Electrical Transmission Line and Substation Structures: Structural Reliability in a Changing World

This paper covers structure deflections and their overall impact on transmission line system designs. Structure deflections can affect anything from simple cable blowout calculations to the complex effect on sags of conductors under unequal spans. Structure deflection requirements can be merely aesthetic from desiring plumb poles under everyday conditions to required deflection limits to prevent excessive conductor sags under extreme weather events. Strict deflection limits can result in very expensive structures where they are not required, while inattention to deflection limits may cause more significant problems in other places. Finally, line construction using flexible structures often presents many difficulties in the field. The author will discuss the measurable changes in sags and tensions of conductors due to structure deflections under various loading conditions and examine various deflection limits and their impacts on optimized tubular steel pole structures. This paper will conclude with a proposed method designated by the author as Dynamic Stringing Charts whereby stringing charts are developed in consideration of these flexible structures which will theoretically allow line constructors to sag each conductor once as it is pulled in on a flexible pole instead of the iterative process that is often required.

Transmission lines in service today in the US have been designed using a multitude of design approaches and structural loading criteria. The principal cause of structural failures is associated with weather events that produce loads that exceed the structural loading design criteria. In some cases, failures have been the result of inadequate design, construction and/or maintenance practices, airplane or vehicle accidents and criminal activities. The cost of storm-caused transmission outages is significant, costing utilities and users on the order of $270 million per year and $2.5 billion per year (2003 $'s) respectively. The cost of storm damages may be under-appreciated by utilities and regulators since standard industry reliability indices (SAIDI & SAIFI) omit the costs of large storm related outages. Currently available data suggest that the frequency and severity of hurricanes and ice storms will increase in the future. There has been a doubling of Category 4 and 5 Atlantic hurricanes from 1970 to 2004 which is the same time period during which ocean temperatures have increased. If this trend continues, it will have a significant impact on utility and user costs due to structural failures. Studies have shown that increases in CO₂ levels in the atmosphere could increase hurricane wind velocities by about 10%, resulting in an increase in wind loading of about 20%. Under current policy, there is a lack of financial incentives for transmission line owners to upgrade/uprate, refurbish and/or build new lines. For example transmission line owners in restricted jurisdictions do not incur penalties associated with user costs caused by storm outages.

In September 1996, Manitoba Hydro reported damages of about $10 Million (US) due to failure of 19 transmission towers during a downburst event. As a result, a research program has been initiated in Canada to study the performance of transmissions tower under high intensity wind loads associated with localized events such as downbursts and tornadoes. This study focuses on assessing the failure of one of the towers that collapsed in Manitoba, Canada, during the 1996 downburst event. The study is conducted numerically using a computer code that was developed in-house at the University of Western Ontario specifically for the analysis of transmission towers under the effect of downbursts. The numerical model combines wind field data for downbursts generated using a Computational Fluid Dynamic (CFD) simulation together with non-linear finite element formulation that accounts for the large displacement behaviour of the conductors. Failure criteria for the tower members associated with both compressive and tension forces are incorporated into the model. As such, the non-linear finite element model is capable of predicting the progressive failure of a tower that might initiate once one of its members reaches its ultimate capacity. A parametric study is conducted to determine the downburst. parameters, such as its radius and location relative to the tower, that are most critical for the structure in terms of failure. Using these parameters, the velocity of the downburst is gradually increased till one of the tower members reaches its ultimate capacity. Failures of other members progressively occur till the structure looses its overall stability. The study provides an insight about the mode of failure of transmission tower structures under downburst loading.

Electric power utilities are familiar with and skilled in preparing for and responding to almost-routine natural hazard events such as strong wind and ice storms and seasonal floods, as well as intentional human acts such as vandalism. Recent extreme weather (hurricanes Katrina and Rita), extremely destructive international earthquakes (in Sumatra and Pakistan), and nation-wide concerns regarding future terrorist attacks have increased the pressure on utilities to take appropriate steps to avoid being overwhelmed by such infrequent and exceedingly severe events. Determining what constitutes the appropriate steps to take requires various levels of understanding of the specific hazards and the risks faced by the utility. The American Lifelines Alliance (www.americanlifelinesalliance.org) has prepared a Guideline that provides clear, concise, and nationally-applicable guidance on determining the scope and level of effort necessary to assess power system performance in the wide range of natural hazard or human threat events. Included in this Guideline are specific procedures to follow and information to consider in performing standardized assessments. With the results of such assessments, utility owners can effectively establish and carry out risk management programs that will lead to achieving appropriate levels of performance in future events. The Guideline incorporates an inquiry-driven process with a two-phase performance assessment that can be applied to power systems of any size. The screening phase enables systems or components that are clearly not at risk to be screened out early. The subsequent analysis phase uses results from the screening phase to prioritize and allocate resources for more detailed assessments of hazard, vulnerability, and system performance. This process helps assure that the scope of the assessment meets the specific performance objectives of the inquiry. A case history is presented to illustrate the type of experience with an inquiry-driven process that was considered in developing the Guideline to meet the diverse needs of utility personnel in engineering, operations, and management.

Electric transmission line conductors experience a large range of dynamic motions due to wind loading. These dynamic activities impress stresses of varying cycles and intensities on the conductor strands, the suspension equipment and structural supports. Some are fairly well known and managed to mitigate problems and failures. The most common among these are aeolian vibration with its influence on conductor fatigue and vortex shedding of conductors creating large amplitude "galloping conductors". There is, however, another cluster of motion dynamics experienced at a different range of amplitudes, frequencies and cycles that is less well known and is not currently routinely included in line design efforts. The fatigue failure of davit arms on structures induced by this lesser-known dynamic activity has not been a frequent or widespread phenomenon, but it has become a reliability and liability concern for several utilities where the convergence of the right combination of conductor properties, sag and tension, line exposure and structural details develop into fatigue failures. This paper addresses the investigation of several fatigue failures, recommends a cause and develops a mitigation program to avoid future failures. In particular, a unique approach to assessing the dynamic influence on stress concentrations and mitigation plans by utilizing Photostress technology is documented. Final, recommendations provide a vibration mitigation strategy and a recommendation for a research effort to isolate what specific converging parameters and their characteristics lead to these failures.

This paper describes a study that was made to explore the compliance of the Alcoa Sag-Tension program, SAG10 with the T/m Theory as applied to overhead electrical conductors. The T/m theory states that the Catenary Constant, C (tension/unit length mass) is a constant between stranded conductors of an overhead line regardless of the conductor type when conductors are strung with equal aeolian vibration behavior. As a secondary interest, the Constant Aluminum Stress Theory — the theory that stress in the aluminum strands is the constant parameter between such conductors — was reviewed. It was found that neither the Catenary Constant parameter nor an aluminum stress value is found as constant between conductors by this program. The result is a lack of support for either of the T/m theory or the Aluminum Stress theory by the Alcoa program. Finally, the paper offers an example calculation that displays the absence of adherence to the T/m theory and the impact of the Alcoa SAG10 program on efficient conductor selection.

In this paper, an actual cross arm failure incident of two transmission towers under unbalanced ice loading is reviewed and discussed. A dynamic simulation through numerical modeling is then presented to reconstruct the failure progress. Also explored in this paper is the question of how unequal ice loads are assessed in mountainous terrain, and the importance of considering dynamic impacts when designing or upgrading a transmission line.

Transmission line conductors and shield wires store energy in the form of gravitational potential energy and strain energy. When a wire breaks, large amounts of energy are released. This large release of energy contributes to large impact loads on the towers adjacent to the break. Rotating crossarms have been used in an attempt to limit broken wire loads on towers. Nonlinear dynamic analysis of typical overhead line sections with rotating crossarms shows that the peak loads on the tower are not substantially reduced by rotating crossarms. Adding dampers to the rotating crossarm does, however substantially reduce the impact loads.

In 1990, American Electric Power (AEP) proposed construction of its Wyoming-Jacksons Ferry 765kV project. After 14 years of seeking State and Federal Permits, construction on the 90-mile line began in 2004 and was completed in 2006. All previous AEP 765kV lines were built with a 4-conductor bundle. The new line utilizes a 6-conductor bundle arrangement. This posed many challenges for the engineering team including the design and development of a new steel guyed-V tower series and the upgrade of AEP's existing self-supporting tower series. This paper highlights the design and construction challenges experienced during the project.

A new rapid deployment tower system has been developed in Canada, with extremely rapid response capability and a high degree of versatility in adapting to a range of circuit configuration requirements. This new generation tower system greatly improves the ability of power utility owners and operators to respond to emergency situations. The tower's light weight makes it easy to handle and its versatility also makes its adaptable to non-emergency temporary and permanent applications. This paper describes the functional capabilities and design innovations of the new tower and describes a case example.

This paper provides an overview of a straightforward method developed for evaluation and study used to up-rate transmission lines by reconductoring. It begins by looking at methods for evaluating solution options for line reconductoring, with particular emphasis placed upon a study of the basic survey and data requirements for modeling existing lines and developing multiple design options that support evaluation of conductor options. These methods were then applied to a 230kV reconductor project where the results of the process are illustrated. The paper concludes with a discussion of lessons learned and the use and effectiveness of the methods for future line reconductoring projects.

Prior to the construction of the new 90 mile line, American Electric Power (AEP) had been operating a 2,025 mile 765 kV network consisting of a 4-conductor phase bundle with either "Rail" ACSR or "Dipper" ACSR subconductors. These lines were energized before 1984. Prior to the siting hearings for this new line it was apparent that audible noise due to corona was a significant issue. To satisfactorily address this issue, a decision was made to build the new line using a 6-conductor phase bundle with "Tern" ACSR conductors. This new phase bundle design required that new spacer-damper units be designed, manufactured and tested. AEP revised their specifications for mechanical testing of spacer-dampers and performed a full range of laboratory testing to ensure that the spacer-dampers met all the necessary requirements to withstand the rigors of aeolian vibration, differential ice loadings, forces due to electrical line faults, etc. This paper describes some of the key mechanical tests performed on the new designs of 6-conductor bundle spacer-dampers.

In 1960 a New England electrical utility company began using aluminum structures to support equipment within its substation yards. After 45 years of design and detailing improvements, revised shop and field assembly practices, periodic cost analysis, review of fastener materials, changes in the supported electrical equipment and conductors, observing performance in the field and predicting future demands, aluminum still remains the preferred support material for a 50-year minimum maintenance-free service life. Recent field reviews of the first aluminum structures erected in 1960 offer conclusive evidence of the versatility and durability of this material when used in outside electrical substations.

In 2002 Georgia Power began final engineering on a new 500kV single circuit transmission line. Given the amount of time that passed since the last major 500kV project, Georgia Power found it necessary to make investigations into areas where technological advances in engineering tools could provide possible cost reductions. In any transmission line project using rigid base towers, for example, foundation costs will most likely be a substantial part of the overall cost of building the line. For Georgia Power's Mosteller Springs-McGrau Ford 500kV transmission line, the estimated foundation costs made up 21% of the total estimated construction cost for the entire project. Although Georgia Power has historically designed foundations using the tower's ultimate design loads, this was identified as an area of potentially significant cost savings. With sophisticated transmission line design software and minimal additional effort by the engineer, the foundations could, in effect, be optimized when designed using site or leg specific tower loads. This paper evaluates Georgia Power's new procedures for specifying foundation design loads for rigid base towers. An analysis of the methods for specifying these loads will determine their respective impact on the foundation's design and overall cost.

American Electric Power Company (AEP) uses bridge strand anchors grouted into rock to support guyed-V towers. The anchors have standardized lengths with some adjustments for variable subsurface conditions. This approach provides installation economy by simplifying construction and minimizing the need for testing during production installations. The standard design is based on an allowable grout-to-rock bond stress of 3 ksf (21 psi). AEP conducted tests of anchors installed in soil, limestone, shale, and sandstone to assess the reliability of the anchors in these materials. This paper describes the anchors, the normal design procedure, and the results of the testing program of six anchors. This information is useful in future designs of these anchors, and in confirming the reliability of the designs.

The Accredited Standards Committee O5 (ASC) of the Alliance for Telecommunication Industry Solutions is responsible for maintaining the ANSI O5.1 Standard Specifications for Wood Poles. In 2002 the ASC modified their interpretation of fiber stress values used to define pole class dimensions for transmission size pole classes. This action was prompted by a growing concern for the engineering reliability of electric power transmission systems. Increased attention given to the probability distribution functions for pole strength required acknowledgment of test data suggesting the existence of a size effect on pole strength. The ASC concluded that a the more rigorous evaluation of transmission pole strength, adopted in the 2002 version of the ANSI standard, accounts for the apparent reduction in strength with pole size. This change enabled the ASC to maintain the pole classification tables based on fiber stress values derived from tests of distribution poles. This paper discusses the basis and design implications of the changes adopted by the ASC in 2002. It provides a brief introduction to the origin and evolution of the ANSI O5.1 standard with primary focus on transmission pole classes. Research conducted in the 1980s suggested the existence of a size effect requiring reduction to the fiber stress values that had been derived for distribution poles. The ASC adopted refinements to the conventional cantilever beam algorithm which changed the meaning of the pole classification for poles in lengths of 50 feet and greater but did not change tabulated values.

An overview is presented of research at Cornell University on reliability-based design (RBD) of foundations for transmission line structures (TLS). Under sponsorship of the Electric Power Research Institute (EPRI), Empire State Electric Energy Research Corporation (ESEERCO), and others, a long-term research effort was conducted to develop a comprehensive RBD methodology for TLS foundations in both soil and rock under uplift, compression, and lateral/moment loading. The results of these studies are described herein in conceptual form, illustrating the basic concepts involved and key issues of geotechnical variability and uncertainty. Many are described in more detail in the geotechnical literature, so references are provided to the pertinent works. These very useful tools are readily available to the TLS design community.

American Electric Power (AEP) has traditionally designed foundations for self-supporting transmission line structures using allowable geotechnical capacities, empirical design procedures, and the same designs system-wide without considering variations in geology. However, the ASCE Manual 74 (ASCE, 1991) and others have encouraged the adoption of the Load and Resistance Factor Design (LRFD) approach. Therefore, AEP adopted the LRFD approach for updating their internal design standards. A significant advantage of LRFD is that the same format is used to design both structures and foundations. In addition, the designer has a useable framework to establish relative levels of reliability among the components of the transmission line system. And finally, the LRFD design procedure for foundations can easily address variations in geology. This paper describes the concepts employed by AEP in the implementation of the LRFD approach for steel grillage foundations for the AEP 765 kV Wyoming-Jackson's Ferry transmission line.

This paper reports a Southern Company-wide research effort to develop a geophysical method (Spectral Analysis of Surface Waves, SASW) for determining the stability design of directly embedded pole structures. For transmission lines, directly embeding of pole structures is a rapid construction process. The conventional design approach recommends a 10% of pole height plus two feet embedment depth, but does not differentiate between pole types, design loads, or inherent soil stiffness and strength. This paper describes the SASW soil analysis and an analytical method for determining the moment equilibrium of a directly embedded pole. The SASW method relies on the generation of small-strain stress wave propagation through soil media utilizing non-intrusive equipment. The tests have proven to be relatively easy to conduct, and inexpensive when compared to soil borings. Hence, it is an ideal technique for rapid site characterization. The analytical method determines pole stability and soil strain by limiting the ground line deflection. The results from five field studies with different site conditions prove that the method makes accurate predictions of pole capacities when compared with actual pull tests.

Shallow foundations placed in sloping ground have less uplift capacity than the same foundations built on flat ground, if the uplift load has a horizontal component toward the downward slope. Since the depth of transmission line foundations to resist uplift loads is usually determined for level ground, maintaining this depth in sloping ground could result in insufficient uplift capacity. Common sense dictates that to achieve an uplift capacity in sloping ground commensurate with the level ground capacity, additional embedment of the foundation is required. The effect of sloping ground on the bearing capacity of shallow foundations is addressed in the technical literature. However, to the knowledge of the authors, no guidance is provided in the technical literature to determine the increased depth. This paper presents an analytical method to estimate the increased depth required for foundations in sloping ground to match the level ground uplift capacity. The procedure is based on changes resulting from the sloping ground in the active and passive pressures on the sides of the soil prism above the foundation. This approach is used to derive simple equations for the required increased depth.

TFlash software was utilized to compare several alternatives in an effort to improve the reliability of a poorly performing 46kV transmission line. These alternatives included lightning arrester installation (in multiple arrangements), grounding modifications, and structure type modifications. Cost-Benefit analysis of these alternatives ultimately resulted in the recommendation to increase the insulation capacity on structures with low insulator Critical Flash Over (CFO) values. This recommendation was implemented, and considered successful a year later due to an 83-percent reduction in lightning related breaker operations when compared to the previous year. The results of this project further validated TFlash for maintenance applications elsewhere on the 46kV system.

Electrical transmission poles are often made of galvanized steel. As a relatively new industry, the steel pole industry is becoming more aware of the potential problems and their solutions associated with underground corrosion of poles. This paper will explore methods to enhance the service life of poles as well as present new findings on how galvanic action can protect the painted portion of a pole in moderately corrosive soils. In general, zinc extra poles, corrosion resistant backfills, maintenance and corrosion monitoring will greatly increase the life of galvanized poles. However, in environments with corrosive reducing soils, stray currents and high water tables, additional protective measures to extend life expectancy are required. Environmental information such as soil resistivity, pH, chemistry, and water tables should be utilized to determine the "hot corrosion spots" and the type of utility pole that should be installed for that particular environment. New findings from field and laboratory work confirm that galvanic action will protect the painted portion of a pole in moderately corrosive soil environments due to cathodic protection by zinc. Case histories for galvanized transmission poles will also be presented.

This paper addresses some issues related to the operation of standard Aluminum Conductor Steel Reinforced (ACSR) conductors, high temperature conductors and compression connectors for high temperature operation. Some concepts and attributes will be examined for operation above 93°C. This paper deals with the some of the technical issues of how to qualify new conductors and their connections. The paper discusses how the industry may have to consider transmission system operations and how the number of thermal cycles with the combination of creep will affect the transmission system reliability. The electric utility industry will have to evaluate conductors and establish new rules to operate, analyze, and qualify these conductors.

This paper describes the state-of-the-art of two different non-destructive evaluation (NDE) technologies currently being used to assess the extent of corrosion in buried portions of steel structure components, namely: steel poles, anchor rods and stub angles. Strengths and limitations of both the Cylindrically Guided Wave Technique (CGWT) and the Magnetostrictive Sensor Technique (MsS) are explored and results of actual field investigations using these techniques are discussed.

A number of articles (including a series by Electric Light & Power) were published in the 1970's on the subject of toe-cracking in base plate weldments. Despite nearly 30 years of experience, toe-cracks in base/flange plate weldments are still being discovered. This paper will review a Six-Sigma study undertaken by Valmont Industries to better identify the causes and the variables that we feel are essential to minimize the risk of toe-crack occurrences.

A key factor in maintaining the Hawaiian Electric Company's high system reliability is the timely and accurate management of its 138kV transmission assets. The use of PLS-CADD with aerial LiDAR data is a key tool in managing these assets. In 2004, HECO contracted John Chance Land Surveyors and Power Engineers to take aerial LiDAR survey data and develop accurate PLS-CADD models of its 138kV transmission system. Data was obtained in a 300 ft. (92m) wide corridor along the 217 circuit miles (350 circuit km) of HECO's 138kV system. Using these models, real time situations and problems involving the transmission system can be simulated, studied and resolved in the office at a fraction of the time and costs when compared to previous traditional techniques used in the field. Within the first month after receipt of the models, four situations were examined and resolved. One of the situations was examined with both LiDAR and traditional techniques. A comparison between the methods was made. The continued integration of the LiDAR based PLS-CADD models into HECO's asset management system will provide additional benefits in the areas of Vegetation Management, Construction and Maintenance, System Operations and Engineering.

In recent years, Southern Company has developed several programs designed to increase the efficiency and accuracy of tasks routinely performed by its transmission line design engineers. Collectively the programs present solutions to many components associated with successfully completing a transmission line design project. These components include managing PLS-CADD projects, creating construction specification packages, creating project estimates, and updating transmission line maintenance databases. This paper describes the specific problems that Southern Company has encountered and the programs created to address the identified issues.

Newfoundland and Labrador Hydro (NLH) maintains approximately 2400 km of wood pole transmission lines operating at 69, 138 and 230 kV. During the past five years, NLH has developed a Wood Pole Line Management Program (WPLM). A framework for systematically analyzing a large volume of inspection data for wood pole transmission lines has been developed using the reliability based analysis technique. The method uses a hybrid approach where the uncertainties in load and strength values and the strength deterioration due to aging are taken into account with the condition rating of each pole and structure to develop a condition matrix table. The methodology is presented with particular application to a section of a HV transmission line with various methods of mitigations.

Burns & McDonnel is providing Program Management and detailed design for the Northeast Utilities (NUSCO) subsidiary Connecticut Light and Power (CL&P) on the Middletown/Norwalk Bulk Transmission Project, a $1.2 Billion project scheduled for completion in December, 2009. The project includes 69 miles of new 345kV transmission line, of which approximately 24 miles will be solid dielectric 345kV underground cable, along with several new and modified substations. Although CL&P serves just more than 1.1 million customers in 149 towns and cities across Connecticut, the Middletown/Norwalk Project (MNP) is the largest transmission capital project in the country. NUSCO is responsible for the completion of 18.5 miles of the underground transmission portion of the project while the neighboring utility United Illuminated (UI) will handle approximately 5.5 miles of the underground transmission portion of the project. NU selected Burns & McDonnell based on their technical expertise, construction management experience, procurement team, project controls experience and overall staffing depth. Burns & McDonnell established an office near the project site and staffed it with nearly 25 professionals to assist NUSCO's Project Manager and Project Director with the complexities of the MNP. The underground transmission segment of this unprecedented program management effort will face multiple technical and construction challenges that have been and are being met by an outstanding team of engineers, lawyers, environmentalists, contractors and manufacturers. The scope of the MN 345kV underground transmission project includes permitting, land acquisition, splice vault design and installation, duct bank design and installation, cable pulling, splicing, terminations, final inspections and testing.

In the summer of 2003, Idaho Power Company (IPC) commenced construction of the Brownlee-Oxbow #2 230 kV transmission line. This construction was the culmination of a two year permitting and design process that presented significant design and construction challenges due to the line's location within the famed Hells Canyon National Recreation area. The double circuit transmission line was designed with tubular steel, base-plated poles set on drilled pier foundations. The construction was performed in the heat of the summer on rocky cliffs. Construction access was difficult and in some locations not allowed, necessitating hand tool and aerial construction techniques. Through all these conditions, the construction contractor completed construction of the project in approximately five months and on schedule to enable IPC to relieve a crowded electrical transmission path.

In 2001, the Bonneville Power Administration (BPA) embarked on an ambitious 500kV transmission line infrastructure development project. Originally intended to design and construct up to 700 miles (1,120 km) of new 500kV transmission line, the BPA designed, constructed and energized 3 new transmission lines, both single and double circuit, totaling 156 miles (250 km). This paper is a case study of the development of these new lines emphasizing on the standardization and development of 23 new lattice steel tower types within a 23 month period.

This paper describes the environmental impact analysis developed by the Universities Study Team (UST) for the Wyoming-Jacksons Ferry 765kV transmission line now nearing completion by American Electric Power (AEP) in southern West Virginia and southwestern Virginia. The analysis was prepared by a multi-disciplinary team to identify and mitigate potential environmental, cultural and visual impacts of the transmission line, and depended on a high degree of interaction with AEP transmission design and construction staff in each phase of the analysis. The analysis relied on and benefited from sophisticated Geographic Information Systems (GIS) technology and computer-based visual simulations to represent potential impacts and proposed mitigations. This paper describes the study process and the accompanying conference presentation will demonstrate some of the analytical GIS and simulation techniques developed by the UST that were employed in the impact analysis.

In late 1997, Gulf Power Company and Mesa Associates, Inc. entered into a contractual agreement for engineering services to rebuild and upgrade 40 miles of 115-kV transmission line to a 230-kV transmission line in the vicinity of Pensacola, Florida. Approximately 30.7 miles of the new line was designed for concrete and steel pole structures on reasonably good terrain with a sandy soil. The remaining 3.3 miles of transmission line was situated across the marshland of the Escambia River Swamp, presenting access problems, environmental issues and potentially insurmountable construction problems. This paper describes the structural design challenges encountered in the transmission line through the marsh area. The existing transmission line through the marshland area was constructed with laced steel towers, using barges to drive pilings and form pile caps upon which the towers were erected. In the 33 years since the towers were installed, the access cut-ins in the marsh had been filled in with silt and eventually vegetation growth. The current environmental regulations ruled out any water access. Marsh Master equipment allowed access for inspection and analysis of both structures and foundations, with a minimum of disturbance to the marshland environment. The Gulf Power Company planning department had indicated a need for future double-circuit 230-kV transmission capacity along the route occupied by the existing single-circuit 115-kV line. A decision was made to limit any future environmental impact by an attempt to design and construct the section of line through the marshlands to meet both present and future needs.

This paper describes the structural design challenges involved in upgrading an existing single-circuit 115-kV transmission line to a double circuit 230-kV line. A portion of the line was located in a salt marsh area. When the original design was constructed about 35 years ago, the marsh portion of the project was built using barges to build foundations and install towers. At the time of the present design, environmental issues ruled out water access. Because of this, a decision was made to use helicopter construction. An engineering analysis revealed that existing foundations could support a double circuit design load and therefore a unique tower concept was developed to utilize those existing foundations. The towers were designed and constructed in sections to allow a helicopter to lift within the maximum capacity and bring each section to the existing foundation. Since the new line was closely located between two energized 230-kV lines, which had to remain in service, clearance between the helicopter blades and the energized lines during construction was very critical. The various design features involved in arriving at an economical solution in concert with difficult conditions is described in the paper.

A new electric transmission line mono pole structure has been developed to provide a more efficient load carrying capability for limited or restrictive diameter pole applications. The primary feature of the new pole is the placement of concrete at strategic locations along the pole, and hence the designation "concrete-filled steel tube" (CFST). Since tubular steel poles have relatively thin walls that eventually buckle under high compressive stresses due to bending loads, the placement of concrete inside the tube will provide lateral stability to the wall as well as added flange area in compression. Additional longitudinal steel reinforcement may be embedded in the concrete to provide additional flexural strength to the cross section. One form of longitudinal reinforcement that was successfully used is standard deformed steel reinforcing bars. This paper addresses the design and testing activities that were performed in developing the CFST transmission pole. Data comparing the strength of the CFST to the strength of conventional tubular steel poles will be presented. Potential benefits of the CFST pole structure include the ability to carry significantly higher loads, using longer spans, and reduction in pole dimensions at ground line.

In power transmission, connectors play an important role in the efficiency and reliability of the system. Due to the increase of power demand and lack of new infrastructure, existing overhead power transmission lines often need to operate at temperatures higher than the original design criteria. However, this had led to the accelerated aging and degradation of splice connectors, which has been manifested by the formation of hot-spots that have been revealed by infrared imaging during inspection of transmission lines operating at elevated temperatures. The implications of connector aging is two-fold: (1) significant increase in resistivity of the splice connector (i.e., less efficient transmission of electricity) and (2) significant reduction in the connector clamping strength, which ultimately results in separation of the power transmission line at the joint.. Therefore, the splice connector has become the weakest link in the electric power transmission infrastructure. The compressive residual stresses induced by the crimping process within the splice provide the clamping forces to secure the conductor and therefore, the determination of the state of residual stresses in splice connectors is a necessary requirement to provide an accurate estimate of their service lifetime. This paper presents a protocol of utilizing finite-element analysis and neutron scattering experiments for evaluating the residual stress fields within a crimped single-stage splice connector assembly.