Articles . . ."Streamlining Metal Poles"Poles that carry transmission lines are so common along urban freeways and in the countryside that most people pay little attention to the sizable loads these poles must bear from wind, ice, and the weight of power or telecommunication lines. West Coast Engineering Group performed several linear stress analyses using software to optimize the insulator bracket (top), which supports the transmission lines on the tangent poles. Physical testing (inset) was used to verify the accuracy of the analysis results.
At West Coast Engineering Group in Delta, British Columbia, Canada's largest aluminum and steel pole manufacturer, ensuring the structural integrity of all types of pole designs is an important part of business. To account for predictable sources of loading, such as the weather, as well as unpredictable sources, like the collision of a vehicle with the base of a pole, West Coast Engineering Group uses finite element analysis and Mechanical Event Simulation software from Pittsburgh-based Algor Inc. Recently, West Coast Engineering Group designed and manufactured poles for a 138-kilovolt transmission line, which was installed by engineering consulting firm Ian Hayward International Ltd. of Vancouver, British Columbia, for an Alberta-based chemical company. West Coast Engineering Group conducted structural analyses of the transmission poles, base plates, and phase connections to assess the stress distribution and deformation under extreme loading conditions. Then the study was expanded to include a dynamic impact simulation of a head-on vehicle collision with a transmission pole. By incorporating the Algor software into the design process, West Coast Engineering Group improved design quality, reduced the prototype testing needed, and eliminated a costly, unnecessary manufacturing process. Most importantly, West Coast Engineering Group was able to pass on these savings to its clients. West Coast Engineering Group manufactures tubular, multisided, and tapered structures for many applications, including light poles, highway signage, telecommunications antennas, ornamental poles, and transmission/distribution structures. The company's recent Ian Hayward International project required two types of poles, dead end and tangent, to support transmission lines from the Alberta chemical company's co-generator power plant to the nearby electric grid of a power company. Both the dead end and tangent pole designs are tapered and 12-sided; however, the placement, phase connection points, and dimensions differ. The dead end poles are used either at the termination or at a right angle bend of a transmission line, which is then fastened to the pole at insulator connector points. The dead end pole design for this project measured 1.04 meters in diameter at the base and 0.32 meter at the top. Each pole stands 25.5 meters tall and weighs 5,647 kilograms (including the base plate). The tangent poles, which are used along the straight, middle sections of a line, feature insulator brackets set perpendicular to the pole. The pole design used for the Ian Hayward International project has a 0.68-meter base diameter and a 0.3-meter top diameter, is 25.5 meters tall and weighs 2,145 kg (including the base plate). A dead end pole requires a larger diameter to handle the higher loads associated with its position at the end or corner of the transmission line. West Coast Engineering Group began the structural analyses by analyzing the shafts of each pole type under ultimate loading, which was determined by Ian Hayward International using standard industry calculations. First, the basic pole shafts were modified using three-dimensional plate elements. Then, the necessary plate element data, including plate thickness and material properties for G40.21-450WT steel, were specified for the models. West Coast Engineering Group obtained these properties from the steel manufacturer. For the tangent pole, static forces in the x and z directions were applied at the ends of eight simplified insulator brackets to represent both the weight of the lines and dynamic loading due to wind and ice. For the dead end structure, static forces in the x and y directions were applied where the insulator connectors would have been attached. The models were fully fixed at the base where the pole shaft and base plate meet. Assessing Stress ResultsWest Coast Engineering Group performed linear static stress analyses on the models and evaluated the von West Coast Engineering Group optimized the design of two types of transmission poles using FEA and Mechanical Event Simulation software from Algor. The poles were part of a 138-kV transmission line. After confirming that the basic pole structure would adequately handle the required loading, engineers at West Coast Engineering Group performed detailed structural analyses of the base plates with welded connections to the shafts. West Coast Engineering Group manufactures all of its base plates using a flame-cut process, in which an intense flame shapes the outside of the base plate and burns a large hole into the center of the plate. At this point, the process can be continued to burn the anchor bolt holes, or the plates can be drilled in a separate process. Small slots are created from the outside edge of the plate inward to each hole when the flame-cut process is used. When the holes are drilled, no material is lost outside the circumference of the holes. In the past, it was widely believed that using the flame-cut process for the bolt holes would weaken the overall base plate structure. West Coast Engineering Group was able to disprove this theory by performing comparative analyses. By flame-cutting both the base plates and the bolt holes, engineers were able to shorten the manufacturing processes for the Ian Hayward International installation and for many other orders, as well. Solid models of the dead end base plate with both flame-cut and drilled holes were created using AutoCAD 14. The models also included 1-meter sections of the shaft with the welded connections. The model geometry was transferred to Algor via IGES files, where solid FEA meshes made of eight-node brick elements were generated. Solid brick FEA meshes are often more uniform, more accurate, and contain fewer elements than solid FEA meshes comprised of tetra-hedra. Engineers applied ultimate loading in the y and z directions to the tops of the shaft sections. The models were fixed at the circumference of each of the 12 anchor bolt holes. After reviewing the von Mises stress results for both models, no significant differences were found in the stress levels or deformation of the drilled and flame-cut holes. The maximum stresses appeared at approximately the same area of the shaft as the previous ultimate loading analyses. In addition, the stresses did not exceed the yield stress of the material; therefore, the thickness of the plate was adequate. Based on these analysis results, West Coast Engineering Group conducted a similar analysis on a base plate with flame-cut holes for the tangent pole design and found comparable results. An MES simulation was conducted of a car hitting the base of a dead end pole. The results helped engineers to determine how impact stresses were distributed in the base plate. The deformation shown here was magnified five times to provide better viewing.
With the pole shaft and base plate structures verified, engineers focused the next analysis on the insulator brackets of the tangent structure to optimize the load bearing capability and material thickness. They built an eight-node brick model of the bracket and insulator, and created a finer mesh for the bracket. By creating a finer mesh on the bracket, which experienced the most loading from the transmission line, engineers could ensure a high level of accuracy without significantly increasing processing times. Analyses of the bracket were conducted with both vertical and horizontal welding configurations and varying material thicknesses, from 9 mm to 16 mm. The vertical weld configuration results showed lower stresses than the horizontal weld. Under vertical loading, the maximum stress in a 12.7-mm plate was well below the yield stress of the material. Based on these results, 12.7 mm was determined to be the optimal thickness for the bracket. West Coast Engineering Group put this analysis to the test. The company created a load test structure, consisting of a full-size bracket welded to a shaft with structural dimensions that correspond to the top connection of the tangent pole. The insulator was simulated using a 12.7-mm-thick flange and a 127-mm outside-diameter pipe with the length and orientation to match the required dimensions. The end of the pipe was gradually loaded, while engineers checked the bracket, shaft, and welded connection for plastic deformation. No cracking occurred in the bracket or shaft. Engineers found that the analysis results closely approximated the actual stress concentrations and deflections at the bracket attachment point. For the first load case, the stress analysis predicted a deflection of 0.06861 meter. The physical test results indicated a 0.07000-meter deflection. Overall, the analysis results used to optimize the pole designs and simulate the physical loading tests corresponded very closely to the results obtained using conventional design methods. This comparison gave engineers a high level of confidence that the models functioned properly and the results are accurate. The engineers concluded that the pole designs met the load capacity specifications and required no full-scale loading test. Distributing Crash StressesWith the predictable loading capacity requirements confirmed for the designs, West Coast Engineering Group expanded the study to include a simulation of the impact loading that can result from a head-on vehicle collision. The goal of the simulation was to check the maximum deformation of the pole shaft and learn how stresses that result from a sudden impact force should be distributed throughout the base plate. The finite element model of the dead end, flame-cut base plate was used as the basis for the impact study. Engineers removed the static loading that had been applied previously because the simulation does not require dynamic loading input. Instead, they modeled a simplified car using Algor's proprietary kinematic element technology. Kinematic elements behave dynamically in the same way as regular, flexible elements and can transmit forces. However, stresses are not calculated for these elements, so processing times for large solid models are reduced. Kinematic elements for the car and flexible elements for the pole model were chosen because engineers were concerned only with obtaining stress and deformation results for the pole. Contact elements were added between the front end of the car and the pole. These elements enabled the software to simulate the complete interaction of the car and the pole, including the transfer of inertia from one object to the other. After the geometry was completed, engineers specified the global parameters of the event, including the duration and an acceleration load curve for the car. The software simultaneously calculated the motion of the car, any buckling that might result from the impact, and the resulting stresses at each instant in time over the course of the event. The software produced a virtual picture of what would happen in an actual impact scenario. The results were very useful in getting a general idea of how stresses were distributed throughout the base plate. By using the Mechanical Event Simulation software, West Coast Engineering Group was able to set up a powerful virtual laboratory, which enables engineers to change their design procedures to the benefit of the company's customers. West Coast Engineering Group is creating better-locking, more flexible structures, while reducing manufacturing costs and improving the competitiveness of its products. West Coast Engineering Group is continuing the use of Algor software in the design of poles and in the development of new pole manufacturing equipment. Currently, the company is using it to simulate and optimize a roll forming process.
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Mises stress criteria for ductile materials to assess the stress results. The maximum stresses were located on the pole shafts slightly above the welded connections between the base plates and shafts. This was expected due to the added strength of the welded connection. The maximum stresses were within the allowable range for the materials used. These results matched very closely with the calculations performed using conventional design methods.