 132 kV, 220 kV and 400 kV transmission line lOwers due to the following advantages: (i) Tower materials can be supplied to site in knocked down condition which facilitates easier and cheaper transportation. (ii) It does not require any heavy machinery such as cranes etc. (iii) Tower erection activity can be done in any kind of terrain and mostly throughout the year. (iv) Availability of workmen at cheap rates. This method consists of erecting the towers, member by member. The lOwer members are kept on ground serially according to erection sequence to avoid search or time loss. The erection progresses from the bottom upwards. The four main comer leg members of the fIrst section of the tower are fIrst erected and guyed off. Sometimes more than one contiguous leg sections of each comer leg are bolted together at the ground and erected. The cross braces of the first section which are already assembled on the ground are raised one by one as a unit and bolted to the aIreadyerected comer leg angles. First section of the tower thus built and horizontal struts (belt members) if any, are bolted in position. For assembling the second section of the tower, two gin poles are placed one each on the top of diagonally opposite comer legs. These two poles are used, for raising parts of second section. The leg members and braces of this section are then hoisted and assembled. The gin poles are then shifted to the comer leg members on the top of second section to raise the parts of third section of the tower in position for assembly. Gin poles are thus moved up as the tower grows. This process is continued till the complete tower is erected. Cross-arm members are assembled on the ground and raised up and fixed to the main body of the lOwer. For heavier towers, a small boom is rigged on one of the tower legs for hoisting purposes. The members/sections are hoisted either manually or by winch machines operated from the ground. For smaller base towers/vertical configuration towers one gin pole is used instead of two gin poles. In order to maintain speed and efficiency,· a small assembly party goes ahead of the main erection gang and its purpose is lO sort out the tower members, keeping the members in correct position on the ground and ,assembling the panels on the ground which can be erected as a complete unit. Sketches indicating different steps or erection by built up method are shown in Annexure-'L' 11.7.1.2 Section Method In the section method, major sections of the tower are assembled on the ground and the same are erected as units.

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is approximately 10 m long and is held in place by means of guys by the side of the tower to be erected. The two opposite sides of the tower section of the tower are assembled on the ground. Each assembled side is then lifted clear of the ground with the gin or derrick and is lowered into position on bolts to stubs or anchor bolts. One side is held in place with props while the other side is being erected. The two opposite sides are then laced together with cross members and diagonals; and the assembled section is lined up, made square to the line. After completing the first section, gin pole is set on the top of the first section. The gin rests on a strut of the tower immediately below the leg joint The gin pole then has to be properly guyed into position. The first face of the second section is raised. To raise the second face of this section i~is necessary to slide the foot of the gin on the strut of the opposite of the tower. After the two opposite faces are raised, the lacing on the other two sides is bolted up. The last lift raises the top of the towers. After the tower top is placed and all side lacings have been bolted up all the guyes are thrown off except one which is used to lower the gin pole. Sometimes whole one face of the tower is assembled on the ground, hoisted and supported in position. The opposite face is similarly assembled and hoisted and then the bracing angles connecting these two faces are fitted. 11 .7.1.3 Ground Assembly Method This method consists of assembling the tower on ground, and erecting it as a complete unit. The complete tower is assembled in a horizontal position on even ground. The tower is assembled along the direction of the line to allow the crQSsarms to be fitted. On slopping ground, however, elaborate packing of the low side is essential before assembly commences. After the assembly is complete the tower is picked up from the ground with the help of a crane and carried to its location. and seton its foundation. For this method of erection, a level piece of ground close to footing is chosen from the tower assembly. This method is not useful when the towers are large and heavy and the foundations are located in arable land where building and erecting complete towers would cause damage to large areas or in hilly terrain where the assembly of complete tower on slopping ground may not be possible and it may be JiiffIcult to get crane into position to raise the complete tower.

yards where these are fabricated and then transported one by one to line locations. Helicopter hovers over the line location while the tower is securely guyed. The ground crew men connect and tighten the tower guys. As soon as the guy wires are adequately tensioned the helicopter disengages and flies to the marshalling yard. This method is adopted where approach is very difficult or to speed up the construction of the transmission line. 11.7.2 Tightening of Nuts and Punching of Threads and Tack Welding of Nuts All nuts shall be tightened properly using correct size spanners. Before tightening it is ensured that filler washers and plates are placed in relevent gaps between members, bolts of proper size and length are inserted and one spring washer is inserted under each nut. In case of step bolts, spring washer shall be placed under the outer nut. The tightening shall be carried on progressively from the top downwards, care being taken that all bolts at every level are tightened simultaneously. It may be better to employ four persons, each covering one leg and the face to his right The threads of bolts shall be projected outside the nuts by one to two threads and shall be punched at three positions on the top inner periphery of the nut and bolt to ensure that the nuts are not lossened in course of time. If during tightening a nut is found to be slipping or running over the bolt threads, the bolt together with the nut shall be changed outright. 11.7.3 Painting of Joints For galvanized towers in coastal or highly polluted areas, the joints shall be painted with zinc paint on all contact surfaces during the course of erection. 11.7.4 Checking the Verticality of Erected Towers The finally erected tower shall be truly vertical after erection and no straining is permitted to bring it in alignment Tolerance limit for vertical shall be one in 360 of the tower height

In India, this method is nOl generally adopted because of prohibitive cost of mobile crane, and non-availability of good approach roads to tower location.

11.8 EARTHING 11.8.1 Each tower shall be earthed after the foundation has been cast. For this purpose, earth strip shall be fixed to the stub during concreting of the chimney and taken out horizontally below the ground level. In normal circumstances, the earth strip shall be provided on No.1 stub leg as given in Figure 3, i.e. the leg with step bolts.

I! .7.1.4J1elicopter Method In the, helicopter method, the transmission tower is erected in sections. For example bottom section is first lifted on to the stubs and then the upper section is lifted and bolted to the first section and the process is repeated till the complete tower is erected. Sometimes a completely assembled tower is raised with the help of helicopter. Helicopters are also used for lifting completely assembled towers with guys from the marshalling

11.8.2 Tower Footing Resistance The tower footi~ resistance of all towers shall be measured in dry v.:eather after tJ:teir erection and before the stringing of earthwire. In no case the tower footing resistance shall exceed 10 ohms. In case the resistance exceeds the specified values, multiple pipe earthing orcounterpois'e earthing , shall be adopted in accordance with the following procedure, but withQut interferring with the foundation concrete even

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Figure 3: Designation of Tower Legs, Footing and Face

1. 2. 3. 4.

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represents leg or pit No.1 represents leg or pit No.2

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represents leg or pit No.3

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represents leg or pit No.4

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A. represents near side (NS) transverse face B. represents near side (NS) longitudinal face C. represents far side (FS) transverse face D. represents far side (FS) longitudinal face NOTE 1: Danger and number plates are localed on face 'A' NOTE 2: Leg 1 represents .the leg with step bolts and anti-climbing device gate, if any.lftwo legs with step bolts are required, the next is No.3 leg.

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though the earth strip/counterpoise lead remains exposed at the tower end The connections in such case shall be made with the existing lattice member holes on the leg just above the chimney top. 11.8.3 Pipe Earth The installation of the pipe earth shall be in accordance with IS : 5613-19'&"1Part II/£ection 2). A typical example of pipe type of earthing is given in Annexure- 'M' 11.8.4 Counterpoise Earth . Counterpoise earth consists of four lengths·of galvanized steel stranded wires, each fitted with a lug for connection to the tower leg at one end. The wires are connected to each of the legs and taken radially away from the tower and embedded horizontally 450mm below ground level. The length of each wire is normally limited to 15m but may be increased ifthe resistance requirements are not met. Galvanized steel stranded wire preferably of the same size of the overhead ground wire may be used for this purpose. A typical example of counterpoise type earthing of tower is given in Annexure- 'N'. 11.9 STRINGING OF CONDUCTORS 11.9.1 Mounting of Insulator Strings, and Running Blocks 11.9.1.1 Suspension insulator strings shaIl be used on $USpension towers and tension insulator strings on angle and dead end towers. The strings shall be fixed generally on the tower just prior to the stringing of conductors. Damaged insulators and fittings, shall not be used in the assemblies. Before hoisting: all insulators shall be cleaned in a manner that will not spoil, injure or scratch the surface of the insulator, but in no case shall any oil be used for the purpose. Security clips shall be in position for the insulators before hoisting. Arcing horns or guard rings, if required, shall be placed along the line on suspension, and facing upwards on tension insulator string assemblies.

11.9.1.2 Traveller/Running Block Installation Installation of travellers, including finger lines where used, requires consideration of traveller attachment methods and the need for and location of traveller grounds and uplift rollers. For single conductor venical insulator assemblies, the travellers are normally connected directly to the insulators, and with 'vee' string insulator assemblies, to the yoke plate. For most bundled conductor lines, the travellers are connected to the yoke plate. With post type insulators, the travellers are connected to the end of the insulators. Where travellers are installed to string through tension towers, the travellers are normally connected directly to the tower. If substantial line angles are involved, two travellers in tandem may be requUed to reduce the bending radius of the conductor or the load on each traveller, or both.

Where bundled conductor traveUers are used atline angle locations ~rover 5 degrees, it is advisable to change to individual··single conductor travellers after the passage of the running board to facilitate accurate sagging. When adequate quantities of travellers are available, it is com mon practice to install travellers alongwith the insulators. Under some situations traveUers may be attached to slings or rods in place of the normal insulator assembly. Sketch of travellers is shown in Annexure- '0' ~

Use of travelling grounds and choice oflocations must be based on the degree of exposure to electrical hazards. When such hazards exist, as a minimum, traveUer grounds should be installed at the first and last tower between tensioner and puller. When stringing in proximity to energized lines, additional grounds shall be installed as required, but at a maximum distance not exceeding 3 km. Additionally, grounds shall be instaIled within a reasonable distance on each side of an energized crossing, preferably on the adjacent structure. Travellers with grounds are usually sensitive to direction and care must be exerfLsep in hanging the travellers. Usually the grounds ~~~UllThg end. Each traveller with grounds must be connected with temporary grounding sets to provide an electrical connection between the traveller and earth, or to some conducting medium that is at earth potential. Personnel should never be in series, with a ground lead. Traveller grounds should have a suitable grounding stub located in an ac.cessible position to enable placing and removing the ground clamps, with hot sticks when necessary. Traveller grounds also help protect the sheave linings. At the time the travellers are hung, finger lines, when used, should be installed and tied off at the base of the structures. If the helicopter method of pilot line installation is not to be used, the pilot line could be installed at this time in lieu of finger lines. 11.9.2 Paying out of Earthwire and Conductor

11.9.2.1 Paying out of Earthwire Normally earthwire drums are mounted on a turn table. Pulling machine/tractor are employed to pull the earthwire. Earthwire running blocks are hoisted on the towers prior to taking up of this operation. The earthwire while paying out passes through theearthwirerunning blocks. Earthwiresplices shall be made in such a way that they do not crack or get damaged in the stringing operations. It should be noted that no earthwire joints are·allowed within 30m from the tension or suspension clamp fittings. 11.91.2 Paying out of Conductor 11.9.2.2.1 Slack Layout or Direct Installation Method: Using this method, the conductor is payed out over the ground rollers by means of a pulling vehicle or the ~l carried along the line on a vehicle. The conductor reels are positioned on reel /

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20 stands or jacks, either placed on the ground or mounted on a transporting vehicle. These stands are designed to support the reel on a shaft permitting it to rotate as the conductor is pulled OUl Usually a braking device is provided to prevent overrunning and backlash. When the conductor is payed out past a tower pulling is stopped and the conductor placed in travellers are attached to the structure before proceeding to the next structure. This method is generally applicable to the construction of new lines in cases where maintenance of conductor surface condition is not critical and where terrain is easily accessible' to a pulling vehicle. The method is not usually economically applicable in urban locations where hazards exist from traffic or where there is danger of contact with energized circuits, nor it is practical in mountainous regions inaccessible to pulling vehicles. Major equipmem required to perform slack stringing includes reel stands, pulling vehicles and a splicing cart.

11.9.2.2.2 Tension Stringing Method Multi-conductor lines shall generatly be strung with the help of tension stringing equipment. Using this method, the conductor is kept under tension during the stringing process. Normally, this method is used to keep the conductor clear of the ground and obstacles which might cause conductor surface damage and clear of energized circuits. It requires pulling of a light pilot line through the travellers, w~i_ch in tum is used to pull in a heavier pulling line. The pulling line is then used to pull in the conductors from the reel stands using specially designed tensioners and pullers. For lighter conductors, a light weight pulling line may be used in place of pilot line to directly pull in the conductor. A helicopter or ground vehicle can be used to pull or layout a pilot line or pulling line. Where a helicopter is used to pull out a line, synthetic rope is normally used to attach the line to the helicopter and prevent the pulling or pilot line from flipping into the rotor blades upon release. The tension method of stringing is applicable where it is desired to keep the conductor off the ground to minimise surface damage or in areas where frequem crossings are encountered. The amount of right of way travel by heavy equipment is also reduced. Usually, this method provides the most economical means of stringing conductor. The helicopter use is particularly advantageous in rugged or poor! yaccessible terrain. Major equipment required for tension stringing includes reel stands, tensioner, puller, reel winder, pilot line winder, splicing cart and helicopter or pulling vehicle. While running out the conductors, care shall be taken such that the conductors do not touch and rub against the ground or objccts which could cause scratches or damage to the strands: The conductor shall not be over-strained during erection. The -- -

Construction a/Transmission Lines

Wherever required jointing of conductor during paying out will be carried out.

11.9.2.2.2.1 Typical Procedures/or Stringing Operations 11.9.2.2.2.1.1

Site Selection, Equipment Location, Anchor and Equipment Grounding

11.9.2.22.1.1.1 Sile Selection

The selection of pull, tension, anchor and splicing sites must consider accessibility, location of deadments, length of conductor to be strung, available conductor and line lengths, puller capacity, including placement of pullers, tensioners and conductor anchor locations, placemem of reel stands, pilot line winders, reel winders and the ability to provide an adequate grounding system. 11.9.2.2.2.1.1.2 Equipmenl LocatiollS

The locations of the puller, tensioners and intermediate anchor sites must be selccted so that the structures are not overloaded. A pulling line slope of three horizontal to one . vertical from the traveller to the site is considered good practice. It is also necessary that the puller be positioned so that the pulling line enters the machine at the smallest horizontal angle thereby minimizing the possibility of damaging the line. When a bull wheel type puller is employed, the reel winder to recover the pulling line is located at the pulling site. The pilot line winder is located at the tensioner site. "The arrangement of the tensioner and reel stands should be such that the lateral angle between the conductor as it approaches the bull wheel and the plane of rotation of the wheel is not large . enough to cause the conductor to rub on the sides of the groove. For example, birdcaging problems were eliminated in large conductors by using a maximum fleet angle of 1.5 degree from the plane normal to. the conductor reel axis and a back tension of approximatel y4500 N. Problems of birdcaging are normally more acute in the case of large conductors having three or more aluminum layers. 11.9.2.2.2.1.1.3 Anchors

Anchors are normally required for holding equipment in place and snubbing conductors against tensions imposed. The type of anchor is dependent upon the soil conditions and stringing and sagging tensions. Portable equipment as well as ground type anchors are often used for this purpose. Slack should be removed from all anchor lines prior to loading to minimize the possibility of equipment movement or impact loads to the anchors. 11.9222.1.1.4 Equipment Grounding

Adequate grounding most be established at all sites. The methods required and equipment used will be deteqnined by the degree of exposure to electrical hazards and the soil conditions at the site. All equipment, conductors, anchors and

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Once the ropepulIing lines have been installed prior to pulling in any conductor or conduct.ivc type p~lIing lines, a running ground must be installed betwccn the reef stand or tcnsioncr for conductor, or puller for pulling line, and the first tower. This ground must be bonded to the ground previously established at the site. Pulling lines are usually pulled in under tension. The pulling line is then connected to a single conductor through swivel link, or to bundle conductors through swivel links and a running board. Swivel links should not be used on a three strand synthetic pulling line. Pulling lines may be synthetic fibre or wire rope. When wire rope is used, it is tecommended that swaged type or braided type be used since it has less tendency to rotate under load, which minimizes spinning problems. A ball bearing swivel link is usually used for the connections betwee~ conductors, pulling lines and running boards. Swivel links must be sufficient rated worked load to withstand loads placed on them during tension stringing. They should also be compatible with the travellers being used so that they can pass through without spreading or damaging the sheaves. These special line stringing swivel links are clevis type and compatible with woven wire grips and swaged steel pulling lines. It is recommended that swivel links not be passed over bullwheels under significant tension since they may be weakened or damaged due to bending. When reeving the bullwhccls of a tensioner with the conductor entering and leaving the wheel from the top facing in the direction of pull, the conductor should enter from the left and leave from the right for right hand lay (standard for aluminium conductor) and enter from the right and leave from the left for left-hand lay (standard for groundwire). The procedureeliminates the tendency of loosening of outer layer strands while conductor passes around the bull wheel. It is recommended that conductor of only one manufacHIrer be used in a given pull, and preferably in any given ruling span. This precaution helps in minimizing the.possibility of difference in sag characteristic of conductor significantly. Attachment of the conductor to the pulling line, running board or to another reel of conductor to be pulled successively is accomplished by the use of woven wire grips. These grips should be compatible strength wise and sized as close as possible for the conductor or pullil\g line on which they are .used. 9verall diameter of the grip over the conductor or rope should be small enough to pass over the sheaves without damage to the sheave or its lining and the grip must also be capable of mating with a proper size swivel link. Metal bands should be installed over the grip to prevent it from accidentally coming off and dropping the conductor. The open end of the grip should be secured with two bands. This should then be wrapped with tape to prevent accidentally

stripping the grip off the conductor if the end were to snag OJ catch. This is particularly important when these grips are used on pulling lines or between lengths of conductor when more than one reel is strung. The grips will then pass through the travellers backwards and if the ends are not banded and taped, they may slip off.

.

Experience has shown th.at pulling speed is an important factor in achieving a smooth stringing operation. Speeds of 34 kmlhour usually provide a smooth passage of the running board or connecting hardware, or both, over the travellers, whereas slower speeds may cauSe significant swinging ofthe traveller and insulator hardware assemblies. Higher speeds . create a potential hazard of greater damage in case of a malfunction. The maximum tension imposed on a conductor during stringing operations should not exceed than necessary to clear obstructions on the ground. This clearance should be con frrrned by observation. In general, stringing tension of about one-half of the sagging tension is a good criterion. If greatertensions are required, consideration must be given to any possible prestressing of conductors that may result, based on the tension and time involved. Consideration must also be given to the fact that when long lengths of conductor are strung, the tension at the pulling end may exceed the tension at the tensioner by a significant amount. Difference in tension is caused by the length of conductor strung, number and performance of travellers, differences in elevation of supporting structures, etc. Light and steady back tension should be maintained on the conductor reels at all times sufficient to prevent over run in case of a sudden stop. It must also be sufficient to cause the conductor to lie snugly in the first groove of the bullwheel and to prevent slack in the conductor between bull wheels. It may be necessary periodically to loosen the brake on the reel stand as the conductor is payed off. As the reel empties, the moment arm available to overcome the brake drag is reduced, and the tension therefore rises. This may cause the conductor to wedge into the underlying layers on the reel. . The reel should be positioned so that it will rotate in the same direction as the bullwheels. looSening of the stranding' that often occurs between the reel and the bull wheels of the tensioner is caused to a great extent by coil memory in the conductor. As the conductor is unwound from the reel and straightens out, the outer strands become loose, a condition that is particularly noticeable in a large diameter conductorandcan be best observed at the point at which it leaves the reel. As the conductor enters the bull wheel groove, the pressure ofcontact tends to push the loose outer strands back towardsrJie reel where the looseness accurg,u!ates, leading to the condition commonly known as birdcaging. If this condition iSoot controlled, the strands can become damaged to the extent Hi'at the damaged area of conductor must be removed. lbls'pr6blem can be remedied by allowing enough distance between the reel and tensioner to permit the strand looseness to distribute along

I 22 _____ . _. __ . the intervening length of conductor and simultaneously main-

taining enough back tcnsion on the reel stretch the core and inner strands to sufficiently tighten the outer strands. The maximum time conductors may safely remain in the travellers depends on wind induced vibration or other motion of the conductors. Wind blown sand can severely damage conductors in a few hours if clearance is less than about 3m over loose sand with little vegetation. Damage from vibration at sagging tensions is quite possible and, when required, dampers should be installed promptly. However, at lower tensions generally used for initial stringing, damage to conductors or sheave bearings, or both, is not likely to occur from vibration. Even for travellers having lined sheaves with root diameters 20 times the conductor diameter, it is important to complete conductor stringing, sagging, plumb marking, c~ip­ ping, spacing and damping operations as soon as possible to prevent conductor damage from weather, particularly wind. Conductor should not be strung if adverse weather is predicted before the entire sequence can be completed. Sub-conductoroscillation may occur in bundled conductor lines and tie-down methods. Temporary spacers, or other means may be required to prevent conductor surface damage prior to installation of spacers. Temporarily positioning of one sub-conductor above another to prevent conductor clashing is undcsirable since different tension history will produce subconductor mismatch unless the tensions are low and duration short enough so that creep is not a factor. Conductor clashing can mar the strands and produce slivers which can result in radio noise generation. If a bull whccltype puller is utilized, the pulling line must be recovered during the pulling operation on a separate piece of equipment. This function is usually performed by a reel

windcr which is placed behind the puller in an arrangement similar to the reel stand at the tension site. These coils shall be removed carefully and if another length is required to be run out, a joint shall be made according to the recommendation of the manufacturers. Drum battens shall be removed just prior to moving drums on drum stands. Drums will be transported and positioned on station with the least possible amount of rolling. The conductors, joints and clamps shall be erected in such a manner that no birdcaging, over-tensioning ·of individual wires or layers or other deformation or damage to the conductors shall occur. Clamps or hauling devices shall, under erection conditions, allow no relative movement of strands or layers of the conductors. Scaffolding shall be used where roads, rivers, channels, tclecommunication or overhead power lines, railway lines, fences or walls have to be crossed during stringing operations. It shall be seen that nonnal services are not interrupted or damage caused to property. Shut-down shall be obtained when

The sequence of running out shall be from top to downwards i.e. the earthwire shall be run out first, followed by the conductors in succession. In case of horizontal configuration tower, middle conductor shall be strung before stringing of outer conductors is taken-up. A sketch of Tension stringing operation is shown in Annexure-'P'

11.9.3 Repairing of Conductor Repairs to conductors, in the event of damage caused to isolated strands of a conductor during the course of erection, if necessary, shall be carried out during the running out operations, with repair sleeves. Repairing of conductor surface shall be done only in case of minor damage, scuff marks etc., keeping in view both electrical and meChanical safe requirements. Repair sleeves may be used when the damage is limited to the outer layer of the conductor and is equivalent to the severances of not more than one third of the strands of the outer most layer. No repair sleeve shall be fiued within 30m of tension or suspension hardware fittings, nor shall more than one repair sleeve per conductor normally be permitted in any one span. 11.9.4 Jointing The fullest possiblc usc shall be made of the maximum conductor lengths. in order to reduce to a minimum number of joints. All the joints on the conductor shall be of compression type, in accordance with the recommendations of the manufacturers for which all necessary tools and equipments like compressors, die sets etc., shall be arranged. The final conductor surface shall be clean smooth and shall be without any projections, sharp points, cuts, abrasions etc., Conductor ends to be joined shall be coated with an approved grease immediately before final assembly. Surplus grease shall be removed after assembly. All joints or splices shall be made atieast30 metres away from the structures. No joints or splices shall be made in tension spans. No tension joint shall be used in any span crossing other major power lines. The compression type fitting used shall be of self-centering type or care shall be taken to mark the conductors to indicate when the fining is centred properly. During compression or splicing operation the conductor shall be handled in such a manner as to prevent lateral or vertical bearing against the dies. After pressing the joint the aluminium sleeve shall have all corners rounded, burrs and sharp edges removed and smoothened. 11.9.5 Final Sagging of Conductor and Earthwire The final sagging of the conductor shall be done by sagging winches.

After being rough sagged the conductor/earthwire shall

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The tensioning and sagging shall be done in accordance with the approved stringing chans before the conductors and earthwireare finally attached to the towers through theearthwire clamps for theearthwire and insulatorslrings fortheconductor. The sag will be checked in the first and last span of the Section in case of Sections upto eight spans and in one intermediate span also for sections with more than eight spans. The sag shall also be checked when the conductors have been drawn up and Iran sported from running blocks to the insulator clamps. The running blocks, which are suspended from the transmission Slructure for sagging shall be so adjusted that the conductors on running blocks will be at the same height as the suspension clamp to which it is to be secured. At sharp vertical angles, the sags and tensions shall be checked on both sides of the angle, the conductor and earthwire shall be checked on the running blocks for quality oftension on both sides. The suspension insulator assembly will normally assume vertical positions when the conductor is clamped. Tensioning and sagging operations shall be carried out in normal weather when rapid changes in temperatures are not likely to occur. Sag board and dynamometers shall be employed for meac;uring sag and tension respectively. The dynamometers employed shall be periodically checked and calibrdted with a standard dynamometer. Attempts to sag conductor on excessively windy day should be avoided since serious error can result due toconductor uplift caused by wind pressure on the conductor. Should severe wind conditions occur when sagging is in progress, the sagging must be stopped till peaceful conditions prevail to resume sagging. Once a Section hac; been sagged, the sub-conductors of the bundle should be checked for evenness. Unevenness, if any, shall be rectified as far as possible with the help of sag adjuster.

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The travellers which are used to string conductor are not frictionless and therefore, can cause problems during a sagging operation. If one or more of the Iravellers becomes jammed, sagging can become very difficult A Iraveller which swings in the direction. of the pull may be an indication of a defective traveller. Should unexplainable sagging difficulties occur, the traveller should be checked. Tensions applied to the conductor to overcome sticky or jammed travellers can eause sudden, abrupt movement of the conductor in the sagging spans and . quickly cause change of sag, particularly, if the conductor is already tensioned to the required value. During sagging care shaJl be taken to eliminate differen-_ tial sags in the sub-conductor ac; far as possible. However, in no case sag mismatch of more than 25mm shall be allowed. 11.9.6 Clipping in/Clamping in of Conductors The clipping portion of the conductor stringing operation

involves thework foil owing sagging and plumb marking of the conductors. This entails removing the conductors from the travellers and placing them in their permanent suspension clamps attached to the insulator assemblies. When clipping is being done, care must be exercised to ascertain that the conductors are grounded prior to clipping despite the fact that the lines being clipped are not attached to any electrical source. This involves placing a locaJ ground upon the conductor at the location of work. After the conductors have been marked, the erection crew will lift the weight of the conductors, allowing the travellers to be removed and the suspension clamps, and armour rod, if any used, to be placed on the conductors. Lifting is nonnally done by use of a hoist suspended from the structure and a conductor lifting hook which is designed so as not to notch or severely bend the conductors. After placing the suspension clamps on the conductor, the hooks are lowered thereby placing the weight of the conductor on the suspension clamp and completing the assembly. Where bundle conductors are used, the multiple conductors may be lifted simultaneously by using a yoke arrangement supporting the hooks and a single hoist or other lifting means. 11.9.7 Installation of Spacers Following the clipping operations for bundled conductor lines, spacers must be installed. This is done by placing the erection crew on the conductors in the 'conductor car' normally known as spacer cycle to ride from structure. Depending on the length of line Lo be spacered and the equipment available, cars may be hand powered, towed by persons on the ground or in adjacent slrucLures wiLh ropes, or powered by a small engine on the car itself. Care must be exercised to ensure thaL the concentrated load of the man, car and equipment does not increase the sag appreciably to cause a hazard from obSlructions over which the car will pass. The installation of the spacers on the conductor varies with the type and manufacture of the spacer and is normally done in accordance with the manufacturer's recommendations. The load of the man, car and equipment should be equally diSlributed to all sub-conductors of the phase. This is particularlyimportant at the time each spacer is attached. Number of spacers. per span and the spacings are provided as per the approved spacer placement chart 11.9.8

Installation of Vibration Dampers/Spacer Dampers Vibration Dampers/Spacer Dampers are nonnally placed on the conductors immediately following clipping to prevent any possible wind vibration damage to the conductors which at critical tensions and wind conditions can occur in a matter of a few hours. The number of dampers/spacer dampers and spacing are provided as per the design requirement and instructions of the manufacturers.

24

Construction of Transmission Lines

11.9.9 Jumpering The jumpers at the Section and angle towers shall be fonned to parabolic shape to ensure maximum clearance requirements. Pilot suspension insulator string shall be used, if found necessary, to restrict the jumper swings to the design values. Clearance between the conductors and ground and between jumpers and the tower steel work shall be checked during erection and before handing over the line. 11.9.10 Ground Undulation The provision of 150mm shall be made to account for any undulations in the ground in final still air sag at maximum. 11.10

HOT-LINE STRINGING OF E.H.V. LINES

11.10.1 General Hot line stringing means stringing of second circuit on the same tower with first circuit electrically & mechanically loaded. This is shown in Figure A. 11.10.1.1 With the available techniques, the hot-line stringing is done in this country only upto 220 kY. The advantage of stringing second circuit at a later'date (with hot-line method) is saving in initial capital investment in the form of conductors, insulated hardware. Besides, with provision of Double circuit towers from the beginning saves way problems as second corridor is not required for second circuit 11.10.2 Precautions 11.10.2.1 Hot-line stringing is a specialised job and calls for special precautions. All the crew members are provided with rubber shoes and hand-gloves and are compelled to use them during the stringing. 11.1 0.2.2. All the drums of conductor and pilot wires are solidly earthed. All the tension locations, where the conductor ends are terminated, are solidly earthed. 11.10.2.3 In addition to above, during final sagging and clipping operation, standard earthing rods are used for con, necting each conductor to the tower body.

Circuit No-1 strung and energised

11.10.3 Operations 11.10.3.1 Arrangement for earthing the conductor drums and pilot wire drums is made at both the ends of the section under stringing. The hoisting of insulators, clamping of pilot wire and the conductor and rough sagging of conductor is done as per nonnal stringing method. 11.10.3.2 Before marking and clipping the dead ends, each phase conductor is solidly earthed in two separate sets. One set is earthed by means,of droppers and earthing rods and second set is by earthing of conductor end to tower body. This is shown in the Figure B. While removing the second set of earthing, the conductor end is removed first and the tower end later. Similarly in case of the first set the cable is disconnected from canductor end first and the rod end later. 11.10.3.3 Similarly, before clipping the canductor on the suspension towers, each canductor on both the sides of the clamp is earthed to tower body. After the clipping is aver, the earthing cable is first removed from the conductor end and later from the tower end. This is shown in the Figure C. 11.10.3.4 In arder to limit the parallelism and induced voltages, it is advisable to do thejumpcring work at the end. While daing the jumpering work also the earthing cables are required to' be pravided.

11.10.4 Earthing 11.10.4.1 Solid earthings are provided by driving one or mare G.I. SPIKES in the soil as dane in pipe type of earthing. If required, more pipes are driven at the same place. In any case the soil resistance should not be more than 5 ohms. 11.10.4.2 In case of rocky soils, counterpoise type earthing system is used. The length of the wires is decided by trial & error till the earth resistance is lowered to 5 ohms or less. 11.10.4.3 For earthing a t1exible copper cable having 10 sq. mm area (20 Ampere capacity) is used. The cable is generally armoured type for rough use. Proper clamps/connectors are used to connect the cable to the conductor and to the earth.

Circuit No.-2 to be strung as hot line

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First set

H/(

B/C

2 10 mm flexible copper cable Tension tower Standard earthing rods

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Earth

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TIC

Suspension insulator string 10 mm 2 flexible earthing cable

H/C

B/C

Suspension tower

FIGUREC

26.

. - - - - - - - - - - - - . ------------- -- - - _ .... _. . . -

11.11 PROTECTION OF TOWER FOOTINGS The woi"k includes all necessary stone reveunem, concreting and earth filling above ground level and the clearance from )tacking on the side of all surplus excavated soil, special measures for protection of foundations close to or in nallahas, river beds, etc., by providing suitable reveunenl or galvanised wire nelling and meshing packed with boulders. A typical revetment drawing is shown in Annexure- 'Q' 11.12

TESTING AND COMMISSIONING

11.12.1 General Before the line is energised, visual examination of the line shall be carried out to check that all nuts and bolts are tight and insulators and accessories ar9 in position. The earth connections shall also be checked to venfy that these are in order. 11.12.2 Testing Before commissioning of the lines, the following tests may be carried out:

(a) Conductor continuity test-The objective of this test is to verify that each conductor of the overhead line is properly connected electrically (that is, the value of its electrical resistance docs not vary, abnormally from that of a continuous conductor of the same size and length). The electrical resistance of the conductor shall be measured with a Wheatstone bridge or other suitable instrument. (b) Insulation resistance test-This test may be carried out with the help of a 5 kV megger preferably driven to ascertain the insulation condition of the line. 11.12.2.1 The line may then be kept charged on no load at the power frequency voltage preferably for 72 hours. for the purpose of full scale testing. 11.12.3 Statutory Requirements

The statutory authorities shall be informed before commissioning the lines and their approval obtained in accordance with Indian Electricity Act, 1910 and Indian Electricity Rules, 1956. (For details see Rules 63 to 69 of Indian Electricity Rules, 1956). 11.13 REFERENCES 1. IEEE Guide to the Installation of Overhead Transmission Line Conductors. (lEE Std. 524-1980). Published by the Institute of Electrical and Electronics Engineers, Inc. 345 East 47th Street, New York 1.0017, Dec' 18,1980. .

Construction a/Transmission Lines

2. The following papers published by the Association of Indian Engineering Industry Transmission Line Division Published on the occasion of International Conference on Trends in Transmission Line Technology during 17th18th April, 1985. (i) "Latest Erection Techniques for Tranmission Line Construction" by Shri R. K. Madan, MIs National Hydro-electric Power Corporation .. (ii) "Tower Foundation design practice" by Shri S.D. Dand, MIs KEC International Limited, Kurla. 3. Overhead Line Practice-by John Mc-COMBE. 4. Manual ofTransmission Line Towers-Technical Report No.9 of Central Board of Irrigation and Power. 5. Text book on "Surveying and Levelling-by Shri T.P. KaneLkar. 6. "Company Standard Guide for Transmission Line Surveying"-EMC Ltd., Calcutta. 7. Indian Standard Codes (a) IS: 5613 (Part II/Section I)-1976-Code of Practice for Design, Installation and Maintenance of Overhead Power Lines-(Lines above 11 kV and upto and including 220 kV). (b) IS: 5613 (Part II/Section 2)-1976-Code of Practice for Design, Installation and Maintenance of Overhead Power Lines-(Lines above 11 kV and upto and including 220 kV). (c) IS : 4091-1979-Code of Pmctice for Design and Construction of Foundations for Transmission Line Towers and Poles. (d) IS: 456-1978-Code of Practice for Plain and Reinforced Concrete. (e) IS: 3043-1966-Code of Pmctice for Earthing. (f) Draft "Indian Standard Code of Practice for Design,

Installation and Maintenance for Overhead Power Lines" -Part 3 (400 kV Lines)-Section I-Design-"IS : 5613 (Part III/Sec. 1.)".

ANNEXURE 'A'

CLEARANCES

1. 1.1

The minimum clearances shall be in accordance with Indian Electricity Rules, 1956 and are given in Table I TABLE·I Minimum Clearances

VOLTAGE CATEGORY (IE RULES, 1956) Nominal System-Voltage Clearance

HIGH VOLTAGE 33kV

66kV

1l0kV 220kV 132kV (Minimum value in m)

(i) Clearance to Ground (a) Across street 6.1 6.1 6.1 (b) Along street 5.8 6.1 6.1 (c) Olher areas 5.2 6.1 S.5 (ii) Clearance to Buildings (a) Vertical (*) -from 3.66 highest ,object 3.97 4.58 (b) Horizontal (+) -from nearest point 2.14 2.75 1.83 (iii) At Crossings with (a) Tramway/trolley bus 3.05 3.36 3.76 (b) Telecom lines 2.44 2.75 (c) Railway # 1 Category 'A' and 'C' Groad Guage Inside station area 10.6 10.0 10.3 Oul

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ANNEXURE.

Typical Sa g Template Drawing

Ground clearance curve (3) Tower footing curve

(4)

Normal span 400 m

Scale Hor. 1 cm = 20 m Ver. 1 cm = 2 m

PARTICULAR 1. CONDUCTOR MOOSE ACSR 2. ULTIMATE STRENGTH 16434 Kg 3. TEMPERATURE RANGE 00-370-750 4. NORMAL SPAN 400 m 5. SAG OF CONDUCTOR AT MINIMUM TEMPERATURE AT NORMAL TEMPERATURE NOWIND . 6. MAXIMUM SAG CONDUCTOR 12.865 m EARTHWIRE 10.196 m 7. TENS'ION AT MAXIMUM TEMPERATURE STILL WIND 8. TENSION AT MINIMUM TEMPERATURE STILL WIND GROUND CLEARANCE 8.840 m GROUND UNDULATIONS 0.150 m

Construction o/Transmission Lines

34

ANNEXURE·E STRUCTURE LIMITATION CHART/TOWER SPOTTING DATA (FOR 400 KV TRANSMISSION LINES)

Tower Type Max. Angle of Deviation Vertical Load Limitations on Weight Span. Groundwire effect (a) Both Spans (b) One Span Conductor effect (a) Both Spans (b) One Span Weights Groundwire effect (a) Both Spans (b) One Span Conductor effect (a) Both Spans (b) One Span Permissible sum of adjacent span for various deviation angles.

'C' MKD. 'c'

'D'MKD. 'D'

15°

15° to 300

6OO/O.E.

Max. (Min.)

Max. (Min)

.Max. (Min.)

Max (Min.)

600 (200) 360 (100)

600 (0) 360 (-200)

600 (0) 300 (-200)

600 (0) 360 (-300)

600 (200) 360 (100)

600 (0) 360 (-200)

600-(0) 360 (-200)

600 (0) 360 (-300)

350 (117) 210 (58)

350 (0) 210 (-117)

350 (0) 210 (-117)

350 (0) 210 (-175)

2405 (802) 1443 (401)

2405 (0) 1443 (-802) 15°-800 14-876 13-956 12-1034 11-1112 10-1190

2405 (0) 1443 (-802) 30°-800 29-874 28-952 27-1028 26-11()4

2405 (0) 1443 (-802)

'A'MKD. 'A'

'B'MKD. 'B'

2°

2°-80'0 1-838 0-878

60°-800 59-868 58-936 57-1004 56-1074 55-1144

25-1182

Design (a) Groundwire

6.

(i) 32° Full wind

1574

1561/1574

1520/1574

(ii) 00 x 2(3 Full wind

1525

1521/1525

1473/1525

1363/1574 1321/1525

4470

8864/8940 9086/9164

8635/8940 8852/9164

7742/8940 7936/9164

(b) Conductor (i) 32° Full wind (ii) 00 x 2(3 Full wind TOWER TYPE 18m and 25m Extension for Towertype 'A' marked 'A'

4582

(a) Maximum Wind span 300m (b) Deviation Angle odegree (c) Vertical load Limitation on Weight span of Conductor/Groundwire: Minimum Maximum (i) Both spans

600

200

(ii)

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ANNEXURE I • (Contd, 6A.

18m and 25m Extension for Tower type 'D' marked 'D'

400m 40 degree (c) Vertical load limitation on weight span of Con ductorlGround wire: Minimum Maximum

(a) Maximum wind span (b) Deviation Angle

(i) Both spans

\ 7. 8.

(-) 600

(ii) One span (-) 360 Way leave clearance 26 metres either side from centre of line of tower. Electrical clearance for Railway crossing 17.9m,

o (-) 300

------ -

9.

Minimum clearance between power line to power line crossing

5.490ml

NOTES:

Vertical loads on individual spans are acting downwards for suspension towers. 2. Broken wire condition: As per specification requirement. 1.

3.

4. 5.

6.

7. 8.

Maximum sum of adjacent spans for various angles of deviations are subjected to the condition that maximum live metal clearance and minimum ground clearance are available. Limit of Highway crossing span: 250 metres Maximum deviation angle for dead end tower: (a) Line side and Slack span side: 15 degree on either side. (b) For River crossing Anchoring with longer wind span with 0 degree deviation on crossing span and 30 degree deviation on either side. Angle tower types 'B', 'C' & 'D' are designed for following unbalanced tension resulting from unequal Ruling spans of 200 m and 400 m on each side of the towers for nonnal condition only. Temperatures Unbalanced Tension Groundwire Conductor At 32 degree Celsius (Without wind) 80 983 At Zero degree Celsius (Without wind) 85 376 Tower type 'C' to be u'sed as Transposition tower with 0 degree deviation. Tower type' B' to be used as Section towers. The number of consecutive spans between two section points shall not exceed 15.

\~ ANNEXUREF TOWER SCHEDULE NAME OF THE LINE: Tower No. Final mst.

)/4

Length Span (m)

of Section (m)

05

XX

XX

Angle of

XX

XX

XX

XX

Wt. Span (m)

Type

deviation

of T.ower

L

13°32' OO"L T

B+9

107

R

260

XX

XX

Type

Total

367

of Fdn.

FS

Details of Earthing Type Resistance{ohm} Type IniFinal (P/CP) tial Pipe

5

2

450 )/5

11 kY crossing

06

A

190

190

380

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Pipe

3

2

A

190

189

379

Wet

Pipe

4

2

380 ./6

Nala crossing

07 395

';7

Remarks

2 Nalas crossing

08

A+3

206

217

423

WBe

Pipe

3

2

A

198

194

392

Dry

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5

3

B

196

196

392

WBe

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3

415

1/8

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APPENDIX :A

MODERN METHODS OF SURVEYING (Reference to the clause: 11.5.2) 1.1 Satellite Doppler Technique Accurate and flexible survey data are necessary to achieve the minimum cost transmission line routing with the minimum environmental impact. Precise and reliable topographic data arc obtained including detailed and accurate horizontal and vertical terrain information by compiling large scale ''Orthophoto'. maps of the proposed transmission corridors. These give a 'Picture' of the route which is geometrically correct and overlayed on this are contour lines which depictthe changes in elevation of the land. By studying these maps,tmnsmission corridors are selected which are most attractive for tower installation purposes. Within these corridors, specific line routes can be defined on .the map and profiles of these lines are automatically generated for detailed analysis. Before mapping is produced points with known coordinates are established throughout the area to control the photographs both horizontally and vertically. Each of the various components of route survey under this technique are discussed in following paras.

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1.1.1.' Initial Survey Under initial survey, one or more preliminary transmission corridors are established. These are established with the help of Topo sheets of the region and after having a walkover survey along the tentative route alignment. 1.1.2. Controls Control points are fixed along the route for which the latitude, longitude and elevations are accurately known. An initial reconnaissance will establish the most suitable sitesfor the control points based on terrain conditions. Control points need not be proposed along the transmission line corridors, they can be at the sides of roads or elsewhere they cause the minimum impact on the land owners. Each of these points is to have a permanent marker placed on the ground. This is because the field staff is required to return to the same points again and again during the execution period of the project. Two types of permanent markers are used. For the preliminary control, a concrete cylinder is placed approximately 6 ft in the ground with the top of the cylinder flush with the surface. This is used for the 8 to 10 points which are surveyed using doppler satellite techniques. Concrete markers are installed along the proposed route to provide the overall basis for the control net work. A receiver is placed on each control point to monitor the position of satellite. From this information, position coordinates are calculated for the receiver locations on the ground. The remaining points are surveyed using the Inertial

Survey system which coordinate the control points (in x, y and z) between any two of the previously established doppler points. For these points, a 4 ft long steel bar is driven in the ground so that the top is flush with the surface. Inertial Survey System is operated from a helicopter in order to produce large number of coordinated points in a minimum amount of time.

1.1.3

Orthophoto Mapping

Aerial survey mapping (photogrammetry) has a definite application to the planning and design of transmission lines and is used in the advanced countries both in the preliminary stages of line routing and in the preparation of plan and profile maps for structure plotting. Aerial photography is taken immediately after fixing the control points along the tentative route alignment in order to minimise the loss of targets due to weather or any other problems. Here it is necessary that these control points show up very clearly when the aerial photography is taken. Orthophoto is a photograph of the area which is true to scale in all respects. It gi~es the transmission line engineer a complete picture of all ground features with the added bonus of the required vertical pata. It is produced from aerial photography using compJier technique. :"

A band, approximately,2 kms wide is generally mapped along the preliminary corridors. The horizontal scale for the mapping is 1: I0,000 with 1 m contour intervals in the plain section and 5 mcontour in the mountaneous terrain. This gives a gOod basis for selection of tower site with spot height accuracy to within 1 to 2 metres. Some of the specific advantages of using photogrammetry techniques for transmission line survey-are as under:

1.1.4 Advantages Determine:J.the best route: The broad coverage provided by aerial photographs facilitate selection of best line route. Potential routing difficulties can be recognised and avoided before any field activity begins. Also angles can be selected easily for efficient and economical use of structures.

1.15

Economical Aerial surveying has definite economic advantages-both in respect of time and cost. Where mountaineous/rugged terrain, inaccessible swamp land or heavily populated areas are encountered, even greater economies can be realised.

1.1.6

Saves\ Times Data that could take months to obtain by ground survey can be obtained by aerial survey in a much shorter period of time.

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54 1.1.7 Greater Visual Details. The use of photogmmmetry techniques provides tisual detaiis as well as pennanent visual record of existing features which can not be obtained byany other means.· 1.1.8 More Accurate Engineering. Design & Construction

Bids Accurate plan and profile maps can be prepared from .photographic enlargement; which hel p the designers to spot the tower~ and design the footing with greater accuracy and " economy.

1.1.9 Flexibility All necessary.line data, including tower spoUing profiling etc. can be detennined from theorthophotos for any number of ro.ute· variation~ withOl,lt returning to the actual site. In fact, changes in the rOute alignment can be made with the mini·mum . difficulty.

1.1.10 .Confidential Aerial surveys are confidential and therefore help in minimising the way leave problems. 1.1.11

Equipment required and their cost

Equipment required/or Satellite Doppler Technique are: Equipment for control surveys i.e., Satellite doppler global position system, Inertial survey system and Electronic distance measurement system. Equipment for aerial photography i.e. Aeroplane, Camera & PhoLOmechanicallaboratory. Mapping equipment-Analytical stereo compilers. Cost of these equipments is definitely substantially high and as such initial investment for acquiring the same is much more. In regard to the operational cost, it may vary due to geographic location, distance from aerial survey station to job site, type of aircraft employed, quality of photography and degree of accuracy required.

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The e'quipoise

A mandate for balance To strike ahead for a more optimum system of bulk power distribution, POWERGRID was incorporated in October,1989. The formation of POWERGRID is merely the reorganisation of the Power Sector in the pursuit of a more efficient. planning, implementation and development of power for the country. With the amalgamatjon of available expertise inJhe areas of Transmission, Load Despatch and Communications, . POWERGRID is poised to set the milestones towards a reliable, economic and secure National Power Grid.

(A Government of India Enterprise) Regd. Off. Hemkunt Chambers, 10th Floor, 89 Nehru Place,New Delhi _ 110019. ",.~----------~-.-

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