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TRANSMISSION LINE TOWER DESIGN

DIFFERENT STAGES OF THE TRANSMISSION LINE PROJECTEngineering Activities-Line Design.Tower Designs.Foundation Designs.Stringing Charts preparations.Tower Testing.*Procurement / Manufacturing Activities Procurement of bought out items such as conductors, Ground Wire, Insulators, Fittings etc.Fabrication of Towers & Galvanization.Shipping of all material to site.Site Activities Survey works.Tower spotting.Access Roads & Platform works.Soil Investigation.Foundations.Tower Erection.Stringing.Testing.Handover

*TowersSelf - Supporting

Lattice Structure

Members made of Angles, Plates (Tubular sections are also used)

Bolted connections

Types of Tower (Based on kV)HVAC TOWERS33kV Tower66kV Tower132kV Tower220kV Tower400kV Tower765kV Tower

HVDC TOWERS500kV Tower1200kV Tower*

Tower DesignTypes of Towers --Classification according to number of circuits Single Circuit , Double Circuit & Multi Circuit Towers.Classification according to function Suspension Tower, Angle Tower & Dead End Tower.(35 to 39)Tower Configuration (Geometry) is governed by following --Length of insulatorElectrical clearancesShield angle (Protection of conductors from lightening)Ground clearance*

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*Towers

Design - Material

Mild Steel High Tensile Steel Super High Tensile Steel

*TowersDesign - Span Lengths

Basic or Normal SpanRuling or Equivalent SpanAverage SpanWind SpanWeight Span

*TowersDesign - Span Lengths

Basic or Normal Span

The most economic span for which the line is designed over level ground giving the necessary ground clearance at maximum temperature

*TowersDesign - Span Lengths

Ruling or Equivalent Span The assumed designed span that will produce between dead ends the best average tension throughout a line of varying span & climatic conditions Stringing charts are for these value of span in each section

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*TowersDesign - Span Lengths

Average Span

Mean Span Sag Tension Calculation based on this

*TowersDesign - Span Lengths

Wind Span Sum of half the span on the two sides of a tower Usually 10 - 15% higher in excess of the normal span to cater to slight increase in angle of deviation for optimizing

*TowersDesign - Span Lengths

Weight Span

Horizontal distance between the null points on two adjacent spans

*TowersProfile and Conductor Loads - Sagtension calculation

Parabolic Formula

Catenary Formula

*TowersSag Tension Calculations :Parameter for Calculation

Conductor / Ground Wire details Diameter Youngs Modulus Coefficient of Thermal Expansion Weight per meter Factors of safety for various climatic condition Cross sectional area

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*TowersSag Tension Calculations :Parameter for Calculation

Climatic Details Temperature & Corresponding wind pressure Temperature of Conductor /Groundwire at particular ambient temperature

Other Details Parallel factor for Ground wire

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*TowersOutline Tower Height - Governed by clearance, sag, insulator lengths

Cross- Arm spread - Governed by clearance,type of insulator string assembly - I or V

Tower widths at base & waist

*TowersLoad Transfer from conductor points to foundation

Cross Arm tips Legs & Lattice Foundation

Different Lattice Patterns Lead to different distribution for loads.

Most efficient pattern to be chosen.

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*Towers

Design Philosophy

Changed from Deterministic Method to Probabilistic concept / Reliability based method in line with IEC :826

*Towers

Design Parameters :

Service Conditions

Reliability Climatic loads under normal conditionsSafety Loads under maintenanceSecurity Failure containment loads under broken wire condition

*TowersDesign - Loads - Broken wire Condition

Single Circuit Tower - Anyone phase orground wire

Multiple Circuit Tower Suspension Towers - Anyone phase or ground wire Small and Medium Angle Tower - Any two phases or one phase and a ground wire on the same side and span Large Angle / Dead End Tower - Any three phases or two any phases and a ground wire on the same side and span

*TowersDesign Parameters : Climatic Conditions

WindTemperatureSeismic IntensityIce Formation

*TowersDesign Parameters - Loadings

Conductor Loads - Deviation , Wind, Broken WireWind on TowerSeismic LoadsSelf Weight of Tower, InsulatorsErection / Stringing LoadsTemperatureIsokeran LevelIce Formation

*TowersDesign - Loads - WindBasic Wind Speed - Gathered from meteological station and studiesReference Wind Speed - Extreme wind speed over an average period of 10 minutesDesign Wind Speed - Includes the following factors Risk Coefficient Terrain Roughness Coefficient

*TowersDesign - Loads - Wind

Basic Wind Speed Reference Wind Speed (Vb) (VR)

Design Wind Speed Design Wind Pressure (Vd) (Pd)

*TowersDesign - Loads - Wind Load on Tower

For a panel, the wind load in Newton at the C. G.of the panel = Pd Cdt Ae GT Where, Cdt - Drag coefficient - depends on solidity ratio Ac - Total net surface area of legs and bracings of the panel GT - Gust response factor - depends on terrain roughness & height of panel above ground

*Towers

Design - Seismic Consideration Not Significant in a majority of cases Reasons Light Structure - inertial forces are less Flexible & free to vibrate The critical condition of maximum wind occurs rarely simultaneously with earthquake

*Towers

Design Parameters :

Nature of Loads

TransverseVerticalLongitudinal

*Towers

Design - Analysis

Graphical Method - 2 - D Analysis

Analytical Method (manual calculations) - 2 - D Analysis

Stiffness Matrix Method - Computer Aided - 3 -D Analysis

Tower DesignLoading on tower (Refer IEC 826) --Transverse Loads (X Direction) Transverse Wind on Tower members, Wind on Conductor, Earth wire, Insulators.Transverse component of conductor & Earth wire tension.Wind pressure calculation is based on Basic wind speed, Gust factor, shape factor & wind zones, characteristics of the area.(78)Longitudinal Loads (Z Direction) --Longitudinal Wind on Tower members, Wind on Conductor, Earth wire, Insulators.Longitudinal component of conductor & Earth wire tension. (Usually in broken wire condition).

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Tower DesignLoading on tower --Vertical Loads (Y Direction) Self weight of Tower, Insulators, Conductor & Earth wire.Sag Tension Calculations Conductor and Earth wire are cable structures.Tension in conductor is inversely proportional to sag.Sag and Tension are calculated based on --Basic span.Conductor properties such as cross section area, unit weight, modulus of elasticity, coefficient of linear expansion, ultimate breaking strength.Wind loading on conductor.Factor of safety.*

Tower DesignLoading conditions --Normal Condition All conductors and Earth wire are intact. The tower will be in this condition for most of design life span.Broken wire condition Unbalanced loading resulting the breakage of specified number of conductor or/ and Earth wireConstruction & Maintenance loadsAnti cascading loadsSeismic loads are considered as per Equivalent static approach (Load = Horizontal seismic coefficient x weight of structure).Factor of Safety FOS = 2.0 For Normal condition loads FOS = 1.5 For Other Loading conditions.However as per IEC 826, Loads are multiplied by load factors and strength of tower is multiplied by strength factors.

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Tower DesignAnalysis --Transmission line tower consists of linear structural members connected to one another by bolts.For purpose of analysis, the tower is idealized as Space Truss.Assumptions Influence of gusseted connection in transmitting moments is neglected.Leg members which are continuous are assumed to be hinged at joints.Loads are acting only at joints.Above assumptions lead to a conservative estimate of forces and deflections.Limits for deflections --Although there is no specific code binding on deflections, it is controlled within L / 100.Deflections are so small that generally first order analysis give satisfactory results.*

Tower DesignTower Design --All members are checked for axial compression and tension loas.Check for Compression load The capacity of member is calculated based on Cross section area of member, slenderness ratio, end restriction and material grade.Check for Tension load The capacity of member is calculated based on Net Cross section area of member after deduction of hole area and material grade.Capacity of bolts against shearing and bearing Capacity of joint is calculated based on dia of bolts, grade of bolts, planes of shearing, connected member / gusset thickness and member/gusset grade.*

*TowersDesign Assumptions

Analysis by 3 D Stiffness matrix method

Pin jointed structure : axially loaded members

Fixed support of Rigid Foundation

*Towers Design - General Stipulation - Member Dimensioning

Minimum Flange width vis--vis bolt diameterMinimum ThicknessLimits of L/ R ratio

*TowersDesign - General stipulations - Member dimensioning

Minimum Flange width vis- -vis bolt diameter

BOLT DIA FLANGE WIDTH 12 mm 40 mm 16 mm 45 mm 20 mm 50 mm 24 mm 60 mm

*TowersDesign - General stipulations - Member dimensioningMinimum Thickness

a) Leg members : 5 mmb) Ground wire peak and External members of Horn Peak : 5 mmc) Lower members of Cross - Arm : 5 mmd)Upper members of Cross- Arm : 4 mme) Bracings & Inner members of Horn peak : 4 mmf) Other members : 4 mm

*TowersDesign - Bracings - L/R

L/R ratios for bracings depend on the

Hip bracing patterns

The length of equal or unequal parts into which the bracings are divided

*TowersDesign - General stipulations - Member dimensioning

Limits of L/R ratio

-Leg members, G. W. Peak, and X-arm = 120 lower member- Bracings = 200-Redundants/Secondary Members = 250 carrying members- Tension members = 375

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*Towers

Design - Compression Members

Dictated to by buckling consideration Affected by

Concentric or Eccentric load transfer at either or both ends - for KL/ r < 120

Restraint of joints at either or both ends - for KL/r >120

*Towers

Design - Reduction of allowable stress due to b/t ratio

Due to local buckling of individual flanges of angle section

(Depends on the b/t ratio of flange; b is measured from the end of the fillet to the extreme fibre)

*TowersDesign - Tension Members

Net Area Calculation

Limit on L/R ratio < 375

*Towers

Design - Redundant - Criteria

L/r Limit < 250

Adequate support to the main members - percentage of force in the main member

Adequate to support a man with tools (Say,150 Kgs at mid point)

*TowersDesign - Bolts & Nuts -Governing Criteria

Shear stress on gross area (nominal area) of boltBearing stress on gross diameter of boltBearing on memberTension

*Towers

Design - Stubs - Criteria

Same size as the bottom most leg member

Effective transfer of loads Check against punching shear failure Check against pull out while in tension (Cleats to be provided) Minimum cover distance in concreteProvision for earthing

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TRANSMISSION LINE FOUNDATION DESIGN

*FoundationsParameter and Details

Soil Type and ConditionFoundation TypeLoads

*FoundationsTypes of Soil and Rock

Non-cohesive Soil

Cohesive Soil

Rock

*FoundationsTypes of Soils

Non- Cohesive Soil

Sandy soils with very little clay /Silt Soft and Hard Murrum Easy Excavation

*FoundationsType of Soil

Cohesive Soil

Normal Soil having mixture of silt & t clay ( 15%) Clayee Soils - High clay percentage (> 15%) (Black Cotton Soil) Marshy Soil having sea mud SPT results not necessarily dependable Lab test of undisturbed soil samples preferable

*FoundationsTypes of Soil

Rocks

Soft / Fissured Rock - Excavation without blasting

Hard Rock - Chiselling, Drilling & Blasting required

*FoundationsSoil Parameter

Limit Bearing Capacity of Soil

Density of Soil

Angle of Earth FrustumWater Table

*FoundationsLoads - Types

Compression or downward thrust

Tension or uplift

Lateral forces or side thrust - transverse & longitudinal direction

*FoundationsLoads - Resistance from soilUplift Resistance Shear strength of the surrounding soil Weight of the foundation Rock Anchors

Lateral Force Resistance Passive earth pressure of adjoining soil

Bearing Capacity

*FoundationsClassification

Based on Ground Water Table

Structural Arrangement

*FoundationsType - Based on GW Table & Type of SoilNormal Dry Soil foundationWet Soil foundationPartially submerged foundationFully submerged foundationBlack Cotton Soil foundationPartial Black Cotton Soil foundationSoft/Fissured Rock foundationHard Rock foundationSandy Soil foundation

*FoundationsType - Based on Structural ArrangementPCC type Pyramidal SteppedRCC Spread TypeBlock Type Under - Cut TypeGrouted Rock and Rock Anchor TypeAugur TypeUnder- Reamed Pile TypeSteel Grillage TypeSteel Plated TypePile Type Well Type

*FoundationsDesign Procedure

Stability Analysis

Structural Design

*FoundationsDesign Procedure

Stability Analysis- against tilting, over turning , uprooting and sliding

Check for bearing capacity Check for uplift resistance Check for side thrust Check for overturning` Check for sliding

*FoundationsDesign Procedure

Structural Design Chimney Base Slab Pyramid Block Pile Well

*FoundationsStructural Design

Chimney

Safe against combination of side thrusts & uplift or compression

Passive resistance of soil can be availed except for fissured rock

*FoundationsStructural Design

Base Slab

Bending Adequacy Shear Adequacy Limit State Design

*FoundationsDesign - Material

Concrete - M 20, M15 for pad

Steel - HSD for main steel ; MS for stirrups

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THANK YOU

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