ANALYSIS AND DESIGN OF FOUR LEGGED STEEL TRANSMISSION TOWER WITH DIFFERENT BRACING SYSTEMS

e-ISSN: 2582-5208 International Research Journal of Modernization in Engineering Technology and Science Volume:02/Issue:09/September-2020

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ANALYSIS AND DESIGN OF FOUR LEGGED STEEL TRANSMISSION TOWER WITH DIFFERENT BRACING SYSTEMS T. Sunil Kumar*1, T. Gireesh*2, U. Suneel Kumar*3, S. Krishna Chaitanya*4, Sri. K. Venkateswara Rao*5 *1,2,3,4Student, *5Associate

Department of Civil Engineering, Gudlavalleru Engineering College, India

Professor, Department of Civil Engineering, Gudlavalleru Engineering College, India.

ABSTRACT Transmission costs of electricity can be made more economical and effective by using light weight configuration of transmission line tower. Four legged transmission tower are most commonly used as transmission line tower. In this study an attempt is made to design, analyze and compare four legged model with different bracing system using common parameters such as constant height. This study includes designing the tower as 132KV single circuit transmission line tower by considering load conditions and wind forces as per IS: 802(1995), IS: 800(2007), IS: 875(1987).Design parameters such as basic wind speed, influence of height above ground and terrain, design wind speed, design wind pressure, design wind force will be considered according to the above mentioned codes. This work is focused on selecting the most effective and economical section by conducting comparative study of both X and K bracing system towers.

KEYWORDS: Transmission towers, Geometry of tower, Self-supporting tower, lateral loads, axial force, deflection, design.

I.

INTRODUCTION

The requirement of electricity uses has continued to grow in every country, the amount of requirement being bigger in the developing countries. The transmission line towers are considered one of maximum important life-line structures that help in transmitting electric powered energy. The Transmission towers are essential for the cause of providing electricity to diverse areas of the nation. In present situation, there may be growth in building of power stations and consequent increase in energy transmission traces from the producing stations to the distinct corners. Interconnections between structures also are growing to enhance reliability and financial system. Transmission line should be solid and punctiliously designed so that they do now not fail all through herbal catastrophe and should agree to the countrywide and global popular. The planning and designing of a transmission line encompass some of requirements of both structural and electric. From the electrical point of view, the most important requirement is insulation and safe clearances of the strength sporting conductors from the ground. The cross-segment of conductors, the spacing between conductors, and the area of ground wires with appreciate to the conductors will decide the design of towers and foundations. Transmission line is an incorporated device inclusive of conductor subsystem, ground cord subsystem and one subsystem for each category of guide structure. Mechanical supports of transmission line represent a considerable portion of the price of the road and that they play a crucial position in the reliable power transmission. They are designed and built in huge form of shapes, types, sizes, configurations and materials. The supporting shape types utilized in transmission lines normally fall into one of the three categories: lattice, pole and guyed. The supports of EHV transmission traces are usually steel lattice towers. The value of towers constitutes approximately sector to half of transmission lines and for this reason ultimate tower design will bring in substantial savings.

II.

TRANSMISSION TOWER COMPONENTS

The following parameters for transmission line and its components are assumed from I.S. 802: Part 1: Sec: 1:1995, I.S. 5613: Part 2: Sec: 1:1989.

1.

Voltage of transmission = 132 kV

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Power conductor

30 mm diameter A.C.S.R (aluminium cable steel reinforced conductors (Consisting of 54 strands of 3 mm diameter of aluminium and 7 strands of 3 mm diameter of steel) shall be used.

Unit weight of conductor = 16.76N/m (0.01676kN/m) Permissible axial tension =35.60kN Young’s modulus of elasticity =0.842 x 105 N/mm2 Coefficient of expansion =0.00001992/℃ Shape factor for conductor = 0.67 3.

Ground wire

10mm diameter galvanized steel wire shall be used Permissible axial tension = 25.40kN 4.

Variation of temperature

Range = 5 to 60ᵒc 5.

Wind

Uniform intensity of wind =1.50kN\m2 6.

Snow

Snowfall is not expected Tangent of the tower with not more than 2: line deviation shall be erected, Weight span of tower = 240 m

III.

SAG TENSION FOR CONDUCTOR AND GROUND WIRE

w2 =16.76N/m.

T1 = Permissible tension in the conductor = 35.60kN T2 = Tension in the cable at mid-span, A = Effective cross-sectional area (It is calculated on basis of net area of each stand) A = (54 + 7) π/4 x 32 =431.8 mm2 (t2 - t1) = Variation of temperature = (60-5) = 55ᵒc E = 0.842 x 105 N/mm Since, the wind gusts are not likely to cover complete span, and the swinging of the conductors continues, the intensity of wind is decreased to 75 percent.

Wind load = (0.75 x 1.50) x 0.667 x 0.03 kN/m w1 = 0.0225kN/m Weight of the conductor at minimum temperature with wind, w2 = [0.02252 + 0.016762]1/2 = 0.02806 kN/m Horizontal component of wire pull, p T22 [(T2-T1) +w12.L2EA/24T12+ (t2-t1) α. EA] = (w12.L2EA/24) PH = T2 = 17.918kN Maximum sag of the conductor at mid span d= (wL2/8PH) = (0.01676×240×240/8×17.918) = 6.7347m

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IV.

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HEIGHT OF THE TOWER

Vertical height of conductor above ground = 6.7 m Vertical spacing between power conductors = 4 m Height of the ground wire above top-most Power conductor = 3.12 m Maximum sag for power conductor = 6.7347 m Total height of the tower = 21 m.

V.

LATERAL LOADS

1.

Normal operating condition

2.

Top most power conductor in broken wire condition.

3.

Ground wire in broken condition.

Table 1: Lateral forces resisted by tower for X bracing Under the Condition

Lateral Force

1(kN)

2(kN)

PH

0.5×4.33

PG

Table 2: Lateral forces resisted by tower for K bracing Under the Condition

3(kN)

Lateral Force

1(kN)

2(kN)

3(kN)

0.5×4.33

0.5×3.610

PJ

0.5×4.330

0.5×4.33

0.5×3.61

0.5×11.05

0.5×8.89

0.5×11.05

PI

0.5×11.05

0.5×8.89

0.5×11.05

PF

0.5×11.05

0.5×11.05

0.5×11.05

PH

0.5×11.05

0.5×11.05

0.5×11.05

PE

0.5×10.61

0.5×10.61

0.5×10.61

PG

0.5×10.41

0.5×10.41

0.5×11.05

PD

0.5×2.27

0.5×12.27

0.5×2.27

PF

0.5×1.85

0.5×1.85

0.5×11.05

PC

0.5×4.67

0.50×4.67

0.5×4.67

PE

0.5×2.18

0.5×2.18

0.5×10.61

PB

0.5×7.10

0.5×7.10

0.5×7.10

PD

0.5×2.72

0.5×2.72

0.5×2.27

PC

0.5×3.18

0.5×3.18

0.5×4.67

PB

0.5×3.16

0.5×3.16

0.5×7.10

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VI.

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AXIAL FORCE AND DEFLECTION OF THE MEMBERS OF THE TOWER:

Fig-3: Axial force diagrams for x bracing tower

Fig 4 Deflection diagram for X-bracing tower

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Fig 5 Axial force diagram for k bracing tower

Fig 6 Deflection diagrams for K bracing tower Table-3: Axial Force For X Bracing Tower MAXIMUM AXIAL FORCE HEIGHT

MINIMUM AXIAL FORCE

AXIAL FORCE

HEIGHT

AXIAL FORCE

(m)

LOAD CONDITION

KN

(m)

LOAD CONDITION

KN

6

1

50.62

7.6526

1

-49.481

6

2

55.735

7.6526

2

-55.358

6

3

50.058

7.6526

3

-48.966

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60 40 20 Maximum Minimum

0 -20

Condition 1

Condition 2

Condition 3

-40 -60 Fig-7: Bar Chart of Axial forces in X-Bracing Table-4: Axial Force For K Bracing Tower MAXIMUM AXIAL FORCE HEIGHT

MINIMUM AXIAL FORCE

AXIAL FORCE

HEIGHT

AXIAL FORCE

(m)

LOAD CONDITION

KN

(m)

LOAD CONDITION

KN

3.023

1

75.607

3.023

1

-75.555

3.023

2

71.607

3.023

2

-71.559

3.023

3

74.9

3.023

3

-74.849

70

20

-30

Maximum Condition 1

Condition 2

Condition 3

Minimum

-80 Fig 8 Bar Chart of Axial forces in K-Bracing TABLE 5 Displacements For X Bracing

TABLE 6 Displacements For K Bracing

MAXIMUM DISPLACEMENTS HEIGHT

MAXIMUM DISPLACEMENTS

DISPLACEMENT

HEIGHT

(mm)

(m)

LOAD CONDITION

DISPLACEMENT

(m)

LOAD CONDITION

21

1

23.786

21

1

35.772

21

2

23.877

21

2

33.034

21

3

23.373

21

3

35.22

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(mm)

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VII.

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DESIGN OF MEMBERS OF THE TOWER

Critical section of X-bracing tower Design charts tension member The charts have been prepared based on IS 800:2007 for tension members. The procedure is shown below. Assumed material properties: fy = 250MPa, fu = 400MPa, fub = 410MPa Design chart for critical section ISA 120×120× 8 Tension strength of single Angle ISA 120×120× 8(As per IS 800:2007) with single row bolted connection as shown in figure The no. of bolts considered for the design of tension members for end connections based on minimum no. of bolts required for the full strength of the angle of block shear.

Fig 10: Design Details of leg member (All Dimensions are in mm).

Design strength due to yielding of gross section Tdg = fy Ag/ γmo Ag= 1856 mm2, γm0 = 1.1 Tdg= 250×1856/1.1 = 421.8 kN Design Strength due to rupture of critical section e = 30 mm, p = 70 mm Tdn= 0.9fu Anc /γml + βAgo fy /γm0 Anc = (120 – 18 – 8/2) ×8 = 784 mm2 Ago = (120 – 8/2) ×8 = 928 mm2 β = 1.4 – 0.076 (w/t) (fu/fy) (bs/Lc) ≤ (fu γm0 / fy γm1) ≥ 0.7 Lc = 70×2 = 140, bs = 120+30-8 = 142 β = 1.4 – 0.076(120/8) (250/410) ((142)/140) =0.7 0.7< (fu γm0 / fyγm1) = (410×1.1)/ (250×1.25) =1.44 1.44≥0.7≥0.7 Therefore, Tdn= (0.9×784×410)/1.25 + (0.7×928×250)/1.1 = 379.07 kN Design strength due to block shear The block shear strength Tdb, at an end connection is taken as the smaller of Tdb1 =

( Avgfy /(√3γm0) + 0.9fuAtn /γm1 ) or,

Tdb2 =

(0.9fuAvn /(√3γm1) + fyAtg /γm0 )

Avg = (30+70×2) ×8 = 1360 mm2 Avn = (30+70×2-18×2.5) ×8 = 1000 mm2 Atg = (90×8) = 720 mm2 Atn = 720-(9×8)= 648 mm2 Tdb1 = ((1360×250)/ (√3×1.1)) + ((0.9×648×410)/1.25) = 369.7 kN or, www.irjmets.com

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Tdb2 = ((0.9×1000×410)/ (√3×1.25)) + ((720×250)/1.1) = 334.7 kN Therefore, the block shear strength is Tdb = 334.7 kN Now, Strength of the single angle Tension member should be least of the above three values (i.e. 421.81 kN, 379.07 kN and 334.7 kN) which is equal to 334.7 kN. As per our calculation we get that the maximum tension force is in the leg member of the ground panel which is 55.735 kN i.e. factored load = 55.735×1.5 = 83.60 kN is lesser than the above three values. Therefore our design is safe for maximum tension.

Critical section of K bracing tower Design charts tension member The charts have been prepared based on IS 800:2007 for tension members. The procedure is shown below. Assumed material properties: fy = 250MPa, fu = 400MPa, fub = 410MPa Design chart for critical section ISA 125×95× 8 (X Bracing) Tension strength of single Angle ISA 125×95× 8(As per IS 800:2007) with single row bolted connection as shown in figure The no. of bolts considered for the design of tension members for end connections based on minimum no. of bolts required for the full strength of the angle of block shear.

Fig 11: Design Details of leg member (All Dimensions are in mm).

Design strength due to yielding of gross section Tdg = fy Ag/ γmo Ag= 1698 mm2, γm0 = 1.1 Tdg= 250×1698/1.1 = 385.9 kN Design Strength due to rupture of critical section e = 30 mm, p = 70 mm Tdn= 0.9fu Anc /γml + βAgo fy /γm0 Anc = (125 – 18 – 8/2) ×8 = 824 mm2 Ago = (95 – 8/2) ×25 = 728 mm2 β = 1.4 – 0.076 (w/t) (fu/fy) (bs/Lc) ≤ (fu γm0 / fy γm1) ≥ 0.7 Lc = 70×2 =140, bs = 95+30-8 = 117 β = 1.4 – 0.076(95/8) (250/410) ((117)/140) =0.94 0.7< (fu γm0 / fy γm1) = (410×1.1)/ (250×1.25) =1.44 1.44≥0.94>0.7 Therefore, Tdn= (0.9×824×410)/1.25 + (0.94×728×250)/1.1 = 398.77 kN www.irjmets.com

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Design strength due to block shear The block shear strength Tdb, at an end connection is taken as the smaller of Tdb1 =

( Avgfy /(√3γm0) + 0.9fuAtn /γm1 ) or,

Tdb2 =

(0.9fuAvn /(√3γm1) + fyAtg /γm0 )

Avg = (30+70×2) ×8 = 1360 mm2 Avn = (30+70×2-18×2.5) ×8 = 1000 mm2 Atg = (95×8) = 760 mm2 Atn = 760-(9×8)= 688 mm2 Tdb1 = ((1360×250)/ (√3×1.1)) + ((0.9×688×410)/1.25) = 381.55kN or Tdb2 = ((0.9×1000×410)/ (√3×1.25)) + ((760×250)/1.1) = 343.16 kN Therefore, the block shear strength is Tdb = 343.16 kN Now, Strength of the single angle Tension member should be least of the above three values (i.e. 385.9 kN, 398.77 kN and 343.16 kN) which is equal to 343.16 kN. As per our calculation we get that the maximum tension force is in the member of the given panel which is 75.605 kN i.e. factored load = 75.607×1.5 = 117.41 kN is lesser than the above three values. Therefore our design is safe for maximum tension.

VIII.

CONCLUSION

As all the towers are analyzed and designed, the following conclusions are made: The steel weight for the four legged tower with K bracing is found to be 28.16 kN, less than that of the tower with X bracing which is 55.26 kN. K Bracing system shows a saving of 49.04% steel by weight when compared to X Bracing Tower. The tower with X bracing is found to have lesser amount of displacement throughout the height of the tower as compared with K bracing tower. The tower with X bracing is found to have lesser amount of axial force throughout the height of the tower as compared with K bracing tower. Axial load at critical section in manual calculation is 60.37 and in STAAD analysis is 55.736 is differs by 7.98% for X bracing tower. Axial load at critical section in manual calculation is 68.85 and in STAAD analysis is 70.60 is differs by 9.34% for K bracing tower. From the whole analysis it is concluded that tower with X bracing is more effective when compared with K bracing.

IX.

REFERENCES

[1]

Archana R, Aswanthy S Kumar “Analysis and Design of Four Legged Transmission Tower” International journal of science and research, Volume 5, Issue 7, July 2016.

[2]

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[3]

D.B.Sonowal, J.D.Bharali “Analysis and Design of 220 kV Transmission Line Tower(A conventional method of analysis and Indian Code based Design)” IOSR Journal of Mechanical and Civil Engineering, Department of Civil Engineering Tezpur University, Napaam 784028, Assam, India, eISSN : 2278-1684, p-ISSN : 2320–334X.

[4]

Shivam Panwar et. al. (2016)”Structural Analysis and Design of Steel Transmission Tower in Wind Zones II and IV- A Comparative Study” Department of Civil Engineering, Amity University, Uttar Pradesh, Noida , India , Volume 4, Issue 5, ISSN 2349-4476

[5]

IS 802 part 1 sec 11995 code of practice for use of structural steel in overhead transmission line towers.

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