Steel Lattice Towers Research Articles

Design recommendations for the stability of transmission steel lattice towers

Lattice towers are extensively built in Europe and worldwide to serve telecommunication or power transmission purposes. Their members are usually made of equal leg angle profiles that are bolted at their extremities. Such types of towers are mainly designed to EN 1993-3-1 and EN 50341-1, based on a first-order linear elastic structural analysis of a truss structure. In this paper, an assessment of the current design approach is performed, where the tower has been simulated with a full non-linear finite element software, considering relevant imperfections as well as geometrical and material non-linearities. The importance of the second order effects in the analysis is underlined while the existence of an instability mode not properly covered directly by the norms, and usually therefore not checked, is highlighted. Two analytical models for the prediction of the critical load of this buckling mode are proposed and validated numerically. Both proposed models are rather easy to apply and may fill the gap in the existing design recommendations for lattice towers.

Performance based assessment of transmission lines to seismic events

Typical transmission line steel lattice towers are not designed for seismic excitations, based on the assumption that their ultimate limit state is governed by climatic actions (wind and ice). However, it is not unusual for line inspection reports to indicate the observation of moderate to severe structural damages after intense ground motions, which can ultimately reduce the capacity of tower elements. Performance-Based Earthquake Engineering (PBEE) assessment is a modern technique to earthquake resistant design frequently adopted for building frame structures, but not yet employed in the design of transmission line supports, to the best of the authors’ knowledge. The present article evaluates the seismic performance of two typical 230kV transmission towers in a PBEE framework, considering artificial and actual ground motions based on Chilean seismicity. Time-domain nonlinear dynamic analysis is employed to numerically obtain tower responses considering distinct earthquake loading scenarios according to two model configurations: stand-alone and tower-line section. Significant structural damage was detected even for moderate ground motions, which might reduce the tower capacity to future climatic events. The estimated failure probability for the ultimate limit state was found greater than the recommended values in the standard IEC 60826 (2017).

Structural effects of unequal leg lengths in lattice steel towers with the D-type bracing system

This paper presents a structural modeling analysis of transmission towers with varied leg length combinations to examine the structural effects of unequal leg lengths. Power transmission towers in mountainous areas often use unequal leg lengths to accommodate the changing slope of the terrain. In this study, three types of 345 kV steel transmission towers with the D-type bracing system were considered as analytical examples to discuss the internal force distribution of the tower structure subjected to a load under a fully wired condition. Different tower structural types exhibit similar structural characteristics. The upper part of the force distribution of a tower structure is less affected by varied leg length configurations. The central parts affected by unequal leg lengths are the extension part and the tower leg component. Analytical results indicate the critical elements of tower structures with unequal leg lengths, which can serve as a valuable reference for disaster prevention and reduction. In this study, the primary elements of the towers were all under the permissible strength even with the leg length differences exceeding the current specifications. This implies that adjusting the regulation of leg length differences of a transmission tower with unequal leg lengths can serve as the basis for future research, highlighting the advantages of unequal leg length tower applications.

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.

Influence of external loads to wind turbine tower

One of the most important parts of a wind turbine is a tower. There are various designs of the wind turbine towers, and they are most often made of steel pipes, lattice towers or concrete towers. In order to increase energy density to meet the growing electricity needs, larger wind turbine projects have been developed. Larger wind turbine towers can generate more electricity, but such large sizes also create higher costs in terms of development and maintenance. This research sets up a model of a wind turbine tower, where the load to the tower is calculated by its relation to the wind velocity. Analytical approach coupled with a finite element method (FEM) is used to analyse the distribution of tower stresses under these loads. The fatigue analysis of the column is performed using the load from its own weight, the weight of the housing and the distribution of the wind velocity. The effects of different loads are also compared. The results show that the main loads of the tower are the wind force acting on the area of ??rotation of the wind turbine blades and the moment caused by the uneven wind velocity. Construction is modelled using SolidWorks modelling package, where the analysis was performed using FEM in ANSYS software. As a result of the analysis, the stress distribution in the support was determined and compared with analytical calculations.

Aeroelastic modeling to study the wind-induced response of a self-supported lattice tower

The results from a 1:50 scale aeroelastic model of a self-supported steel lattice tower subjected to simulated hurricane winds are presented. The lattice tower considered is a typical structure that is used as part of a tower-insulator-conductor system for electrical transmission infrastructure. The aeroelastic tests were conducted at the NSF Wall of Wind Experimental Facility (WOW EF) at the Florida International University (FIU). The tower was tested at various wind speeds ranging from 50 m/s to 92 m/s (equivalent full-scale speeds) for varying wind directions. Two system identification (SID) techniques were utilized to evaluate along-wind aerodynamic damping and compare with theoretical estimates. The SID techniques were also utilized to evaluate crosswind aerodynamic damping. A buffeting analysis was conducted to estimate the response of the tower and compare it to measured values at the WOW. Drag and moment coefficients were calculated from the measured responses, and the dynamic amplification factors (DAF) as well as gust effect factors were computed. The analysis required consideration of the variation of the turbulence intensity along the height of the tower in the buffeting analytical equations. The drag coefficients are shown to agree with values proposed in the current standards. However, there might be a need to introduce base moment coefficients in lattice tower design. The resonance contribution is shown to reach a maximum of 18% of the peak response of the tower.

Reliability assessment of existing transmission line towers considering mechanical model uncertainties

Reliability assessment of transmission line towers (TLTs) becomes particularly important in the context of the growing demand for increasing the energy transport capacity of existing lines. However, a comprehensive literature survey in this field reveals that important discrepancies may exist between the original design model analysis results and the actual lattice steel tower behavior. For instance, it has been demonstrated (Kaminski-Jr, 2007; CIGRE, 2009; Souza et al., 2019, 2020) that bolt slippage effects, which are disregarded in the current industrial practice, is importantly impacted by the tower topology that is defined by the tower engineer based on experience. Therefore, because several existing structures have topologies prone to connections’ slippage, they may be operating with a lower reliability when compared to modern recommended values. Within this context, the main goal of this paper is to assess the structural reliability of existing TLTs considering mechanical model uncertainties induced by connection details. For this purpose, the failure probability of two real TLTs built in Brazil are investigated employing different levels of structural modeling complexity. The first tower was designed 40 years ago and, several TLs operating in Brazil have adopted a similar design. The second is a newer structure, designed 15 years ago. In addition, to better understand the impact on the results when following the classical design procedure, the progressive collapse of both TLTs is modelled through a full material and geometric nonlinear analysis with and without the consideration of bolt-slippage effects. The results show that the older tower design, which presents a topology highly impacted by bolt slippage, is associated with a lower reliability than the current target values.

Static and dynamic approach to the analysis of wind gusts in case of a tower H=110 m

Steel lattice towers have large application in meteorology, and are regularly exposed to loads difficult to determine reliably, like wind and ice, and especially wind gusts and accompanied structural vibrations. EN 1993-3-1 treats such structures and requires checking of vibrations, but does not supply methodology for it, neither allowed values for deformation and vibrations. The paper presents analysis of a tower 110 m high using the Finite Element Method (FEM), in case of wind gust, for iced and non-iced structure, both statically and dynamically. The results were compared and recommendations for future treatment of such sensitive structures were given.

Evaluation of Seismic Performance of Steel Lattice Transmission Towers

The electricity transmission systems are an important lifeline for modern societies. They are used for overhead power lines as supporting structures. Transmission towers are designed to meet electrical and structural requirements. They are designed according to the weight of conductors and environmental effects such as wind and ice loads. They also considered other extraordinary stresses such as cable breakage and ice-breaking effects. Because of a common perception that transmission line (TL) towers show low sensitivity to earthquakes, the effects of the earthquake in TL tower construction are not considered. For this reason, TL towers are investigated with regard to the seismic performance in this study. The principal objectives of this research are: i) to assess the sensitivity of typical TL towers to earthquake loads, ii) to retrofit an existing steel lattice tower using a new section Centre To Center (CTC). In this study, a finite element model of a representative 154 KV transmission tower in Turkey was performed using a set of 10 recorded earthquake ground movements. The four-legged square TL tower has been analyzed and designed for Turkey, Eskisehir seismic zone considering 42.95 m height using finite element (FE) software. Therefore, a new section Centre To Center (CTC) type has been designed and the failed sections have been replaced with a designed section using the SAP2000 section designer. The results show that the load of failure increased after retrofitting. The retrofitting method was effective and easily conducted in fields. Doi: 10.28991/cej-2020-03091600 Full Text: PDF

Analysis and Design of Transmission Tower with Isolated Footing

A transmission tower is a tall structure, usually a steel lattice tower used to support an overhead power line. They are used in high voltage AC and DC systems. Steel tower structure resting on footings are currently becoming popular in any geographical location for electrical transmission, microwave transmission etc. It is necessary to have an understudy of the behavior of such structures and provide an economical design. To understand the performance of such high rise structures, it is necessary to know their behavior considering the soil structure interaction. Modeling of Transmission tower for the validation purpose is done. The basic soil properties of laterite were also found accordingly. Here the entire transmission tower with the isolated footing is modeled and analyzed in STAAD Pro V8i and STAAD Foundation software. The total load transmitted to each tower leg is found using STAAAD Pro and the size of isolated footing under each tower leg is determined using the STAAD Foundation software respectively. If required the soil structure interaction effects (SSI) can also be found easily

Reliability Assessment of Steel Lattice Tower Subjected to Random Wind Load by the Stochastic Finite-Element Method

This work concerns the stochastic finite-element method (SFEM) reliability analysis of a skeletal, very slender steel telecommunication tower calibrated with a full-scale pushover experiment carried out for the same tower. The basic SFEM implementation is completed with common application of the generalized stochastic perturbation technique (SPT) and of the response function method with polynomial basis of the statistically optimized order. Verification of the proposed stochastic technique is carried out for the first four probabilistic moments by a comparison with traditional crude Monte Carlo simulation and, alternatively, with semianalytical methodology using the same polynomial structural responses as SPT and the well-known probabilistic integral definitions. Characteristic mean wind speed is introduced here as the input random Gaussian variable, while the computational model is subjected to the wind profile linearly dependent upon this mean speed consistent with applicable European design codes. The SFEM analysis is carried out here including large deformations of the tower elements and dynamic excitations in each node induced by the given trial wind spectrum. Finally, second-order reliability method (SORM) analysis is provided on the basis of the resulting extreme normal forces in the tower legs and their counterparts taken directly from the full-scale pushover experiments. The obtained results show that reliability indices for these towers according to the engineering codes have larger magnitudes than these corresponding to the experimental statistics and SFEM.

Experimental research on bolted joint strength and dispersion in steel lattice transmission tower legs under low temperatures (−20°C to −45°C)

AbstractCold regions with subzero temperatures (−20°C to −45°C) have important impacts on the mechanical properties of structural steel used in lattice steel towers of overhead transmission line systems. The results from regular material tests are not appropriate for the accurate analysis of joint strength in cold weather conditions. This paper presents the tensile test results of 18 coupons of steel material and eight groups (18 specimens per group) of bolted joints with Q345 and Q420 steel under temperatures of 20°C, −20°C, and −45°C. The results show that the yield‐to‐ultimate strength ratio of the joints under low temperature conditions is beyond the range of 0.60 to 0.75 for Q420 structural steel. Suggestions are made on how to improve the accuracy of joint design for both the partial resistance factor and the design value of joint yield strength in cold regions.

Torsional Capacity of Slip Joints with Polygonal Hollow Sections

ABSTRACTPolygonal steel sections are widely used for lighting towers and are more and more adopted for transmission line towers. They convince with an inconspicuous appearance compared to steel lattice towers. Therefore, instead of flange connections “invisible” joints are preferred, such as slip joints. The European design rules for overhead electrical lines (1) cover the use of these joints very generally by a constant ratio of the overlapping length L and the section's diameter D. It is neither distinguished between load conditions nor between cross‐sectional shapes. The ratio value L/D = 1.5 is based on bending tests without regard to torsional moments. In the design of transmission line pylons, torsional loading occurs in the load case of a single‐side loss of a transmission line. Thus, there is a need to improve the knowledge on the load bearing effect of a slip joint connection within this special load case.In this paper the torsional capacity of slip joints is investigated to identify the capability of polygonal cross sections to carry this additional load, particularly in comparison with circular hollow sections. On the basis of numerical and analytical analysis, an adaption for the design of the slip joints is proposed.

Structural failure analysis of a river-crossing transmission line impacted by the super typhoon Rammasun

Steel lattice towers are commonly used as supporting structures for overhead transmission lines. Extreme wind conditions like typhoons can cause significant damage on those steel towers and consequently jeopardize the safe operation of transmission lines. In the line sections where a river crossing is unavoidable, the steel lattice towers are designed to be much taller and stronger than regular towers owing to safety and operation considerations. In previous typhoon events in Hainan Province of China, none of the installed river-crossing towers have collapsed or experienced significant damages, except in the aftermath of super typhoon Rammasun in 2014, when two 95-m tall river-crossing suspension towers collapsed. This paper presents the structural failure analysis of these two collapsed lattice steel transmission towers: The improved genetic algorithm (IGA) was applied to solve the typhoon wind field model for improved resolution of wind records measured during the Rammasun event and based on of 10-min average wind speeds. The IGA used the field measurement data to generate time-varying typhoon key parameters that lead to a more accurate prediction of the wind field than the conventional method. Then a systematic procedure was developed to investigate the failure mechanism of steel lattice towers under fluctuating wind loads combined with rain loads with consideration of the aerodynamic characteristics of the transmission tower-line system. The results show that the root-cause of the structural failure of the two transmission towers was overstressing of the tower main leg members in the second panel above the ground. Based on the numerical simulation of typhoon wind loads and the structural failure analysis procedure applied in this study, a general reinforcement method of river-crossing towers is proposed and the implementation of a typhoon warning method is recommended to identify the potential risks in transmission lines and formulate corresponding typhoon emergency plans to reduce the losses caused by typhoons.

Hysteretic Model of Single-Bolted Angle Connections for Lattice Steel Towers

AbstractSingle-bolted angle joints are widely used on secondary bracings of lattice steel towers due to their low cost and ease of construction. It has been observed that the hysteretic behavior of...

Experiment-based reliability analysis of structural joints in a steel lattice tower

The main aim of this paper is to present a reliability estimation procedure for steel lattice telecommunication towers based on tensioned joint reliability. The experiment, which involved doing a full-scale pushover test of a 40 m high lattice tower, constituted the starting point for further analyses. Experimental data served as bases for finite element model calibration and assignment of steel mechanical properties based on tensile tests. A computational investigation was performed for random wind loads. Numerical considerations included a detailed dynamic analysis with respect to the specific wind velocity–time function, modeling of two particular tower joints by shell finite elements, and elastic-plastic range determination of joint reliability using the first-order and second-order reliability methods.

Static and dynamic analysis of guyed steel lattice towers

Guyed steel lattice towers (or guyed masts) are widely used for supporting antennas for telecommunications and broadcasting. This paper presents a numerical study on the static and dynamic response of guyed towers. Three-dimensional nonlinear finite-element models are used to simulate the response. Through performing static pushover analyses and free-vibration (modal) analyses, the effect of different bracing configurations is investigated. In addition, seismic analyses are performed on towers of different heights to study the influence of earthquake excitation time-lag (or the earthquake travel distance between tower anchors) and antenna weight on the seismic response of guyed towers. The results show that the inclusion of time lag in the seismic analysis of guyed towers can influence shear and moment distribution along the height of the mast. Moreover, it is found that the lateral response is insensitive to bracing configurations. The results also show that, depending on the mast height, an increased antenna weight can reduce the tower maximum base shear while other response quantities, such as cables tension force are found to be insensitive to variation in the antenna weight.

Capacity assessment of existing corroded overhead power line structures subjected to synoptic winds

The physical infrastructure of the power systems, including the high-voltage transmission towers and lines as well as the poles and wires for power distribution at a lower voltage level, is critical for the resilience of the community since the failures or nonfunctioning of these structures could introduce large area power outages under the extreme weather events. In the current engineering practices, single circuit lattice steel towers linked by transmission lines are widely used to form power transmission systems. After years of service and continues interactions with natural and built environment, progressive damages accumulate at various structural details and could gradually change the structural performance. This study is to evaluate the typical existing transmission tower-line system subjected to synoptic winds (atmospheric boundary layer winds). Effects from the possible corrosion penetration on the structural members of the transmission towers and the aerodynamic damping force on the conductors are evaluated. However, corrosion in connections is not included. Meanwhile, corrosion on the structural members is assumed to be evenly distributed. Wind loads are calculated based on the codes used for synoptic winds and the wind tunnel experiments were carried out to obtain the drag coefficients for different panels of the transmission towers as well as for the transmission lines. Sensitivity analysis is carried out based upon the incremental dynamic analysis (IDA) to evaluate the structural capacity of the transmission tower-line system for different corrosion and loading conditions. Meanwhile, extreme value analysis is also performed to further estimate the short-term extreme response of the transmission tower-line system.

Mitigation of conductor line galloping by a direct cable-connection to non-conductive composite power pylons

Steel lattice towers with suspended insulator strings are typically used to carry high-voltage overhead transmission lines. The installation of non-conductive power pylons made of glass fibre reinforced plastics enables a direct cable-pylon connection, as the composite structure acts as an unibody insulator. At the same time, wind-induced vibrations, such as the severe cable vibration phenomenon galloping, will consequently be directly transferred to the slender composite mast structure, potentially leading to extensive damage. The aim of the study is therefore to investigate the galloping behaviour of iced conductor lines with regard to different cable support conditions. Furthermore, additional damping in the composite power pylon structure is assumed to mitigate conductor line galloping and therefore reduce the risk of phase flash-overs between adjacent conductor lines. A numerical galloping simulation is carried out in order to evaluate the effect of a rigid cable-pylon connection with enhanced damping properties on the cable vibration amplitudes. A pylon-cable system, consisting of 3×300 m spans, is investigated. It was found that the support conditions of the conductor lines have a significant influence on the galloping mode, the vibration amplitudes and the orientation of the characteristic galloping ellipse. The addition of damping to the pylon decreases the vibration amplitudes slightly and leads to a re-orientation of the galloping ellipse.

Probabilistic capacity assessment of single circuit transmission tower-line system subjected to strong winds

The safety and resilience of physical infrastructure of the power systems, including the transmission towers and lines for high-voltage transmission as well as the poles and wires for power distribution at a lower voltage level, is critical to avoid and recover from power outages under the extreme weather related loads such as strong winds from hurricanes, tornados, downbursts or isolated thunderstorms. In the current engineering practices, single circuit lattice steel towers linked by transmission lines are widely used to form power transmission systems. Failure of either a structural component (local failures, such as buckling of a local member or breakage of a single wire) or the entire structure (global failures, such as instability of the entire tower) could propagate to their adjacent structural members or systems (such as neighboring towers or lines). Therefore, there is a strong need to identify the root cause of the failure of the transmission tower-line system and identify the weakest link under various extreme weather scenarios. This study is to develop a probabilistic assessment approach for a typical transmission tower-line system subjected to strong wind loadings under synoptic winds (atmospheric boundary layer wind). Due to the complicated structural details and complex wind-structure interactions, wind tunnel experiments were carried out to obtain the static wind load coefficients for different panels of the transmission towers as well as for the transmission lines. Incremental dynamic analysis (IDA) is carried out to obtain the capacity curve for the transmission tower-line system. Uncertainties from the meteorological parameters are also considered in the analysis. The probabilities of failure for the two predefined limit state functions are obtained considering the meteorological uncertainties. Meanwhile, the probabilities of failure at different return periods are presented for a transmission tower-line system subjected to strong winds in a coastal region. Finally, the effects of the aerodynamic damping for the transmission lines on the dynamic responses of the tower-line system are discussed, as well.