Lattice Transmission Towers Research Articles

Buckling behavior of 420 MPa HSSY columns: Test investigation and design approach

To meet the requirement of larger bearing capacity and better material economization than traditional main members in latticed transmission towers, a novel Y-section column is put forward in this paper. A series of full-scale tests is carried out to investigate the buckling behavior of 420MPa high strength steel Y-section (HSSY) columns. 57 columns including three different sectional dimensions and nine slenderness ratios ranging from 30 to 80 are designed for axial compression tests. The width-to-thickness ratios, reduction factors accounting for the design stress or effective cross section area, material mechanical properties, initial geometric imperfections and longitudinal residual stresses are measured and discussed in detail. Based on the validated finite element results taking the initial imperfections into account, the buckling deformations and ultimate strengths are obtained and demonstrate a good agreement with the test results. The buckling factors are calculated and compared with different code results such as Eurocode 3, GB 50017-2003, ASCE 10-97, ANSI/AISC 360-10, CSA S16-09, AS4100-2012, BS 5950-1 and AIJ 2010. Related buckling curves and formulae are derived through nonlinear fittings and compared with the code design curves. It is concluded that the HSSY columns characterize very strong stability and high buckling strengths. Finally, the recommended buckling curves and design formulae are proposed to predict the buckling strengths of HSSY columns.

Bolt Joint Effects on the Ultimate Bearing Capacity of a Lattice Transmission Tower with Non-uniform Settlement

The importance of bolt joint effects on lattice transmission towers has been observed and studied for a long time. Foundation subsidence, which happens in the frozen ground and coal mined-out domain, may bring damages or destruction of lattice transmission towers due to non-uniform settlements. However, some towers can still work well even though their actual deformations exceed the predicted ultimate values. This is because the bolt joints are simplified as rigid or pinned connections in the conventional analysis. In this paper, a finite element model considering bolt joints is established and the effects of bolt joints on a lattice transmission tower with three cases of non-uniform settlement are numerically investigated. This work can provide a better reference to the safety evaluation for lattice transmission towers with non-uniform settlement.

Structural behaviour of lattice transmission towers subjected to wind load

Hurricane Wilma hit Yucatan Peninsula in 2005, causing substantial damage to local electrical transmission towers. Based on the failure characteristics observed on such towers, an analytical study is performed to reproduce such failures and assess their vulnerability. Two latticed transmission towers are analysed under the action of 14 different wind velocity patterns corresponding to several national and international wind design codes. Displacement-controlled pushover analyses are performed to reproduce impending failure mechanism for considered wind patterns, and associated gradient wind speed is computed. Results illustrate that consistent cyclonic wind speed patterns lead to better estimates of failure mechanism and gradient wind values than traditional non-cyclonic patterns.

Direct Analysis of an Ultrahigh-Voltage Lattice Transmission Tower Considering Joint Effects

AbstractTransmission towers play an important role in transmitting electricity in a power grid safely and reliably. In traditional design practice, the second-order P-Δ (frame side sway) and P-δ (m...

CORROSION PROCESSES ON WEATHERING STEEL RAILWAY BRIDGE IN PRAGUE

This contribution deals with experimental corrosion tests carried out on the weathering steel railway bridge in Prague. The basic specific property of the weathering steel is an ability to create in favourable environment a protective patina layer on its surface. Since 1968 weathering steel is used under the name “Atmofix” in the Czech Republic and can be used as a standard structural material without any corrosion protection. The weathering steel Atmofix is mostly used for bridge structures and lattice transmission towers. Some of these constructions built in the Czech Republic have been assessed and inspected in the last few years. Authors of this paper developed a new methodology of non-destructive experimental testing of corrosion processes. Testing specimens are installed on typical surfaces of steel bridges in such a way to simulate real conditions of the examined surface. This paper describes results of corrosion tests (thickness of corrosion products, corrosion losses and X-Ray analysis) after one-year exposure, correlation factor and dependence of location.

Progressive collapse analysis of power transmission towers

Lattice transmission towers are critical components of power infrastructures. Collapse of a transmission tower may cause major economic and social impacts. The present study aims to evaluate the progressive collapse vulnerability of a lattice tower in a 400kV power transmission line. The load increase factors (IF) after an instantaneous element removal are determined for application in static analyses and for design purpose. The capacity-to-demand ratios (C/D) are proposed to identify critical members after different removal scenarios. This proposed parameter is examined by comparison with Overload Factor (OF) from pushdown analysis. In addition, the pushdown analysis is used to determine the remaining capacity of the structure after removing an element. It is found that tower may resist progressive collapse because of the possible alternative load paths. It is also observed that the studied structure has less susceptibility to progressive collapse in scenarios of element removal at higher elevations.

Experimental and numerical analysis of a bolted connection in steel transmission towers

This paper presents an integrated numerical and experimental study on a bolted splice connection used in main legs of steel lattice transmission towers. At specific locations, where the number of angle sections in built-up cross section of main leg members changes, the complex geometry around the connection region results in eccentricities in the load path and indirect load transfer. Such complex configurations and uncertainties in the load path have led to overdesigned connections with increased number of bolts and redundant connection reinforcing members. The current study was conducted in an attempt to gain a better understanding of the load-flow mechanism at this specific location where the cross section of main leg members changes. The experimental part included tensile load testing of six specimens with different connection details. The main parameters used in the testing program were the number of bolts used in the connection as well as the presence of connection reinforcement angles and tie plate. For all connection configurations studied, the failure occurred due to net section fracture of upper main member angle near leading bolt holes. The calculated load capacity based on the measured material strength closely predicted the measured load capacity of specimens. The experimentally determined response of each connection configuration was better predicted by the FE model that incorporates bolt slip as compared to the model that assumes no slip. The experimental and numerical results also indicate that major differences among the investigated connection details do not cause any appreciable difference in behavior under tensile loading.

Structural Analysis of Lattice Steel Transmission Towers: A Review

Lattice Transmission Towers (LTT) and the associated transmission line systems are important infrastructure in modern society. Due to the complicated load conditions and the nonlinear interaction among the large number of structural components, accurate structural analysis of the LTT systems has been a challenging topic for many years. Still today there are some gaps between research and industrial practice. This paper presents a summary of research outcomes from current literature. Recent developments in structural modelling and failure prediction technology are reviewed in terms of connection joints, individual members and structural systems subjected to static and dynamic loads. The advantages and disadvantages of different approaches have been compared. Finally, the knowledge gaps, associated challenges and possible research directions are highlighted

Probabilistic Capacity Assessment of Lattice Transmission Towers Under Strong Wind

Serving as one key component of the most important lifeline infrastructure system, transmission towers are vulnerable to multiple nature hazards including strong wind and could pose severe threats to the power system security with possible blackouts under extreme weather conditions, such as hurricanes, derechoes, or winter storms. For the security and resiliency of the power system, it is important to ensure the structural safety with enough capacity for all possible failure modes, such as structural stability. The study is to develop a probabilistic capacity assessment approach for transmission towers under strong wind loads. Due to the complicated structural details of lattice transmission towers, wind tunnel experiments are carried out to understand the complex interactions of wind and the lattice sections of transmission tower and drag coefficients and the dynamic amplification factor for different panels of the transmission tower are obtained. The wind profile is generated and the wind time histories are simulated as a summation of time-varying mean and fluctuating components. The capacity curve for the transmission towers is obtained from the incremental dynamic analysis (IDA) method. To consider the stochastic nature of wind field, probabilistic capacity curves are generated by implementing IDA analysis for different wind yaw angles and different randomly generated wind speed time histories. After building the limit state functions based on the maximum allowable drift to height ratio, the probabilities of failure are obtained based on the meteorological data at a given site. As the transmission tower serves as the key nodes for the power network, the probabilistic capacity curves can be incorporated into the performance based design of the power transmission network.

Modeling of retrofitted steel transmission towers

To meet the increased design loads from upgraded wind load standards and higher power demands, existing aging transmission towers are often in need of retrofitting. One effective way to increase the load carrying capacity of a tower structure is to reinforce its critical leg members through attaching additional elements with bolted cruciform connections. The effectiveness of the leg retrofitted method has been verified experimentally in both leg segments and tower systems. In this paper, a new modeling method for lattice transmission towers with retrofitted leg members was developed in STRAND 7 through the use of truss-beam elements to represent tower members, and spring elements to simulate bolted connections. Numerical results based on the new modeling method agreed well with those from previous experimental retrofitted tower tests. A parametric study showed that: (a) the cruciform connections have a high load-transfer performance in retrofitted steel lattice towers and thus are effective in enhancing the strength of the whole structure, (b) pre-loading in tower members delays the occurrence of the peak load-sharing rate and excessive pre-loading causes a significant reduction of structural strength, and (c) extra mid-panel connections, cruciform connections with higher slip-resistance and increased retrofitting member size can significantly enhance the structural load carrying capacity.

Failure analysis of guyed transmission lines during F2 tornado event

Motivated by many failure incidents observed worldwide of transmission line structures during tornadoes and by the lack of appropriate design procedures accounting for loads associated with such events, an extensive research program focusing on this subject initiated at The University of Western Ontario, Canada, few years ago. The current study is a part of this research program, where a numerical model that can predict the capacity and the failure mode of a transmission line system under tornadoes is developed and validated. In this numerical model, the tornado wind field is obtained from previously conducted and validated computation fluid dynamics simulations. Two different lattice transmission towers are considered for failure analysis under the developed numerical model. Due to the localized nature of tornadoes, the forces acting on a structure depends on the location of the tornado relative to the structure. As such, a parametric study is first conducted by moving the tornado in space in order to identify the critical tornado locations leading to peak forces in the tower members. Failure analysis of each system is conducted under the two most critical tornado locations identified from the parametric studies. The analysis is conducted incrementally by gradually increasing the tornado velocity till reaching the peak F2 tornado velocity. The critical velocity at which a full collapse of a tower is determined. The effects of inclusion of geometric nonlinearities and of the assumptions made regarding post yielding behavior of tension members on the failure capacity are also assessed in the study. While both transmission tower systems are shown to fail at critical velocities less than the maximum F2 tornado velocity, significant difference in those critical velocities is observed between the two considered systems.

Theoretical and Experimental Research on Joint Slippage Effects of Lattice Angle Steel Tower

In lattice angle steel transmission tower, as the bolt diameter is small and member connection is relatively thin, lower clamping force is commonly used in bolted joint. It is common in lattice tower during full tower tests, joint slippage occur even under design load. However, traditionally trussed beam hybrid finite element model without explicitly considering slippage effects has been widely used in the analysis of the tower. In this paper, the HD-1 tower was experimentally studied under various static load cases, and several numerical models with including joint eccentricity and slippage are established. After comparing the theoretical analysis results and the experimental results, the following conclusions are presented: joint eccentricity almost has not effects on leg member axial force; Among all the studied load cases, joint slippage effects the leg member force most under torsional condition than the other load conditions; Numerical model with including joint slippage effects yield much better axial force results in leg member compared with experimental test results.

Effect of wind direction on the response and capacity surface of a transmission tower

Electrical transmission structures play a vital role in power transmission networks throughout the world and are often subject to strong wind loads. The present study investigates the inelastic response of a self-supported lattice transmission tower under different wind events, including traditional atmospheric boundary layer wind (ABL) and downburst wind, and for wind loading at different directions relative to the tower. The nonlinear static pushover (NSP) analysis is used to obtain the capacity curve of the tower, defined by the force–deformation relationship, at each considered wind direction. The results show that the yield and maximum capacities vary with wind direction. The deformation trajectory is projected on the horizontal plan for each wind direction, illustrating the out-of-plane loading deformation. Since the trajectories are sufficiently smooth and do not overlap for closely spaced wind directions, it is suggested that the capacity curves obtained for different wind directions can be used to form the capacity surface of the tower. Moreover, the results indicate that the capacity for ABL and rectangular (uniform) winds could provide an approximate envelope for the capacity estimated for different sizes of downburst events. These findings could assist in the evaluation of the adequacy of existing towers under downburst events, as well as in the design of new towers.

Seismic Control of Power Transmission Tower Using Pounding TMD

Lattice transmission towers are vital components of transmission line systems, which play an important role in the operation of electrical power systems. This paper proposes a new type of tuned mass damper (TMD), the pounding tuned mass damper (PTMD), to upgrade the seismic resistant performance of a transmission tower. In the PTMD, a limiting collar with viscoelastic material laced on the inner rim is installed to restrict the stroke of the TMD and to dissipate energy through collision. The pounding force is modeled based on the Hertz contact law, whereas the pounding stiffness β is estimated in a small-scale test. A multimass model of a 55-m tower is established to verify the effectiveness of the PTMD numerically. Harmonic excitation and time-history analysis demonstrate the PTMD’s superiority over the traditional TMD. Finally, a parametric study is performed for the optimal design.

Effects of Different Initial Imperfections including Residual Stress on the Compressive Bearing Capacity of Steel Angles in Towers

Angles are extensively used in latticed transmission tower. Initial imperfections have a significant impact on the compressive instability and failure of angle components. To study the impact of different initial imperfections on the compressive bearing capacity, general-purpose finite element software ANSYS is used. With shell element SHELL181, selecting angle L200×20 of the slenderness ratio of 20, 40, 60, 80, 100 and 120, the bearing performance analysis with initial imperfections is conducted, including initial bending, section defects, initial eccentricity, residual stress, and a combination of imperfections. At the same time, for the angle components of the optimal slenderness ratio of 40 and slenderness ratio of 80, under the single influence of initial bending, section defects and residual stress, how the size of the value of the imperfections affects the bearing performance is investigated.

Experimental study on multi-panel retrofitted steel transmission towers

Due to the increasing demands on power supply and telecommunication services, existing transmission towers are frequently being required to carry extra loads above their initial design limits. A range of methods have therefore been used to increase the capacity of existing towers by retrofitting them in some way. This paper addresses steel lattice transmission towers with main leg members retrofitted by steel angles through bolted double steel angle connectors, a method that is widely used in practice but to date with little experimental research to support it. Three unreinforced tower models and four groups of retrofitted tower models with and without preloading have been tested in the structural laboratories at the University of South Australia. The experimental results verify the effectiveness of the retrofitting method. Load sharing analysis shows that axial loads can be effectively transferred between original tower members and reinforcing members through the bolted-splice system. Preloading reduces the load sharing in reinforcing members in the early loading stage but does not have significant influence on the ultimate strength of the whole structure.

Modelling of steel lattice tower angle legs reinforced for increased load capacity

As a result of changes in design code requirements for wind loads, increased demand for electrical power, and the growing communication industry, retrofitting of existing steel lattice towers has become an important strategy in the transmission tower industry. At present there are two main reinforcing methods that are used in practice for lattice transmission towers. The first method involves adding a number of horizontal braces (diaphragms) to the mid-length of slender, inadequately braced members along the height of the tower; while the second method is focused on strengthening the capacity of the existing legs only by retrofitting them with additional members. In this paper, the most effective leg retrofitting method has been investigated through an experimental testing program using a one-panel angle leg retrofitting model and a nonlinear finite element (FEM) model. A 3D continuum element was developed to model the panels and bolted connections and the results are compared with those obtained from experiments, with good agreement. The model was then simplified using a simple locking system where a special technique was developed to simulate the bolt slip, which was found to be a critical issue from the experiments. The simplified model can then be extended to multi-panel modelling.

Accurate modeling of joint effects in lattice transmission towers

Lattice Transmission towers are vital components of overhead transmission lines which play an important role in the operation of electrical power systems. Accurate prediction of the structural capacity of lattice towers under different failure modes is very important for accurate assessment of the reliability of transmission lines and power grids, and for design of efficient failure containment measures. Traditionally, lattice towers are analyzed as ideal trusses or frame-truss systems without explicitly considering loading eccentricities and slippage effects in bolted joints. Such effects are always observed in full-scale tower tests and introduce great differences in the ultimate bearing capacity and failure modes obtained from classical linear analysis models. In this paper experimental results available from full-scale prototype tests of a single-circuit 110 kV and a single-circuit 220 kV lattice transmission towers subjected to different load cases are presented and compared with those obtained from four series of numerical models that include joint eccentricity effects and different joint slippage models. The numerical simulation results confirm that joint slippage dramatically increases the deformation of the lattice towers, while its influence on load-bearing capacity will vary in different load cases according to the magnitude of vertical loading and the tower failure mode. Results from the pushover nonlinear static analysis of the towers considering both joint slippage and eccentricity are found in agreement with the experimental results. This type of analysis can be used to model joint effects in lattice towers.

Failure Analysis of Failed Wire Rope

Failure of an old rope from a stringing lattice transmission towers occurred in winter while the rope was being removed to make way for a new rope. Fracture took place around mid-span. At that time, ambient temperature was −22 °C. Wire rope was in service for nearly 50 years. We were given the mandate to determine the reasons for the fracture of the wire rope and also to suggest measures to prevent such failures from occurring. The study involved laboratory testing (mechanical and metallographic) of representative wire rope samples. The effect of low temperature (from room temperature to −40 °C) on the tensile behavior of wires and wire rope samples was evaluated. In addition, we designed an instrumented impact test to assess the effect of notches, low temperatures and dynamic loading on the fracture behavior; however, no standards were available for direct comparison. Optical metallography was used to judge the extent of corrosion and the nature of microstructure and the cleanliness of the steel. The fracture morphology of broken tensile and impact specimens was carried out using scanning electron microscopy to establish relations between test parameters and fracture modes. Results indicate that considerations have to be given to the occurrence of corrosion, notches, low temperatures, and dynamic loading conditions when replacing wire ropes and this may necessitate the replacement of wire rope earlier than the time dictated by the criterion of 10% loss in breaking strength. Results also indicate that impact testing is a better evaluator of the susceptibility of wire ropes to brittle fracture than tensile tests.

Numerical simulation of structural behaviour of transmission towers

Transmission towers are a vital component and management needs to assess the reliability and safety of these towers to minimise the risk of disruption to power supply that may result from in-service tower failure. Latticed transmission towers are constructed using angle section members which are eccentrically connected. Factors such as fabrication errors, inadequate joint details and variation of material properties are difficult to quantify. Consequently, proof-loading or full-scale testing of towers has traditionally formed an integral part of the tower design. The paper describes a nonlinear analytical technique to simulate and assess the ultimate structural response of latticed transmission towers. The technique may be used to verify new tower design and reduce or eliminate the need for full-scale tower testing. The method can also be used to assess the strength of existing towers, or to upgrade old and aging towers. The method has been calibrated with results from full-scale tower tests with good accuracy both in terms of the failure load and the failure mode. The method has been employed by electricity utilities in Australia and other countries to: (a) verify new tower design; (b) strengthen existing towers, and (c) upgrade old and aging towers.