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The following information is excerpted from this journal.
[A photo of a lineman working on repairs after an ice storm:]
[...]
The following information is excerpted from this book.
During the last decade, the use of steel pole structures for electrical transmission lines has increased very rapidly. This use has been prompted by a need to provide a new visual concept for these structures. As this demand has grown, a similar increase has occurred in the number of manufacturers, the new configurations available, the manufacturing techniques employed, and the design refinements.
As with all transmission line structures, the design engineer has practically unlimited freedom in the selection of the structure configuration, and wide latitude in the determination of the design criteria. This report has been developed to bring together the more pertinent aspects of steel pole design and its direct application to electrical transmission structures. These recommendations reflect the experience of good practice and presently available test data, which have been carefully reviewed by the committee. The guidelines cover loadings, design recommendations, fabrication, testing, erection, foundations, quality assurance, and pertiennt reference information.
This report does not include requirements for the electrical components of transmission lines, e.g., safety clearances, insulators, sags, and spans. These items are presently covered in the National Electrical Safety Code (NESC).
These recommendations have been developed to relate primarily to higher voltage transmission lines (100 kv and above), where reliability and continuity of service must be provided. As the need for electric power increases and available right-of-way becomes more critical, the demand for pole type structures will create an increasing usage.
Development of Loading Criteria
The loadings on steel pole transmission structures are the same as those on latticed towers, except for the effects of the special features of poles, e.g., flexibility and shape factors. Therefore, it is felt that Section A on Loading of the ASCE Guide for Design of Steel Transmission Towers essentially should be repeated in this document. Changes are made where necessary to account for the differences between poles and towers. [...]
The primary loads for transmission pole structures are weather element and erection loads. The weather element loads are wind or ice, or both. When considering extreme weather element conditions, a calculated risk versus cost factor should be established. Where winds from hurricanes exist and cover large fronts, the design winds should reflect these conditions.
Where high velocity winds and special gust conditions can be determined, they should be evaluated by the engineer. Comprehensive wind information for general areas is available. Normally the tangent or light angle suspension structure design for high winds is controlled by a combination of wind on wires and structures. Maximum wind gusts do not affect the entire wind span on a long span structure instantaneously, but gusts from wind can engulf a structure.
There is considerable information available on snow, but little general data on ice formation on conductors and ground wires. Meteorological consultants can predict probable amounts of ice formation on wires in combination with various wind conditions. This can be done from a study of past records of nearby weather stations, showing precipitation, temperature, and wind velocities. Studies of this type can be done for either specific lines or a specified area. With this knowledge, load conditions and overload factors can be established.
Erection loads should be determined by a study of the procedures followed for structure erection and stringing.
Basic loadings, overload factors, and member design should be covered in the same design criteria. [...]
A horizontal wind velocity, at right angles to the directon of the line, of 39.5 mph, on all wires when covered with a layer of ice 0.5 in. in radial thickness, and on the surfaces of the supporting structure without ice covering, is called heavy loading.
A horizontal wind velocity, at right angles to the directon of the line, of 39.5 mph, on all wires when covered with a layer of ice 0.25 in. in radial thickness, and on the surfaces of the supporting structure without ice covering, is called medium loading.
A horizontal wind velocity, at right angles to the directon of the line, of 59.3 mph, on all wires without ice covering, and on the surfaces of the supporting structure without ice covering, is called light loading. [...]
The vertical loads on supporting structures and foundations should be their own weight plus the superimposed weight they support, including all wires, ice coated where specified. The effective vertical span for wires should be determined with proper consideration of the effect of support at different elevations. [...] The weight of ice on the supports is normally ignored. [...]
Vertical loads in excess of those specified previously may be produced on the structure during construction or maintenance. Construction and maintenance techniques should be determined, to establish realistic loads that may exist, and the vertical load capability of the structure should be increased if required. [...]
Where a change in direction of wires occurs, the transverse load on the supporting structure should be a resultant load equal to the vector sum of the transverse wind load and the resultant load imposed by the wires due to their change in direction. [...]
The longitudinal loads on supporting structures at dead ends for line terminations should be taken as the pull equal to the longitudinal component of tensions of all wires under the condutions of wire loadings specified, but with spans in each direction from the dead-end structure, the pull should be taken as the difference in longitudinal component of tensions.
Where longitudinal loads can be created by the difference in tensions in the wires in adjacent spans caused by unequal vertical loading, the structures should be capable of supporting this longitudinal loading.
Proper allowance should be made for longitudinal loads that may be produced on the structures by wire stringing operations and construction techniques.
Longitudinal loads imposed by the breaking of a wire should reflect experience under realistic conditions. [...]
Deflections
Transmission structures constructed of one or several steel poles must be analyzed for deflection to provide adequate strength requirements. The larger deflections usually resulting from horizontal loads cause additional stress due to the vertical loads being applied in the final deflected position. [...]
The deflection of a structure can affect the vertical clearances to ground, or horizontal clearances to edge of right-of-way, if they should become excessive at certain locations. Since in many causes the use of steel pole structures is dictates by esthetic considerations, the effect of deflection on appearance is a very important factor to consider. [...]
Deflections can play an important part in the appearance of a structure. At line angles or where all vertical conductors are on one side of a pole structure, the constant load in one direction will cause the structure to bow and appear to be near failure. There are several methods that can be used to design against this. One method is cmabering the pole so that it will appear straight and plumb. To do this, a special loading case normally considered [to be] the everyday load on the structure should be specified. The pole is then bent (or cambered) to offset the deflection under this load case. Another method is to rake the pole when setting it. The deflection at the top of the pole is determiend for the everyday loading. The pole is tilted this predetermined amount so that under the everyday loading, the top of the pole is at a specified position in relation to the bottom. In this case, the pole will be curved. Finally, the structure can be designed to limt deflections, but this can be expensive because of the extra-heavy section of pole required.
The deflection of guyed structures must also be considered. Elongation of the guys and guy anchor creep should be considered in the design. Deflection in any direction adds mo[ve]ment and stress to the pole due to the vertical loads and lateral movement. The stretch of the guys can be ignored if the stretch does not exceed 0.2% for guys up to 200 ft in length. Often, anchor creep can be minimized by preloading the anchor. [...]
The length of suspension insulator strings can greatly influence the structure oading under unbalanced longitudinal loading conditions. The decrease in tension caused by the swing of long insulator strings can be significant. For pole-type self-supporting structures, the deflection of the structure may provide a tension reduction in the wires. Both of these factors may be included in the unbalanced loading condition, as long as proper consideration is given to any impact loading imposed on the structure. [...]
Handling of Material
Pole type structures should be erected in accordance with the manufacturer's recommendations. The methods of handling and erection, and the equipment used, should be such that the poles, crossarms, and other components will not be damaged and will not be subjected to loads in excess of those for which they were designed. Special handling instructions, if required, should be fully specified on the erection drawings. Normally, nylon or polypropelene rope can be used without special padding. Wire rope slings may require padding to prevent damage to the surface finish of the poles or accessories during erection and installation. Wood blocking and cribbing should be used for supporting poles and pole sections during storage, and assembly if poles are assembled on the ground. Bent, twisted, damaged, or misfabricated parts should not be installed until reviewed by qualified personnel. Any field straightening or repair should be approved by the responsible engineer, and the manufacturer notified of the corrective measures. No field holes or alterations should be allowed in a pole or accessories unless approval is obtained from the manufacturer.
Subassembly
When assembling poles with slip joint sections, care should be taken to assure correct alinement and orientation. Foreign matter, e.g., dirt, gravel, or vegetation, must be removed from the slip joint surface. The two sections should be pulled together using approved procedures to achieve the specified overlap and assure a tight joint. Fabrication tolerances are such that the tightness of slip joints will vary. For single pole structures, a slip joint can be considered satisfactory if it is tight and if the overlap is equal to or greater than the minimum overlap specified. [...] These tolerances may not suffice when greater accuracy in the elevation of connections is required, e.g., in H-frame structures. In this case, the poles should be fabricated to assure that the specified overlap is maintained, or a different type of joint splice should be installed. If poles are of slip joint construction, each section (both male and female) should have mating marks for proper section assembly, axis orientation, and splice overlap. These match marks should be properly alined. If like pole sections in an order of structures are not interchangeable, each pole section should be clearly marked, identifying it with the other units of the same structure and the sections with which it must be assembled to form a complete structure.
Bolted Connections
When pole sections are equipped with flanges, a small angular error in attaching the flanges can result in appreciable variation in the alinement of the completed pole. It may be found necessary to install shims between flanges, or to take other means to assure acceptable alinement. Care should be taken by the manufacturer to avoid supplying flanges with a convex connection face. Flange bolts can be overstressed in trying to pull the outside portions of the flange plates together. If flange plates have a gap in excess of 1/32 in., they should be returned to the manufacturer for grinding or other suitable finishing operations. When this is not possible, properly designed shims may be installed. All bolts should be installed with proper torque procedures. Locking devices should be installed as specified.
Field Tolerances
Whether vertical, raked, battered, or precambered, poles must be set to the alinement and orientation specified. Normally, the controlling factor will be the acceptable visual limitations. The following suggested tolerances may be adjusted if visual considerations are not a critical factor. When poles are set on anchor bolts, the pole should not be displaced horizontally from the specified alinement more than 1% of the height. It is essential that this check be made at a time when the sun does not cause an unequal temperature on each side of the pole. [...] After acceptable pole erection, the hold-down nuts on the anchor bolts should be secured with a suitable locking device, a second nut, or by applying a predetermined torque value. [...] Direct ground burial of steel pole structures has limited service experience. Where this method of installation is used, it is suggested that the previously specified tolerances should be followed for pole alinement. For multiple pole structures, e.g., H-frames, design considerations may require greater accuracies.
The location accuracy of guy anchors is not critical to the proper function of the structure, as long as the angularity of the guy in relation to the structure is maintained. The specified guy angle should be within a tolerance of 2°. Guys of 1/2-in. diam and larger should be prestressed to a value exceeding the everyday load condition prior to installation. Guys should be installed within 5% of the specified tension, through proper sagging or a suitable tensioning device. All attachment hardware should be provided with suitable locking devices to discourage vandalism or removal.
Other Considerations
New erection techniques, e.g., helicopter erection, are being used today. The use of special techniques and the applicable erection procedures should be closely coordinated with the pole manufacturer. Some of the previously mentioned recommendations on specific items, e.g., slip joints, should be carefully reviewed in light of the special erection procedures to be followed.
There are other worthwhile suggestions under "Transmission Line Construction" in the ASCE's Guide for Design of Steel Transmission Towers that apply equally well to pole type structures. The major consideration is that the construction forces be provided complete information on procedures and limitations during the construction of the line.
Climbing Provisions
Generally, provision should be made so that all parts of pole structures and insulator and hardware assemblies can be reached for maintenance. Steps and ladders can be either fixed or detachable, and should be sufficiently strong so as not to deform permanently under the weight of a man with tools and equipment. Steps and ladders should start at a specified point above the ground, generally 8 ft, and should extend to the top of the pole. Steps and ladder runs have been spaced from 12 in. to 18 in. apart vertically, and should be located on the pole to provide adequate clearance between the climber and energized parts, allowing for insulator movement under normal weather conditions. On large diameter poles, two or more sets of steps or ladders may be required. Detachable ladders should be fabricated in lengths that can be handled by a man on the structure. In some cases, special provisions, e.g., permanent walkways or handrails, may be required to assure accessibility to the ends of crossarms for erection and maintenance.
Grounding
Grounding lugs should be provided at the base of each pole and on each pole at each conductor level. The upper lugs should be suitable for grounding equipment during live-line maintenance.
Safety Devices
Safety of personnel climbing poles is of paramount importance. An appropriate safety clamp and belt should be supplied for a group of structures. Any safety clamp and belt should be supplied for a group of structures. Any safety clamp or safety device that allows free fall of any kind, whether it is in the clamp itself or in the connecting hardware, or in the belt, should not be accepted. To assure satisfactory operation, it is suggested that the safety clamp be constructed of stainless steel. The requirements of Title 29 Part 1926 of the Occupational Safety and Health Administration should be met.
Comparison with Laced Towers
In considering proper foundations for pole type transmission structures, the designer has a multitude of options to satisfy the loading conditions of the structures and the varied soil conditions of the structures and the varied soil conditions encountered at the site. The great majority of transmission structure foundations in prior years have normally dealt with laced structures with considerable base width. For this type of evaluation, the loads supported by the foundation were created by statically determinate structures and generally resulted in large uplift or downthrust loads with limited shear loads perpendicular to the leg of the structure. [...] In pole type structures, configurations that provide similar conditions are used, e.g., guyed structures and multiple pole structures with internal bracing designed to carry the horizontal shear loads to the foundations. However, the majority of pole type structures rely on the horizontal shear loads being carried to the foundation through flexural loading in the structure. This creates small horizontal shear loads and high moment loads at the top of the foundations.
Comparison with Wood Poles
Large single pole transmission structures normally carry service loads in excess of those encountered for wood poles and the foundation analysis should reflect a proper balance. Arbitrary setting depths, such as those established for wood poles, hsould not be used unless proper analysis shows that such depths are adequate to provide reliability for the designing loads. [...]
Design Considerations
The design of the structure should be closely allied tot he design of the foundation. A good structure design can be made worthless by a poor foundation design. The most common structure is the single cantilever pole. The deflection, or tilting, of the foundation does not greatly influence the load capabilities of this structure. For this condition, the primary tilting limitation is based on the allowable permanent set that can be tolerated within acceptable visual limitations. Under some conditions, electrical clearances within the structure or to adjacent structures may be the controlling factor. The structure normally has deflections, under load, far in excess of that created by the foundation movement, but the structure has the ability to recover when the load is relieved. For a guyed pole, the movement of the base foundation or the guy anchor foundation, or both, may affect the load distribution as well as the visual appearance. Normally the downthrust load in the pole is of considerable magnitude, and excessive movement of the guy anchor foundation allows the downthrust load to create a mo[ve]ment in the pole. [...] Multiple pole structures incorporating a combination of the preceding factors should be reviewed in light of foundation movements within the range of acceptable visual limitations and stress redistribution within the structure.
As large steel pole structures are now being utilized on high voltage circuits requiring large expenditures and maximum reliability, the structure and foundation design should incorporate adequate design factors to minimize future plumbing or adjustments of the structure. [...]
Foundations or anchors for transmission pole structures are generally of concrete. The design should provide for the resultant of all dead and live load reactions and proper evaluation should be made of all horizontal shears, overturning moments, and uplift or downthrust reactions. [...]
In establishing the maximum design values for the foundations, consideration should be given to the type of loading producing the foundation reactions. The greater part of the combined maximum reactions on pole type structure footings is usually from temporary loads, e.g., broken wire, wind, and ice. With the exception of heavy angle, dead end, or terminal structures, only a part of the total reaction is of a permanent nature. As a consequence, the permissible soil pressures as used in the design of building foundations, where the greater part of the design load on the foundation is of a permanent nature, may be considerably exceeded for foundations for pole structures. [...]
The cost of soil borings is small compared to the cost per mile. Some take the viewpoint that it would be more economical to omit borings and design a more conservative foundation. The primary purpose of soil borings is to assure an adequate and safe foundation. Whether this is more expensive than a haphazard guess at the foundation is a secondary consideration. The primary concern should be one of safety. No foundation could be designed for the worst possible soil condition. There is no limit to what the soil condition would be.
The 'Preface' states:
During the past decade, substantial effort has gone into improving methods for determining structural loads on transmission line structure. The Committee on Electrical Transmission Structures is charged with the responsibility to report, evaluate, and provide loading requirements of transmission structures. The initial results of this effort were presented in a committee report at the ASCE Annual Convention and Exposition, [in] San Francisco, Calif., on October 1, 1984. Comments received on the 1984 committee report were incorporated into a set of draft guidelines, which were presented for review and comment at teh ASCE's Sixth Annual Structures Congress, in Orlando, Fla., on August 19, 1987. This document, which incorporates comments received on both the 1984 committee report and the 1987 draft guidelines, was prepared by the Task Committee on Structural Loadings of the Committee on Electrical Transmission Structures. The recommendations presented herein are the consensus of opinion of the task committee members. [...]
The following information is excerpted from this book.
Systems, Subsystems, and Components
It is convenient to consider that a transmission line is an integrated system consisting of subsystems. The subsystems include the conductor subsystem, the ground wire subsystem, and one subsystem for each category of support structure that is to be designed for a different reliability level. If dead-end structures are to be made more reliable than tangent structures, then all dead-end structures are part of one subsystem that is different from that of the tangent structures.
Each subsystem in turn is made up of components. For example, the conductor subsystem includes the conductor itself, as well as strain or dead-end insulator assemblies. A structure subsystem may include many different components, including suspension insulators. For a lattice structure, components are angle members, bolts, foundations, etc.
Types of Load-Producing Events
When describing loads in a transmission line system, it is convenient to distinguish between the events that produce the loads and the resulting loads in the components of the subsystems. The loads are direct forces on the conductors and ground wires, or the structures. The events producing the loads can be classified as weather-related, accidental, and construction and maintenance (C&M) events.
Weather-Related Events
Weather-related events of interest are the occurrences of wind velocity, ice thickness, and temperature. [...] They can only be described by probability distributions. [...]
Weather-related events and corresponding loads [...] constitute the reliability requirement.
The probability that an event or load with a [one per year] return period will be exceeded at least once in the planned lifetime of a line (say 50 years) is given in table 1.2-1. This probability is a useful indicator, but it does not correspond to the probability of failure of the line or to that of any of its components.
Table 1.2-1 - Probabilities of Having at
Least One Event that is More Severe than
the Event with RP-Year Return Period
During an Estimated Life of 50 YearsReturn period
RP (years)Probability
50
0.64
100
0.39
200
0.22
400
0.12
[...]
Accidental Events
Some of the events that produce loads in a transmission line cannot be described statistically because of their nature and/or lack of data. Accidental events such as breaks of components from defects, wear, fatigue, or impact; or failures of entire structures from landslides, tornadoes, sabotage, or any other unforseen phenomena, fall into that category. Design procedures do not control the occurrence of these events, but attempt to minimize their consequences. Because of this, the designer has to make sure that if a failure is triggered by an accidental event or weather-related event, it will not propagate without control. This security requirement can be accomplished by designing for special loadings (longitudinal and torsional loads) at all or some structures, or by load-limiting devices such as mechanical fuses.
Construction and Maintenance Events
Some line components may be subjected to their critical loading during construction and maintenance operations. Once the magnitudes of the loads produced by the operations are established, they should be multiplied by a load factor to provide an adequate level of safety. Design for these factored loads, in addition to those specified by national regulations or codes of practice, constitute the safety requirement.
[...]
[...]
Pole Structures
[...] Surface roughness (e.g., rough for wood, smooth for steel) will influence the force coefficients for these shapes. Attachments on pole structures, such as steps, ladders, arms, and brackets, will also influence the force coefficients. The effects of attachments and surface conditions can be significant on highly streamlined shapes, such as circular members. [...]
Recommended force coefficients for structural shapes [ (circular, 16-sided polygonal, 12-sided polygonal, 8-sided polygonal, 6-sided polygonal, and square/rectangle) ] and surfaces (e.g., galvanized, weathered, or painted steel, and wood) commonly used in transmission pole structures [are listed in a table, not reproduced here]. [...]
Ice and Wind Loading
[...] Ice can be classified either by its method of formation or its physical characteristics. Precipitation icing is the most common icing mechanism, and can occur in any area subject to freezing rain or drizzle conditions. In-cloud icing, on the other hand, is caused by the impingement of supercooled cloud droplets on a surface. This icing mechanism usually occurs in mountainous areas where clouds exist with liquid water droplets at subzero temperatures.
Ice may also be classified into four types based on its physical characteristics, and noted here in order of decreasing density: glaze, rime, wet snow, and hoarfrost. Rime ice is usually associated with in-cloud icing conditions, whereas glaze ice may occur under either precipitation icing or in-cloud icing situations. Though recorded thicknesses of glaze ice are evident throughout most of the United States, a more severe icing condition may result from wet snow loads. Due to the ability of wet snow to accrete large radial thicknesses, even moderate winds may impose governing transverse loads on the structure. Hoarfrost is not usually a major problem in transmission line icing, since the amounts of ice accretion by this process are small compared to freezing precipitation icing, in-cloud icing, or wet snow.
It is extremely important that the transmission line engineer be aware of the icing conditions (i.e., freezing precipitation, in-cloud icing, or wet snow) that may occur along the route of a proposed transmission line. Ice accretion produced by a freezing rain rarely exceeds a thickness of a few inches, whereas that due to in-cloud icing and wet snow conditions can build to thicknesses of a foot or more. Furthermore, in-cloud icing and wet snow conditions can produce significant unbalanced accretion loadings on adjacent spans that have different wind exposures. The designer would benefit from the advice of a meteorologist in situations where there is a potential for in-cloud icing or wet snow. [...]
The four broad categories of icing cover the spectrum of the icing phenomenon. The distinctions made by definition of each category are not often identifiable in practice. It has been shown, for example, that ice density may vary over the duration of the icing event. This is particularly true of rime ice, which may approach glaze ice densities near the collector surface, gradually changing to soft rime near the outer surfaces of the ice. Furthermore, in some cases there can be an overlap of more than one type of icing condition, such as wet snow and freezing rain. In specifying ice loadings, the density should be noted and is typically assumed to be uniform with thickness.
Radial thickness of ice is assumed in design to be uniform throughout the wire's circumference and length. Observations of ice accretion have noted that this assumption is not always a naturally occurring phenomenon. Ice buildup may be uniform, elliptical, crescent-shaped, pennant-shaped, or have icicles attached. In calculating the vertical load component, the generalized assumption of uniform thickness is valid if the actual ice buildup has been converted to an equivalent radial thickness. All ice thickness magnitudes referred to herein are equivalent radial thickness. The assumption of uniform ice loading (radially and along the span) represents an envelope of the actual icing conditions. [...]
Ice accretion on the structural members themselves is typically not included directly in the design. For the design of bracing members of lattice structures and crossarms, the construction and maintenance loads recommended [...] will generally impose design stresses greater than the bending stresses resulting from the vertical weight of ice-coated members. For vertical supports (e.g., pole shaft or leg angle), the additional axial load due to ice on the member does not add significantly to the member stress. [...]
Ice loading is unique in that it may not occur on all wires on the transmission structure to the same degree. Although the principal design loading combination is with all wires on the structure having the same radial ice thickness, unequal ice loading should also be considered in the design. Due to varying exposures, it is possible for adjacent spans to experience different severity of conditions. These conditions, along with the possibility of differential ice shedding, may develop significant imbalances in the longitudinal wire tensions. Generally, tangent structures with suspension insulator strings will not experience significant longitudinal conductor loads due to ice imbalances; however, shield wire attachments with short hardware assemblies may transfer most of the imbalance to the structure. [...]
Introduction
Transmission line design must include loadings from many sources in addition to the more common weather-related events described. This section on special loads outlines some of the other loadings that transmission structures may encounter. The section is not all-inclusive; conditions requiring special investigation and design, such as landslides, ice flow, flooding, and many other possible load-producing events, are not discussed
Longitudinal Loads
[...] The risk of cascading of transmission line structures can be reduced by one of three methods. The method selected is governed primarily by the type of structure and, to some extent, by local conditions and practices. These methods are sometimes referred to as "security requirements".
Rigid square-based latticed towers; typical guyed (four guys) V, Y, Delta, or Portal; and single-shaft pole structures can resist the applied longitudinal loads required for containment at relatively low cost. Thus, it is a common practice to specify longitudinal loads that will provide strength to resist cascading at every structure. All structures are expected to resist cascading, although localized structure loss adjacent to the origin of the failure can be anticipated. [...]
Some structure types, such as H-frames and narrow-based rectangular latticed structures, have little inherent ability to withstand the longitudinal loads of a cascading line. The cost of strenthening each structure to resist cascading could be prohibitive, and the required structural modification, such as the addition of longitudinal guys, might be undesirable.
Very early users of wood poles or wood H-frame structures adopted the practice of creating stop or anchor structures at intervals along the line, so that a cascade failure would be limited to what the users decided was a manageable number of failures. The stop structures would often be an ordinary suspension structure with extra longitudinal guys, the structure being at a site where local conditions of soil or land use readily permitted the guys.
With structures such as H-frames, there is a tendency to rely on tightly attached ground wires as components that will act to resist cascading. This perception may be valid if the ground wire system remains intact and the disturbance arises in the conductor system. However, very long cascades have resulted when the ground wire system was interrupted, as by failure of a ground wire or of an angle structure, and the longitudinal pulls of the ground wires became major contributors to the cascade.
A third method of protecting against cascading has been used with evident success in a few countries whereby slip- or release-type suspension clamps act as fuses and limit the longitudinal loads that can be applied by the wires. The design of the slip or release mechanism must ensure consistent operation under extremes of temperature or when wires are coated with ice, and not be subjected to change due to buildup of corrosion or oxide products. If the clamps perform as expected, structures with reduced longitudinal strength may be adequate in preventing cascading.
There may be justified restrictions on the use of release clamps and reduced strength towers in areas where heavy ice buildups are frequent. A clamp release under unbalanced ice could result in a dangerous and undetected clearance to ground, or present a danger to linemen. Therefore, the use of this method is not recommended in areas of heavy ice loadings.
Galloping
Galloping (or dancing) is a dynamic condition that occasionally occurs in transmission line ground wires and conductors. Galloping usually occurs with a moderate wind blowing across an ice-coated wire. The wires move at amplitudes ranging from a few feet to more than the full sag. Galloping may cause one or a combination of the following:
- flashover or direct contact between phases or between phase and ground wire, resulting in line outages and possible conductor damage;
- excessive conductor sag due to inelastic stressing;
- failure or wear damage of the ground wire or conductor support hardware; and
- failure in the supporting structure.
Flashover and direct contact are not loading related; they are service related. These events can be minimized by spacing the individual wires so that the assumed wire paths during galloping do not cross.
Galloping conductors produce vertical, transverse, and longitudinal loads. There have been several studies performed to quantify the loads produced by galloping. Load magnitude is dependent on many factors. These include, but are not limited to, span length, wire type, rigidity of the wire support, weather-related conditions, and wire tensions. In a paper on the dynamics of galloping loads, Fang and Simpson (1978) conclude that:
- The maximum dynamic tension for single-loop galloping is usually considerably less than the rated strength of the conductor; however, fatigue failure may occur.
- Except during extreme galloping, the dynamic vertical load does not exceed the capacity of the conductor support hardware; however, fatigue failure may occur.
- The dynamic tension is inversely proportional to the number of loops per span length, and the increase in this tension is proportional to the system stiffness.
- Due to the large influence of conductor support rigidity on dynamic tension, galloping is potentially more damaging to dead-end structures and structures with rigid conductor supports, than most suspension structures having flexible conductor supports.
The potential for support structure damage exists when the fluctuating loads produced by galloping are in resonance with the structure. Approximate natural frequencies of galloping are 0.08 to 3 Hertz. Some types of structures, such as running angle suspension structures and guyed heavy angle or dead-end structures, have been reported to be susceptible to damage from the loads produced by galloping. However, there have been few reported cases of galloping loads presumed to have caused complete structure failure.
Measures to reduce or eliminate galloping include detuning pendulums, interphase spacers, airflow spoilers, and modified conductor design. These methods are currently being evaluated by field investigation.
Construction and Maintenance Loads
Unlike weather-related loadings, construction and maintenance loadings are to a large extent controllable, and are directly related to construction methods. Workers can be seriously injured as a result of structure overstress; therefore, personnel safety should be a paramount factor when establishing construction and maintenance loadings. [...]
Construction loads are those loads that act upon the structures due to the assembly and erection of the structures themselves, and due to the installation of ground wires, insulators, conductors, and line hardware.
Special erection methods, such as lifting or rotating a structure, may be critical to some structure members. In these cases, specific loading conditions simulating the construction methods must be added to the structure loading agenda. These loadings may be as simple as the case of lifting wood, concrete, or steel poles. On the other hand, they may be significantly more complex, as in the case of truss actions developed by tilting up a ground-assembled lattice tower, or truss action resulting from pickup of large sections of a lattice tower for crane or helicopter erection. These erection loadings result from supporting the weight of the structure in some manner different from the manner of weight support in a completed structure. [...]
Ground wire and conductor installation loads result from the application of cable tensions in different locations and directions than those applied to the completed transmission line. For example, at the ends of a wire pull, the wire passes over the stringing blocks, then downward to the pulling or tensioning equipment at ground level. A load is produced at the location where the stringing blocks are mounted to the structure. [...]
As stringing progresses, the wires may be transferred from tensioning equipment to temporary anchors, so that additional wires may be pulled in the same spans; consequently, there may be several loads acting simultaneously on the structure. [...]
It is not usual practice to make temporary dead ends to unguyed suspension structures or to crossarm support points. Any such attempts must be carefully planned and checked against the design capacities of the structures.
During a tension stringing operation, the running board may sometimes jam in the block, and sturctures have been pulled over when there was no control to stop the pull. Although a few utilities have designed suspension structures to resist such possible loads, a more practical solution is to control the stringing operation so that these loads to not exceed the strengths required for other service conditions.
Maintenance loads are those loads that act on the structures as a result of scheduled or emergency inspection and/or replacement of all or part of a structure or all or part of the ground wire, insulator, conductor, and conductor hardware system.
Structure maintenance loads consist of the effects of workers on the structure being maintained, and of loads at adjacent structures due to temporary modifications, such as temporary guying, to permit the repair or replacement of the structure being maintained.
The most common kinds of maintenance performed on a transmission line relate to the ground wires, conductors, and insulators. At times, it is necessary to remove the wires from their supports, and either lower them to the ground or transfer them to some temporary alternate support location. Unless care is taken, these operations can greatly magnify the ordinary weight loading imposed on the structures. [...] Also, if the sag if the wires is changed (as by lowering), there is a companion change in wire tension acting at the structures adjacent to the lowered span. [...]
With level spans, the lowering of wires at one structure will cause an increase in tension in the wires that would almost double the original value, unless there was swing inward at the adjacent structures. This very simple maintenance operation can impose dangerous combined vertical and longitudinal loads on the adjacent structures in some conditions.
Structure Vibration
Transmission line structures can be subjected to dynamic forces caused by the wind, conductor motions, and earthquakes. These forces have the potential to initiate complete structure or individual member vibration. Industry experience has demonstrated that structure and member vibrations generally do not occur, or have not caused design problems, and only isolated occurrences have been reported.
The majority of reported problems have been with wind-induced vibration of individual members. These events have occurred with both tubular and structural shapes (such as single angles and double angles back to back), and members with reentrant cuts. The result of this type of vibration can cause fatigue failure of the member or connection bolts, or loosening of bolted connections. Design and detailing practices have been used to mnimize individual member vibration. [...]
Introduction
A transmission structure is designed to support a continuous system of condctor and overhead ground wires. A transmission line has a large number of structures located at varying environmentally exposed and geographically different sites. The potential of a transmission struture being placed in an unknown vibration-prone environment is much greater than that for a typical civil engineering structure.
Dynamic behavior of a structure can be characterized by using two parameters: naturla frequency and vibration mode shape. These vibration characteristics are controlled by the structure's mass, stiffness, and damping. The structure's mass includes that of its members, attached wires, and hardware. The structural stiffness is affected by the structure member sizes, material, geometry, and foundation supports. The structure damping is a function of the member connections, aerodynamic drag, foundation conditions, and hysteresis behavior of the members. [...]
Vibration of a transmission structure can consist of complete structure vibration modes, structure component modes, or individual member vibration modes. The initiation of these modes can be caused by induce vibration forces from the wind acting directly on the structure, from conductor and overhead ground wire vibrations (aeolian, subconductor oscillation, and galloping), or by induced ground motion such as that caused by an earthquate.
Structure Vibrations
Conductor motions (aeolian, subconductor oscillation, and galloping) can be coupled with the structure vibration modes when the frequency of the vibrating conductor corresponds to one of the natural frequencies of the structure. Approximate natural frequencies of conductor vibration for aeolian motions are 3 to 150 Hertz, for subconductor oscillation 0.15 to 10 Hertz, and for galloping 0.08 to 3 Hertz.
In most cases, conductor vibration is not a problem, because conductor systems can be designed, using dampers and spacer dampers, to prevent and/or reduce the effect of wind-induced vibration behavior. Of the three types of conductor vibrations, galloping has the potential to cause the most structural damage. Lattice steel running angle suspension towers, guyed-mast dead-end structures, heavy-angle towers, and flexible "narrow-base" pole structures, have been reported to be more susceptible to damage caused by conductor galloping motions. [...]
Typically, transmission structgures are not designed for ground-induced vibrations caused by earthquake motion. The strandard transmission structure loadings caused by wind/ice combinations and broken wire generally exceed design earthquake liads. This may not be the case if the transmission structure is partially erected, or if the foundation fails due to earth fracture or liquefaction. [...]
Longitudinal Loads Produced By Weather-Related Events
Unequal wind or ice loadings on adjacent spans, and even extremes of temperature with unequal adjacent spans, can result in differential tensions, but these potentially dangerous imbalances are usually reduced to inconsequential values by the swing of the suspension strings. The longitudinal loads transmitted to the structures by the inclined strings will seldom exceed 10 to 20 percent of the conductor bare wire tension, except in hilly or mountainous terrain where in-cloud icing is a hazard. The "in-cloud" ice load can be very large and of greater significance, [and] the loads can differ greatly from span to span.
Long unloaded adjacent spans with large available 'slack' can allow insulator string swing that effectively turns the suspension assembly into a strain support that transfers the full tension differential to the structure. [...]
The buildup of in-cloud icing is very dependent on exposure to moisture-laden winds and, in mountainous or very irregular terrain, it is not uncommon to go from very heavy ice loading in one span to negligible loading in the next. If the unloaded span is a long span, the slack therein may be pulled into the loaded span and may produce large longitudinal loads on the intervening suspension structure. [...]
Longitudinal Loads Resulting From Stringing and Maintenance Practices
Large but temporary loads may be imposed on structures during stringing and maintenance operations, when conductors or ground wires are being lowered from their support points. It is a generally observed principle that these temporary loads should not govern the design of the structures. Therefore, the field operations should be controlled so that imposed loads are within the capabilities of the structures. [...]
Longitudinal Strength Requirements for Failure Containment (Anticascading)
The infrequent failure of a few structures or components must be accepted as a result of buiding transmission lines with many miles of exposure to the extremes of wind and ice and other causes of mishaps, such as aircraft, vehicles, footing washouts, and tornadoes. A line design should be based on the anticipation that such mishaps will occur, and therefore should include strengths that will ensure that damage is limited to within a few structures, at the most, of the site of the original failure. [...]
Experience will demonstrate that it is almost impossible to anticipate the manner and form of the initial failure. A train derailment, a major tornado, a low-flying aircraft, or a freak ice storm may bring several structures to the ground, accompanied by no or many wire breakages, and with dynamic energy inputs or releases that cannot be assessed. [...]
Many "flexible" or low-longitudinal-strength H-frame-type structures have been installed with the underlying theory that the ground wires, firmly attached at the tops of the structures, would provide longitudinal restraint in the event of a broken conductor or some other imposed loads resulting from a failure of another structure. Some of the longest cascades of recent decades occurred on the flexible ground-wire-supported lines, when a heavy angle structure failure or similar mishap introduced excessive slack into both conductor and ground wire systems. The pulls of the ground wires were major contributors to the cascading that followed. Reliance on ground wire support may be a very false security measure. [...]
[...]
Some Causes of Failure
The following list represents some of the more general causes of transmission line failures.
- Natural phenomena (exceeding design conditions):
- extreme wind;
- extreme ice (on structures and conductors);
- combination of (1) and (2);
- landslides;
- snowslides;
- ice movement on rivers and lakes ([for] structures located in the water); and
- flooding (causes damage to the structure or foundation).
- Unnatural causes:
- sabotage, vandalism, or theft of members and bolts; and
- damage caused by equipment and vehicles (airplanes, barges, cranes, tractors, trucks, etc.).
- Structure deficiencies (where design conditions were not exceeded):
- design inadequacies of structure;
- missing members or bolts, caused by vibration or omitted during erection;
- misfabricated members, resulting in incorrect size, length, or grade of steel; and
- inadequate or improperly set footings.
- Conductor, ground wire, and hardware deficiencies:
- improper wire splices;
- faulty or inadequate hardware; and
- fatigue failure of wire or hardware components.
- Construction-related causes:
- excessive vertical load during stringing;
- excessive longitudinal load during stringing, caused by the running board hanging up in the stringing block; and
- improper stringing sequence during reconductoring.
[...]
(Additional information from this book is located on the Power Systems page, the Interference Between Power and Telecom Lines page, the Overvoltage and Flashovers page, and the Insulator Usage page.)
The following information is excerpted from this book.
[...] When the sag is a maximum, the minimum height of the conductor shall not be less than a certain specified distance above the ground. The British Regulations specify the following minimum ground clearances at 122° F. :
Working Pressure in kV.
Minimum Ground Clearance.
Not exceeding 66
20 ft.
66 to 110
21 ft.
110 to 165
22 ft.
Exceeding 165
23 ft.
For voltages not exceeding 650 D.C. or 325 A.C., the minimum ground clearances of any line conductor (other than a service line), earth wire, or auxiliary conductor, at any point of the span are as follows:
- 19 ft. across a public road.
- 17 ft. in other positions.
- 15 ft. in positions inaccessible to vehicular traffic for service lines.
- 19 ft. across a carriageway.
- 17 ft. along a carriageway.
[...]
(Additional information from this book is located on the Power Systems page, the Interference Between Power and Telecom Lines page, and the Insulator Usage page.)
The following information is excerpted from this book.
[...]
Factors Affecting Line Design
The function of the overhead three-phase electric power transmission line is to transmit bulk power to load centers and large industrial users beyond the primary distribution lines. A given transmission system comprises all land, conversion structures, and equipment at a primary source of supply, including lines, switching, and conversion stations, between a generating or receiving point, and a load center or wholesale point. It includes all lines and equipment whose main function is to increase, integrate, or tie together power supply sources. [...]
The basic configuration selection depends on many interrelated factors, including esthetic considerations, economics, performance criteria, company policies and practice, line profile, right-of-way restrictions, preferred materials, and construction techniques.
Figure 2.9 shows typical compact configurations.
Figures 2.10 - 2.13 show typical structures used for EHV transmission systems.
[Ref. 14 = Electric Power Research Institute. "Transmission Line Reference Book: 345 kV and Above." EPRI, Palo Alto, California, 1979.]
Figure 2.14 shows a 345-kV line with a single circuit and wood H-frame, whereas Figure 2.15 shows a 345-kV line with a double circuit and steel tower.
[...]
Introduction
Overhead construction is only 15 to 60 percent as costly as underground, and is therefore more economical. The first consideration in the design of an overhead line, of course, is its electrical characteristics. [...] The electrical design of the line must be sufficient for the required power to be transmitted without excessive voltage drop and/or energy losses, and the line insulation must be adequate to cope with the system voltage. The mechanical factors influencing the design must then be considered. For example, the poles supporting the conductors must have sufficient mechanical strength to withstand all expected loads. Another example is that the material chosen for the conductors must be strong enough to withstand the forces to which it is subjected.
The conductors and poles must have sufficient strength, with a predetermined safety factor, to withstand the loads due to the line itself, and stresses imposed by ice and wind loads. Thus, the overhead line should provide satisfactory service over a long period of time without the necessity for too much maintenance. Ultimate economy is provided by a good construction, since excessive maintenance or especially short life can easily more than overbalance a saving in the first cost.
The overhead line must have a proper strength to withstand the stresses imposed on its component parts by the line itself. These include stresses set up by the tension in conductors at dead-end points, compression stresses due to guy tension, transverse loads due to angles in the line, vertical stresses due to the weight of conductors, and the vertical component of conductor tension. The tension in the conductors should be adjusted so that it is well within the permissible load of the material. This will mean, in practice, that one must allow for an appreciable amount of sag.
The poles must have sufficient height and be so located, taking into account the topography of the land, as to provide adequate ground clearances at both maximum loading and maximum temperature conditions. The conductor ground clearance for railroad tracks and wire line crossings, as well as from buildings and other objects, must meet the requirements of the National Electric Safety Code (NESC).
A proper mechanical design is one of the essentials in providing good service to customers. A large majority of service interruptions can be traced to physical failures on the distribution system, broken wires, broken poles, damaged insulation, damaged equipment, etc. Of course, many of these service interruptions are more or less avoidable. But their numbers can be reduced if the design and construction of the various physical parts can withstand, with reasonable safety factors, not only normal conditions but also some probable abnormal conditions.
Therefore, the overhead line must be designed from the mechanical point of view to withstand the worst probable, but not the worst possible, conditions. For example, the cost of an overhead line that would withstand a severe hurricane would be tremendous, and thus from the economical point of view, it may be justifiable to run the risk of failure under such conditions. [...]
Factors Affecting Mechanical Design of Overhead Lines
In general, the factors affecting a mechanical design of the overhead lines are:
- Character of line route.
- Right-of-way.
- Mechanical loading.
- Required clearances.
- Type of supporting structures.
- Grade of construction.
- Conductors.
- Type of insulators.
- Joint use by other utilities.
Character of Line Route
The routes of overhead transmission lines are usually selected across the country on private right-of-way in order to obtain the most direct route and proper space for towers, as well as to avoid buildings, roads, highways, and low-voltage lines. Lower voltage overhead distribution lines are run along streets and highways as much as possible, in order to reach customers more easily and to make the lines accessible for maintenance. In urban and suburban areas, poles are spaced 100 to about 150 ft apart, to provide convenient points for service attachments or service drops, and to keep the service lengths to a minimum. Usually, poles are set from half to one foot inside the curb when along streets. Transmission lines may have spans of several hundred feet. Of course, the general character of the country in which the overhead line is to be located affects the design, primarily in terms of selecting the conductor and [the] type of supporting structures. [...] In general, the factors affecting the length of a span are:
- Character of route.
- Proper clearance between conductors.
- Excessive tensions under maximum load.
- Structures adequate to carry additional loads.
It is usually not recommended, especially in mountainous country or in heavily populated areas, to choose a direct route or try to locate the line on long tangents.
Right-Of-Way
[...] Higher voltage transmission lines on private right-of-way are usually built with long spans, and the type of terrain covered by the line has impact on the selection of the construction type. Of course, existing right-of-way should be utilized whenever possible, for especially augmented transmission systems, and in many cases this is done with less environmental disruption than would occur with the acquisition of new right-of-way. Advance planning and scheduling of road, pipeline, telephone, and electrical transmission is imperative for the future. In general, rather than purchasing the right-of-way in a fee, a permanent easement is obtained in which the owner or owners permits the necessary rights to construct and operate the line, but keeps ownership and use of the land. The easement secured must stipulate the following:
- Permission to build all supporting structures.
- Permission for a means of access to each supporting structure.
- Permission to clear all trees and brush over a width of at least 10 ft larger than the spread of the conductors, in order to allow sufficient working space for construction.
- Permission to remove all trees which might violate the minimum required clearance to the conductors if they were to fall.
- Permission to remove all trees which might violate the minimum required clearance to conductors if the conductor were to swing out under maximum wind.
- Permission to remove all obstacles -- for example, buildings, lumber piles, haystacks, etc. -- which might cause a fire.
As a rule, trees that may interfere with conductors should be trimmed or removed. Normal tree growth, the combined movement of trees and conductors under adverse conditions, voltage, and sagging of a conductor at elevated temperatures, are among the factors to be considered in determining the extent of trimming required. Where trimming or removal is not practical, especially in distribution lines, the conductor should be separated from the tree with suitable insulating materials or devices to prevent conductor damage by abrasion and grounding of the circuit through the tree.
Mechanical Loading
The term mechanical loading refers to the external conditions that produce mechanical stresses in the line conductors and supports; that is, poles or towers. Mechanical loading also includes the weight of the conductors and structures themselves. Structures are subject to vertical and horizontal loads. Vertical loading includes the dead weight of equipment such as crossarms, insulators, conductors, transformers, etc. It also includes ice and snow clinging on the structures and the conductors.
Poles supporting overhead conductors and other equipment are subjected to strains from the tension with which they are strung. When a force is applied against an object, it produces stress within the object. There are five kinds of stress:
- Tensile Stress. Caused by the forces acting in opposite directions, away from the body and along the same straight line to elongate, or stretch, the body beyond its normal length. For example, a conductor strung between two poles, or a guy wire, under tension is subjected to tensile stress.
- Compressive Stress. The opposite of tensile stress. Caused by the forces acting toward the body to shorten the body. For example, a distribution transformer hung on a pole subjects the pole to a compressive stress.
- Shearing Stress. Caused by the forces, not in the same straight line, that tend to cut the body in two. For example, bolts attaching a cross-arm to a pole are subjected to a shearing stress between the cross-arm and the pole.
- Bending Stress. Caused by the forces acting along a body. For example, a pole supporting a corner in the line, and not guyed, is subject to a bending stress.
- Twisting Stress or Torque. Caused by line tensions that are not equal on the two sides of a pole. For example, a pole may be subjected to a twisting stress when a conductor breaks between supports.
[...]
Required Clearances
In general, the following clearances need to be considered: ground, tracks, buildings, trees, conductors and structures of another line, other conductors on the same structure, the structure itself, guy wires and other equipment on the structure, and the edge of the right-of-way. The NESC gives the minimum required clearances. [...]
Briefl, the location of poles must be chosen to provide sufficient clearance from driveways, fire hydrants, street traffic, railroad tracks, buildings, fire escapes, etc. Table 9.1 gives the clearance of conductors passing by but not attached to buildings and other installations except bridges. The given clearances are taken from the NESC, 1984 edition.
Conductors of one line should not be less than 4 ft from those of another and conflicting line. If conductors pass near the pole of another overhead line, providing that they are not attached, they should not interfere with the climbing space.
Table 9.2, taken from the NESC, 1984 edition, presents vertical clearances. The given values are applicable to crossings where span lengths do not exceed 175 ft in heavy-loading districts, 250 ft in medium-loading districts, or 350 ft in light-loading districts. The given clearances are based on a temperature of 60° F with no wind, and voltages not over 50 kV to ground. For longer spans and higher voltages, larger clearances are required, depending upon sag and tension in the span.
Crossings [of wires] should be made on a common crossing structure where practical. If not practical, the clearance between any two wires, conductors, or cables crossing each other and carried on different supports should not be less than the values given in Table 9.3 in order to prevent the possibility of accidental contact under varying wind, temperature, and ice loading. The given clearances apply at 60° F with no wind, and for spans not exceeding 175, 250, or 350 ft, in heavy-, medium-, and light-loading districts, respectively. For longer spans and higher voltages, greater clearances are required, depending on sag and tension in the span.
The NESC requires that for supply conductors of the same circuit, at voltages up to 8.7 kV, the minimum horizontal clearances between conductors should be 12 in., and for higher voltages should be 12 in. plus 0.4 in. per kilovolt over 8.7 kV.
It is required that for supply conductors of different circuits, at voltages up to 8.7 kV, the minimum horizontal clearances between conductors should be 12 in.; for voltages between 8.7 and 50 kV, the clearances should be 12 in. plus 0.4 in. per kilovolt over 8.7 kV; and for voltages between 50 and 814 kV, the clearances should be 28.5 in. plus 0.4 in. per kilovolt over 50 kV. [...]
In addition to those clearance requirements included here, the NESC provides other minimum requirements, such as for climbing space through lower wires on a pole to gain access to wires on upper arms, or for vertical separation of cross-arms. [...]
Type of Supporting Structures
There are basically four different pole types:
- wood poles,
- concrete poles,
- steel poles, and
- aluminum poles.
In general, wood poles are preferred over others for overhead distribution lines because of the abundance of the material, ease of handling, and cost. Concrete poles reinforced with steel have been used for street lighting because of their neat appearances. Steel poles have been used to support trolley overheads, and street and parkway lighting. Both concrete and steel poles have been used to a limited extent for distribution. Aluminum poles are used basically for parkway lighting.
The life of wood poles is materially extended by impregnation with wood preservatives. Wood that has been properly treated for the environment in which it will be used will resist decay and maintain its mechanical strength for many years. A minimum life expectancy of 35 years has been accepted by the wood industry. Cedar, pine, and fir are best suited by their proportions and properties for use as distribution poles. Besides their usage in distribution systems, wood structures have been utilized for many years as a means of supporting single- and double-circuit transmission lines at voltages of 115 through 230 kV, and single circuit of 345 kV. As a result of developing technology, wood structures have recently been designed for applications up to 765 kV, and tested for 500 kV. Wood structures design is based on an assigned or calculated ultimate stress for the species used. The inherent flexibility of wood adds a certain degree of cushion when severe loadings are imposed. This property provides wood construction the ability to absorb shock loads, and longitudinal load capability not found in rigid structures.
Figure 9.6 shows some typical single-pole wood structure designs used in distribution systems. Figure 9.7 shows typical single-column [transmission] structure designs. Single wood column designs have been used for double-circuit lines through 230 kV, and appear feasible for 345 kV. Structures using two columns, as shown in Figure 9.8, provide the basis for conventional H-frame designs with variations. Wood cross-arms are normally used, although metal arms are sometimes specified. Double-circuit structures have been built using two columns of voltages through 230 kV, and appear feasible for double circuits of 345 kV.
In distribution systems, single poles are widely used to support three-transformer banks and their fused disconnects and surge arresters. A-frame poles are used where greater strength is required, and H-frame poles are used where it is necessary to support switching equipment and/or a transformer as well as the line.
The poles must have sufficient height and be so located as to provide adequate ground clearance at either maximum-loading or maximum-temperature condition. The conductor ground clearance for railroad tracks and wire line crossings, as well as from buildings and other subjects, must meet the requirements of the NESC and other local rules and regulations. In essence, the height of the pole required for a particular location is determined by the following factors:
- Length of vertical pole space required for wires and equipment.
- Clearance required above ground or obstructions for wires and equipment.
- Sag of conductors.
- Depth of pole to be set in ground.
In distribution systems, the most commonly used pole is the 35-ft pole, and poles shorter than 30 ft are generally not used. The 30 ft pole may be used in alleys and on rear-lot lines. The larger sizes are, of course, used for providing clearance over obstructions, for heavier loads, etc.
The size (i.e., class), or diameter, of the pole is determined by the strength required to endure the mechanical loading imposed upon it. The critical point of strength for an unguyed pole [is] at or near the ground line -- therefore, the circumference of the pole at this point determines the resisting moment of the pole when bending as a cantilever. However, if a pole is guyed, the diameter of the pole at the point of attachment of the guy is the measure of its strength. The resisting moment at the point of the guy attachment must be sufficient to endure the bending stresses caused at that point. Also, the top of the pole must be of adequate circumference to permit the attachment of cross-arms without excessively weakening the pole near the top. The wood poles are divided into several classes, according to top circumference and the circumference 6 ft from the butt end for each nominal length. The wood class refers to the dimensional classifications set up by the American Standards Association. The classes are numbered from 1 to 10. Class 1 provides the largest ground circumference, and class 7 the smallest. Classes 8 to 10 inclusive specify minimum top circumferences only. All poles in a given class, regardless of length, have approximately the same strength against load applied horizontally at the top. Table 9.6 gives the standard pole dimensions for yellow pine, chestnut, and western cedar. In order to identify any particular wood pole, its class, pole length, and wood type should be given.
A stable pole must have sufficient setting depth. Table 9.7 provides minimum depth of pole settings. However, the distribution engineer chooses the depth of settings as the situation dictates. For example, corner poles should have about 6 in. deeper settings. Of course, the stability or rigidity of the pole depends not only on the depth of setting, but also on the type of earth, moisture content of soil, size of pole butt, and setting technique used. Figure 9.9 shows some of the setting techniques.
Table 9.7 - Minumum Required Setting Depths
Pole Size (ft)
30
35
40
45
50
55
60
70
Setting Depth (ft)
5
5.5
6
6.5
6.5
7
7
7.5
Earth can be classified into eight different groups, as given in Table 9.8, for the purpose of settings. Table 9.8 also gives the resistance Se, as percentage of pole ultimate resisting moment, that the earth around the pole base shows to displacement for various earth types. The values given in the table are somewhat arbitrary, and based on the assumptions that the pole setting is standard, the hole diameter is minimum, and the backfilling is properly tamped.
Table 9.8 - Various Earth Resistance to Displacement
Class
Earth Type
Percentage of Pole
Resisting Moment, Se1
Hard rock
50
2
Shale, sandstone, or soft rock
50
3
Hard, dry hardpan
50
4
Crumbly, damp
40
5
Firm, moist
35
6
Plastic, wet
30
7
Loose, dry; or loose, wet
25
8
Swamps, marshes
20
Mechanical Calculations
In general, the forces acting on a given supporting structure -- for example, the pole -- are:
- Vertical forces due to weight of pole, conductors, ice clinging to conductors.
- Vertical forces due to downward pull of guys.
- Lateral forces due to wind across line pole, conductors, ice, etc.
- Longitudinal horizontal forces due to unbalanced pull of conductors.
- Torsional forces due to unbalanced pull of conductors.
[...] If there is an angle in the line, an additional stress is imposed upon the supporting structure at the angle point because of the tensions in the conductors. [...]
If the angle departure of the line is less than 60°, the resultant side pull force is less than the maximum tension of the conductors in the adjacent spans. Therefore, one single guy installed in the opposite direction of the resultant side pull force will be sufficient. However, if the angle departure of the line is larger than 60°, the resultant side pull force is larger than the maximum tension of the conductors in the adjacent spans. In order to stop a tendency to displace the pole if the angle does not exactly bisect the line angle and not use a guy of extreme strength, install two guys each located in the opposite direction of the line. [...]
It is almost impossible to build an overhead line of any considerable length, especially transmission lines, without several angles, which may vary in magnitude from only a few degrees to 90° or more. [...]
Whenever a pole is not strong enough to endure the bending stresses imposed on it by unbalanced forces, it should be guyed. For example, at the pole where the direction of a line changes, tension of the conductor should be supported by guying to other poles, to a ground anchor, or to a stub. Another common usage of guys is at dead-ends.
The strength of a guy should be large enough to take the entire horizontal stress in the direction in which it acts, the pole acting only as a strut taking only the vertical component of guy tension. Special structures such as A-frames, push braces, etc., are sometimes used instead of guys in some applications; but the most common technique is to install guys of steel wire or other high-strength material to take the stress. Figure 9.14 illustrates various guying techniques. Figure 9.15 shows plan and elevation views of a guy installation at an angle. Figure 9.16 shows a dead-end guy installation.
Guys are firmly attached to poles by wrapping the end of the guy wire twice or more around the pole and clamping the free end to the main section of the guy, usually by means of one or more guy clamps. However, nowadays the guy is usually attached to the pole by a thimble-eye or by a guy eye bolt and a stubbing washer, as shown in Figure 9.17. The attachment point of a guy should be as close as possible to the point where the resultant side pull is imposed upon the supporting structure. If there are several cross-arms mounted on the pole at different elevations, then the load at those elevations has to be converted to an equivalent load applied at the level where the guy is attached.
Usually one or two strain insulators are installed in guys to prohibit the lower part from becoming electrically energized by contact of the upper part with conductors or by leakage. Figure 9.17 shows the basic components of an anchor guy wire, which include the guy wire, clamps, the anchor, and a strain insulator. The guy wire is usually copperweld, galvanized, or bethanized steel.
Burring logs, which were called dead-men, [set] in the ground to anchor the guy wire, as shown in Figure 9.14(c), have been abandoned since the soil conditions often deteriorated the wood. Instead, in the present practice, metal anchors are used in any type of ground, from swamp to solid rock. [...]
Grade of Construction
The criterion used for the strength requirements of a line is called the grade of construction. The grades of construction are specified on the basis of the required strengths for safety. The NESC designates the grades for supply and communication lines by the letters B, C, D, E, and N. Grade B is the highest, and requires the greatest strength. Grade D is specified only for communication lines, and it is higher than grade N. The grade used depends on the type of circuit, the voltage, and the surroundings of the line. For example, a power line of any voltage, crossing over a main track of a railroad, requires grade B construction, but under certain other conditions may be as low as grade N. In addition to the NESC requirements, there are also local rules and regulations for the grades of construction.
Line Conductors
Copper and aluminum are the metals most frequently used as conductors in distribution systems. The selection criteria include conductivity, cost, mechanical strength, and weight. According to these criteria, copper conductor is the best [conductor] and aluminum is the second-best conductor, in terms of conductivity and availability. Aluminum has the advantage of about 70 percent less weight for a given size, but its conductivity is only about 61 percent that of annealed copper. Its breaking strength is about 43 percent that of hard-drawn copper. In general, aluminum conductor is rated as equivalent to a copper conductor two AWG sizes smaller, which has almost identical resistance. [...]
Joint Use By Other Utilities
There are advantages in the use of joint poles. However, when supporting structures of overhead lines are used jointly by other utilities, such as telephone or other communication systems, additional factors are introduced into the problem of line design beside those needing consideration in the case of power lines alone. For example, often a higher grade of construction is necessary, and consideration must be given [to] the required separations between the conductors and equipment of the two utilities.
The cost of providing the pole is borne jointly by the companies that share in its ownership. In general, the allocation of the expense is made in proportion to the space assigned to [the] owners. The cost of the clearance between higher voltage and lower voltage power circuits is usually charged to the [owner of the] higher voltage circuits. However, the required clearance space between power and communication circuits or between the lowest attachment and ground is disregarded in determining percentage ownership. It is also possible that poles are used jointly under a lease agreement, in which case the leasee has only the right to occupy a designated space.
In general, conductors are placed in such an order so that the higher voltage conductors are at the higher levels. As a result of this, the highest voltage circuits are near the pole top, and communication circuits are at a lower level, as shown in Figure 9.21.
Conductor Vibration
The failure of conductors under tensions that are much below maximum design stresses has been caused by fatigue due to very fast vertical vibrations of the conductor (from 15 to 100 Hz) caused by steady nonturbulent winds blowing across the line. [...]
Since two factors can cause conductor motion -- namely, conductor shape and weather -- Kaiser Aluminum engineers deduced that wind-induced motion can be controlled by changing the shape of the conductor. It is obvious that a round conductor presents the same profile to the wind along its entire length between structures. Therefore, it was reasoned that a twisted double conductor would present a continually changing profile to the wind, thus preventing the buildup of resonant vibrations. [...] A T2 conductor is made up of two round aluminum conductors twisted at the factory to make one complete 360° revolution gradually over approximately every 9 ft of length. Its profile is like a figure 8. [...] Figure 9.23 illustrates the installation of T2 conductors. Figure 9.24 shows a corner tower carrying T2 conductors.
[...]
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