Lattice Transmission Towers Research Articles

Component-based modelling of single-leg bolted steel angle connections at elevated temperatures

As a vital part of contemporary society, steel lattice transmission towers and communication towers play an important role in the power transmission grid and telecommunication, respectively. Being often located in rural areas, they are prone to bushfire attacks due to heatwaves and droughts, which are becoming increasingly commonplace. However, unlike the steel members found in a lattice tower, the behaviour and material properties of which at elevated temperatures have been comprehensively studied and codified in design standards, the semi-rigid behaviour of the bolted connection between such members at elevated temperatures has not been investigated thoroughly. Therefore, this paper aims to investigate both the axial and rotational behaviour of single-leg bolted steel angle connections at both ambient and elevated temperatures. A theoretical model using the component method is firstly proposed to model the axial load-slip behaviour incorporating the temperature increase, with the effects of the bolt shear, plate bearing, friction between clamped members, bolt slip, and member elongation being considered. Once the axial load-slip model is proposed, the component-based model for describing the moment-rotation behaviour of the bolted connection is derived based on the load-deformation relations of individual bolts in the bolt group. The reliability of the theoretical model is then validated by modelling the connection behaviour obtained from previous connection test results. A three-dimensional finite element model is also constructed using ABAQUS software to further validate the accuracy of the proposed theoretical model for predicting the axial and rotational behaviour of single-leg bolted angle connections at elevated temperatures. It is demonstrated that the proposed component-based theoretical model for the bolted angle connection accounting for axial and rotational semi-rigidity as well as temperature elevation is capable of effectively capturing the load-deformation relationship for the single-leg bolted angle steel connections, and would be applicable to the study of steel lattice towers under bushfire attack.

Fire resistance behavior and failure analysis of transmission tower exposed to wildfires

This paper deals with the fire resistance behavior and failure mode of transmission towers exposed to wildfires. A nonlinear thermo-mechanical coupling model of a latticed transmission tower was established by the finite element (FE) program ANSYS, which was validated from steel members’ perspectives. Wildfire analysis was conducted to determine the key parameters, including the number of legs in fire, fire intensity, and residence time, and the temperature–time curve was further proposed for parametric analysis. It reveals that the transmission tower may collapse under serviceability state loads in the wildfire, and the final failure mode was determined by the number of tower legs in the wildfire. A higher wildfire intensity will significantly reduce the wildfire resistance time of the transmission tower. Therefore, combustible materials around transmission towers need to be regularly cleaned to reduce the intensity and residence time of wildfires, which is an effective way to prevent wildfires. Moreover, a wildfire protection layer design process was proposed based on the environment surrounding the transmission tower, which proved reliable to provide valuable insights for the fire protection design of transmission towers in areas prone to wildfires.

Ultimate bearing capacity of T-shaped retrofitting diagonal members for angle steel transmission towers

Collapse of steel latticed transmission towers under strong winds is a significant safety concern for power systems. Attaching additional members to the tower members can enhance the wind resistance of transmission towers. To establish systematic retrofitting schemes and methods for predicting the ultimate bearing capacity of T-shaped retrofitting diagonal members, static experiments and finite element analysis on various cross-sectional dimensions and member lengths were utilized in this study. The experimental results indicate that retrofitting can effectively increase the bearing capacity of diagonal members. Finite element models are developed and verified based on the discussion of load-displacement curves and failure modes. The finite element analysis reveals that the increase in capacity achieved by reducing connector spacing is relatively small when compared to the ultimate bearing capacity obtained with the spacing of two connectors, and the arrangement of connectors at both ends meets the requirement of connector spacing. The relationship between the retrofitting bearing capacity and the slenderness ratio of diagonal members can be divided into two linear stages, which correspond to different failure modes. Furthermore, a λk coefficient method based on the axial compression equation is proposed to predict the ultimate bearing capacity of T-shaped retrofitting members. The bearing capacity based on the λk method shows agreement with finite element analysis results.

Buckling resistance of star-battened angle members made of high-strength steel

Recent upwards developments of the power grid requests lead to use of high strength steels (HSS) for the design and construction of higher and heavier loaded steel lattice transmission towers, or to strengthen the existing ones. Usually star-battened sections are preferred for the strengthening of individual members or in case of very high compression loads. Thus, the investigations focus here on the bearing capacity of a star-battened member made of two S460 L250x250x28 angles. The objective was to use a star-battened member made of two S460 L300x300x35 angles with a member length of 4486 mm that will be used in a specific project of 240 m high power transition tower. However, as the request in terms of length capacity cannot be met by the testing labs in Belgium, the studies have concentrated on the profile made of L250x250x28 angles.Firstly, a critical review of the current European normative documents (EN 50341, EN1993-1–3, prEN1993-3) has been made and the buckling resistance of the above-mentioned profile has been evaluated. Then, an experimental compression test has been performed for this member. The test has been complemented by a full non-linear finite element simulation by means of ANSYS software. Experimental, numerical and analytical results have been compared and discussed. Finally, conclusions were drawn considering the design of S460 star-battened members in compression. These studies are part of an ongoing project entitled “New steel” funded by Elia and ArcelorMittal and involving the University of Liège.

Load carrying capacity of star‐battened angle members made of S460 steel in compression

AbstractLattice towers are extensively built in Europe and worldwide to serve power transmission purposes. Their members mainly consist of single angle profiles, while built‐up sections, such as star‐battened ones, are usually preferred for the strengthening of individual members or in case of very high compression loads. Recent upwards developments of the power grid requests also lead to the need to use high strength steels (HSS) for the design and construction of higher and heavier loaded steel lattice transmission towers. Within the present paper, investigations conducted on the bearing capacity of a star‐battened member made of two S460 L250×250×28 angles are presented, even if the conclusions may be extended to any angle size and steel grade.Firstly, a critical review of the current European normative documents (EN 50341, EN1993‐1‐3, prEN1993‐3) is made and the buckling resistance of the above‐mentioned member is evaluated. Secondly, the results of an experimental compression test performed on the studied member are presented; these results are complemented by a full non‐linear finite element numerical simulation. All the obtained results are finally compared and discussed.

Numerical analysis of insulated and conventional cross arms for transmission line tower

The development of the world power industry has led to studies focused on high-performance materials to enhance the durability and prolong the service life of existing structures. In this context, cross arm is one of the main components of an electricity-transmission structure. The application of composite units (insulated cross arm) on lattice transmission towers was evaluated using the software Ansys 2020 R2 in terms of efficiency, cost, and mechanical performance, compared to conventional transmission towers. Finite-element analysis results demonstrated reductions in internal forces, displacements, and support reactions when the insulated cross-arm (IC) tower structure was employed instead of the conventional cross-arm (CC) tower structure. This could lead to minor structural costs. The IC tower exhibited a better stability under free vibrations owing to its higher stiffness than that of the CC tower. The main reason for these results was the height reduction proportioned by IC structures.

A simplified analytical method for lateral dynamic responses of a transmission tower due to rockfall impact

Transmission tower structures support high-voltage power lines that carry electricity over long distance and rockfall is one of critical disasters during its safe operation. This paper presented a simplified analytical methodology for lateral dynamic responses of a transmission tower structure due to rockfall impact. At first, the lateral dynamic displacement of a lattice transmission tower structure can be represented by a second-order partial differential equation and half sine wave was used for rockfall impact. Then, the solution can be approximated by a set of specified shape functions multiplied by time-dependent generalized coordinates. And the partial differential equation is discretized into a set of single degree of freedom system. And then the shape function can be determined by solved an eigenvalue function and the fundamental frequency of a transmission tower can was derived based on the energy method and combination synthesis method. Finally, the lateral dynamic displacements can be approximately obtained. A numerical study of a transmission tower was conducted. Parametric study of the effect of impact location height, impact duration, peak impact force, as well as the distribution of cross-arms on dynamic responses were also carried out. And the results show that the discrepancy between the analytical and the computed of fundamental frequency is less than 3%, the error of dynamic displacement is within 10%, and the fundamental frequency of the structure decreases with the increase of the tower top additional mass ratio.

INFLUENCE OF BOLTED SPLICE CONNECTIONS ON THE GLOBAL BEHAVIOUR OF STEEL LATTICE TELECOMMUNICATION TOWERS

The bolted joints in the leg and the bracing members of the lattice transmission towers are always subjected to predominant axial forces, which will cause joint slip that greatly affects the global be-haviour of the whole structure. The paper shows the results of the numerical modelling of the re-sponse of the steel lattice communication tower, with height h = 40.5 m located in Rzeszów. A comparison was made of five tower models, differing in the characteristics of the joint force-elongation relationship, including stiffness of the components and also joint slippage, coming from Category A joints. The paper presents the difference in displacements and rotations of chosen tower panels, internal forces in leg members, as well as in the fundamental flexural frequency obtained without considering the force-displacement characteristic and with four different ways of modelling of joints behaviour

Recent Advances of GFRP Composite Cross Arms in Energy Transmission Tower: A Short Review on Design Improvements and Mechanical Properties.

Currently, pultruded glass fibre-reinforced polymer (pGFRP) composites have been extensively applied as cross-arm structures in latticed transmission towers. These materials were chosen for their high strength-to-weight ratio and lightweight characteristics. Nevertheless, several researchers have discovered that several existing composite cross arms can decline in performance, which leads to composite failure due to creep, torsional movement, buckling, moisture, significant temperature change, and other environmental factors. This leads to the composite structure experiencing a reduced service life. To resolve this problem, several researchers have proposed to implement composite cross arms with sleeve installation, an addition of bracing systems, and the inclusion of pGFRP composite beams with the core structure in order to have a sustainable composite structure. The aforementioned improvements in these composite structures provide superior performance under mechanical duress by having better stiffness, superiority in flexural behaviour, enhanced energy absorption, and improved load-carrying capacity. Even though there is a deficiency in the previous literature on this matter, several established works on the enhancement of composite cross-arm structures and beams have been applied. Thus, this review articles delivers on a state-of-the-art review on the design improvement and mechanical properties of composite cross-arm structures in experimental and computational simulation approaches.

Creep Properties and Analysis of Cross Arms' Materials and Structures in Latticed Transmission Towers: Current Progress and Future Perspectives.

Fibre-reinforced polymer (FRP) composites have been selected as an alternative to conventional wooden timber cross arms. The advantages of FRP composites include a high strength-to-weight ratio, lightweight, ease of production, as well as optimal mechanical performance. Since a non-conductive cross arm structure is exposed to constant loading for a very long time, creep is one of the main factors that cause structural failure. In this state, the structure experiences creep deformation, which can result in serviceability problems, stress redistribution, pre-stress loss, and the failure of structural elements. These issues can be resolved by assessing the creep trends and properties of the structure, which can forecast its serviceability and long-term mechanical performance. Hence, the principles, approaches, and characteristics of creep are used to comprehend and analyse the behaviour of wood and composite cantilever structures under long-term loads. The development of appropriate creep methods and approaches to non-conductive cross arm construction is given particular attention in this literature review, including suitable mitigation strategies such as sleeve installation, the addition of bracing systems, and the inclusion of cross arm beams in the core structure. Thus, this article delivers a state-of-the-art review of creep properties, as well as an analysis of non-conductive cross arm structures using experimental approaches. Additionally, this review highlights future developments and progress in cross arm studies.

Seismic response study of a steel lattice transmission tower considering the hysteresis characteristics of bolt joint slippage

Ignorance of the influences of bolt slippage effects on transmission towers will lead to the underestimation of the deformations of the transmission towers. Therefore, the responses of transmission towers under different load scenarios cannot be accurately predicted. This will result in an incorrect estimation of the ultimate load-bearing capacity and the failure mode of transmission towers. Recently, studies of the influences of bolt slippage effects on transmission towers have been limited to static analysis only. This paper presents a dynamic response study of transmission towers considering bolt slippage. This paper first presents the test results of the mechanical properties of typical bolt joints in transmission towers under cyclic and monotonic loads. The numerical models of typical joints and the hysteresis curves of each joint under cyclic loads are then proposed, and the numerical models of hysteresis curves are validated through laboratory tests. From the various simulation results on the different types of joints, the hysteretic curves of each joint are obtained, and the skeleton curves are extracted. The numerical models of the slippage effect of transmission tower joints are then integrated into the tower–line system model for seismic analysis. The numerical simulation results indicate that neglecting the dynamic effect of bolt slippage will underestimate the seismic response of transmission towers, which will directly lead to an overestimation of the dynamic resistance capacity of the transmission towers. Furthermore, the higher the earthquake magnitude is, the more serious the influence.

Revisit of underestimated wind drag coefficients and gust response factors of lattice transmission towers based on aeroelastic wind tunnel testing and multi-sensor data fusion

Accurate estimation of extreme wind-induced loads is key to the cost-effective and reliable performance-based design of overhead lattice transmission towers. Particularly, drag coefficient and gust response factors are among the main aerodynamic properties of these structures for the estimation of equivalent static wind loads. Wind design standards recommend values for these key properties for generic lattice frames based on the solidity ratio and height of the frames. However, the geometric properties of lattice transmission towers often vary along the height of the structure. Therefore, local drag coefficients and gust response factors of transmission towers must be assessed to reliably analyze the extreme wind performance of towers. This study estimates the drag coefficients and gust response factors of a double-circuit lattice transmission tower using an approach based on Kalman filtering. Multiple along-wind and crosswind responses of the tower were obtained from a series of aeroelastic wind tunnel tests that were conducted at the National Science Foundation (NSF) Natural Hazard Engineering Research Infrastructure (NHERI) Wall of Wind Experimental Facility (WOW EF) at Florida International University (FIU). The developed Kalman filtering model facilitates the fusion of noisy measurements from multiple sensors of the same and different types that were implemented in the wind tunnel experiments. This approach is integrated with an optimization technique to provide estimates of wind load parameters of interest with high spatial resolution and accuracy from measured responses. The derived drag coefficients and gust response factors are treated as new evidence along with the ASCE manual recommendations for these properties as priors in a Bayesian regression model to provide new recommendations. The findings of this study indicate larger drag coefficients and gust response factors than those suggested by ASCE No. 7 and No. 74 by up to 12% and 13%, respectively.

Analytical and numerical analysis of angle steel joints for conductors in lattice transmission towers

This paper presents a theoretical and numerical study of angle steel joints for conductors in transmission tower structures in the medium and heavy icing area. The results show that the deformation of the angle steel joint in suspension towers is controlled by the vertical load, and the deformation of the angle steel joint in strain towers is controlled by the tension load. The bolt of the angle steel joint for conductors is tension-controlled. A theoretical method considering the bending stiffness ratio of the angle steel plate and the bolt is derived. The proposed theoretical method quantifies the bending stiffness difference in the angle steel plate and bolt group, and the effects of bolt layout and load cases are considered. The numerical results are in good agreement with theoretical results by validating the internal forces and neutral axis depth. The accuracy of the proposed theoretical method is higher than that of theoretical methods based on steel structure principles. The proposed theoretical method is of great significance to the development of transmission tower structure design.

Load-slip behaviour of single-leg bolted angle steel connections at elevated temperatures

Steel lattice transmission towers and communication towers are vital infrastructure components of power grid and telecommunication systems, respectively. They are usually located in rural or urban wildland areas and can be prone to bushfires or wildland fires in the current climate of heatwaves and droughts. However, an understanding of the resilience of steel lattice towers in fire has not received due attention. Such lattice structures are commonly made from steel angle members with bolted connections in which slippage might occur inevitably due to relatively larger size of bolt holes for erection purpose. Therefore, this paper investigates the axial load-slip behaviour of the single-leg bolted angle steel connections at elevated temperatures for the steel lattice structures. A three-dimensional finite element model is developed for the investigation by using ABAQUS software. Temperature-dependent material nonlinearities and the interaction of structural components at elevated temperatures are taken into account in the model. An effective approach of generating bolt pretension and the quasi-static analysis method using the dynamic explicit procedure are adopted for the analysis. The reliability of the developed finite element model is validated by comparing its predictions with available experimental results reported in the open literature. By using the validated finite element model, extensive parametric studies are conducted to elucidate the effects of the change in the angle section dimension, the direction of applied loading, the grade, size and number of the bolts on the load-slip responses of the connections at elevated temperatures. A simplified theoretical model in component formulation form is proposed and verified for predicting the axial load-slip curves of the single-leg bolted angle steel connections at elevated temperatures and would be applicable to advanced analysis of steel lattice structures considering joint slip.

Numerical and full-scale test case studies on post-elastic performance of transmission towers

Post-elastic capacity of transmission tower is critical for the realistic assessment of tower vulnerability under the extreme loading conditions. This paper first presented a series of numerical simulations of a lattice transmission tower section under torsional (longitudinal) or bending (transverse) loading in both static and dynamic scenarios. From the results of the numerical simulations, four prototypes of lattice transmission towers were constructed and tested for the investigation of the post-elastic performances of the towers under different loading conditions. The tests results showed that there existed significant post-elastic strength reserve of the tower under investigation. For the towers tested in this research, the post-elastic strength reserve was 1.22 for flexure-torsion (i.e. tower under longitudinal loading) governed by diagonals, and 1.37 for bending (i.e. tower in transverse loading) governed by inelastic buckling of the main legs. Diagonal members affected the failure modes of transmission towers and their connection design may be a weak link in the development of their post-elastic capacity.

Soil-Structure Interaction Effects on Dynamic Behaviour of Transmission Line Towers

As inferred from earthquake engineering literature, considering soil structure interaction (SSI) effects is important in evaluating the response of transmission line towers (TLT) to dynamic loads such as impulse loads. The proposed study investigates the dynamic effects of SSI on TLT behavior. Linear and non-linear models are studied. In the linear model, the soil is represented by complex impedances, dependent of dynamic frequency, determined from numerical simulations. The nonlinear model considers the soil non-linear behavior in its material constitutive law and foundation uplift in a non-linear time history analysis. The simplified structure behavior of a typical lattice transmission tower is assessed. The analysis of frequency and time domain are followed through varying soil stiffness and damping values. Three different shock durations are investigated. The soil-structure system with equivalent dynamic properties is determined. The behaviors achieved utilizing a rigid and a flexible base for the structures is compared to estimate the impact of taking SSI into account in the calculation. The current mainstream approach in structural engineering, emphasizing the importance of the SSI effect, is illustrated using an example where the SSI effect could be detrimental to the structure. Furthermore, the non-linear analysis results are analyzed to show the linear approach’s limitations in the event of grand deformations.

Weight optimization of steel lattice transmission towers based on Differential Evolution and machine learning classification technique

Transmission towers are tall structures used to support overhead power lines. They play an important role in the electrical grids. There are several types of transmission towers in which lattice towers are the most common type. Designing steel lattice transmission towers is a challenging task for structural engineers due to a large number of members. Therefore, discovering effective ways to design lattice towers has attracted the interest of researchers. This paper presents a method that integrates Differential Evolution (DE), a powerful optimization algorithm, and a machine learning classification model to minimize the weight of steel lattice towers. A classification model based on the Adaptive Boosting algorithm is developed in order to eliminate unpromising candidates during the optimization process. A feature handling technique is also introduced to improve the model quality. An illustrated example of a 160-bar tower is conducted to demonstrate the efficiency of the proposed method. The results show that the application of the Adaptive Boosting model saves about 38% of the structural analyses. As a result, the proposed method is 1.5 times faster than the original DE algorithm. In comparison with other algorithms, the proposed method obtains the same optimal weight with the least number of structural analyses.

Wind load investigation of self-supported lattice transmission tower based on wind tunnel tests

Through a series of wind tunnel tests on a self-supported lattice transmission tower model, the wind loads on the tower were investigated in this paper. According to similarity laws, the scaled tower model was designed and fabricated, and the atmospheric boundary layer wind field of Category-B specified in Chinese load code was simulated in the wind tunnel laboratory. Based on the aeroelastic wind tunnel tests and the high frequency force balance tests, the dynamic vibrations, the nondimensional wind force coefficients and the generalized power spectral densities (PSDs) of dynamic wind forces were analyzed and discussed. For the convenient application, the empirical formulas of the generalized PSDs of dynamic wind forces were established and the nonlinear mode shape correction was introduced, which were verified via comparison between the aeroelastic tests and the calculated dynamic responses of tower top. It is indicated that the unfavorable wind attack angles may happen around θ = [15° 30°] and θ = [60° 75°] for this type of tower; the generalized PSDs of wind forces are unimodal in shape, and the corresponding effective bandwidths and the nondimensional frequencies of peaks vary with the wind attack angle θ, especially those of the cross-wind forces are larger than those of the along-wind forces obviously; the established empirical formulas of the generalized PSDs of dynamic wind forces and the derived nonlinear mode shape correction factor are applicable.

Reliability-Based Approach for Fragility Analysis of Lattice Transmission Tower in the Type Test

Precise prediction of the structural capacity of lattice transmission towers under various loads is essential to assess the reliability of the transmission network accurately. In doing so, the uncertainties inherent in the modeling parameters of the towers need to be taken into account. In this paper, a probabilistic framework is developed to analyze the failure of a 230 kV double-circuit tower in full-scale type test accounting for the uncertainties including eccentricity at the connections, joint slippage, and initial imperfection in the members. Three loading patterns are applied to the manufactured full-scale tension tower. A finite element model of the tower with consideration of mentioned uncertainties is built and verified by the test results. The importance vectors derived from reliability analysis clarifiy the effect of each of these parameters on the target points' displacement, as well as the maximum load carrying capacity in towers' members for these load patterns. Besides, the additional moments due to eccentricity at the connections are considered by a proposed regression-based equation. The failure probability of the tested tower is determined for various load factors, and the results are presented in terms of fragility curves. Besides, the effect of eccentricity on the tower’s failure is quantified.

The design of a steel lattice transmission tower in Central Europe

AbstractThe current paper resumes the design of a steel lattice transmission tower carried out in the framework of the RFCS‐funded research project ANGELHY. The scope of ANGELHY is the establishment of analysis methods for lattice towers and the development of design rules for steel and hybrid angle sections, as well as built‐up members composed of angles. In Europe, the design of transmission towers for overhead electrical lines can be carried out according to EN 1993‐3‐1: 2006 or for electrical lines exceeding 1 kV according to EN 50341‐1: 2012. Both standards prescribe linear elastic global analyses for the design of steel lattice towers. In the current paper, the linear elastic design of the suspension transmission tower according to the German National Annex of EN 50341‐1:2012 is presented. In a first step, common typologies of lattice transmission towers used in Europe are described. In a second step, the chosen tower typology is explained in detail. The Danube tower represents the widest spread tower typology in Europe. Finally, the detailed design procedure of the tower according to EN 50341‐1:2012 is presented. The design is carried out with the software TOWER – Version 15.0 from Power Line Systems.