Lattice Towers Research Articles

Trihedral lattice towers with optimal cross-sectional shape

The article discusses a new design of triangular lattice supports and suggests a technique for optimizing their cross-sectional shape. Optimization is carried out from the condition of the cross-section being equally stable in two planes. Such relationships are found between the geometric parameters of the section, at which the moments of inertia take the maximum value.

Failure Analysis of a Cruciform-Leg Transmission Line Tower

Design of latticed steel power transmission towers is a challenging task, which involves lengthy modeling and detailing of significant amount of steel members and connections. The tower is analyzed and designed as a space truss and various members of tower are checked for Tension, Compression and Slenderness Ratio. Sometimes, due to unavailability of required sections or economical point of view, single angle sections are replaced by Cruciform Sections (made of two angles connected toe to toe). The cruciform section, when connected with single angle sections, generate bending moments due to eccentric loading at connection on application of loads on tower, which subsequently results failure of the tower. In the present work, a full-scale testing of a 400 kV tower with built-up leg (Cruciform Leg connected with Single-Angle Leg) has been conducted and the behavior of joint on application of loads has been studied. While testing, one of the legs buckled at the connection. The failure pattern of the leg has been validated using FEM analysis, and the reason of failure has been discussed. In order to confirm the quality of steel, mechanical testing of steel used in leg has been carried out using UTM and results have been explained in the papers. The paper also gives some suggestions for the connection of this type of built-up sections used in tower in order to avoid such failures.

Trihedral lattice towers geometry optimization

The problem of trihedral lattice towers geometry optimization, the width of which varies linearly with height, has been considered. The variable parameters were the support width at the base and top point, as well as the cross-sectional areas of the lattice and chords. A restriction was introduced on the mass constancy. The objective function was the potential strain energy, the maximum displacement and the first frequency of natural vibrations. In the first and second cases the optimum corresponds to the minimum of the objective function, and in the third - to the maximum. The solution was performed by the finite element method in combination with four nonlinear optimization methods: the interior point method, surrogate optimization method, genetic algorithm and pattern search method. The efficiency of each of the listed optimization methods has been compared by the authors.

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.

Retrofitted built-up steel angle members for enhancing bearing capacity of latticed towers: Experiment

Many existing transmission or communication towers designed several decades ago have undergone nonreversible performance degradation, making it hardly meet the additional requirements from upgrades in wind load design codes and extra services of electricity and communication. Therefore, a new-type non-destructive reinforcement method was proposed to reduce the on-site operation of drilling and welding for improving the quality and efficiency of reinforcement. Six built-up steel angle members were tested under compression to examine the reinforcement performance. Subsequently, the cyclic loading test was conducted on a pair of steel angle tower sub-structures to investigate the reinforcement effect, and a simplified prediction method was finally established for calculating the buckling bearing capacity of those new-type retrofitted built-up steel angles. The results indicates that: no apparent difference exists in the initial stiffness for the built-up specimens compared to the unreinforced steel angles; retrofitting the steel angles by single-bolt clamps can guarantee a relatively reasonable reinforcement effect and is suggested for the reduced additional weight and higher construction efficiency; for the substructure test, the latticed substructure retrofitted by the proposed reinforcement method significantly improves the lateral stiffness, the non-deformability and energy dissipation capacity; moreover, an apparent pinching behavior exists in the hysteretic loops, and there is no obvious yield plateau in the skeleton curves; finally, the accuracy validation result indicates that the proposed theoretical model achieves a reasonable agreement with the test results. Accordingly, this study can provide valuable references for the design and application of the non-destructive upgrading project of steel angle towers.

Operational modal analysis of a self-supporting antenna mast

This paper presents the results of an operational modal analysis on a self-supporting antenna mast. The structure is instrumented with seismic accelerometers, which are moved in different setups for a detailed characterization of the mode shapes. In addition, eight strain gauges are installed, two at the bottom of each leg of the tower. Two particular challenges in the modal analysis are highlighted. First, due to the symmetry of the tower, the bending modes occur in pairs with near to identical natural frequencies and orthogonal mode shapes. These mode shapes change from one setup to another, which complicates the merging of partial mode shapes obtained from different setups. Second, the acceleration signals are polluted by radio signals. It is shown how these two issues are solved, resulting in the identification of five bending modes. The identified natural frequencies and mode shapes are very close to those predicted by a detailed finite element model of the tower, where the vertical columns are modeled using beam elements and the connections between the columns and the braces are assumed pinned. This confirms the validity of such detailed finite element models for the assessment of the dynamic response of these lattice towers under wind loading.

Mean wind loads on equilateral triangular lattice tower under skewed wind loading

Compared with lattice towers with square or rectangle cross sections, triangular lattice towers have the advantage of smaller foundation footprint. Triangular lattice towers have been widely used in transmission towers and communication towers, yet only a few studies have been conducted on their wind loads. Therefore, a series of wind tunnel tests on a triangular lattice tower were conducted using direct force measurements in this study. Effective drag coefficients and effective skewed wind load factors normalized with the “true” windward projected areas were proposed and obtained from the test results. Piecewise functions were suggested to calculate the effective skewed wind load factors and skewed wind load factors. It is found that the skewed wind load factors show an approximate “W” profile which the British standard formula does not fit. The effective skewed wind load factors show a strong regularity and are only related to yaw angles and solidity ratios. The analytical effective skewed wind load factors have a good agreement with fitting results with a maximum error of 3.1%. The recommended formula for skewed wind load factors is concise and can effectively illustrate the changing tendency of drag coefficient curves.

Lattice and Tubular Steel Wind Turbine Towers. Comparative Structural Investigation

Renewable energy is expected to experience epic growth in the coming decade, which is reflected in the record new installations since 2010. Wind energy, in particular, has proved its leading role among sustainable energy production means, by the accelerating rise in total installed capacity and by its consistently increasing trend. Taking a closer look at the history of wind power development, it is obvious that it has always been a matter of engineering taller turbines with longer blades. An increase in the tower height means an increase in the material used, thereby, impacting the initial construction cost and the total energy consumed. In the present study, a numerical investigation is carried out in order to actively compare conventional cylindrical shell towers with lattice towers in terms of material use, robustness and environmental impact. Lattice structures are proved to be equivalently competitive to conventional cylindrical solutions since they can be designed to be robust enough while being a much lighter tower in terms of material use. With detailed design, lattice wind turbine towers can constitute the new generation of wind turbine towers.

Tower structure optimization through finite element analyses

In this study the objective is to obtain an optimal configuration for an antenna lattice tower which has an imposed height of 30 m. For this purpose, a variable geometric model which considers specific working parameters is created, and a 3D finite element (FE) model is generated by using beam and shell elements for an optimization study. The 3D FE analysis is updated automatically for each variant of the geometric model. The response surface methodology (RSM) is used together with the central composite design (CCD) approach as to optimize the response of the lattice tower. The model of the tower is subject to static and dynamic loadings, including earthquake analysis. A response spectrum analysis based on Rosenblueth's Double Sum Combination (ROSE) is performed because the natural frequencies resulting from the modal analysis have close values. The optimization solution algorithm used for the tower optimization is the nonlinear programming by quadratic Lagrangean (NLPQL), which is based on the gradient algorithm for models with only one objective function and several optimization restrictions. Out of three final candidate design solutions the one which gives the minimum mass is proposed.

Experimental study on failure modes and retrofitting method of latticed transmission tower

Recent years have witnessed an increasing number of transmission towers collapsing under strong winds. Therefore, a retrofitting technique on diagonal bracings is proposed in this paper to enhance the load-carrying capacity of existing transmission towers based on a thorough exploration of the failure mechanism of transmission towers in working conditions. A pair of double-panel subassemblages of a transmission tower with one’s diagonal members retrofitted and the other not were statically tested to study the failure modes and to evaluate the retrofitting effects. The experimental results reveal that in the original tower subassemblages the diagonal members are vulnerable to buckling, which could lead to structural failure, while in the retrofitted specimen the most vulnerable member is the leg member rather than the diagonal bracings, which is accompanied by a change of the failure mode of the subassemblage. The ultimate lateral and vertical loads applied on the retrofitted subassemblage increase significantly compared with the original subassemblage.

Structural damage assessment in lattice towers based on a novel frequency domain-based correlation approach

The goal of this experimental study is to validate a novel vibration-based damage identification methodology for lattice structures. To this end, this work has determined experimentally the degradation state of a lattice tower under different damage conditions by assessing variations to its vibration response, which lead to the identification of spectral-domain indicators that correlated with the structure’s severity of damage. These indicators are derived from the complex frequency domain assurance criterion (CFDAC) matrices, which are obtained by comparing the vibration response of the structure between damaged and undamaged scenarios. The results showed that there exists a correlation between the proposed spectral correlation indices (SCI) and changes to stiffness and mass in the lattice tower. The results obtained in this work further demonstrate the viability of this technology as an innovative tool for autonomous and remote structural monitoring.

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

Wind tunnel measurement of overall and sectional drag coefficients for a super high-rise steel tube transmission tower

Wind tunnel tests on models of a 1000 ​kV super high-rise tubular steel transmission tower were carried out under two wind fields. In order that the boundary conditions for the tower section models be consistent with those of the actual structure, a new experimental method is proposed. The results show the drag coefficients in a thick turbulent boundary layer are generally less than those in uniform flow, as expected, the drag coefficient of a lattice tower section is related not only to the solidity ratio but also to its geometric shape and oncoming flow turbulence intensity. The effective projected areas (EPA) calculated by wind tunnel tests were compared to the regulations in some applicable codes. The results for the wind loads on the superstructure show that the calculation method should take into account the geometric shape, both in the transverse and longitudinal direction, and be distinguished from the methods for cross-arm and tower body. The test values for the cross-arm are in best agreement with the data from the Chinese code, while the results for the whole tower and tower sections match better with the values from the USA code. The European code generally underestimates the wind loads for all the test models.

Comparative analysis of offshore support structures for two-bladed large wind turbines (10+MW) in deep waters

The offshore wind market is developing towards exploiting wind resources in deeper water sites. Inevitably, this fuels new research and feasibility studies on alternative solutions for the wind turbine support structures. Within this context, this work aims at comparing three different support structure design concepts for a 14 MW two-bladed downwind wind turbine: an XXL monopile, a hybrid jacket-tower and a lattice tower. To ensure a fair comparability of the three design concepts, a load capacity analysis is first performed to assess the yield strength of each concept and guarantee a similar material utilization. The comparative analysis is then carried out in terms of total mass, dynamic behaviour of the interaction between rotor and support structure, soil-structure interaction and resulting hydrodynamic forces. Based on the given design constraints, the preliminary results of this study favour a lattice tower solution. The study also highlights peculiar dynamic phenomena such as veering, mode hybridization and mode coalescence for the dynamic interaction between the lattice tower and the two-bladed rotor which need to be taken into account during the design phase.

Tower shadow excitation of a downwind rotor blade of a turbine with a tubular tower

The downwind configuration of wind turbines offers benefits regarding the blade-tower clearance, as during operation the blade primarily bends away from the tower. Consequently, the blades can be designed with lower stiffness. For tubular towers, however, a significant deficit of the wind speed in the tower wake occurs, resulting in fatigue-inducing vibrations. For this reason, full-height lattice towers are considered the preferred support structures for wind turbines with a downwind rotor. This work estimates the tower shadow excitation of a downwind rotor blade from a tubular tower. To this end, the blade of a commercial 6 MW downwind turbine is modelled with finite-elements. The tower wake is described on the basis of Madsen’s model and for the unsteady aerodynamic interaction Küssner’s function is adopted. At below- and above-rated wind conditions, the tower wake-induced vibrations are compared with the response of a blade of an equivalent upwind rotor, considering both the tip deflections and the root moments, the latter on the basis of damage-equivalent moments, to obtain an indication of the expected difference in fatigue damage. The downwind blade experiences vibrations with considerable larger amplitudes, especially in the out-of-plane direction. From the damage-equivalent moments it can be inferred that the blades of the downwind rotor encounter a much faster accumulation of fatigue damage.

Topology optimisation of lattice telecommunication tower and performance-based design considering wind and ice loads

With increasing demand of infrastructure to support power transmission and telecommunication systems, the need of erecting more towers has also been rising significantly. For many years, these towers were designed by using a conservative approach and the opportunities lying in the design optimisation of the towers were not leveraged. This paper presents the application of structural topology optimisation to lattice self-supported telecommunication towers in developing an improved solution in terms of weight-to-stiffness ratio. 2D and 3D topology optimisation studies were performed with highly optimised bracing systems reducing the amount of steel material used, thus its carbon footprint. The new exoskeleton structure is representing a lattice tower composed of ‘high-waisted’ bracing type and elliptical hollow sections (EHS). Comparative modal analyses demonstrated the structural performance of the optimised tower models. In addition, a research-led design was carried out for optimising the geometric cross-sectional properties of the optimised lattice tower subjected to quasi-static analysis followed by regression analysis. The cross-sectional parameters were progressively changed; explicitly the diameter and thickness of the members. The performance-based analysis and design of a topologically optimised lattice tower present alternatives to onerous approaches such as wind tunnel testing or finite element modelling. The results were further analysed to understand their viability in different loading design cases and the effect of cross-sections. Conclusions highlighted the benefits gained by introducing the structural topology optimisation process in the design of slender support structures.

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

Safe Practice Guidelines and Risk Assessment for Lattice Tower Assembly

Safe Practice Guidelines and Risk Assessment for Lattice Tower Assembly

Shaking table test on 1000 kV UHV transmission tower-line coupling system

Ultra high voltage (UHV) transmission tower-line coupling system consists of transmission lines (conductors and ground wires) supported by a series of lattice towers. However, in the current seismic design code, the transmission lines are just simplified as additional mass while the effect of nonlinear vibration of transmission lines on the seismic responses of transmission towers is not taken into consideration. To analyze the influence of the ignorance of the coupling effect between towers and transmission lines, two types of model structures, i.e. tower-line coupling system model and single tower model with lumped masses, should be tested for comparison. But normal reduced-scale model design method cannot complete the shaking table test by a uniform geometric similarity ratio for the coupling system in the limit of table size, hence this paper proposed to rescale the geometric similarity ratio for the model design of transmission lines and meanwhile satisfied the requirement in dynamic properties similitude and inertia force similitude separately, which successfully solved the problem that the span of transmission lines in the test was too large to fit within the shaking table. This method may offer reference to experimental studies for other large-span structures and the experimental results show that the nonlinear vibration of transmission lines plays a role as seismic reduction by dissipating some seismic energy to towers under earthquake.

Earthquake failure mode and collapse fragility of a 1000 kV outgoing line frame considering interactions in the tower line system

This study investigated the seismic responses, earthquake failure modes, and collapse fragility of a 1000 kV outgoing line frame (OLF1000, a supporting-equipment structure used in substations), by considering the interactions in the tower line system (ITLS). First, three analytical models of the OLF1000 were established, in which transmission lines were represented as massless springs based on their dynamic stiffness, and the lattice transmission tower was simplified to an elastic beam tower. Additionally, 40 real seismic records with similar spectral characteristics were used to analyze the seismic responses of the OLF1000 by considering the ITLS under minor, moderate, major and extremely major earthquakes. Moreover, a time history analysis was carried out to investigate the failure modes, and the collapse risk of the OLF1000 was considered from a probabilistic perspective. According to our results, the OLF1000 remains mostly elastic under minor and moderate earthquakes, as the energy consumption and elastic constraints of the transmission lines reduce the seismic response of the OLF1000. However, amplification of excitations by the transmission tower increases the previously reduced responses. Additionally, in the case of major and extremely major earthquakes, when considered in isolation, the OLF1000 remains intact or is only slightly damaged, but it becomes seriously damaged and can even collapse when the ITLS is considered. Furthermore, the failure mode of the OLF1000 is that of an in-plane global collapse, whereas the inclusion of the ITLS in the model suggests that an out-of-plane continuous collapse will be caused by a local failure. The ITLS significantly reduces the collapse load of the OLF1000. Finally, the ITLS substantially increases the probability of collapse of the OLF1000 under strong earthquakes.