INTRODUCTION

The aim of this lecture is to introduce engineers to various types of transmission line supports. The lecture is divided in two parts viz. Part A for pole supports & Part B for transmission tower support.

PART A :

I ) POLE SUPPORTS :Under this category following supports are included.

1. Wooden Poles2. Reinforced concrete Poles ( PSC poles) 3. Tubular poles4. Latticed poles5. Girders6. Rails7. Wide based tubular hollow PSC Poles

The above poles are detailed one by one as follows.

1. WOODEN POLES :These are poles ranging from 100mm to 200mm in diameter. In

Gujarat they are very rarely used except in the places where there is too much of oceanic pollution & steel support are not able to stand. But, they are widely used in places where forest is very wide & wood is available at very cheaper rate for example Jammu & Kashmir, Himachal Pardesh, Karnataka, Kerala, etc. The main drawback of these poles is their uncertain life, the tendency to get eroded from within the soil.

2. REINFORCED CONCRETE POLES ( PSC POLES) :These are pre cast in factories having length from 8 meters &

breaking capacity of 140 Kg to 210 Kg. They are mainly used to support 11 KV, 22KV & 33 KV feeders.

3. TUBULAR POLES :These can be used for voltage up to 33 KV but they are not being

used in GEB for the simple reason of high cost of fabrication & transport.

4. LATTICED POLESThese are single, tapered poles fabricated from angles which in use

in the pre independence era. Now they are not used. They can be used in 33 KV transmission lines.

5 & 6. GIRDERS & RAILSGirders at size 150 x 150 mm & rails of 90 pounds can be used for

the transmission of 66 KV by making H frame. The tower size of girders & rails can be utilized for the transmission line of the 33 KV & 22 KV.

7. WIDE BASED TUBLAR HOLLOW PSC POLESThese are tubular type structures which are pre stressed & pre

fabricated section by section are erected at site as a single structure on which cross arms can be laid & a wire stung. These type of pole have been manufactured on trial basis & are likely to be in market soon but are not presently used in GEB.

II DESIGN ASPECTS:Out of the single poles structures described above, the tubular hollow PSC

poles can withstand transverse, longitudinal, vertical stresses whereas other type of single poles can withstand only vertical load and transverse load (to some extent only) and as such for these type of poles wind guys have to be provided on every 5th/ 6th locations. Also at the angle points and cut points where the tension and deviation loads are coming, special guys are required to be provided.

Fig a1 – sag in equal supports

Fig a2 - sag in unequal supports

The design of these poles mainly based on the sag & tension of the conductor which is to be used as shown in the Fig.a1 & a2 above. The sag in case of fig. a1 will be (WL2/ 8T) where T is the tension of the conductor in the Kg. and W weight of metre of the conductor and L is the span. In fig. a2 where unequal supports are used, the sag D1 & D2 will be worked out by formula

D1 = ( W x 12/27), D2=(W x 22/27)

where, x1 = (1/2)+(Th/W1)x2 = (1/2) (Th/w1)

where h= difference in height of supports.The tension of the conductor as to be worked out depending upon average

span, temperature, size of the conductor and factor of safety under various conditions which is again a separate topic all together and not discussed here.

The height of the two towers thus will totally depend upon the sag and the requirement of the clearance. The number of guys to be provided will depend upon the maximum tension of conductor under worse condition and has to be evaluated separately in each case. The angle of the guys will also depend on which type of wire is to be used.

III) (a) DESIGN OF SINGLE INTERMIDIATE WOOD SUPPORTS:Single wood poles at intermediate positions are subjected to single

bending due to the windage on the conductors and on the pole. In determining the diameter of the wood pole for any overhead line, two sets of loading must be considered.

(1) The total windage on the ice-loaded conductors in lb, adding at a particular height above the ground, knows as the centre of the gravity of the conductor loading; exerts the bending moments on the poles of the ground level.

(2) Windage acting on the pole for its whole lengths above the ground and exerting a bending moment on the pole at ground level.

Considering windage is acting on bare conductors only & the windage of the pole is ignored.

H= Height of poles above ground level in ft.h= Height of center of gravity of conductor loading in ft. above the ground level.P= Total horizontal conductor in loading in lb, acting at height h ft. above the ground level.Dg= Diameter of pole at ground level in inches.Da= Average diameter of pole in inches.W= Total windage on pole in lb. assumed to be acting at H/2 ft. above ground level.

Total bending moment of pole at ground level BM = ( P x h + w x H/2 ) lb. ft.

Max. fibre stress for red fir. = 7800 lb./sq.in.Thus, max. Allowable fibre stress = 7800/F.ofs. lb/sq.inch.Moment of intertia for circular pole I = 7IDg4/64 in4.Section modulus Z = I/(Dg /2) in3 = (II dg3) / 32 in3.Allowable stress = ( BMx12)/z lb./sq in.

= (BM lb.ft. x 12 x 32) / 3.14 Dg3 lb./ sq in.Pole diameter Dg = √(BM x 122.3/f) in.

(b) DESIGN OF STAYED WOOD SUPOORTS :At angle and terminal positions the supports are stayed to take the pull of the conductors, & consequently the support acts as a strut, due to the vertical loading imposed by stay tension.

Calculating angle between stay & support.

z Let P= Horizontal conductor pull on

pole in lb. θ= Angle between stay and pole in degree. S= Tension in stay in lb.

Tension in stays S= P/Sinθ ib.Vertical load on support = S cosθ lb.

P

s

stayθ

(c) CIRPPLING LOAD ON STRUTS:In determining the crippling load of struts.

Following formula used.

1) P = π² EI/L² 1b. for struts with ends pinned.2) P = 4 π² EI / L² 1b. for struts with ends fixed.

Where P = Buckling of failing load in 1b. L = Length of strut in inches. I = Moment of intertia of cross section about the neutral axis.

a) Centre of gravity of the conductor loading is in the region of 2 ft. from the pole top. This is normally the stay position.

b) In case of foundation strengths, the fulcrum point of the pole is taken as approximately 2/3 of the depth of planting.

C = Crippling load of strut in lb. L = Length of strut in ft.

C = π²EI LB L²

The moment of intertia I for a Circular section= D 4 in4

64

C = π² E 2 X π D 4 Lb.

(Lx12) 64

Safe working load = C Lb F.S.

IV) DESIGN OF ‘H’ POLES :

(A) Two types of design for ‘II’ poles.

i. ‘H’ pole (umbraced).ii. ‘H’ pole (braced)

i. ‘H’ POLE (UNBRACED) :

H = Height of pole in ft. above ground level.h = Height of centre of gravity of conductor load in ft. above ground level.P = Total horizontal conductor loading in lb Acting at height h ft. above ground level.Dg = Dimm. Of pole in inch. At ground level.Da. = Average diam. Of pole in inch.

W = Total wind load in lb on poles.H = Height above ground level in ft. at which 2 windage on pole acts.

It should be noted that the windage is taken 1.5 time the windage on one leg.

P

H

h

Total bending moment on pole = (Pxh + W H/2) lb. ft.At ground level

Max. stress for red fir. = 7800 lb/sq.in.

Max. alloable fibre stress = 7800 lb/sq. inchF. of. S

Moment of incertia for a single = I = π D 4 in4. 64

Section module = IDg/2 in3 .

Substituting value of I.

Z = π D 4 x 2 = πDg³ in³ 64 Dg. 32

Allowable fibre stress = { = BM x 12 lb/sq.in2

= BMX12X32 lb/sq.in.Dg3

Assuming that each leg of the { = BM x 12x32 lb/sq.inPole tales half the load, then 2 π Dg3

Dg = 3√(BM in lb ft. x 61.15) / f in

ii) ‘H’ POLE (BRACEL)

X bracing is generally used because it may be assumed that the two poles will defect identically and that half the total loads will be carried by each pole.The wind load on conductor is F, which is carried as F/2 on each pole at the appropriate level.The wind on two poles (due to the sheltering effect) can be taken as 1.5% mean diameter x height ; and this is allocated to the various “ pane points.”

1 Level of conductors.2 Top of brace.3 Bottom of brace.4 Ground level.

This gives the forces.

1) P (which includes F & the wind on half of panel A).2) O (the wind on half panel A + half of panel B)3) R (the wind on half panel B + half panel C + D).

Force in each cross-brace.

= + [ p (A+B+C) + Q (B+C) K R (C) ] E 2 x B

Position of point of contraflexure

D = (C+D) (3d2 – d1) (3d2 + d1)

where d1 = diameter of pole at bottom of cross-brace.

P/2

Q/2

R/2

A

B

C

D

x x

d2 = diameter of pole at ground level. D = height above ground level of point of contraflexure.

PART ‘B’ :

(1) INTRODUCTORY :The aim of this Lecture is to introduce engineers to the transmission line

tower, salient features of design, with a particular reference to loading.

Towers in general means a multi storied skeletal structure. The material to be used in construction depends upon the use to which they are put. Usually towers are made out of timber, cement, concrete, steel and aluminium.

(2) USE OF TOWERS :

1) Monumental features of town (like EIFFEL tower of paris.)2) To support micro-wave antennas, transmission line cables.3) To give signals in geodetic survey.

(3) TOWER MATERIALS :

(a) Timber:Temporary purpose when the tower to be erected, timber is the best

material. The durability of timber is largely affected by many natural factors and hence usage of timber as construction material is out dated.

(b) Concrete :Comparatively durable cost of construction is less.

Disadvantages :

(1) Height is restricted.(2) Large height

(3) Concrete cannot withstand tensile stress which are developed due to the pulling of cables.

(4) Reinforced & pre-stressed towers can not be transported conveniently.

(c) Steel :Looking to the disadvantages of concrete steel can be used as the

tower construction material. It can be erected as high as upto 200metres with comparative others. The member can be assembled at site and has less dead weight which facilitate the erection.

(4) TYPE OF STEEL TOWERS(1) Steel pole ( tubular type) (2) Self supporting fabricated steel pole (3) Flexible towers(4) Self supporting widebase towers(5) Guyed towers ( Portal and V Types)(6) Narrow based towers.

Transmission line towers are self supporting (broad) wide base type.

(5) CLASSIFICATION OF TOWERS:(1) By IS : 802 (Part – II).(2) By circuit.(3) By configuration of cable ( Horizontal & Vertical).

Configuration depends upon the following factors:

(1) Insulator assembly.(2) Minimum clearance from ground & conductor to the tower.(3) Minimum clearance under dynamic conductors.(4) Geometric distance between the conductors.(5) Economic use of the materials.

(6) NEED FOR DESIGNS:

Electric power is an important part of the Industrial Development & its economy. Before world war – II, 115 KV was considered as high voltage. This high voltage was generated near the industrial area itself, unlike today, the distance between the generating station & the industrial area where it is put to use was farely less. For this voltage to be transmitted was solely a problem of electrical engineering side which they have overcome by providing timber poles or simple poles. The recent development in electrical field had given rise up to 1100KV. In 1969, 1100KV voltage test was carried out in WALTSHILLE’S

PENNESYLVANIA in U.S.A. by EDISON Research council. The level & distance between industrial area & generation station grew faster. With this EHV several hundreds Amp. Current is also accommodated & hence necessary clearance & insulation become a vital part for electrical engineers. The necessity of high multi-storied structures to support these EHV cables becomes a problem for structural engineers. The supporting structures are designed after consideration of economy, durability & fluency of voltage transmission. The stability & safety of such multi-stored structures are checked theoretically & by proto type testing.

(7) TRANSMISSION TOWER :

The transmission towers are used to support power lines of 66KV and above.

Towers are used to reduce the number of locations from maintenance point of view (longer span) to get proper ground clearance & to have strong support to withstand wind load etc.

The tower is a balanced structure with four legs. The spans of 250 meter and above can be adopted for tower line. Towers are generally fabricated from galvanized steel sections to avoid shutdown for painting & maintenance and for costal area. They are pre-fabricated in the workshop & subsequently assembled & erected at site. Hence, degree of accuracy in design & fabrication should be very high. Entire structure is bolted (no welding or riveting is allowed).

(8) TYPES OF TOWERS:

The towers are divided into the following categories.a) Tangent tower. : Used on straight runs or with a line deviation upto 2ºb) Small Angle : Used on line deviations upto 15º Tension Tower.c) Medium Angle : Used on line deviations upto 30º Tension Tower.d) Large Angle or : Used on line deviations upto 60º and for dead end and Dead End Tower. Anchor tower on River crossing sections of the line.e) Special Tower. : Used mainly for River crossing span.

(9) BROKEN WIRE CONDITION OF TOWERS:The following broken wire conditions may be assumed in the design of

towers.a) Single Circuit : Any one power conductor broken or one ground wire

broken, whichever is more stringent for a particular member.

b) Double Circuit

Towers : i) Tangent tower : Any one power conductor broken or one ground wire

broken, whichever is more stringent for a particular member.

ii) Small angle : Any two of the power conductors broken on the same Tension tower side and on the same span or any one of the power (upto 15º) conductor and any one ground wire broken on the same iii) Medium angle : span whichever combination more stringent for

Tension tower a particular member. (upto 30º) iv) Large Angle : Three power conductors broken on the same side on the

Tension tower same span or any two of the power conductors & any (upto 60º) one ground, wire broken on the same span, whichever

combination constitutes the most stringent condition for a particular member.

v) Cross Arms. : In all types of towers the power conductor supports and ground wire supports shall be designed for the broken wire conductor also.

(10) LOADING :Load on towers are worked out with two conditions.

a) Normal condition.b) Broken wire condition.

10.1 NORMAL CONDITION :10.2

This is the condition in which all the conductors and ground wire/wires are in their position.

In this condition, the tower has three loading.a) Transverse load.b) Vertical load.c) Wind on tower frame.

10.2.1 TRANSVERSE LOAD :

This will be on the tower during normal as well as broken wire conditions. :

a) Load due to wind on wire.b) Load due to wind on insulator.c) Load due to wind on tower frame.d) Deviation component of load.

a) Load due to wind on wire is calculated by p x d x l,

Where P = wind pressure (Kg/m²).d = conductor or ground wire diameter. l = normal span.

b) Load due to wind on insulator is calculated by 50% of the area of the cylinder formed by the insulator string, i.e. (1/2) x D x n x h x p :

Where,D = diameter of insulator disc.n = No. of insulators.h = height of one insulator.P = wind pressure.

c) load due to wind pressure on tower frame is calculated by taking the area of the fabrication material (angles) facing the wind and multiplying it by wind pressure.

d) Deviation load :The tower is erected in such a way that it bisects the angle of line deviation say (D) if the conductor or ground wire tension is T, the deviation load due to horizontal component shall be 2T sin θ/2.

10.2.2 VERTICAL LOAD :

This will be worked out as under :

1) load due to conductor & eartwire. (i.e. weight span x weight).2) Load due to insulators hardware & accessories.3) Man with Tools (150 kg.).

LOAD DUE TO WIND PRESSURE ON TOWER FRAME:

The frame of the tower faces wind pressure ‘p’. The total force is worked out by taking the area of windward face of each & every angle member. For rough estimate 15 % to 20 % of the projected area (known as shadow area) is

considered at designing stage and figures are checked with actual ones after the designing is over.

The wind force on tower frame at various parts of tower is generally added to the transverse loads of conductors or groundwire.

In case of special towers having height above 40 meters, the wind forces are resolved sectionswise.

10.3 BROKEN WIRE CONDITION :

- In this condition, one or two power conductors or groundwire are presumed to be broken.

- Single circuit & Double circuit tower different “Broken wire condition” are specified in IS: 802 (part – I) 1973.

- Under this condition, an additional load known as “longitudinal load” is added to the tower loading.

LONGITUDINAL LOAD:

When a wire is broken the tension ‘T’ of the wire tries to pull the tower on the other side and hence the structure has to offer a balancing force equivalent to T x Cos (θ/2) here θ is the line deviation angle.

In case of suspension towers, however, the tension under broken wire is taken on as 50 % of normal tension ‘T’.

10.4 DEAD END TOWER In case of dead end tower the longitudinal loads of all the wires are

considered in “Normal Conditon” & the same are absent under broken wire conditions.

10.5 DESIGN STRESS :a) Tensile stresses :

The elastic limit of mild steel is 6500 kg per m² stresses in the member under normal condition should not exceed 2600 kg per m² factor of safety 2.5 is to be maintained.

b) Bending Stresses :A member subjected to bending should return to it’s original condition after a load equal to 2.5 time the maximum working load has been applied.

Safe working bending stress = Elastic limit of the material Factor of safety.

c) Shear stress:Shear stresses on gross area of bolts should not exceed 2220 kg/cm² under maximum condition.

or

5550 kg/cm² when factor of safety of 2.5 is applied.

d) Bearing Stress:Bearing stress on the gross diameter of bolts should not exceed twice the shear stress i.e. 2220 x 2 = 4440 Kg/cm².

e) Compressive Stress :Ultimate compressive stress is depend upon it’s slenderness ratio i.e.

(L/r) WhereL = Length (unsupported).r = Radius of gyration.

If the compressive stress increase, the slenderness ratio decrease, and in the case of low ratio the ultimate compressive stress approaches the elastic limit.

The limiting values of (L/r) values shall be as follows:

1) Leg members : 1502) Member carrying : 200

computed stress.3) Redundant members : 250

and those carrying nominal stresses.

4) Member carrying axial : 500tension only shall not be exceed.

10.6 DESIGN SPAN LENGTHS :

(1) Normal Span :The normal span length is the distance between standard towers on

level ground so that the required ground clearance is obtained at the maximum specified temperature. It is necessary to initially fix a normal span, in order to determine the height of the bottom conductors’ support position for the standard towers.

(2) Wind Span :The wind span length used in calculating the load on tower due to

conductor. Wind loading is taken as one half the sum of the two spans adjacent to the support.

In order to take the full advantage of towers located on elevated ground, it is usual to allow a wind span of 10 % to 15 % in excess of the normal span and this additional strength can be used in taking a small

angle of deviation on an intermediate tower, where the actual wind span is less than the actual designed wind span.

(3) Weight span :The weight span is the horizontal distance between the lowest

point of the conductors on the two spans adjacent to the tower.The effect of weight span allowance on the cost of a transmission

line tower is not excessive. Generally an allowance of last times that of the normal span is made to cover the conductor weight at hill top positions.

For design of uplift foundations, the calculation of the tensile stresses in corner legs, and also in some members of the structure, the critical condition for design is that of corresponding to the minimum weight span.

10.6.1 a) ELECTRICAL CLEARANCES :

The tower height must be such that the clearance between the lowest conductor and the Ground level in still airmeets the requirements of IS.

The lines will operate in a such a way that the clearance must be allowed between live metal & tower steel work.

The special attention is given to jumpers at section positions, as allowance must be made for the swing of the conductor due to wind.

Swing of the Suspension = Wind span Insulator weight span

It is usual to allow for a swing of 45º for both conductor and jumpers.

b) MECHANICAL STRENGTH :The mechanical design of supports must be such that under adverse

conditions of wind load i.e. or under conditions of conductor tension, or line deviation, the support clearance should not be reduced.

11) SPECIFICATIONS :Basic Data for preparing specification is derived from IS : 802 (PART –

I) 1973. However, keeping in view the importance and locations of various transmission lines are made, generally to increase the safety and to reduce the cost.

12) BASIC TECHNICAL PARTICULARS :

12.1 Technical particulars for 66kv D/C towers : a) Climate – 1. Maximum Temperature - 67º C. 2. Minimum Temperature - 0 º C. 3. Everyday Temperature – 32 º C.

a1) Wind Pressure i) For conductor – 75 kg/m² acting and ground wire on full and groundwire projected area. ii) For tower - 150 Kg/ m² acting on 1.5 times the

projected area of wind ward face.

b) Conductor : 1) Material : ACSR 2)Size and stranding : 64.52 mm² cu.equ. “DOG”.

6/4.72mm Aluminium 7/1.52mm steel.

3) Ultimate tensile : 3310 kg. Strength

4) Weight : 394 kg/km. 5) Overall diameter : 14.15 mm. 6) Area of cross : 118.00 m² section 7) Maximum working

tension at i) 32º C. with : 1505 kg.

full wind ii) Minimum temperature : 1550 kg

with 2/3 full wind – 8) Maximum sag under : 5310kg. maximum temperature

and still wind -

c) Earthwire: 1) Material : Galvanised stranded steel wire. 2)Size and stranding : 7/3.15mm (70 kg/mm²quality)

3) Weight : 427 kg/km. 4) Overall diameter : 9.45 mm. 5) Area of cross : 54.53 m² section 6) Maximum working

tension at ii) 32º C. with : 1490 kg.

full wind ii) 0º C with 2/3 full wind : 1545 kg

7) Maximum sag with : 3970kg. maximum temperature

and still wind -

d) Insulation and Insulator strings : 1) Size of Discx. – 255 x 146 mm 2) Number of Disc. – Maximum Minimum i) Suspension string 6 5 ii) Tension string 7 6 3) Insulator string : i) Electro : kg for single susp. Mechanical : kg for Double susp

Strength : kg for single tension. Kg for Double tension

ii) Length : 1170mm for single suspension.1470mm for double suspension.1310mm for single tension.1450mm for double tension.

iii) Weight 75 kg for single suspension. 150 kg for double suspension.

85 kg for single tension.170 kg for double tension.

e) Towers : 1) Number of circuit : D/C 2) Configuration : Barrel type 3) Normal span : 260 mt. 4) Weight span : Maximum – 390 meter.

Minimum – 195 meter 5) Wind span : 290 meter. 6) Shield angle : 30º

Clearances : i) Minimum ground clearance : 6000mm ii) Clearance of suspension : 915mm

string deflection upto 20º iii) Clearance of Suspension : 800mm string deflected upto 35º iv) Clearance of jumper : 915mm

deflected upto 20º v) Clearance of jumper deflected : 800mm upto 30º

Factor of Safety

Normal Condition : 2.00Broken wire condition : 1.5

f) Strength of steel & bolts : Steel (Axial Strength) In Tension : 2600 kg/m²

In compression : 2600 – (K L/r) ² 12

Where KL < 120 and r 2 x 10 7 Where KL > 120

(K L/r) ² r

Where K is constant depending upon the type of connection (specified in IS – 802).

Where: L is unsupported length r is radius of gyration.

Bolts : i) Shear on gross area of bolt : 2220 kg/cm² ii) Bearing stress on gross : 4440 kg/cm² diameter of bolt iii) Axial tension stress on the : 1980 kg/cm² root area of the Threaded bolt.

g) Foundation (Normal Type) : 1. Weight of earth : 1440 kg/m² 2. Weight of concrete : 2300 kg/m² 3. Safe bearing pressure of soil : 11.00 MT /m² 4. Factor of safety : Normal condition – 2.2

Broken wire – 1.65 5. Minimum Chmney size : 300mm 6. Angle of internal friction : 30°

13) Calculation of Tower Loadings : The loading worked out on the basis of technical date given in 12.0 and method of calculation indicated in for 30°, 66kv D/C tower are as follows, considering tension t 32°C and full wind.

Load type Ground wire condition

Normal condition

Broken wire condition

Normal condition Broken wire condition

TC MC BC TC MC BCTrans-verse 1056 589 1240 1246 1471 707 713 938Vertical (Max) 332 282 644 644 644 597 597 597Vertical (Min) 98 98 247 247 247 247 247 247Longitudinal - 1439 -- -- -- 1531 1531 1531

14) DESIGN

14.1 Tower Components : The tower structure consists of i) Leg members, ii) Lattices iii) Cross arms iv) Ground wire peak v) Redundands vi) Members are joined by lap joint, butt joint or gussets.

14.2 Design of Leg Member :

Refer sketch ‘A’. If, we want to design the leg member marked ‘j’, we have to take the moment’s of all the transverse load on the towers at the centre of lattice connecting these sections which is 4.287 meters in width.

Normal condition Broken wire conditionG.W. 1056 x 17.974 = 18981 (589 + 1439) x 17.974 = 36451T.C. 2 x 1240 x 13.974 = 34655 (707+1240+1531)x13.974 = 48602M.C. 2 x 1246 x 11.274 = 28095 (2 x 1246 x 11.274) = 28095B.C 2 x 1471 x 8.74 = 25225 (2 x 1471 x 8.574) = 25225 ------------ --Total … …. 106956 Total … …. 138373

Stress (T & L) = 106956 138373 2 x 4.287 2 x 4.287

(width) = 12474 = 16139

Unbalance Vertical load. (644-597) x 2.3 = 13 2 x 4.287

Vertical load. 644 x 6 + 332 = 1049 597 x 6 + 282 = 1025 4 4

Tower Weight. = 600 = 600 Total Comp. 12474 + 1049 + 600 = 14123 17777

Total Tension 12474-1049-600 = 10825 17777-1025-600 = 14514

Section proposed 90 x 90 x 8

Properties of Section Y = 1.75 cm.

Gross Area. = 13.79 cm².

Net area after deducting = 11.0 cm².Hole for bolt. The unsupported length of this leg member

Is 122.9 cm. L = 122.9 = 70 r 1.75

This is less than 120 hence ultimate compressive strength can be worked out from 2600 – (K L/r)²/12 x Gross area. = 2600 – (70)²/12 x 13.79

KL/r = L/r for leg member as per IS (802). = 30228 kg.Ultimate tensile strength = 2600 x 11 = 28600 kg.

Check for factor of safety :

N.C B.W.C

Compression 30228 = 2.14 30228 = 1.7

14123 17777

Tension 28600 = 2.64 28600 = 1.97 10825 14514

It is seen that F.S is more than required i.e. 2 in N.C. and 1.5 in B.W.C. Hence, the section is safe.

14.3 Design of Lattice :

Under stressed conditions, the forces in lattice form a couple which is counter balanced by reaction of the member. Tension & Compressions are equal.

For example in case of lattice a (Ref. sketch ‘A’) the total force due to ground wire shall be resisted by members of the lattice.

c Ac = √ ab² + bc²

= √ 0.5² + .65² 650 = 0.820 820 cosά = 500 = 0.6097 α 820

a b 4 x W X cosά = 4 x 1 x 0.6097 ( width) 500 = 2.44

N.C B.W.C

(a) Force = T.R load x width = T.R load x width4W cosά 4W cosά

= 1056 x 1 = 589 x 12.44 2.44

= 433 = 241

(b) Longitudinal Force = 1545 x cos 30 o /2 x width 4W cosά = 1545 x 0.9659 x 1

2.44 = 613

The proposed section is 40x30x5.

14.4 Design of cross-arms :Top cross arm design is worked out for illustration as under :-

L = √ ((2.3-0.5)2 + (0.5)2) = 1.868 L = √ ((1.868)2 + (0.65)2) = 1.977

DESIGN : 1) Lower Member: N.C B.W.C Transverse stress 1240x1.868 = 643 707 x1.860 = 367

2x1.8 2x1.8 Stress Longitudinal _____ 1531x1.868 = 2059

2x0.5 Stress Vertical 644x1.868 = 925 597x1.868 = 858

2x0.5 2x0.5_________ ________

Total 1568 4084

2) Upper Member: Stress Vertical 644x1.977 = 979 597x1.977= 908

2x0.65 2x0.65

14.5 DESIGN OF FOUNDATION : (a) Normal Soil:

Foundation of a tower leg with frustum earth condition G.W. & T.C. Broken.(a) Effect of transverse load :

N.C B.W.CG.W. 1058 x 26.41 = 27942 ( 1.39+589) x 26.41 = 53559T.C. 2x1240x22.41 = 55577 ( 1240+707+1531)x22.41 = 77942M.C. 2x1246x19.71 = 49117 2492 x 19.71 = 49117B.C. 2x1471x17.01 = 50043 2942 x 17.01 = 50043

Wind load at 760x6 = 4560 760x6 = 45603mt. Ext.

Wind load at 447x3 = 4560 447x3 = 45606mt. Ext. ______ _______

Total 188580 236562

Stress = Load/2W Stress = Load/2W = 188530/2x7.023 = 236562/2x7.023

= 13422 = 16842

(b) Vertical load N.C B.W.C= 644 x 6+ 332 = 644 x 5 + 597 + 282

4 4 = 1049 = 1025

Min : 247 x 6+98 4

= 395

(c) Self weight of tower N.C B.W.C 800 800

(d) Overload due to concrete ( taking place of soil ).

( 2300 – 1440 )(wt. of concrete) – ( wt. of each) x {Total concrete volume (of pad, pyramid, chimney)} = 700 kg.

Total Thrust N.C. B.W.C

Transverse load 13422 16842Vertical load. 1049 (395) 1025(395)Self wt. of tower. 800 800Overload due to concrete 700 700 ________ _________

15971 19387

Total uplift. 13422-800-395 16842-800-395 = 12227 = 15647

a) Thurst Bearing Area (M ² ) :1.35² = 1.825m²

Bearing Strength = Bearing pressure x area = 11000 x 1.825 = 20075 Kg.

Ultimate Load (Kg.)Bearing strength x F.S. (N.C.)

= (11000 x 1.825) x 2.2= 44104 Kg.

Factor of SafetyN.C. B.W.C.

= Ultimate Load = 44104 44104 Total thrust 15971 19367

= 2.76 2.20

b) Uplift :

Weight of earth frustrum (Kg.).B= 2.45 tan 30A= 2xB + 1.350 = 4.179 meter.

Earth Volume = H ( a² + b² + ab) = 2.45 [ (1.35)² + (4.179)² + 1.35 x 4.179] = 20.358

Weight of Earth frustrum = Earth volume x Wt. of earth = 20.358 x 1440

= 29315 Kg.

Additional Wt. of concrete = 700 Kg.

Total Anchorage = 29315 + 700

= 30015 Kg.

Factor of Safety : N.C. B.W.C.

= Total Anchorage = 30015 30015 Total uplift 12227 15347

= 2.45 1.92

15) Type of Soil :

1) Cohisive2) Non Cohisive

16) Genearl Classification : Angle of repose

1) Normal Soil 30º2) Submerges soil 15º3) Black Cotton soil 0º4) Hard rock soil --5) Soft rock soil ---

17) Type of Foundation :

1) Normal soil foundation2) Hard rock & soft rock foundation.3) Black Cotton soil foundation.4) Wet foundation.5) Pile foundation.6) Special tower foundation.

18) Foundation Cost :Cost of foundation usually 40% of total transmission project cost.

19) Other type of foundations

The route of the transmission line is usually encountered with different types of soil. Where normal yellow soil is encountered the design indicated in the 14.5 is sufficient to keep the tower in position. However, if the soils like black cotton, soft rock, hard rock or wet are encountered the foundation designs have to be different the description in short is given below.

1) Black Cotton Soil :This is soil which has bearing for direct pressure and direct thrust

equal to 40% of the normal soil but in the uplift the angle of repose is taken zero and hence the foundation volume increases or otherwise reinforcement has to be provided.

2) Soft Rock and Hard Rock : This type of soil is having very high bearing capacity/ and hence the

foundation depth is restricted some times to 1.5 meters only against the normal depth of 3 meters. Besides to take care of the uplift mass concrete is adopted i.e., excavation concrete volumes are equal. In this case since the foundation is shallow, the stub is required to be cut to suit the foundation depth. Figure 8.8 give an idea of such foundation.(4 to 8 times that of normal soil)

3) Wet Foundation :

This type of foundation is adopted whenever wet or submerged locations are encountered or otherwise at the locations which are likely to be submerged along River or along ponds and irrigation projects. In this type of soil R.C.C. foundations are used and they are generally in the form of steps of 250 to 300mm. Needless to emphasis that foundations have to be self supporting under worst condition to take care off all the moments of the tower due to all types of loads.

20) DESIGNH OF 400KV TOWER :

20.1 The outline diagram of 400kv tower is given in sketch. 20-A.

Since this tower is not symmetrical above waist level, the stress analysis by analytical method becomes extremely difficult and hence to analysis stresses on tower members, two separate methods are adopted. From ground wire peak to waist level the analysis is done by graphical method and from waist level to ground level analysis is done by the usual method indicated in 14.0 above.

20.2 Graphical Method :

In this method the load diagram draw separately for each type of load under each condition.

20.3 Load Diagram for Normal Condition :

Under the normal condition 3 separated load diagram for vertical load, transverse load and wind load are required to be drawn.

20.4 Load Diagram for Broken wire condition :

Under this category, the vertical, transverse and longitudinal loads under broken wire condition are indicated on load diagrams. The wind diagram indicated in 20.3 above holds good for this condition.

20.5 Specimen load diagram :

The figure 20 indicates vertical loads on the tower and the reactions under normal condition. Figure 20d indicate the transverse load on the tower left conductor broken condition including the reactions.

20.6 Stress Diagram:

Corresponding to Figures 20b and 20d are the two stress diagram given in figure 20b and 20d.

20.7 Working out stresses :

Once all the stress diagrams are drawn i.e., stress diagram for vertical, transverse and longitudinal loads under normal condition and under broken wire condition (i.e. ground wire broken) or each and individual member under normal condition and broken wire condition are tabulated and totaled

separately and whichever combination makes highest stress for a particular member are adopted.

It is needless to mention that stresses of member due to different stress diagrams is nothing but the stress diagram corresponding to each member, multiplied by scale of the stress diagram.

20.8 Once the total stress for a member is worked out the remaining design work is carried out as indicated in 14.2 i.e., selecting angle section and No. of bolts that will stand to the total stress.

20.9 Design of Cross-Arms : This also can be done by analytical method, if required, as indicated in

14.4 However, the method indicated in 20.8 above will also help in evaluating the member stresses for cross-arm.

20.10 Design of boom :The elevation members of boom in elevation are covered in 20.8

However, the top plan and bottom plan of boom is to be designed keeping in view various loads acting on them. Hence load diagram for the top plan and bottom plan of the boom are also separately prepared and stress diagram for them also prepared to evaluate stresses on members. Figure 20f & 20g indicate the bottom plan of boom and corresponding stress diagram under left wire broken condition.

20.11 Design of members below waist level : As already indicated earlier the members below waist level i.e., leg members and lattices are evaluated as indicated in 14.2 & 14.3.

20.12 Design of foundation : Design of foundation is also identically evaluated as discussed in 14.5 and 19.

21) TESTING OF TOWER : 21.1 General :

The tower to be tested is erected in test bed with adjustable jaws substituting for stubs. The tower is surrounded by tower mast on three sides & on the front side there is elevated observation cabin. The arrangement for testing is as per the rigging arrangement shown in sketch. Transverse and longitudinal loads are applied by means of wires supported by tower masts and pulled by electric motors installed in the winch house. The motors are controlled from the observation cabin. Load cells are fastened in the stretching wires for measuring the applied load. The vertical loads are applied by 20 kg. dead weight in crates attached to cross arms by wire.

The applicable loads for each case are worked out depending upon the rigging arrangement.21.2 Design load Testing :

The design loads multiplied by the relevant factor of safety (2.0 in 11.c. and 1.5 in B.W.C.) are applied to each loading point and it is seen whether tower stands or not. In case of failure, the critical member is replaced by higher section.

21.3 Destruction Test :This test is carried out taking the H.O. loading starting with loads corresponding to F.S. 2.0. The loads are gradually increased in steps of F.S. of 0-.25 (i.e., F.S. 2.0, 2.25, 2.5) till the tower fails. This tests helps in knowing the maximum strength of the tower.

21.4 Test Beds : Testing beds with the modern testing facilities are situated at

(a) Jaipur and Bombay owned by M/s. Kamani Engg. Corpn. Ltd.

(b) Bangalore owned by C.P.R.I.(c) Jabalpur owned by S.A.E.

22) CONCLUSION :

The entire chapter discussed above gives detailed account of transmission line supports in general & elaborate details of transmission towers including that of the highest voltage viz. 400 KV which is being taken up by G.E.B. and will therefore give an idea to Engineers regarding the loadings, testing & capacities of the transmission towers being utilized by G.E.B.