Electrical Transmission and Substation Structures 2018: Dedicated to Strengthening our Critical Infrastructure

In 2013, AltaLink initiated work on the South and West of Edmonton Area development project comprised of multiple 138-kV and 240-kV circuit modifications. Scopes included greenfield construction, rebuilding existing lines, and tapping existing lines to new substations. The complicated project scopes were challenging to construct due to outage constraints, environmental restrictions, and the need for multiple foundation solutions. Proper outage planning with the AltaLink Control Center was imperative and directly impacted the construction sequencing of the project. Environmental concerns affected the project in multiple ways: work restrictions during bird nesting and migration seasons, requirements for rig matting in environmentally sensitive and wet areas, and the use of vehicle wash stations. A variety of structure types were implemented on the project including double-circuit latticed steel towers, single-, double-, and triple-circuit tubular steel poles, and wood poles. Foundation types, including drilled shafts, helical foundations, and direct embed foundations, were selected based on structure type and location. Construction commenced in early 2017, and the project was placed in service in December 2017.

When critical weld repairs and an overall shortening of a foundation caisson threaten to slow down construction crews on an important transmission line construction project, several activities must be quickly and effectively completed to get the issue resolved in a safe and effective manner. This paper will review the steps taken in one such instance, including: developing an overall problem statement; engineering analysis of the shortened caisson depth; organizing the site specific and project specific constraints that might impact execution of the repair; assuring appropriate and effective weld procedures to execute the field repair and ensuring qualified and experienced welder/fitters are utilized; developing an effective solution for the finish/coating repairs that would be needed following weld repairs; and identifying all quality assurance steps that will be needed to be executed throughout the entire repair process.

In the age of rebuilding transmission, outages are shorter and more difficult to obtain, causing the construction of new transmission lines to become more difficult. Therefore, de-energizing transmission facilities to accommodate construction tasks requires careful planning, often affecting the line design. American Electric Power (AEP) and DiGioia Gray, Inc. co-developed a methodology to increase construction productivity of drilled shaft concrete foundations and reduce the overall duration of transmission outages. This methodology incorporates mechanical (bolted) splices into full length #18J anchor bolt cages so that multiple but shorter sections may be safely set underneath energized lines. Tensile testing of these splices was performed to insure conformity to current codes and establish cage assembly tolerances. The electrical clearances available at each foundation location were evaluated considering the drilling equipment and rigging, which helped determine anchor bolt cage lengths and special detailing and fabrication considerations to accommodate splicing. Collaboration between the engineers, anchor bolt supplier, and contractor was critical to ensuring a successful foundation installation. This paper provides an in-depth discussion of the development of spliced anchor bolt cages, including the testing performed and solutions to unique design challenges. Furthermore, lessons learned from actual installations and the project benefits realized from this approach will be presented.

Transmission lines across the country are plagued with aging infrastructure and Hawaii is no exception. Replacing the transmission structures on an island is more challenging than just being 2,400 miles from the mainland. This paper details the challenges faced when Hawaiian Electric Company, Ltd. (HECO) and HDR partnered to replace the most critical of the roughly 1,300 138 kV transmission structures on Oahu. This paper highlights how issues with remote access and environmentally and culturally sensitive areas in the mountains of Oahu were addressed using structures designed to be installed using helicopters, how system outage limitations and topographical extremes influenced construction and design sequencing, and how the challenges associated with being isolated from the mainland pool of available labor with similar project experience and specialized construction equipment and materials were overcome. This paper also details how HECO and HDR successfully replaced HECO’s aging transmission grid while developing new strategies to enhance future structure replacements in remote and challenging terrain.

A recently completed industry study summarizes best practice guidelines for evaluating and selecting appropriate transmission line foundations with the least impact to the environment where various sensitive and difficult conditions exist. The effort is based on a review of published case studies supplemented with selected utility and consultant surveys along with the author’s personal files to gather unpublished case history information. The report details the advantages and disadvantages of traditional foundation systems (driven piles, drilled shafts, direct embedment poles, steel grillages, spread footings, and anchored structures) and alternate foundation systems (helical anchors/piles, vibratory caissons, micropiles, rock socketed anchors, and auger cast piles). Local practices, economy, available equipment, and site access generally control the selection of foundation alternative for projects in sensitive environments. The document presents a great deal of information, options, and alternatives that must be assessed to select the optimal foundation alternative in sensitive environments. Organizing this information is critical in performing a logical assessment to arrive at the best foundation alternative for a project. A rational step-by-step model is presented where information is organized and numerical values are assigned to criteria for each foundation option.

After investigating cost savings, risk reduction, and design optimization, the project team has decided to utilize alternative foundation designs on their transmission line project. Now what? This paper will discuss the approach that was taken by the ATC Bay Lake project team to incorporate steel vibratory caisson foundations into the project plan. The paper will highlight the reasons for choosing this specific alternative foundation, the considerations for design and construction, testing and pre-work investigations, and the methods of communicating project decisions and progress among all stakeholders. The paper will incorporate considerations and lessons from the planning, design, and execution phases of a major transmission project.

Transmission line structure foundations are often designed without soil borings at each structure due to schedule and/or budget constraints. This paper presents a statistical analysis using soil boring data obtained at more than 2,000 structures for a high-voltage transmission line, encountering variable subsurface conditions over its 375-mile length. The analysis compares foundation designs based on structure-specific boring information to theoretical designs resulting from presumptive design parameters or parameters from borings obtained at various intervals. Soil and rock parameters for hundreds of structure locations have been regenerated, applying assumptions to reflect those that would be made if boring data was not available at every structure. The results reveal substantial schedule and cost savings when borings are obtained at each location, due to foundations that would have been over-designed or under-designed with fewer borings. In some cases, foundations could have been designed without accounting for adverse subsurface conditions, such as shallow rock, which would have resulted in construction delays if encountered without structure-specific boring information.

There are presently no comprehensive guidelines for foundation design of electrical system and power delivery structures. In recognizing this deficiency, industry professionals formed the Electric Power Systems Foundation Working Group in 2012 under the auspices of the Deep Foundations Institute (DFI) with the intent to lay the groundwork for future guidelines and to identify the state-of-the-practice. This paper provides a synopsis of findings and recommendations identified by the working group on four topics: (1) foundation design models, methodologies, and practices, (2) structure loads and reactions, (3) subsurface conditions and geotechnical parameters, and (4) limits state and performance expectations. The lack of a consistent analysis and design methodology has resulted in transmission line foundation designs that vary significantly from utility to utility. The goal of the Electric Power Systems Foundation Working Group is to quantify the variability in the current design practice and recommend modifications in an effort to generate a more unified approach to foundation design. Various electric power industry codes exist for defining structure loads; however, there is a lack of guidance for the selection of the governing case for foundation design, how to handle extreme loading and appropriate factors related to foundations, and overall chain of failure of the system (component strength). The state of the practice for limit state methods use target reliability indexes that minimize risk; however, there is variability in indexes used for each structure type. Standards on the compatibility of loads and resistance are needed and should also include performance parameters that are consistent with non-linear load-deformation responses observed with transmission line structure foundations.

Composite transmission towers are hybrid systems that have been introduced to practice. Being composed of steel and concrete, composite transmission towers present many advantages such as: economy, rigidity, fire resistance, durability, high stiffness, high strength, high ductility, and easy to assemble and erect. Recently, composite transmission towers have been replacing the timber and steel transmission towers worldwide, yet few research studies have been performed in this field. Being composed of steel and concrete makes these structures’ behavior challenging to predict. Therefore, this study presents the analysis and behavior of composite transmission towers. Three-dimensional non-linear finite element models were created using ABAQUS to compare and validate the behavior of the towers and materials against experimental work. The models include detailed analysis and investigation up to failure using concrete damage plasticity and bilinear elasto-plastic models. Good comparison between the FE results and the experimental work has been achieved which allowed to studying and investigating multiple forms of composite transmission towers. The analysis is expanded to study the slip between concrete and steel by comparing non-composite and composite assemblies using various number of stud arrangements. The analysis is expanded to obtain interaction diagrams for various diameter/width to thickness ratios beyond known code limits and practices.

Typhoon Mujigae struck the province of Guangdong in China which caused tremendous death and economic losses in October 2015. A large area was hit by a blackout for the tropical storm caused 206 power transmission lines to fail, among which 80 transmission towers collapsed. One typical collapsed tower hit by the storm was analyzed to estimate its ultimate bearing capacity and to find out the weak parts. After eigenvalue buckling analysis, dominant buckling mode was imposed on the tower as initial imperfection, then nonlinear buckling analysis was carried out to investigate the tower’s static load bearing capacity as well as the buckling members’ distribution. By applying dynamic wind loads to the tower-line coupled system, with stabilities of the members considered, to evaluate the tower’s dynamic load bearing capacity, and simulate the coupled system’s behavior. Results are well consistent with site investigation findings. Using the original design drawing, experimental research was carried out with the originally designed members to estimate the snap buckling limit point, and some retrofitting measures were suggested based on the findings of the analysis.

Latticed steel towers have been widely used to support overhead transmission lines worldwide. In a tower, leg members are intended to carry the major loads. It is a common practice to design a leg member based on the assumption that its effective length factor is unity (1.0). While this practice has been working well over time, no report is available in the literature to validate this assumption. In this paper, a nonlinear buckling analysis approach is first proposed by using a geometrically nonlinear finite element (FE) code in conjunction with the concept of trigger load. The proposed approach is able to re-produce the ASCE-10 column curve very nicely, proving its practical usefulness. This approach is then used to rigorously examine the unity effective length factor assumption for the leg members. For this purpose, leg members are modeled as a continuous beam-column with multiple linear elastic springs representing the supports of the associated bracing members. As a result, it is found that the unity assumption is valid only if a leg member is adequately supported on its both ends, and would be risky otherwise. Accordingly, a simple, semi-empirical equation is derived, based on the numerical parametric study, to correlate a leg member’s buckling load with the support stiffness.

Transmission lines and their major components are critical infrastructure for electrical power transmission. The state-of-the-art for the design of transmission line structures is commonly governed by wind/ice combinations and broken wire loads. However, earthquake effects are not commonly considered in transmission line structural design, even in high risk seismic areas. This paper presents information from a CEATI project, 2017, to investigate the seismic performance of transmission line structures. A survey conducted as part of this project shows that the majority of utilities do not consider seismic loading and associated geotechnical hazards in the design of new or retrofitting existing transmission line structures. This paper reviewed numerous case histories regarding the seismic performance of transmission line structures that documented the primary source of damage to transmission lines as a result of geotechnical-related seismic hazards.

The various software suites for concrete drilled shaft foundation designs utilized in the electric transmission line utility industry make different modeling assumptions for analysis. The objective of this study was to conduct a comparative analysis of drilled shafts, subject to transmission line monopole structure loading conditions, designed using the programs LPile by Ensoft, Inc., moment foundation analysis and design (MFAD) by Electric Power Research Institute (EPRI), maintained by DiGioia Gray and Associates, and CAISSON by Power Line Systems, Inc. While the programs' main difference is the modeling of the drilled pier behavior, this study focused on how each program accounted for the interaction of the foundation shafts with the surrounding subsurface. Additionally, the effect of the submerged unit weight of the subsurface due to the presence of groundwater, is also considered as an input variation parameter in this study. Multiple foundation shaft sizes, with varying aspect ratios, were analyzed for a combination of different subsurface material properties and structural loads. The performance of each of the foundation designs was assessed based on the strength and deflection criteria. By comparing the resulting designs, this study provides guidelines to help optimize a drilled shaft design for a given set of parameters.

Substation structure design in flood plains are a unique design situation not accounted for in the current version of ASCE 113. Appropriate flood loads and load combinations for substations can be obtained utilizing ASCE 113 and incorporating the design standard ASCE 24. This paper proposes modification to the equations in ASCE 113 to apply appropriate effects of loads due to flooding effects in combination with typical characteristic loads of substation structures. The effects of flood loading can be demonstrated using three case studies of structural designs for a typical substation bus structure. The first case study is located within the riverine flood hazard area. The second case study is in a coastal A zone, designated on the FEMA’s FIRM inside the limits of moderate wave action (LiMWA). The final case study is a coastal V zone, also designated by FEMA. The case studies are evaluated to show the loading impacts of inappropriately selecting flood design locations, categories, and load combinations.

Fiber reinforced polymer (FRP) utility poles are used to support overhead utility lines for the electric power industry. Manufacturers commonly determine ultimate strength of their product using full-scale physical testing of poles under short-term loading and then base the design strength on the 5% lower exclusion limit, as outlined in the National Electric Safety Code (NESC). This is adequate for characterizing the strength of FRP poles under short term load conditions, but does not adequately address cases of long-term, or sustained loading. FRP materials are affected by creep, which is a time-dependent deformation under sustained (long-term) loads. For most FRP materials, design for sustained loads is limited to a fraction of the short-term strength. The NESC, however, does not provide guidance on the effects of creep on FRP structures, and the NESC strength factors do not explicitly account for creep. Utility providers and their engineers must therefore interpret manufacturer data to understand if creep has been considered in the manufacturers’ published design strengths. This paper reviews the existing industry standards and guidelines currently applicable to FRP utility pole design in the United States, discusses approaches used by manufacturers to obtain design strengths for FRP poles, and recommends code clarifications to assist manufacturers, designers, and specifiers in understanding how sustained loads and creep effects are incorporated into FRP pole design.

A vehicle veering off the road has the potential to crash into an overhead transmission pole, compromising the operation and delivery of electric utility services. Although many full-scale crash tests have been developed to meet transportation-related needs, there are few crash tests for distribution-class poles, which compared with transmission-class poles are typically narrower and lower to the ground. In this study, the first full-scale test was developed for transmission-class poles. To fully understand the impact of a vehicle crashing into a transmission pole, a full-scale crash test using a popular passenger vehicle was conducted on a transmission-class (H6) wood pole. The vehicle traveled at 36 mph, and the center of its front bumper directly collided with the pole. Results showed that the crash exerted a large impact force on the transmission pole, which, in turn, led to a transient pole deflection—an effect that is not typically accounted for in the design of roadside transmission poles. However, the force of impact dissipates quickly such that the wood pole can withstand the impact without significant structural damage. The test setup and photographs are presented together with an estimate of the impact force on the pole during the crash event. These results provide guidance to engineers designing transmission poles in terms of ensuring that the latter can withstand the impact of a vehicle and remain safely in operation following a collision event.

Steel structure design requirements in high seismic zones are clearly defined in ASCE 7 in conjunction with AISC 360 and AISC 341, but these standards are more applicable to buildings. For substation steel structure designs, utilities often refer to ASCE 113 and IEEE 693. Many design aspects outlined in ASCE 113 are similar to those in ASCE 7 but were modified to better suit substation steel structures. IEEE 693 is also referenced in ASCE 113 for substation seismic requirements; however, there are some differences in seismic design requirements between ASCE 113 and IEEE 693. In this paper, the authors will first present and compare the design philosophies of each seismic design component of substation steel structures, e.g. load development, load combinations, detailing requirements, etc., as outlined in IEEE 693, ASCE 113, and ASCE 7 standards. Then, the differences in design requirements among the standards will be noted. The main focus of this paper is to make engineers aware of substation steel seismic design requirements as outlined in those standards as well as to offer opinions of how each requirement may affect design results or how to comply with an aspect of design requirements that one standard may not offer for a safe, effective, and optimal design of substation steel structures in high seismic areas.

As modern utility structures move toward taller and more flexible designs, the problems of wind effects on these structures have become increasingly apparent. Vortex-induced vibration (VIV) is more complex than a mere resonant forcing problem. The complexity and uncertainty of the wind field and its interaction with structures that necessitate such an interdisciplinary approach, involving scientific fields such as meteorology, fluid dynamics, statistical theory of turbulence, structural dynamics, and probabilistic methods. Many structures used in substations can be prone to this problem. Based on some field reports of structural vibration, a study is performed to better understand the VIV issue and find ways to predict and mitigate/prevent it. This paper presents observed vibration cases, theory behind the phenomenon, and a numerical analysis to predict and investigate the VIV effects using modal analysis and ANSYS 3D two-way fluid structure interaction (FSI) models. The cases are updated and validated using field structural vibration and wind monitoring data. The paper concludes with some recommendations on how to identify critical structures, in addition to considerations that need to be taken into account during the design of these substations structures including fatigue design, and mitigation approaches to new or in-service structures.

For electric utilities, limited outage availabilities and harsh environments are common. Preserving, maintaining, and retrofitting aging structures supporting critical assets is key to their longevity. In this paper, a twofold approach that is being developed and implemented by San Diego Gas & Electric (SDG&E) for managing their aging structures will be discussed: 1. Analysis of existing structures: A case study of how severe outage limitations impacted phasing the rebuilding of Point Loma substation and motivated preservation and retrofit of a 50-year old steel structure to accommodate all new electrical equipment. A case study of SPS1 substation steel rack structural analysis and repairs will also be presented. 2. Ongoing inspections and maintenance: It is important to have a program to perform routine inspections and maintenance of critical structural assets. This is especially important when faced with highly corrosive environments. A discussion of how this multi-year inspection and maintenance program is being implemented at SDG&E’s SONGS substation will be presented. This proactive approach to inspections and maintenance manages the high risks associated with deterioration. Aging infrastructure is a reality in the industry. Planning for it and accounting for it in project planning is key to a robust and sustainable utility infrastructure management program.

This paper describes chronological mitigation efforts (before, during, and after) to the Southern California Edison (SCE) electrical grid (transmission, substation, and distribution system) after major earthquakes in Southern California. “Before the earthquake” describes the seismic standard and designs at that time. “During the earthquake” describes real time mitigation efforts to continue emergency power flow of the electrical grid even in the midst of chaos, and documents how the grid can operate during an emergency situation and help guide future methods of operation during emergency. “After the earthquake” illustrates how SCE and the industry changed seismic standards for the electrical grid, in the interest of increasing seismic resiliency, and summarizes actual mitigation. The history behind a lot of today’s (2018) utility specific seismic design practices for the electric grid are not typically published in the literature. This paper provides a historical perspective of how real-world earthquakes help shape industry codes, company standards, and emergency protocol.

Over the past several years, large magnitude earthquakes have inflicted significant damage on lifeline infrastructure, including electric power systems. These events serve as reminders of the importance of implementing good seismic design practices in order to deploy resilient systems that sustain limited damage and can be restored within reasonable time frames. IEEE Std. 693, Recommended Practice for the Seismic Design of Substations is the standard of choice for the seismic qualification and design of substation equipment and their supports. At the time of this writing, the new edition of the standard was in the final stages of approval. This paper discusses basic concepts of IEEE Std. 693 and describes the most significant changes expected in the approved version. These changes include guidance for the use of seismic protective systems (e.g., base isolators and dampers), qualification of DC equipment, insulator seismic strength criteria, the explicit consideration of conductor load effects in equipment qualification procedures, guidance for nonlinear dynamic analysis of equipment/supports, revised procedures for developing input time histories for testing and analysis, and changes to transformer bushing shake table test protocols. Improved consistency and coordination with IEEE Std. 1527 (Design of Bus Work in Seismically Active Areas) and ASCE Manual of Practice 113 (Design of Substation Structures) have also been built into the revised IEEE Std. 693.

Worried about the big one? Preparing and upgrading facilities for earthquakes doesn’t have to break the bank. Learn about how Seattle City Light’s innovative tiered approach to screening, evaluating, and upgrading is building resiliency throughout its substations including, buildings and nonstructural components, yard structures, and high voltage equipment. Find out what City Light has done to efficiently manage and minimize upgrade costs as part of ongoing asset management and facility programs. Learn about resiliency technology already codified, tested, and constructed in earthquake regions for decades as well as the useful tools engineers can employ.

Power delivery projects are a product of the progression of design choices that satisfy structural and electrical purposes, however this process typically lacks built-in steps for evaluating the visual characteristics of the results. This paper will establish the importance of addressing aesthetics systematically in overhead line design (OHL). Structural and line design engineers can integrate aesthetic practices and principles as a valuable part of the design process, without compromising the functional, safety, and economic goals of the project. The industry offers very little in terms of training and resources to equip engineers with principles for use in the aesthetics arena. The intention of this paper is to introduce aesthetic understanding and appreciation to engineers in the OHL industry.

High intensity wind events, in the form of tornadoes and downbursts, cause the majority of transmission line system failures at many locations worldwide. The current case study investigates the structural response of two different transmission line systems under the Stockton, Kansas, 2005 EF2 tornado. Multi-span self-supported and guyed transmission line systems are considered in the study. Nonlinear three-dimensional finite element models are developed for both systems. The finite element models simulate six spans and five towers. Computational fluid dynamics simulation is used to develop the matching EF2 tornado wind fields. Using proper scaling method for geometry and velocity, full-scale flow field of Stockton, KS, 2005 tornado is simulated. The velocity field is incorporated into the three-dimensional finite element models. By varying the location of the tornado relative to the transmission line systems, peak internal forces in tower members are obtained. Sensitivity analysis is conducted to assess the variation of the members’ peak forces associated with the location of the tornado relative to the transmission line systems. The transmission tower members’ peak internal forces due to the 2005 Kansas tornado are compared with corresponding values evaluated using the ASCE-74 guidelines, which currently do not account for the tornado loading.

In the regulated environment of California, Southern California Edison (SCE) faces many challenges in designing overhead lines. It must meet regulatory requirements and seek cost effective solutions while ensuring long term reliability in all environmental conditions. Governmental agencies are concerned about the aesthetics of a project and will require that SCE detail aesthetic considerations prior to approving a project. Due to the varying terrain (deserts, coastal regions, mountains) and environmental conditions (hot, cold, high fire zones), traditional pole materials—steel or wood—may not be acceptable from either an aesthetics or reliability perspective. SCE developed a rigorous program to evaluate and implement the use of alterative pole materials into its transmission and distribution system. Fiber reinforced polymer (FRP), ductile iron, weathering steel, and concrete poles as well as various pole coatings are being considered. While these pole materials have been available, the latest technological advances make them suitable for SCE’s consideration. Each material type has its advantages and performs well in certain niche applications. Our paper will discuss the aesthetic and reliability advantages of each material type as well as the rigorous evaluation process which SCE uses to ensure that only high-quality suppliers and materials are approved for the SCE system.

With the national interest in critical lifeline infrastructure system performance and resiliency it may be time for the high-voltage transmission line industry to consider the current reliability level of an electric transmission line system. ASCE 7-10 and 7-16 standards have introduced target reliabilities for buildings and other structures. The question maybe asked, in the near future, what is the current design reliability level of a high-voltage electric transmission line and how does it compare with ASCE 7 target reliabilities? Probability/reliability based design (P/RBD) of transmission lines is not new to transmission line designers. In the 1980s, and into the 1990s, probability based design was being investigated for application to transmission lines. A significant number of studies were completed to better understand the reliability provided by transmission line structural design practice. Also, there is data collected on transmission line failures, and full-scale tower test results that can be used to consider the question of current reliability levels. Based on available information, are existing levels of design reliability acceptable and if not where should the industry target transmission lines for performance base design concepts? This paper will introduce the question, what is the acceptable target reliability for a transmission line, to current and next generation of transmission line engineers.

Tubular steel poles are a popular structure choice for high voltage transmission line applications because of their simple appearance, small footprint, and ease of installation (as compared to latticed towers). With modern welding technologies, they can also be a good option for extra high voltage lines. But because of the inherent slenderness and the low self-damping characteristics of the davit arms, the unloaded arms can be prone to vortex-induced vibration (VIV). In this context, “unloaded” means the arm is installed on the pole, but no weights, tie-down cables, insulators, conductor, or other physical load have been placed on the arm. This condition is not uncommon during the construction phase of a line and can last from a few hours to several months. Industry guidance recommends that all unloaded arms be secured, immediately after the arm is installed to mitigate VIV induced damage, by hanging weights, or tying the arm tips back to the pole shaft with cables. The details (cable types and sizes, weights) of this guidance are nonspecific. Experience has also shown that not every arm need be secured; the actual requirements depend on arm geometry and local wind conditions. This paper describes an AEP analytical study supplemented by testing that investigates the effects of wind-induced vibration on (various sizes and shapes of) davit arms to determine critical wind speeds (CWSs). These, in turn, lead to guidelines for when and when not to secure an arm. This paper also proposes an engineering method to determine the appropriate weights and cable sizes to use when necessary.

There has been a significant increase in the number of EHV (345 kV and above) tubular transmission lines in the United States in the last decade. At EHV levels, these structures are tall (for clearance reasons) and slender (for economic reasons and lack of appropriate criteria) structures. These structures are often located in rural areas with flat terrain and significant wind exposure and their dynamic behavior is not well understood by transmission line designers. For these reasons, there have been more reported incidents of wind-induced vibrations and damage to transmission line structures, substation A-frame structures, and static masts. This study focuses on lines and stations in the midwest and Texas areas that are relatively flat and exposed to steady-state winds. Historically, those that have experienced these phenomena have had to seek assistance from aeronautical experts outside our industry or consider the expensive option of wind-tunnel testing. However, with the advent of commercially available super-computing, this is no longer the case. Software providers now offer Cloud based computing platforms wherein a user pays hourly for computation time on a 32-core computer. The computation allows a transient airflow analysis on the subject structure to study the flow characteristics in the wake region. With this computational fluid dynamic (CFD) analysis, aeronautical damping (strakes or other geometrical additions) can be designed and implemented. Within the same software, once the aeroelastic response is found, the model can be analyzed with the finite element method to provide vibration amplitudes and stress levels in various members.

The transmission industry is increasingly specifying what is sometimes referred to as “notch tough” steel for lattice transmission towers. Notch-tough is a type of steel, which meets certain specified strength requirements as well as a specified impact property (Charpy V-Notch) requirement. Historically, lattice towers have performed well, even in extreme cold, without notch-tough steel being specified. Proponents of notch-tough steel for transmission towers offer that such a specification provides dynamic resistance to impact loading and provides better low temperature performance. While this cannot be disputed, one must consider the historical performance of their structures, the expected loading on their lines, and the cost of introducing notch-tough steel. As notch-tough steel is not widely used in the steel industry for general steel construction, it is worth considering why it is increasingly being specified for transmission structures. As notch toughness provides material the ability the absorb energy in the presence of a defect, it may not provide the cold temperature impact strength desired when specified. Understanding what notch toughness can provide and what it cannot is the responsibility of the transmission line engineer. While it may provide piece of mind, it should not be used to replace the design and detailing practices set forth in ASCE 10-15, rigorous quality control program, or experienced engineering judgment. This paper provides a set of recommendations to identify if the supplemental toughness requirement for steel is to be specified on lattice steel towers, based upon experience and knowledge from four different perspectives: the asset owner’s perspective, a metallurgical perspective, a manufacturing perspective, and an engineering perspective.

Within the power delivery industry, it is traditionally assumed that a lattice steel tower will not deflect enough to cause structural concerns during conductor stringing operations. However, on a recent project, an existing tower deflected significantly during stringing-ultimately affecting the stringing sequencing. The deflection was attributed to the tower’s high level of structural utilization combined with construction constraints that resulted from stringing over two major freeways. Engineers used finite element (FE) modeling software to model the tower’s observed deflection and the discrepancy in the sag of the supported spans. The FE model served to determine the cause of the deflection and provide a conductor stringing sequence to mitigate it. Field crews fabricated custom extension links that were temporarily installed to adjust the length of each conductor in the supported spans. After the conductor stringing mitigation sequence was completed, the tower and spans moved to their expected positions per the original design.

In this paper, the results and recommendations of a multi-year study to develop high-level pole standard for transmission line construction across different geographical regions of Duke Energy are discussed. The pole material selection process is very complex considering the multiple options available in different corrosive and environmental conditions. An understanding of how a pole material ages throughout its service life, what options are available for life extension practices, when to implement them, and at what cost are essential for a detailed life-cycle cost study. With the help of EPRI, data was established based on field inspection and associated laboratory testing of 273 steel pole structures across Duke Energy regions. Current Duke Energy cathodic protection designs were evaluated based on a full-scale testing at EPRI’s facility. From the results of life-cycle cost studies, pole material standards were developed for different corrosion severity of subgrade and atmospheric environments and installation conditions.

The purpose of this paper is to identify situations that require non-standard modeling within lattice structures. These situations include the use of non-standard cross-sections, non-standard materials, and members experiencing combined loadings. Further, this paper provides approaches to help engineers move forward with valid analysis of these non-standard scenarios. Some of these approaches include using alternative software aids to fill analysis gaps, referencing alternative guides or standards, and leveraging spreadsheet style formatting to expedite repetitive calculations. Paramount to all of these processes is the engineer’s understanding of what analysis is necessary for accurate results. Coupled with this understanding is awareness of what checks are being performed by any software packages that are used to streamline the analysis effort. This understanding of what needs to be checked versus what is being checked by the software is the key element in eliminating analysis gaps and obtaining accurate results.

Although ASCE Standard 48, Design of Steel Transmission Pole Structures, specifically cautions the designer on the use of so called ‘wood equivalent’ steel poles, the practice of designing a transmission line with wood poles and then doing a one-for-one pre-engineered steel pole replacement continues. This not only can result in an incorrect design, but it also is likely not the most economical design. The cost differential between the lengths and classes of steel or wood poles can vary greatly resulting in very different optimized designs. Often an economical wood pole line may result in smaller poles with shorter spans where and economical steel pole line may result in taller poles with longer spans. This paper will explore three options for the optimized design of a transmission line. First the line design is optimized with wood poles, second, those wood poles are replaced one-for-one with ‘wood equivalent’ steel poles, and then finally, the transmission line is optimized with steel poles. Actual utility costs of wood poles and steel poles were used in the optimization and economic analysis. In addition to comparing the installed costs of each option and in accordance to ASCE Policy Statement 451, a life-cycle cost analysis (LCCA) to include the long-term costs associated with the design, construction, operation, maintenance, and retirement phases as well as a discussion on resiliency of each design will be presented.

The analysis and design of new and existing latticed steel structures are essential engineering tasks for transmission line projects. Optimization of the structural system to meet the expected weather-related and construction loads is especially important in today’s very competitive environment. Engineers must efficiently perform thorough analyses, both productively and accurately, using methods and tool sets currently available. One design responsibility required of the structural engineer is to check the flexural capacity of redundant and other structural members that are climbing-accessible. While performing this task for multiple projects, AEP studied the current methods for calculating the flexural capacity of equal and unequal single angle sections. This study included comparing simple elastic beam theory to alternate methods provided by AISC and ASCE-10, which consider lateral torsional buckling. Interpreting these resources for computations related to specific angle leg orientation and limit states such as yielding, lateral torsional buckling, and local leg buckling can be laborious. So what’s best-“heel” or “toe”? And how does “equal” leg or “unequal” leg impact the flexural capacity? This paper will convey the results of the flexural capacity comparison, along with providing guidance for applying these recommendations to latticed transmission structures.

In the fall of 2013, PECO Energy Corp. (formerly the Philadelphia Electric Company) presented an exciting challenge of developing a design for a 300-foot tall free-standing H-Frame structure required to support a 230 kV single circuit transmission line span crossing the Delaware River. While tall H-Frame structures can present some unusual and complex design considerations, they can become even more so when you add an intricate catwalk and fall protection system. This paper will describe the coordination and collaborative efforts needed to develop the design and details for a 300 ft tall river crossing structure. It will also provide some informative details on the design and fabrication process.

In April 2015 Bonneville Power Administration (BPA) committed to adoption of a 100% fall protection policy on its transmission system. The vast majority of BPA’s transmission system consists of traditional wood pole structures, engineered steel poles, and lattice steel structures; most fall protection efforts to date have centered around these three structure categories. Fall protection on wood pole structures was completed in late 2013 through work practice modification and changes to personal protective equipment (PPE). Updated fall protection efforts for engineered steel poles and lattice structures are ongoing and, in addition to work practice and PPE modifications, structural solutions are being implemented. The structural efforts are divided into two main categories, 1) devising methods that allow linemen to climb and work safely on BPA’s 42,000-plus existing lattice structures while minimizing the need for costly retrofits and 2) developing designed-in fall protection features for BPA’s next iteration of standard lattice tower families and engineered steel poles. For existing structures, efforts include adding cable safety systems in the climbing path and identifying work positioning anchorage points. For new structures, designs include permanent cable safety system in the climbing path, horizontal lifeline anchorage for horizontal movement, and work positioning anchorage.

This paper will explore alternative arm-to-pole connections, including a typical industry standard connection observed on many projects, to resist fatigue from wind induced vibration that has occurred on some slender steel pole transmission line structures. The object of this research study is develop more robust connections with enhanced fatigue resistance from wind induced vibration.

Slender tapered steel pole transmission structures with long cantilevered davit arms can be subjected to vortex induced vibration leading to fatigue problems. This paper will present the results of fatigue tests conducted at Bowen Laboratory at Purdue University to determine the fatigue resistance of a typical U-bracket arm-to-pole connection commonly used in the industry. This paper also discusses a research program that addresses finite element analysis and modeling of industry standard and some alternative arm-to-pole connection details used in slender steel transmission pole structures. These models were developed to better understand their structural behavior and to develop proposed modeling parameters that can be used by designers in standard line element analysis programs.

The fabrication of slender tapered steel transmission pole structures often requires challenging welding procedures in complex details involving longitudinal seam welds, fillet welds, complete joint penetration (CJP) and partial joint penetration (PJP) welds in connections, splices, and base plates. This paper will review some of the challenges in successfully executing these welds and welding procedures as witnessed in shop inspections. A comparison is made between several common welding methods as found in practice and recommendations will be suggested to help ensure performance under the challenging environmental and loading conditions imposed upon these structures.

Slender steel pole transmission structures can be subject to severe wind-induced oscillations due to vortex excitation. Several examples are presented as case studies, including dead-end frames and V-type and monopole transmission towers. Mitigation studies have resulted in reduction of vibration to acceptable levels. Strategies have included vortex spoiling, reduction of lock-in response via increasing the mass-damping parameter (Scruton no.), and addition of tuned mass dampers. Associated design implications are discussed.

Great River Energy has recently experienced two unexpected tower issues that required innovative repair techniques. One incident involved a tractor on autopilot, and the other was due to five inches of radial rime ice on shield wires. The tractor incident required the replacement of an entire tower leg while the 400 kV high-voltage direct current (HVDC) line remained energized. The tower leg was replaced using temporary longitudinal and transverse stabilization and field welding by a certified structural steel welder. The shield wire icing incident caused the lattice arm supporting the shield wires to fail, and unmanned aerial vehicle technology was used to facilitate the damage assessment. Tower repairs are always tricky business with multiple moving parts, so extreme caution is required.

In order to mitigate aging infrastructure that had more than exceeded its life expectancy, Brookfield Smoky Mountain Transmission planned to upgrade the “Long Span”, a 5,010 foot, 161-kV transmission line across the Little Tennessee River in southeastern Tennessee. The first line built in 1917, and the second in 1926, were the longest transmission line spans in the world at the time. The project upgrade posed interesting engineering challenges as well as a look into the past. The original conductor was custom made and most of the hardware was either custom made or something that no one had seen in recent times. We will take a walk through history and look at how this line was originally designed and constructed, discuss the need for the reconductor and the ensuing design, discuss the challenges faced during construction, and ultimately how we were able to teach this old transmission line some new tricks.

High voltage electrical transmission line and substation upgrade projects are placing increased structural demand on existing steel pole support structures. Transmission line and substation engineers face the challenge of finding economical solutions when existing steel poles are determined to be structurally overloaded due to the demands of wire upgrades, code changes, and line routing or substation layout modifications. When existing steel poles are determined to be overloaded, reinforcement solutions are available to increase the structural capacity of the poles. In specific circumstances, reinforcement and modification operations can be performed in the field offering advantages when compared to structure replacement operations. This paper discusses specific pole shaft and baseplate reinforcement solutions that have been designed and installed on existing American Transmission Company steel transmission poles to support upgrade projects. The information provided in this paper will provide a broad understanding of reinforcement options which increase the structural capacity of existing steel poles.

A progressing transmission construction boom has notably affected the availability of qualified construction personnel and equipment along with other resources for transmission construction projects. One solution to the limited outages and resources for foundation construction is prefabricated foundations (PFF). The most common existing applications of PFF focus on the support of transmission line structures primarily in the form of grillages and direct embed foundations. In 2017, our team conducted three pilot projects utilizing PFF designs for both station and transmission line projects. Over the course of these pilot projects, our team observed and documented significant improvements reducing the cost of foundation construction, minimizing construction schedules, and improving field worker safety. PFF provide varying and circumstantial advantages over conventional cast-in-place foundations when optimal site-specific soil conditions are present for the support of station and transmission line structures.

Riser structures are the interface between the overhead and underground sections of electrical lines. Due to limited knowledge of underground cable behavior and its termination, line designers face challenges while designing these types of structures. This paper discusses in-detail the 138 kV and 240 kV riser structures types, mechanical and electrical loading from overhead and underground cables, electrical clearances, and special hardware and construction requirements of riser structures that a line designer should consider. Riser structures are designed to resist electrical and mechanical loads coming from underground systems while keeping the electrical clearances and connections to the bushing intact. Underground conductors exert longitudinal forces during electrical loading and can create thrust on the termination structure. The termination between cable and bushing is the weakest link in the system. Therefore, termination platform design needs to be done in such a way to ensure safe transfer of mechanical loads due to cable expansion during operation to the bushing platform without failure of the termination. Some hardware is unique to these structures and must be ordered well in advance and will be discussed in this paper.

Electrical faults can generate significant electromagnetic forces on substation conductors and stresses in supporting insulators and structures. Typically, IEEE 605 simplified calculations are used to design for these forces. However, simplified methods and conservative assumptions often lead to overdesign or unnecessary retrofit or replacement. Structural engineers’ lack of knowledge of the fault current and force waveforms, force formulation, correct application of forces, and dynamic response of structural systems can lead to design inaccuracies. This paper/presentation summarizes research into standards and papers on electromagnetic fault forces, load application, and structural response to these dynamic forces. Attention is paid to the difference between the common design method, where the peak fault forces are applied in a static analysis, and the realistic dynamic loading and structural response to this dynamic loading. Comparisons are made between IEEE 605-2008 design methods, IEC 60865 methods, and time history analysis. The capacity of station post insulators is discussed as this often drives yard layout. Applicable standards and recommended safety factors are discussed. Analysis shows that simplified methods are often overly-conservative, though they can be unconservative in common cases. Background knowledge of these topics can have immediate impacts on designs and the more sophisticated methods discussed can be used where warranted.