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1. INTRODUCTION
This SECTION describes the basic input data required and terminology used for the design of substation steel structures, overhead line towers and poles.
The industry is currently revising its approach to the general concepts of tower design. The loadings and related strengths required for overhead line design have normally been determined by Statutory Instruments, the client's specifications or the consulting engineer's specifications. The international standards applicable are IEC 60826 'Design criteria for overhead lines', EN 50341 'Overhead lines exceeding 45 kV', and EN 50423 'Overhead electrical lines exceeding AC 1 kV up to and including AC 45 kV.' IEC 60826 gives clear and straightforward guidance using many of the graphs and tables directly from BS 8100 and in its National Normative Aspects (NNAs) EN 50341-3-9 points to loading derived from the same standard for use in the UK. The same criteria are not necessarily applicable in all territories, where, for example, averaging time for wind speeds may be an issue. See also report IEC/TS 61774 'Overhead lines meteorological data for asses sing climatic loads'.
The newer ENs offer the alternatives of a probabilistic approach to design or an empirical approach based upon national/local experience. Such issues are the responsibility of the specialist structural engineer. Therefore this SECTION gives very basic examples to allow the electrical engineer to under stand the fundamental principles and terminology involved rather than the specific methodologies.
It should also be noted that open-terminal substation equipment support structures are nowadays being fabricated more and more from aluminum alloy angle rather than from galvanized steel. The structures may be welded up and drilled to tight tolerances in the factory. The prefabricated structures are light weight and may be transported directly to site. Although there is an initial higher capital materials cost this is largely offset by not having to pro vide special corrosion protection finishes. In addition the aluminum alloy material has a low resistivity. Therefore earth connections from the substation earth mat to the base of the support structures are normally sufficient.
Additional copper tapes to the 'earthy ends' of the insulator supports are not specifically required.
There is a wide range of applicable standards TBL. 1 lists a selection.
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TBL. 1 A Selection of Overhead Line Design Standards
Reference Description IEC 60383 Insulators for overhead lines with a nominal voltage above 1,000 V IEC 60471 Dimensions of clevis and tongue couplings on string insulator units IEC 60720 Characteristics of line post insulators IEC 60797 Residual strength of string insulator units of glass or ceramic material for overhead lines after mechanical damage to the dielectric IEC 60826 Design criteria for overhead transmission lines IEC/TR 60828 Loading and strength of overhead transmission lines IEC/TR 61774 Meteorological data for assessing climatic loads for overhead lines EN 12465 Wood poles for overhead lines durability requirements EN 12479 Wood poles for overhead lines signs, methods of measurement and densities EN 12509 Timber poles for overhead lines determination of modulus of elasticity, binding strength, density and moisture content EN 12510 Wood poles for overhead lines strength grading criteria EN 12511 Wood poles for overhead lines determination of characteristic values EN 14229 Wood poles for overhead lines requirements EN 12843 Precast concrete masts and poles EN 50341 Overhead electrical lines exceeding AC 45 kV. Part 3 covers all the different National Normative Aspects (NNAs) EN 50423 Overhead lines AC 1 45 kV based on 50341 but provides specific simplifications or changes Eurocode 1 EN 1991 Basis of design and actions on structures; 1991-1-3 covers snow loads; 1991-2-4 covers wind loads
Eurocode 2 EN 1992 Design of concrete structures 1992 3 covers concrete foundations Eurocode 3 EN 1993 Design of steel structures Eurocode 7 EN 1997 Geotechnical design Eurocode 8 EN 1998 Design provision for earthquake resistance of structures BS 1990 Wood poles for overhead power and telecommunication lines BS 3288 2 Insulator and conductor fittings for overhead power lines specification for a range of fittings. Other parts of this standard are superseded or becoming so BS 7354 Code of practice for the design of high-voltage open-terminal stations BS 8100 Lattice towers and masts. Part 1 is a Code of Practice for loading; Part 2 is a guide to Part 1; Part 3 is a CoP for strength assessment of tower and mast members; Part 4 covers the loading of guyed masts
Notes:
1. See Table 1 (next section) for standards relating to conductors.
2. A much more extensive schedule of related standards is provided in EN 50341-1.
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2. ENVIRONMENTAL CONDITIONS
--2.1 Typical Parameters
In order to match both the mechanical and electrical characteristics of the overhead line or substation arrangement the environmental conditions and climatic details must first be collected and analyzed. The following parameters are required (for installations in the EU refer to the Eurocodes 1, 2, 3, 5, 7 and 8(EN1991 1998), although where there is conflict EN 50341 prevails):
Maximum ambient shade temperature C Minimum ambient shade temperature C Maximum daily average temperature C Maximum annual average temperature C Maximum wind velocity (3 second gust) km/h Minimum wind velocity (for line rating purposes) km/h Solar radiation mW/m^2
Rainfall m/annum Maximum relative humidity % Average relative humidity % Altitude (for insulation level) m Ice (for loading conditions) Snow (for loading conditions) Atmospheric pollution Light, medium, heavy, very heavy Soil type Clay, alluvial rock, etc.
Soil temperature at depth of cable laying C Soil thermal resistivity C m/W Soil resistivity O m Isokeraunic level Thunderstorm days or lightning flashes to ground per square kilometer Seismic factor
FIG. 1 Wind direction factor (from EN50341). Wind direction (degrees)
--2.2 Effect on Tower or Support Design
--2.2.1 Wind Load
It is normal practice to consider wind loads on structures due to a 3 second gust that occurs over a 50-year period. This basic wind speed figure is, in general, to be obtained from meteorologists (but in the UK, for example, the figures to be used are provided in BS 8100 or the NNAs of EN 50341, unless particular local microclimate features are known to be a problem). On over seas work it may be difficult to obtain data as records may not have been accurately kept over such a period. The wind load is related to the wind speed in accordance with the code of practice applicable to the country where the work is being carried out. In the EU the relevant information is set out in the NNAs of EN50341 and 50423. It describes procedures for calculating wind loads on both structures and conductors, with considerable variation in detail between individual countries. The following example uses the UK NNAs.
The site reference wind speed, r is the mean hourly wind speed at a level 10 m above the effective height of obstructions 'appropriate to the site terrain' and is given by:
Where…
B(ms) 5Basic wind speed obtainable from wind maps in BS8100, Part 1 d 5Wind direction factor see FIG. 1, or site records R 5Terrain roughness factor see TBL. 2 or site records ?v 5Partial safety factor on wind speed51 for reliability level I (50 years)
The variation of wind speed with height for sites in level terrain is given by:
where ... Mean wind speed at height in meters above ground level a=Power law index of speed with height see TBL. 2 e = Effective height of surface obstructions see TBL. 2 The dynamic pressure at height shall be taken as:
where
?a == The density of air, taken as 1.22 kg/m3 for UK It is considered good practice to apply a gust factor at this stage, to derive the 'gust wind pressure' , where:
5 11 comU b and: com =a combination factor to take account of the improbability of maxi mum gust loading on both conductors and towers occurring simultaneously. It may be conservatively taken as 1.0 b 5the basic gust factor for the support, depending on the height of the support. See FIG. 2
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TBL. 2 B Terrain Characteristics (from BSEN 50341-3-9)
Category Terrain Description Terrain
Roughness Factor, R Power Law Index of Variation of Wind Speed with Height, a Effective Height, e (m)
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--2.2.2 Wind Loading Example
The wind load on a structure is calculated by considering the areas of the structure exposed to the wind. The maximum wind load in the direction of the wind for each panel of the body, tw, may be taken as:
FIG. 2 Basic gust response factor for towers ( b) and insulators and
fittings ( i) (from BSEN 50341).
where:
=the total area projected normal to the face of the members in the tower face, for the panel considered, or, for poles, the face area of the pole over the panel considered n 5the overall drag (pressure) coefficient, which for lattice towers is dependent upon the 'solidity ratio,' which takes account of the open nature of the structure (see FIG. 3). The calculation of this ratio is complex (see EN50341) the figure typically lies between 0.1 and 0.6 = the wind incidence factor (unity for a full facing wind) Thus if the basic wind speed for a UK location is determined as 24 m/s, the wind direction factor 0.9, the terrain roughness factor 0.86 (farmland with high hedges), the site reference wind speed is 18.58 m/s from equation (1). Then from equations (2) and (3) the dynamic pressure on a tower panel 40 m above the ground would be:
The drag coefficient for towers composed of flat-sided members, CNf, subcritical circular section members, CNC, and supercritical circular-section members, CNC_:
where C1 _ 2,25 for square towers
C2 _ 1,5 for square towers
_ solidity ratio
_ 1,9 for triangular towers
_ 1,4 for triangular towers
FIG. 3 Overall normal drag coefficients, n, for towers (EN 50341-3-9).
From FIG. 3, assuming a solidity factor of 0.2 the overall drag coefficient would be 2.9 for a flat sided structure and the maximum wind load on a panel of projected area 40 m^2 is given in equation (17.4) as: 333:834032:9 11131:7 315104:5kN
--2.2.3 Wind Load on a Substation Gantry
The design of substation gantry busbar support structures is treated similarly to towers. It must take into account adequate height in order to allow for the maximum conductor sag condition. This will normally occur at maximum ambient temperatures and full busbar load current. A check could also be made for extreme circumstances at no load with maximum ice build-up. The mechanical loading on the gantry will include:
_ wind loading (as described in Sections 2.2.1 and 2.2.2);
_ maximum conductor tension.
The combined effect would be calculated as in SECTION 2.2.4 following.
The conductor tension must allow for the weight of the insulator strings. Also it should be noted that the maximum conductor tension will occur under minimum temperature, minimum sag conditions.
--2.2.4 Wind Load on a Conductor Example
Consider an aerial conductor (for example a moderately smooth earth wire forming a substation lightning screen) with a length of 150 m and diameter 25 mm. Assume the same wind conditions, height, etc. as in Sections 2.2.1 and 2.2.2. The wind loading on the conductor is determined from the formula:
Wind loading, cw 5 c c 11 c sin2
? where =length of conductor (m) c =diameter of conductor (m) z =wind pressure=333.8 N/m^2 (from SECTION 2.2.2) c 5overall drag coefficient for the conductor see TBL. 3
Note
1. This needs the effective Reynold's number to be calculated.
2. That ice loading can affect this.
_conductor gust response factor, the product of the length factor and the height factor (see FIGs. 4a and 4b)
_ the angle of wind incidence to the conductor (assumed 90)
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TBL. 3 Safe Design Tension Taking Account of Terrain Turbulence [5]
Terrain Category Terrain Characteristics ( / )adm (m) I Open, flat, no tree, no obstruction, with snow cover, or near/across large bodies of water; flat desert 1,000 II Open, flat, no obstruction, no snow, e.g. farmland without any obstruction, summer time 1,125 III Open, flat or undulating with very few obstacles, e.g. open grass or farmland with very few trees, hedgerows and other barriers; prairie, tundra 1,225 IV Built-up with some trees and buildings, e.g. residential suburbs; small towns; woodlands and shrubs. Small fields with bushes, trees and hedges 1,425 Notes:
1. This table uses H/w, the ratio of horizontal tension in the span to conductor weight per unit length, as the tension parameter. It is important to note that this horizontal tension refers to the initial tension, before wind and ice loading and before creep, at the average temperature of the coldest month on the site of the line.
2. Valid for homogeneous aluminum and aluminum alloy conductors Ax (AAC and AAAC), bi-metallic aluminum conductors Ax/Ay (ACAR) and steel-reinforced aluminum conductors A1/Syz (ACSR).
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FIG. 4a Length factor L for conductor wind loads (EN 50341-3-9).
FIG. 4b Conductor height factor (EN 504341-3-9).
Thus wind load51503333:830:02531:2 11 0:7531:4 312 53:12 kN
--2.3 Conductor Loads
--2.3.1 Conductor Tensions
The starting point for all conductor sag/tension calculations is the clear definition of the bases and conditions upon which the minimum factor of safety at which the conductor is allowed to operate are set. The following are typical requirements with values given for UK conditions in respect of transmission lines:
_ Maximum working tension (MWT)
Conductor tension shall not exceed 50% of its breaking load (factor of safety of 2) at, say, Temperature 26 C Cross wind pressure 383 N/ m^2
Radial ice thickness 12.7 mm
_ Everyday stress (EDS)
Conductor tension shall not exceed 20% of its breaking load at, say, Temperature 16 C Cross wind pressure Radial ice thickness The figure of 20% has been used above to serve this example, but derivation of safe design tension must take account of a number of factors.
Research into the effect of aeolian vibrations [5] over the life of a conductor has shown that the effect of the local terrain on turbulence is relevant, resulting in recommendations for safe design temperature depending on terrain (see TBL. 3).
Usually either MWT or EDS will be the critical basis for calculations and the other condition will then automatically be met. Often there is a particular span length above which the one basis is critical and below which the other one is. The tension, , in the conductor for a given sag, , is given by the formula:
This is based on the parabolic curve shape for the conductor which, for less than 300 m spans or high span-to-sag ratios (sag is less than 10% of span and generally level topography), is very close to the more mathematically correct catenary formula (see paragraph SECTION 18, Section 5.2.5). The sag/ tension formula is given in SECTION 16, SECTION 4.2.2.
--2.3.2 Conductor Tension Example
A conductor is to have a maximum working tension of 65.95 kN at a temperature of 6 C with 12.7 mm of radial ice and a wind load of 383 N/ m^2
Calculate the sag at 20 C in a span of 400 m. The sag/tension formula is given in SECTION 16, SECTION 4.2.2:
The conductor data is:
The calculation is given in TBL. 4.
--2.3.3 Short Circuit Loadings
Under short circuit conditions lateral mechanical attraction or repulsion forces will occur between the different phase conductors.
The effect of conductor movement during short circuits is erratic and difficult to calculate. Such movement is taken into account in the overall design by allowing adequate clearances between the phase conductors. The conductor short circuit forces are usually ignored in the structural design of over head line towers or substation gantries because of the very short durations of the faults.
--2.3.4 Ice Loading
The build-up of ice on conductors will increase effective conductor weight, diameter and wind loading. Local experience must be used in the application of ice loads to structural design. As an example, EN 50341-3-9 calls for a uniform ice load on all spans of 5 kN/m3 to be considered for UK designs, or 9 kN/m^3 in case of wind and ice.
--2.3.5 Seismic Loads
The application of seismic loads in structural design is a specialist subject.
Eurocode 8 covers it in detail, but the following gives a simplistic overview.
The acceleration due to a seismic event is categorized as a fraction of the gravitational constant, This may be given for both horizontal and vertical effects over a frequency spectrum. TBL. 5 details such acceleration factors for what is loosely described as a 0.2 seismic event. Such an event refers to a surface wave travelling outwards from the epicenter exercising both horizontal and vertical forces on equipment. For simplicity, loadings on substation structures could be taken as an equivalent horizontal load.
An example of an analysis on the stability of a small distribution transformer under 0.2 seismic conditions is given below. Consider the transformer with the physical characteristics given in TBL. 6.
Transformer weight= (kgf) Height from base to transformer centre of gravity=(m) Width of transformer wheel mounting points= (m) Vertical wheel mounting transformer reaction forces, 1 and 2 (kgf)
TBL. 4 Sag and Tension Calculation Parameter Formula Calculation
TBL. 5 Frequency Spectrum Acceleration Factors for 0.2 Seismic Event
Frequency Horizontal Acceleration Vertical Acceleration
TBL. 6 Cast Resin Dry Type Transformer Physical Details and Calculated Stability Results under 0.2 Seismic Conditions
Assume wheel mounted transformer sliding is prevented, then taking moments about point 'X' for the 0.4 horizontal seismic factor and assuming no vertical effects (FIG. 4c):
The worst case for transformer stability is therefore at the maximum downward acceleration (ground cyclic movement falling away beneath the transformer) and the uplift on the rear wheels = (0.36520.4)x100% of nominal transformer weight. If not held down to resist overturning the trans formers will slide because the coefficient of friction, µ, between the trans former steel wheels and the plinth is unlikely to be better than the 0.4 required in practice (for steel on steel µ50.25). In this example all but one of the distribution transformers have an ratio $ 1.2 and will there fore overturn without the effect of vertical acceleration effects provided:
1. they are prevented from sliding, and
2. restraining effects of connecting cables or busbar trunking are ignored.
The transformers may be restrained by fixing arrangements to resist the following forces at wheel:
1: 0:2 longitudinally 2: 0:2 uplift 3: 0:5 deadweight where W=nominal weight of the unit
Switchgear and control or relay cubicles will need to be looked at on an individual basis in the same way.
--2.3.6 Combined Loads
The simultaneous application of individual worst case loads is unlikely to occur in practice and the simple arithmetic addition of all such load cases would lead to an uneconomic and over-engineered solution. The individual loads are therefore factored to arrive at a sensible compromise. For example, wind load plus ice load is often taken as full ice loading plus wind load at, say, 50% basic wind speed. Similarly, wind load plus seismic load is normally taken as full earthquake load plus 50% wind load.
The forces involved on an overhead line tower are shown in FIG. 4d.
A = horizontal conductor wind load, wind span3wind pressure (see SECTION 2.2.4) (note that for simplicity, the wind effect on insulators, fittings, etc. has not been considered in SECTION 2.2, but in practice a designer will consider this, and the standards give suitable guidance)
B = horizontal structure wind load (see SECTION 2.2.2)
C = component of wind loading due to direction of wind and effective structure area normal to the wind (see Eq. (4))
D= vertical conductor weight span x conductor weight, including weight of insulator strings and fixings
E = longitudinal loads due to conductor tension.
This will occur under uneven loading, such as for broken wire conditions or at a terminal tower with a slack span on one side of the tower entering to a substation gantry These forces will, in general, result in a turning moment causing compression on one side of a tower and tension ('uplift') on the opposite side.
Having calculated the forces on structure and conductors, standards such as EN50341 apply a 'partial factor', which depends on the selected reliability level and takes account of possible modeling inaccuracies and uncertainties in the disturbing 'actions'. Such factors will depend on different national experience. EN50341-3-9 recommends for the UK a factor of 1.0 in calculating wind loads, with or without ice, provided the design period is no more than 50 years, but specifies a factor 1.5 when calculating static maintenance and construction loads, and 2.0 on conductor tension when conductors are being pulled by powered winches. Partial factors are also applied to the various material properties such as resistance of steel cross sections and of bucking of sections, compressive concrete strength.
FIG. 4d Forces on an overhead line tower.
--3 STRUCTURE DESIGN
--3.1 Lattice Steel Tower Design Considerations
--3.1.1 General
It should be noted that structural design is not an exact science, but BS5950 covering the subject is comprehensive and will, in general, lead to an economic design. The standard is based on material yield strengths with factors to take account of dynamic and static loads. Eurocode 3 (design of steel structures) is applicable, supplemented by EN50341, with different national criteria provided in EN50341 Part 3.
The l/r ( l=length and r=radius of gyration) slenderness ratios for different steel member sections are obtained from tables in BS5950, Volume 1, Section Properties Member Capacities. Obviously the longer and thinner the steel section the less load it will be able to take before failure along its axis. An equal steel angle will have a radius of gyration equal in both planes whereas an unequal angle will have different radius of gyration values in the and planes and for the design of a steel column the minimum value of ' ' should normally be taken. This gives a higher l/r ratio and correspondingly lower design compressive strength to work with.
The 132 kV substation gantries for an incoming overhead line are shown in FIGs. 5a) and 5b. Figures 6 and 7 show a typical tower general arrangement and associated steel member schedule. Tables 7 and 8 give typical allowable stress capabilities for steel struts with yield strengths, S 5245 and 402 N/mm^2 respectively. Note these figures are for example only; designers should refer to BSEN 10025 (hot rolled steel), and the Eurocodes or other equivalent national structural standards. FIG. 8 gives dimensions and properties of light equal angle steel sections from BS5950, Structural Use of Steelwork in Buildings. FIG. 9 gives bolt capacities for standard 4.6 and 8.8 grade metric bolts.
FIG. 5a Oman 132 kV line entry gantry.
FIG. 5b 132 kV gantry bolted connected Channel Tunnel, Folkestone.
FIG. 6 Typical tower general arrangement.
FIG. 7 (coming soon) Typical tower member schedule.
FIG. 8 (coming soon) Dimensions and properties of steel equal angle (BS 5950).
FIG. 9 (coming soon) Bolt capacities.
TBL. 7 Typical Strut Formulae for Maximum Allowable Stress,
TBL. 8 Typical Strut Formulae for Maximum Allowable Stress
Steel lattice transmission tower design is based on compression formulae such as those in Tables 17.7 and 17.8 for leg members with different length/ radius of gyration ( / ) values. The self-supporting tower design uses steel angle columns supported by stress-carrying bracing and redundant members.
Higher strength steels show their greatest advantage in the lower / range where it can be seen from the tables that the allowable stress is not so sensitive to the slenderness of the member involved. The equivalent area of the member and the radius of gyration is looked up in tables for standard steel sections. See, for example, FIG. 8 taken from BS5950 for a 6036036 mm angle. The minimum radius of gyration is 11.7 mm for the weakest axis.
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TBL. 9 Data Used in the Design of Lattice Steel Towers (Per EN 50341-1)
Symbol Description
A Cross-sectional area; gross cross-sectional area of bolt Aeff Effective cross-sectional area Anet Net cross section at holes AS Tensile stress area of bolt b Nominal width beff Effective width of the leg c Distance between batten plates d Bolt diameter do Hole diameter E Modulus of elasticity e1 End distance from centre of hole to adjacent end in angle e2 Edge distance from centre of hole to adjacent edge in angle F Concentrated horizontal load fu UTS fub UTS for bolt fy Yield strength fyd Design yield strength i Radius of gyration about the relevant axis L System length McRd Design moment of resistance in bending Msd Bending moment at cross section m Number of angles N Axial force Nd Compression force; force in the compression member NR,d Design value of the buckling resistance Nsd Design value of the tensile or compressive force at cross section P Spacing of two holes in the direction of load transfer p Spacing of two holes, measured perpendicular to the member axis Sd Tension force; force in the supporting member (tension or compression) s Staggered pitch, spacing of centers of two consecutive holes t Thickness Weff Effective cross section modulus
M1 Partial safety factor for resistance of member in bending or tension or to buckling
M2 Partial safety factor for resistance of net section at bolt holes
Mb Partial safety factor for resistance of bolted connections
Slenderness ratio for the relevant buckling load
eff Effective slenderness
2 Nondimensional slenderness for the relevant buckling load
p Ratio of width to thickness (b/t) P Reduction factor x Reduction factor
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An accurate analysis procedure is necessary to take into account the tension in the conductors, ice and wind effects. In particular, account must be taken of broken wire conditions (unequal loading on either side of the tower) and also the effect of the insulator strings. Only a very brief introduction to the principles involved is given here.
Such tower design is carried out by specialist structural engineers and is outside the scope of work purely for electrical engineers. Computer techniques are nearly always employed for all but the simplest structures.
TBL. 9 lists some of the factors that have to be taken into account in an accurate calculation, which must allow for bending forces, compression and tension forces, flexural buckling and lateral buckling.
--3.1.2 Adequacy of Steel Angle in Compression
Consider a tower main leg of length 4 m to be designed to carry a maximum compressive force of 400 kN. A 120312038 mm high yield stress equal angle steel, s 5402 N/mm^2 , equivalent area of 1,876 mm^2 and minimum radius of gyration, min 523.8 mm is to be used and checked for adequacy in this application.
Compressive stress on member5 4003103 1;876 5213 N=mm^2 From TBL. 8, the associated / ratio must not be greater than 85 for this condition. Therefore the maximum unsupported length of the leg must not exceed 85323.852,023 mm and therefore a brace support must be pro vided at, say, mid-length of the leg using this type of steel in this application in compression.
--3.1.3 Bolted Connections
There are basically three types of bolt connector in common use:
_ ISO metric black hexagonal bolts, screws and nuts to EN 24016 (strength grade 4.6).
_ ISO metric precision hexagonal bolts, screws and nuts to EN 24016 (strength grade 8.8).
_ High strength friction grip bolts and associated nuts and washers to EN 143399 (grades 8.8 and 10.9 and 10.9 with wasted shank) these are more usually considered for buildings and bridges rather than towers.
The term 'black bolt' is not sufficiently precise to be used without clarification as to the exact bolt grade being described. Both EN24016 grade 4.6 bolts and sometimes 8.8 precision bolts are supplied in the 'black' condition. The term 'precision' refers to the bolt shank dimensional tolerance. Normally an allowance of 2 mm over the bolt size is made for the bolt hole. A 22 mm diameter hole would therefore be drilled for an M20 bolt.
The nomenclature used for bolt types gives the yield strength ( s) and ultimate tensile strength (UTS). Consider a bolt with grade ' ' The first number, , in the bolt grade is a tenth of the ultimate tensile strength expressed in kgf/mm^2 (note this is not in N/mm^2 although 1 kgfB10 N). The second number, , is the ratio of the yield strength to UTS310.
For example, a 4.6 bolt has s 5436524 kgf=mm^2 240 =mm^2 UTS54310540 kgf=mm^2 400 =mm^2 Similarly a high tensile 8.8 bolt has s 5838564 kgf=mm^2 640 =mm^2 UTS58310580 kgf=mm^2 800 =mm^2 There are three main aspects of bolted joints to be considered:
_ Bearing the stress on the inner surface of the bolt hole imparted by the bolt. The thicker the plates being bolted together the larger the bearing area and the lower the bearing stress.
_ Reduction in steelwork material and cross-sectional area due to the presence of the bolt holes.
_ The bending and prying effects of tension in the bolts.
Checking the Effect of Bolt Holes for Connecting Steel Members Together
If structural members are bolted together, then an allowance has to be made for the reduction in steel bulk and therefore stiffness due to the holes required for the bolted connection. Steel plates may be connected together by bolted connections with forces acting in shear across the bolt diameter. Friction grip bolts are normally only used in rigid frame structures where high shear loads and moment loads are involved. In a pinned three dimensional truss structure, such as a steel lattice tower, the design will involve only very slight bending moments. High strength friction grip bolts would not therefore normally be used in a lattice tower structure to clamp the plates together.
Consider 22 mm diameter bolt holes in each right angle 120312038 mm steel gantry leg face for M20 bolts to withstand a 350 kN maximum tensile force. From FIG. 9, an M20 grade 8.8 bolt has an allowable shear value of 91.9 kN. Therefore at least four No. M20 grade 8.8 bolts or at least nine No. M20 grade 4.6 bolts are required. Larger bolts will reduce bearing stresses. It is important to notice that if a design has been formulated around grade 8.8 bolts then they should not be replaced at a later date by a bolt of a lesser grade. Some engineers design on the basis of grade 4.6 bolts and specify grade 8.8 bolts in the material schedule if no major cost disadvantage ensues.
There are standard edge distances and back marks for bolt drilling in standard section steel members. For example, a bolt centre should not be less than 1.43 hole diameter from the edge of the member in the direction of the load, and a minimum distance of not less than 1.253 hole diameter in the direction normal to the load (BS5950). The application of such precautions takes into account the reduction in steel bulk due to the bolt holes.
More conservative guidelines are also given in EN50341, which takes account of whether or not the shear plane passes through the threaded portion of the bolt.
--3.1.4 Bracing
The calculation to confirm the adequacy of a steel brace is similar to that given for the tower leg in SECTION 3.1.1.
A 3.5 m long mild steel, s 5245 N/mm^2 ,6036036 mm angle brace, equivalent area 691 mm^2 and min 511.7 mm (FIG. 8), is to be designed to carry a maximum compressive force of 80 kN.
Experience would show that this is rather a slender steel section for the proposed load. Compressive stress 5803103/6915116 N/mm^2. From TBL. 7 the / ratio must not be greater than 110. Therefore the maximum unsupported length of the brace must not exceed 110311.751287 mm and therefore the brace must have additional supports at 3500/1287B three points along its length when using this type of steel in this application.
--3.1.5 Analysis
The structural analysis may be carried out by:
_ computer;
_ graphical methods.
It is normal to use computer methods to carry out the analysis and often the complete design with simple hand calculations as shown in Sections 3.1.2 to .3.1.4 only to check certain results. The tower or gantry structure is designed to have members either in compression or tension. The computer checks each element to ensure that it is capable of withstanding the applied loads. The checks are carried out in accordance with standard codes of practice applicable to the country involved or as specified by the design or consulting engineer. A most useful reference is the Steel Design Manual.
FIG. 10 Double circuit 400 kV tower undergoing type tests.
--3.2 Tower Testing
New tower designs may be type tested at special open air laboratories.
FIG. 10 shows a double circuit 400 kV tower undergoing tests at Chelms Combe Test Station in the UK (now closed).
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Low-Voltage wood pole Low-Voltage thin wall steel pole Single circuit Single circuit
"TRIDENT" (no earth wire) Single circuit twin earth wire Combined 3 wire 11 kV and 380V Double circuit twin earth wire
FIG. 11 Typical pole arrangements.
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FIG. 12 115 kV twin pole, single circuit line Saudi Arabia.
4. POLE AND TOWER TYPES
--4.1 Pole Structures
Pole structures are especially used for economic household distribution at voltage levels of 380/415 V and 20/11 kV where planning permission allows such arrangements in place of buried cables. Such pole structures are also used at the lower transmission voltage levels, typically at up to 145 kV but also with multi-pole and guyed (stayed) arrangements at voltages up to 330 kV. Low voltage designs are based on matching the calculated equivalent pole head load to the particular type and diameter of wood, steel or concrete to be employed. At higher voltages specific designs are used in order to select optimum size and relative cost.
Some examples of different pole arrangements are given in FIG. 11.
Wood poles must be relatively straight and defects such as splits and shakes are unacceptable. There are various national standards, and TBL. 1 lists some European standards. Commonly used soft woods such as fir, pine and larch require impregnation with creosote, anti-termite repellents if to be used in tropical countries and similar chemicals to prevent decay. Some hardwoods may not need chemical treatment but these are becoming very expensive and their use is considered by some to damage the environment. The poles are usually direct buried with a depth of burial normally equal to one sixth of pole length. If this is insufficient to resist the design turning moment, supplementary blocking or 'Permasoil' is used to provide further resistance to overturning.
FIG. 13 Pole-mounted 11/0.415 kV transformer and LV distribution.
Tubular poles are available in single column circular sections, octagonal shapes made from folded steel and stepped/swaged sections. Very thin steel wall sections, slightly conical in shape, are also available. The poles are shipped with the smaller sections inside the larger ones for compactness.
They are then erected on site by sliding one section over the other to form the pole as shown in FIG. 11.
Concrete poles are available in pre-stressed spun or unstressed cast concrete. Light fiberglass poles are also available for light head loads.
FIG. 12 shows a typical 115 kV twin pole single circuit line in Saudi Arabia with the oil wells in the distance, and FIG. 13 is a pole-mounted 11/0.415 kV distribution transformer.
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Single circuit single earth wire Single circuit double earth wire Double circuit single earth wire Double circuit double earth wire
FIG. 14 Typical tower outlines.
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FIG. 15 400 kV cap and pin insulator set. Low resistance 'ducter' testing
on connections in progress.
FIG. 16 500 kV double circuit tower erection in progress, China.
--4.2 Tower Structures
Steel lattice towers are generally used at the higher voltage levels where longer spans, high wind loads, ice loads and heavy conductors make the use of wood or light steel poles impractical. FIG. 14 gives some examples of typical tower outlines for single and double circuit configurations with single and double earth wires. In order to standardize, towers are categorized typically to fulfill the following duties:
Suspension towers straight line and deviation angles up to approximately 2 10 Angle or section tower angles of deviation up to 10 or at section positions also for heavy weight spans or with unequal effective negative weight spans 30 angle deviation angles up to 30 60 Angle deviation angles up to 60 90 Angle deviation angles up to 90
Terminal tower loading taking full line tension on one side of the tower and none or slack span on other typically at substation entry The terminology adopted to describe such towers varies and must be clearly described in order to avoid confusion. For example, a double circuit 30 angle tower for twin conductor use could be described in short form as D30T, but the 'T' can be confused between 'twin' and 'triple'. Many users would restrict the description to D30. Similarly, a double circuit terminal tower for twin conductor use could be described as DTT, or more commonly just DT. Extensions are described with a further addition; for example D30E6 describes a dual circuit tower with a 30 maximum angle and a six meter body extension. In addition to the conductor and insulator set loadings, tower design must take into account shielding angles (lightning protection).
Further clearances must be maintained as the insulator sets swing towards the earthed tower structure under certain wind conditions. Figure 18 (SECTION 18) indicates the physical size of a 400 kV cap and pin glass insulator suspension set undergoing maintenance on the Lydd/Bolney line in Southern England. FIG. 15 shows 'ducter' (low resistance) measurements being taken on an overhead line clamp during refurbishment work on the Elland/Ferrybridge overhead line in Yorkshire. The tension insulator set is in the foreground.