Electrical Transmission in a New Age

Determination of wind loads involve many parameters. These parameters can be divided into three categories; wind climate, localized wind characteristics and wind-structure interaction. Using the wind loading formulation of ASCE 7-98, the paper discusses each parameter for calculation of wind loads on transmission line structures.

In this paper we discuss the development of the map of extreme ice loads with concurrent wind speeds that is in the latest revision of ASCE Manual 74 and in ASCE Standard 7. This map is based on historical weather data from hundreds of weather stations run by the National Weather Service, Air Force, and Federal Aviation Administration. Equivalent uniform radial ice thicknesses on wires perpendicular to the wind direction in past freezing rain storms were determined from the data at each weather station using ice accretion models. Qualitative damage information was obtained for the storms that appeared to be severe enough to damage trees and power lines. This information was used both to check the modeling algorithms and to group the stations into superstations for the extreme value analysis. Ice thicknesses for long return periods were determined by fitting the generalized Pareto distribution to the sample of largest ice thicknesses for each superstation. Wind speeds concurrent with the extreme ice thicknesses were also calculated. In the West, from the Rocky Mountains to the Pacific, ice thickness zones were extrapolated using qualitative information because extreme ice thicknesses have not yet been calculated from the weather data in this region. For application to overhead electrical wires, the mapped ice thicknesses are adjusted for return period, height above ground, topography, and possibly wire orientation.

Wind speeds and ice thicknesses for a 50-year return period are presented in ASCE Manual 74 for the structural design of transmission lines. In this paper we discuss the occurrence of extremes for 50-year and longer return periods and methods for the estimation of extreme values from a limited number of years of wind or ice data. Plotting positions for the sample of extremes, graphical fits, extreme value distributions, and the methods of moments and. probability weighted moments for analytical fits to the data to determine wind speeds and ice thicknesses for 50-year return periods are discussed. Extreme ice thicknesses for the St. Lawrence Valley are compared to design ice loads specified in the early 1970s for the 735kV system. The effect of wind on the accreted ice thickness is also discussed.

The development of the philosophy for longitudinal design loads to prevent cascades is traced from the beginning of the 20^th century to the present. Using primary sources, evidence is presented that experience with flexible structures in the first two decades of the last century led to prescriptions for periodic deadr end structures. In the late 1950's and early 1960's theories for the behavior of flexible structures similar to those that led to cascades early in the century regained acceptance. This led to construction of lines that suffered extensive cascades and a rediscovery of the necessity of stop structures.

This paper provides an overview of IEC Publication No. 60826 (old number IEC 826) entitled `Loading and Strength of Overhead lines'. This publication has been a milestone in the introduction of improved structural design criteria of overhead transmission lines based on reliability methods. Originally published by IEC in 1991 as a technical report type II (i.e. a pre-standard phase), this document was then extensively reviewed by CIGRÉ and ISC/TC11/WG08, and a revised version of this document is currently being circulated to National Committees of IEC/TC11 for adoption as an IEC Standard. This IEC publication specifies loadings and strengths requirements of overhead lines derived from reliability-based design principles. It is based on the concept whereby a transmission line is designed as a system made of components such as supports, foundations, conductors and insulator strings. This approach enables to coordinate the strengths of components within the system taking into account the fact that in such a series system, the failure of any component could lead to the loss of power transmitting capability. It is expected that this approach should lead to an overall economical design without undesirable mismatch between strengths of line components. Many improvements were introduced in the revised version of IEC 60926 (IEC 2002) such as: dividing the document into a normative section containing all requirements and another section consisting of a commentary to the document and technical annexes, providing default load and strength factors when statistical data are scarce, improvements/simplifications to some loading requirements that existed in the previous version, but may not control the design, etc. The IEC 60826 has already been used in many national standards (ex. CSA C22.3, ex. IS 802, 2002, CENELEC) and utility practices, and has proven to be an essential tool for those migrating toward the more efficient and economical Reliability Based Design (RBD) concepts.

There is a need to provide a design methodology for distribution and transmission poles that yields consistent reliabilities across all material types: wood, steel, concrete, fiberglass and other new materials. Prevailing US practice and most state laws require that transmission and distribution lines be designed, with a minimum, to meet requirements of recent editions of the National Electric Safety Code. (NESC, 2000) The NESC rules for the selection of design load and strength factors are largely based on successful experience, but they do not have a strong theoretical foundation. Some designers find the rules too restrictive, while others adopt more conservative criteria. In fact, individual utilities often develop their own loading agendas to complement the NESC rules. A desire to achieve more consistent reliabilities across materials was the impetus for the development of ASCE Manual 74 Guidelines for Transmission Line Structural Loading and the IEC. While Manual 74 offers consistent methods to calculate loads, there is still a need to provide consistent methods to calculate strength across pole materials.

This section describes a Reliability-Based Design (RBD) methodology for transmission and distribution line structures. The RBD approach is calibrated to yield average reliability levels consistent with years of history and practice with existing deterministic design approaches. Therefore, on the average, designs will be nearly equivalent to what engineers have used. This methodology strives to correct the problem of inconsistent reliabilities among different material types. Minimum reliability levels are recommended for different grades of construction. Simply selecting different return periods in designing the line quantifiably varies reliability levels. The method uses one set of load and load factors regardless of the material type. The strength reduction factors are a function of material type. The equations are based on initial strength before material degradation occurs.

Electrical transmission line structures need to be capable of withstanding loads generated from weather-related events, construction and maintenance events, and provide failure containment to minimize damage from disaster. The design load criteria in this Section addresses only the typical considerations for high voltage transmission systems. These structures most likely have longer span, higher structural profile, and they are more difficult to replace or repair when damaged. They could have substantial impacts when out of service since they cover larger area. The stability of electrical grid could be disturbed by a single unplanned event of structures. Typical structures for lower voltage distribution system are much shorter, less than 60 ft. in height. These structures usually serve a smaller area and do not need sizeable amount of manpower or equipment resources to perform replacement or repair tasks if needed. In addition, electrical system reliability are frequently not depend on the availability of one particular structure since the distribution grids are typically "looped" with multiple degrees of redundancy. Thus, in the past, the design load criteria for distribution structures are considerably different than that of transmission structures. For example, ordinarily, distribution structures do not consider failure containment loads. There are other uncertainties that applied to a lower profile structure that are difficult to taking into account. The wind turbulence is severer and unpredictable in lower elevation. Ice weight from broken tree branches, which lay on the structure, can be many times higher than ice weight accumulated on the wires alone. The debris, from either wind or ice storms, is more frequent and could generate higher impact loads to these structures. Thus, the load calculations are more complicated and may depend heavily on the condition of surrounding environment. Historically, regulatory bodies provide design guidelines based mainly on past performances. ASCE has recently established a committee to study the load effects for this unique type of structure. Structural loads for the distribution system are not covered in this document.

The term "utility structure" covers a very broad range of structural configurations. These structures vary in size, shape, load capacity, material used and importance. However, conventional Allowable Stress Design (ASD) does not account for the interactions between load, resistance and importance. The probability of failure is affected by variability in both resistance and load. While ASD procedures generally do evolve to provide acceptable levels of reliability (1- failure probability), they do not provide a rational approach to quantifying that reliability or extending it uniformly to the broad range of structural configurations. To develop a design algorithm, which provides a uniform level of reliability it is necessary to quantify the reliability for any structure. Quantifying reliability for a variety of conventional structures provides a basis for setting target levels for future design. The resulting algorithms then also provide a means of adjusting conventional designs to common reliability basis. While it may not be possible to quantify the "absolute" reliability, it is possible to provide "relative" value by using a standard "Reliability" basis for derivation of resistance factors to be used with a Load and Resistance Factor Design (LRFD). The greatest challenge to adopting a reliability basis for derivation of resistance factors for an LRFD methodology, however, is characterizing the probability density functions (PDF) for load capacity of a broad range of utility structures. In the case of wood structures, the Accredited Standards Committee 05 (ASC 05) maintains a database of full scale pole test results to provide an empirical basis for the PDF. For manufactured poles (concrete, steel, fiber reinforced polymers (FRP)), however, the PDFs are more likely to be characterized using analytical models, which reference raw material engineering properties. These models range in complexity depending on the pole configuration and the number of failure modes that must be considered. In general their ability to predict performance of a range of pole configurations is verified by full-scale tests, but quantifying the true variability of a structure type using analytical simulation can involve complex interactions and be difficult to verify.

The issue of placing telecommunications equipment on utility structures is becoming more important due to the increasing frequency of these requests. In the early stages of the telecommunication industry's buildout, the issue of co-location on utility structures was not as prevalent. This was due to the fact that it was relatively simple to obtain zoning approval for the placement of a communications tower and the relative cost of constructing a new tower was low. As telecommunication companies become more cost sensitive and as communities become more sensitive to the large number of towers, the ability to locate the telecommunication equipment on an existing structure is an attractive alternative to the placement of a new structure. This paper provide a comparison of the applicable codes for these two diverse industries and provide recommendations for utilities to consider when entertaining a request from a telecommunication company for co-location on one of their structures.

Historically, steel transmission line tower design has been based on truss modeling where the angle members only carry compression and/or tension loads. These axial forces are then compared with calculated strengths based on standards such as ANSI/ASCE-10. This design method has been proven fairly accurate when compared with full-scale testing. However, some towers have experienced premature failure where main leg members have buckled at a load well below their calculated axial resistance. This paper presents a different design philosophy, where towers are modeled with beam elements. By doing so, main leg members may carry bending moments which can significantly reduce the member capacity. Four cases of tested towers are presented where member strengths are compared based on American standards ASCE-10 and AISC-LRFD.

The paper presents the results of tests on six single angles under minor axis bending, thirteen double angles under geometric axis bending, and eight laterally unsupported single angle beams with long legs horizontal. All twenty-seven specimens developed full plastic moment.

We report on two transmission lines that demonstrate the need for more sophisticated analysis of transmission systems. Topics covered include the stability of wood H-Frames, ruling span vs. finite element based sag-tension, and complete finite element line modeling as opposed to the modeling of structures, insulators and conductors in isolation. We demonstrate that traditional ruling span based sagtension can be incorrect resulting in unexpected longitudinal loads and erroneous sags. We discuss a complete system approach that treats an entire line as a large structure, allowing the analysis to consider the effects of unbalanced line loading, structure flexibility and the resulting interactions between structures. We will conclude with a line that experienced flashover failure as a result of the interaction between flexible steel poles, unbalanced line loading and unstable hinged braced post insulators.

In the early years of transmission tower design, the transmission engineer used graphical analysis methods that were developed to a professional-art for application to three dimensional space truss towers. The method of analysis for transmission line towers has advanced since the use of these graphical procedures. With the introduction of the computer, mainframe computer analysis capabilities using structural matrix algorithms were developed. BPA's Tower computer program was one of the original programs developed specifically for transmission line latticed structures. Now with the analytical power of personal computers, PC, the availability of tower analysis computer programs are numerous, such as BPA's Tower, PLS Tower, GT Tower, etc. The standard of practice for analysis of lattice steel transmission line towers is a three-dimensional truss analysis. The structural computer model is based on the tension and compression behavior of the individual tower members. These tower analysis programs are based on linear elastic structural performance, whereby members are assumed to be axially loaded and to have pinned connections. The member forces determined from the computer model are compared to the allowable member capacities. In special cases, a three-dimensional frame model may be required to provide necessary information on the member forces, including bending moments. With the maturity of the transmission tower engineering profession and the advances in PC analytical solution power, the transmission engineer can now push the margin of analysis beyond the simple truss model. The uses of the simple truss model is still the main analysis tool of the transmission engineer, but advanced tools for evaluating the performance of space truss towers, including effects other than simple truss action, are now being utilized. This paper presents three advanced transmission tower analysis computer programs.

The Hawaiian Electric Company is the provider of electrical service on the island of Oahu in Hawaii. In 1998, a structure inspection revealed that two structures on a mountain ridge supporting key lines servicing the East Oahu and the Waikiki areas were significantly weakened. A structure replacement project ensued. Due to the extreme terrain and weather conditions at the sites, there were many physical and design constraints on the project. A team of HECO engineers worked collaboratively with engineers from Valmont Industries to identify the constraints and develop customized tubular steel H-frame replacement structures. Unique solutions had to be developed to ensure that the new structures could be located in the same location as the older structures and be helicopter erected.

In order to report on the latest advancements in the area of Composite Utility Poles and Crossarms, a portion of this paper is taken from the recently completed draft for publication of the "Recommended Practice for Fiber-Reinforced Product Products for Overhead Utility Line Structures" that was prepared by the Subcommittee on Fiber-Reinforced Product Structures for Overhead Lines of the Structural Division of the American Society of Civil Engineers.

The purpose of the substation structure design guide is to provide a comprehensive document for the design of outdoor electrical transmission substation structures. The recommendations of this document apply to substation structures that support electrical equipment, rigid bus, and electrical cables/wires. The electrical equipment can be of significant weight and have attachments of brittle porcelain components. Specific guidelines for structural loads, deflection limits, analysis, design, fabrication, maintenance and construction of substation structures are recommended. Guidelines for the design of the structure to the foundation connections are provided. The design criteria for substation structure foundations are not covered by this document. Foundations in substations should be designed according to accepted engineering practice, the same as foundations designed for other structures. The guide addresses steel, concrete, wood, and aluminum used for the design of substation structures. Design equations are provided when reference to existing structural design documents are not appropriate or convenient. The intent of the design guide is to cover structures that are located inside the fenced area of a substation yard. The guidelines of this document may also be appropriate for structures that serve the purpose of supporting electrical equipment located outside the substation fence that are required to satisfy electrical substation design criteria.

Fiber stresses reported in the American National Standards Institute (ANSI) 05.1 standard are based primarily on tests of small clear samples and tests and performance experience with the smaller distribution-size poles. During the past 20 years, however, tests conducted on transmission, or H-class, poles have generated some question as to the applicability of historic pole values for the design of transmission poles. The primary intent of the ANSI values was to support species interchangeability within any given ANSI size class by providing uniformly reliable groundline moment capacity across size classes and species. The most recent test data, however, suggested that there is a reduction in strength with size and that not all values are uniformly reliable across species. Because utility structures do not require a high degree of design sophistication, the initial response was to acknowledge the new data but keep the old ANSI fiber stress values as the basis for pole size classes. When producers using competing materials began to question the efficacy of the ANSI values for a more rigorous design methodology, the Accredited Standards Committee (ASC) 05 began to discuss incorporating the latest data. An algorithm was proposed as a means of justifying current ANSI values on the basis of the fullscale pole tests. This paper discusses the basis for the proposed algorithm. It highlights the assumptions made in adjusting raw data and discusses how these adjustments affect the derivation of pole design stresses.

Transmission assets of most utilities are being required to transfer greater loads than previously anticipated. Difficulty in obtaining new Rights of Way has tasked utilities with increasing the transfer of power using existing assets. Numerous transmission lines composed of latticed towers are being considered for installation of larger conductors or higher line loads requiring greater line tensions to maintain minimum ground clearance. At the same time utilities are replacing existing shield wires with fiber optic systems as well as adding antenna systems at site specific locations. These assets are now being pushed beyond their original design conditions and often times will now exceed the design limits imposed by the current standards. The American Society of Civil Engineers (ASCE) Standard "Design of Latticed Steel Transmission Structures", ASCE 10-97 (ASCE 10-97) provides recommendations for new designs that at times are not met by existing towers. This paper discusses the key issues that must be addressed by both the engineer and utility during the upgrade/uprate process on older steel transmission towers. Suggested guidelines for existing towers will be presented to deal with issues on slenderness ratio limits, minimum edge distances, bolt shear capacities for unknown bolt types, bearing capacities of bolts and members, use of tower test data in the upgrading/uprating process, and member use ratios which exceed 100% of the member capacity. Also included is a brief discussion of bars used as tension-only members, man-loads on horizontal members, and minimum support requirements of redundant members. A brief discussion on the original compression formulas used in older tower designs and ASTM material specifications in older towers in older towers is also included.

It has been nearly 100 years since the first spun concrete pole was produced in Europe, and more than 30 years since the technology was first introduced here in the United States. However, the technology advancements of the past 8–10 years that have propelled the use of spun concrete poles into an ever increasing, and significant role in today's design and construction of high voltage electrical transmission lines. Significant technology advances have occurred in three key areas. First, the types and quality of the raw materials utilized in the production of high performance concrete have dramatically improved, and in some instances have just recently been developed. Second, scaled up manufacturing methods and equipment to produce high quality, stronger, longer length poles is being realized. And third, with meaningful Research & Development investment in the technology, significant and innovative enhancements to the engineering design technology of spun concrete poles are being utilized. Spun Concrete poles today are not just relegated to small distribution, subtransmission, or wood maintenance applications. Spun concrete poles today are cost effectively being designed and utilized in major transmission line projects up to and including 345kV and 500kV Lines. In addition to a brief history of these interesting structures, this paper will provide a "state-of-the-art" report on the technology of spun concrete poles for electrical transmission structure applications in the United States

It is most appropriate that the 21^st Century has begun with a new and exciting innovation in transmission line structure technology. Interestingly enough, this latest innovation relies on time proven, indisputably reliable technologies. By combining to best advantage, the structural and material characteristics of both tubular steel and spun concrete poles, a new hybrid pole has been developed and is being effectively used to save time, money, and to increase the reliability of both new and existing transmission lines. Oncor Electric Delivery's (formerly TXU Electric) White Rock Lake Project was featured in Transmission & Distribution World magazine's September 2000 issue for applying this new technology to provide an ideal solution to a complex, high profile transmission upgrade project in Dallas, Texas.

This paper describes the design, construction and operation of a guyed suspension tower used in extra high-voltage (EHV) transmission lines, known as the Cross Rope Suspension (CRS) structure. The CRS concept, whereby conductor phases are suspended from a transverse spanning wire rope instead of from rigid crossarms, is a line structure like no other, as different from square-based rigid latticed towers, Guyed-Vs, H-frames and the rest as a jet plane is from a propeller-driven one, or as an aluminum overhead conductor is from a copper one. With a design based on simplicity, flexibility and strength, and utilizing the two most efficient elements of structural engineering, namely a wire rope in tension and a lattice mast in compression, it comes close to being a truly optimum structure. The CRS structural concept has been utilized in difficult and remote areas and across valleys in North America and, as a tower system, in long transmission systems in Canada, South Africa and Argentina. A compact variation of the CRS tower has been used in Europe, taking advantage of its low visual impact, narrow Right of Way and marginally improved electricand magnetic field characteristics. In spite of such strong credentials, including its all-important low impact on the environment and very low cost to strength benefits, designers, utilities and the public are still reluctant or hesitant, or simply decline, to consider the CRS tower in new or existing projects. This may possibly be due to poor understanding of the strength of this design and its behavior, and unfounded reservations that its somewhat larger base area would sterilize more arable land. In the event that inaction is the result of lack of information about this unusual structure, this paper is presented to compile in one place many of the qualities and advantages of the CRS tower, a few perceived disadvantages and a brief review of major applications.

Most utility engineers in the United States use the American National Standard for Wood Poles, ANSI O5.1, to determine the dimensional properties of different speciesof wood poles. ANSI O5.1 defines a groundline distance from the butt for each pole height for use when a definition of groundline is necessary in order to apply requirements relating to scars, straight-ness, etc. In addition, the groundline distance is also used to determine the horizontal load capacity of each class of pole. This paper reviews the adequacy/fixity for different pole heights and species with setting depths based on the ANSI 05.1 groundline location in soils with different properties. The effects on the pole setting depths of backfill diameter and strength as well as groundline forces are also discussed.

An overview is presented of the general accomplishments of the large and comprehensive EPRI Research Project RP1493-4. A brief summary also is included of accomplishments under RP1493-1,2,3. The breadth and depth of these research efforts are described, and the project output is summarized. Basically, this project has led to major research advances, significant analysis/design improvements, and substantially new recommendations in the following areas: analytical modeling, load testing, soil property evaluation and probabilistic modeling, reliability-based analysis and design, drilled shaft behavior under uplift, compression, lateral, moment, and combined loading modes for both static and cyclic loading, and grillage foundation behavior in static uplift loading. Key points on these topics are noted, and the technology transfer to date is summarized.

Steel finned-pipe foundations have excellent overturning resistance and are especially applicable for support of electric transmission line pole structures having high overturning moment loads. These foundations also have excellent grounding capabilities eliminating the need for supplemental ground rods. SAFE^© (SAFE Foundations, Inc.) has developed a procedure for installation of these foundations, which is fast, economical and permits immediate use of the completed foundation. This paper describes the installation procedure, design methods, and full-scale load testing to overturning moments of 2000 kN-m (1500 kip-ft) that demonstrate the usefulness and reliability of this foundation type. The results of three tests are compared with predicted moment capacity by the methods of Broms and Hansen and to predicted moment capacity and deflections using the computer code, COM624, Wang and Reese. For the cases evaluated, the methods by Broms and Hansen tend to overpredict capacity by up to 20%, and COM624 tends to either agree with or underpredict the moment-deflection response.

This paper presents a simple two-spring model for the analysis and design of laterally loaded pole foundations directly embedded in soil and rock (DESRF). This type of foundation is typically constructed by augering a hole in soil and/or rock, inserting the pole in the hole, and backfilling the hole with a well compacted granular aggregate or concrete. When rock is encountered, designers often treat the generally stronger and stiffer rock materials as soil. The model proposed is based on a finite element parametric study and it is also calibrated by comparisons with field measurements of eight tests previously conducted on DESRFs using granular backfill.

This paper presents an overview of the Institute of Electrical and Electronic Engineers/American Society of Civil Engineers (IEEE/ASCE) Guide for Transmission Structure Foundation Design and Testing. The Guide contains chapters on loading and performance criteria, subsurface investigations and selection of geotechnical design parameters, spread foundations, drilled shafts, pile foundations, anchors and full-scale foundation load testing. A brief description of each chapter of the Guide is provided. The paper also provides an overview of the activities of CIGRE Working Group 07 — Foundations. The information presented includes a description of the following Working Group 07 reports: (1) Refurbishment and Upgrading of Foundations; (2) The Design of Transmission Line Support Foundations — An Overview; and (3) Reliability-Based Design of Transmission Line Structure Foundations.

The pole system described in this paper can be used in lengths up to 36 m (120 ft) on a standalone basis or as a component in a structure. Service environments for the system include electrical transmission and distribution, lighting, and communications. Criteria for the pole system development involve ease of manufacture and installation, serviceability, appearance, and cost effectiveness. The resulting pole system contains 1 to 4 segments up to 9m (30 ft) in length each with a tapered static precast concrete hexagonal cross section having 64 mm (2½ in) thick walls and a hollow interior that can be filled with pumped concrete or left empty after installation. Post-tensioned prestressing tendons occur inside the wall of the precast concrete. The numbers and distribution of the tendons vary with the pole applications that allow outside diameters across the flats of the hexagonal cross a section from 250 mm (10 in) to 850 mm (34 in) with a taper of 25 mm (1 in) per 3 m (10 ft). Precast forms have been built for this pole system and are being used for prototype manufacture. Some of the pole prototypes will be installed by a variety of construction techniques. This work, along with suggestions for future implementation steps, will be shown in the paper.

Directional orientation of a transverse load can be an issue of major concern for structures such as poles, towers, platforms, and tall buildings. If the cross section area moment of inertia is constant irrespective of the axis of the transverse loading, these issues are eliminated. This condition exists in cross sections having more than 2 axes of radial symmetry. Radial symmetry exists in a shape when it can be generated by n > 2 rotations of generator shapes repeated at angles of 2π/n around a circle. The generator shape itself need not have any symmetry, but its rotation must be repeated n times in the same direction around the circle. Examples of radial symmetry occur in rotating systems such as wheels, pulleys, and fan and propeller blades. The occurrence initiating the demonstration in this paper arose from the arrangements of prestressing tendons in radially symmetric patterns that varied over the length of a pole structure. The demonstration presented in this paper shows that the area moment of inertia for a radially symmetric structure having n > 2 is constant. Therefore, the designer needs to make only a single area moment of inertia determination using the most convenient orientation of axes for computation.

A full-scale structural test was performed as part of the development of a monopole hybrid pole structure t for use in a double circuit 345 kV line. The test was conducted at the Electric Power Research Institute (EPRI) Research Center at Haslet, Texas. The structure was tested in a vertical configuration to simulate actual conditions and was subjected to an extreme wind load case of 140 mph. Loads and deflections during the test were obtained using advanced computer controlled data acquisition systems available at EPRI Research Center. The test was successful in that the hybrid structure met all the specified design criteria regarding strength and stiffness.

Dynamic tests were performed to determine the characteristics of the structural loads developed by fall protection systems used by electric utilities. The tests were performed to validate present anchorage strength requirements. A series of tests were performed to determine the magnitude of maximum dynamic loading on a fall protection anchorage given a single or simultaneous fall of one or two people under "ideal" conditions. The resulting data was used to determine the total dynamic load for each test configuration. A sufficient number of tests were conducted to obtain a statistical trend of the total dynamic loads resulting from the fall of one or two workers attached at a single anchorage point.

A reliability-based design procedure using the inverse reliability method is proposed in this paper for design of overhead transmission structures. The procedure allows direct and fast determination of design parameters such as wind span and pole groundline circumference, in order to meet pre-specified target reliabilities. Two design applications are illustrated: 1) calculation of the design wind span of a lattice steel suspension tower so that a target reliability is achieved in a compression performance criterion; and 2) determination of design wind span of an H-frame tangent wood pole structure to achieve target reliabilities associated with design criteria in tip deflection (serviceability criterion) and bending (ultimate design criterion). The advantages and implications of these approaches are discussed.

Managers of overhead transmission line assets must increasingly anticipate more and varied threats as well as opportunities, which will have a negative or positive impact on these assets. If you are one who is responsible for managing the assets of overhead transmission lines (OHTL), can you answer the following questions? 1) Can you quantitatively justify investments in your overhead transmission system? 2) Can you predict the probability of your next overhead transmission line (OHTL failure event? 3) Do you collect the right data when you have a failure event? 4) Do you know the present capability of your OHTL components? and 5) Could you manage these data effectively, if you did collect them? In the past, decisions on the management of overhead transmission lines were frequently based on the qualitative judgment of experienced individuals. The guides produced by CIGRE Study Committee (SC) 22, Overhead Transmission Lines, Working Group (WG) 13, are an attempt to quantify this analysis. In order to quantify any analysis of overhead transmission lines (OHTL), unambiguous reliable data must be collected on the causes of any failures and the current capabilities of the OHTL. These data must be made available for analysis by incorporating an appropriate data management system. Using these data, risk management techniques can be used to support management decisions.

The purpose of this paper is to outline a methodology for predicting the foundation corrosion problem by non destructive evaluation (NDE) techniques using half cell measurements supported by some limited excavations and soil tests. Rossum's work is used to estimate the residual life of the corroded foundation tier members of a 30 year old line which is at present undergoing a major upgrading work because of a severe icing problem. These members have either completely or partially lost their galvanization.

The goals of Omaha Public Power District's (OPPD's) Transmission Line maintenance management programs are to: (1) Develop maintenance management philosophies and programs that adequately consider cost, customer satisfaction, and risk to achieve or exceed an acceptable level of reliability for all customers, (2) Accurately depict the overall condition transmission line facilities, (3) Identify the right work to be performed, (4) Assign work appropriately to service centers, area offices and/or contractors, (5) Coordinate related projects, and (6) Maximize effective use of resources. To achieve these goals, several preventative maintenance inspections are performed and various data sources are analyzed. Circuits are physically inspected using aerial, drive-up, ground line, climbing, infrared and various other techniques. Information gathered from these inspections and information gathered from other sources such as equipment failure forms and outage reports are analyzed to determine if circuit sections should be maintained, refurbished, rebuilt, removed, or abandoned. Whether in terms of rebuilding, outage time or lost revenue, a failed transmission structure can prove to be very costly. By implementing a transmission line maintenance management program, OPPD has been able to reduce outages and costs attributed to transmission structure failures. Although structures may still fail in extreme weather conditions, rarely is it attributed to a structural defect that could have been identified prior to the failure (i.e. rotted pole, missing hardware, wash-outs etc.).

Hydro One's extensive network of transmission lines has close to 45000 steel transmission structures. These structures have been built up over the past 95 years. Large numbers of these structures are still serving beyond their original design life of 50 years. To establish an efficient repair and maintenance program for those old structures, it is essential to first evaluate their damage states. Damage assessment of existing structures is, however, a very difficult task due to the lack of accurate information and the complex nature of structural deterioration. Characterization of uncertainties inherent in a transmission line and its operating environment is complicated by the presence of time. As soon as the line becomes energized or operational, it begins to age. As the system ages or goes through various upgrades and refurbishment, the inherent uncertainties will also undergo continuous change. As time passes microstructure will evolve, corrosion will invade steel components and strength will deteriorate. It becomes a constant battle to maintain and support the line system, typically the majority of the costs of a system are incurred during this stage of the system life cycle. Tower inspection results provide critical information for formulating transmission line improvement strategies. The effectiveness of these strategies relies heavily upon the quality of visual field inspection data. Because the procedure of assigning and combining rating information for line components are based on subjective judgement and intuition, the resulting rating for towers with similar condition can vary. Consequently, a procedure that can incorporate the subjective judgement (cognitive uncertainty) inherent in tower inspection would be useful. Cognitive uncertainty associated with visual inspection and assessment can best be handled using fuzzy logic. Fuzzy logic was specifically developed to deal with the fuzziness of human concepts such as those embodied in human perception and decision making. In addition, fuzzy logic provides a systematic framework for dealing with linguistic quantifiers such as "large, small, moderate, rusty, fair, very, many, few, good, bad, etc." which arises during visual inspection and damage assessment of existing transmission structures.

The U. S. Federal Emergency Management Agency (FEMA) and the American Society of Civil Engineers (ASCE) have established the American Lifelines Alliance (ALA) public-private partnership project, with the goal of reducing risks to lifelines from natural hazards and human threat events. Lifelines are those utility and transportation systems, including electric power transmission, whose highly reliable operation is deemed essential for maintaining critical community functions. ALA's objective is to facilitate the creation, adoption, and implementation of design and retrofit guidelines and other national consensus documents that, when implemented by lifeline owners and operators, will systematically improve the performance of lifelines to acceptable levels in natural hazard and human threat events. Electric power transmission system performance, i. e., delivering bulk power to distribution substations, is often viewed by society as paramount to emergency response and recovery activities following disasters resulting from natural hazard or human threat events. ALA is working with partners including Pacific Gas and Electric Company (PG&E) and Bonneville Power Administration on existing guidelines, standards, and industry practices, synthesizing and improving them, as appropriate, and shepherding the resulting documents through a formal consensus process.

High voltage substation equipment has been vulnerable to earthquake events. Substation equipment performance during earthquakes can also be affected by the dynamic interaction between equipments components. This paper first briefly summarizes the performance of substation equipment during recent earthquakes in the United States. Thereafter, the results of an experimental investigation of the dynamic interaction between components of electrical substation equipment interconnected by rigid bus conductors are presented. Shake table tests were conducted on five different pairs of generic substation equipment specimens interconnected by three different rigid bus assemblies. All connectors tested exhibit stable hysteretic behavior with various degrees of energy dissipation capacity. The first connectors incorporates a spring dissipates energy through yielding and generated the highest forces among the connectors tested, and generally caused an amplification of the dynamic response of one of the interconnected equipment components. The second connector includes a bus slider that dissipates energy through friction and generated lower forces than the spring connector did. Its high damping capacity, however, systematically reduced the dynamic response of interconnected equipment components. Finally, a special seismic connector that incorporates three flexural vertical aluminum cables generated the lowest forces. Its minimal damping capacity, however, resulted in higher equipment dynamic response than with the bus slider.

Recently there has been an increase in concern about seismic activity generally and in particular in the Northwest. This interest has been manifested in the upgrading of seismic zones, development of ground motions maps, and updating of the seismic provisions of the building code. The electric transmission grid is by its nature a distributed system. Thus it is reasonable to assume that a transmission tower will be in the vicinity of strong ground motion when a seismic event occurs. Because of this there has been some interest in adding a seismic load case to the traditional load cases already required for the design of electric transmission towers. Structural engineers familiar with the design of electric transmission towers contend that this is not necessary because traditional load cases will control the tower design and produce a safe and reliable structure. It is argued, from a practical point of view, that towers must be the most common structure to be subjected to seismic loads and yet there has been no reported earthquakes that have caused a tower to collapse because of member overstress from ground motion. However, towers have failed due to traditional loads, e.g. wind, ice, and unbalanced conductor tension. The purpose of this study is to determine under what circumstances, if any, a seismic (dynamic) load case controls the size of the primary leg members relative to traditional load cases in an electric transmission tower. The tower leg member, which is the main support for this structure, was chosen because the concern is the collapse of the tower.

This paper has been divided into two sections, both of which focus on transmission line design issues as they affect the everyday practice of line design and engineering. The sections are not comprehensive but highlight items that can and should be applied to the practice and science of line design. The first section is a reflective overview of the industry's attempts to designate loads using a probabilistic approach. Probabilistic based selection of loading criteria is one of the major issues that has dominated discussion of transmission line engineering for almost four decades. The second section offers a brief discussion of High Intensity Winds, Tension/Mass, Failure Investigations, Failure Containment and Focused Maintenance, five diverse topics found in the everyday practice of line design. The focus within these discussions will be on some very pragmatic issues that position our field of work as we look forward to this new era in transmission line engineering.

This paper presents a simple method of analysis capable of calculating sags and tensions of an overhead transmission line cable under different loading conditions. The simplicity of the method is achieved while retaining reasonable accuracy in the computed values. The accuracy of the computed values is tested against those obtained from a more complicated, finite element method. The method follows the traditional Ruling Span Method but satisfies the force balance equilibrium at each cable support. The method can be used for lines with any span lengths and with inclined spans. The effects of unequal spans, inclined spans, and varying support stiffnesses on the cable behavior are studied and discussed.

Uprating of transmission lines has become an issue of major importance to a large number of the electric utility companies around the world. Various techniques are being used to justify increasing the static ratings on critical lines. How good are these techniques in predicting line clearances and/or conductor temperature? This paper will present information that is the product of a recent field experiment in this area. This experiment was done to evaluate the accuracy of some of the different techniques, which include improved weather modeling and actual line monitoring, on an actual transmission line.

As power delivery requirements become more acute, it is imperative that utilities operate their power systems at the highest level while maintaining system integrity. History has shown that utilities have operated conservatively. The increased difficulty in building new lines along with the rising costs for right-of-way, public intervention, and licensing requirements have become real obstacles faced by many utilities. In addition to internal operation difficulties, the lost revenue opportunities from power purchase/sale agreements with other systems because of limited transmission facilities can be substantial. This paper discusses one technique that will permit utilities to target specific locations where line operation may be limited and increase line capacity by 50% or more. More recently, Independent Power Producers are approaching utilities with the intent of developing production and relying on utilities to transport the new generation capacity to the marketplace. One method of delivering the additional capacity is to operate the existing transmission system at temperatures higher than they customarily would. High temperature operation will cause conductor sag to increase. The increased sag may violate minimum clearance requirements. High temperatures for purposes of this discussion are those levels that will not result in conductor damage. The "Maximum Operating Temperature" MOT of a line is the highest temperature that the line can operate, and maintain minimum National Electric Safety Code (NESC) clearance requirements. One approach in determining the MOT of a line is to determine where the conductor is in relation to an object or the ground at a known operating condition. A technique being used in recent years is the GPS Aerial Survey. This process provides the engineer with literally millions of data points to more accurately model the location of a conductor than ever before.