Dynamic reduction-based structural damage detection of transmission towers: Practical issues and experimental verification

Structural damage detection in large-scale three-dimensional spatial structures is a challenging problem. It is impractical to develop a general damage-detection method that is applicable to all types of structural systems and all kinds of damage. A practical and efficient structural damage detection method must consider the characteristics of the target structure and damage in the development stage. In 2009, Yin et al. [33] proposed a damage detection method for the health monitoring of transmission towers. The method was developed based on the dynamical finite element (FE) model reduction technique, which utilizes identified modal parameters, such as natural frequencies and mode shapes, with only a limited number of sensors. In Ref. [33], the proposed method was numerically verified by simulated noisy data from a three-dimensional transmission tower model for single and multiple damage cases. This paper discusses some practical issues related to the proposed method, such as sensor placement and computational efficiency. Rather than proposing a general sensor placement method, a set of preliminary sensor locations is determined based on engineering judgement. This set of sensor locations is then checked against the results of a sensitivity analysis to ensure that the measured data contain information for identifying all of the target damage scenarios. To reduce the required computational power, two simplified versions of the proposed method are presented. The proposed method is then verified with a scaled-down model of a transmission tower (2.4 m high) that was built at the Structural Vibration Laboratory (SVL) of the City University of Hong Kong. This paper reports the detailed experimental setup and the method of extracting the modal parameters from a series of free vibration tests with only a limited number of sensors. The verification results show that the proposed damage detection method identifies the damaged sub-structures in all of the experimental cases considered. It must be pointed out that the transmission tower structure, in its operating conditions, suffers from the effect of the forces transmitted from the cables it carries. The influence of this force on the damage identification result is great and can not be neglected in practice. In the present experimental case study, only a transmission tower-like structure without carrying the cables is investigated in laboratory conditions.     

基于时间序列数据的输电钢塔损伤检测和试验研究

对输电塔进行损伤定位一直是一个研究难点,通过将输电塔简化为分层的10个子结构,利用加速度时间序列对输电塔结构进行损伤检测,提出了用基于预白化改进的AR前3阶系数指标对损伤进行定位.首先使用加速度数据建立ARMA模型,并提取其AR部分的前3阶系数建立基本指标,然后通过预白化来对该指标进行改进,去除各层数据之间的相关性,使...     

Theoretical and experimental modal analysis of the Guangzhou New TV Tower

The Guangzhou New TV Tower (GNTVT) in Guangzhou, China, is a supertall tube-in-tube structure with a total height of 600 m. A complicated structural health monitoring (SHM) system consisting of over 800 sensors has been implemented to the GNTVT for both in-construction and in-service real-time monitoring. By making use of this SHM system, the ambient vibration measurement is carried out in a continuous and long-term manner. This paper presents the analytical and experimental modal analysis of the tower and the field ambient vibration measurement at different construction stages and under different excitation conditions, particularly addressing the following issues: (1) a reduced-order FE model for the GNTVT; (2) field vibration measurement and modal parameter identification of the tower under construction and two environmental excitations (typhoon and earthquake); and (3) comparison of results under different excitation events in the time-frequency domain and correlation between natural frequencies and air temperature using linear regression analysis. The experimental dynamic characteristics of the tower can be used to update the finite element of the tower, so that the updated finite element model of the tower can be obtained, which will serve as the baseline model for future health monitoring and damage detection. They can also be used to verify the effectiveness of vibration control devices installed on the tower.     

Structural assessment of current steel design models for transmission and telecommunication towers

The usual methods of structural analysis involved in the design of steel telecommunication and transmission towers tend to assume a simple truss behaviour where all the steel element connections are considered hinged. Despite this fact, the most commonly used tower geometries possess structural mechanisms that could compromise the assumed structural behaviour. A possible explanation for the structure stability is related to the actual behaviour being close to semi-rigid connections instead of the assumed hinged connections. This paper proposes an alternative structural analysis modelling strategy for the steel tower design considering all the actual structural forces and moments combining three-dimensional beam and truss finite elements. Comparisons of the two above-mentioned design methods with a third method based on the use of spatial beam finite elements to model the main structure and the bracing system on two actually built steel telecommunication towers (40 and 75 m high steel towers) are described. The comparison is based on an extensive parametric study of the tower geometry in order to access the structural tower ultimate and serviceability limit states.     

特高压大跨越输电塔动力特性和风振响应分析

针对特高压大跨越输电塔跨越档距大、塔体高且负荷重的特点,从材料选取、导线排列方式、结构动力特性以及风振响应等几个方面对1 000 kV特高压双回路跨越输电塔进行分析,总结了所研究塔型的动力特点,对其风振系数进行了计算和讨论,并根据动力特性分析提出了结构设计中风振系数的取值方法。结果表明:风振系数具有较大的离散性,不同塔身高度应取不同的值进行计算;该方法为特高压大跨越输电塔的结构设计提供了参考。     

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.     

大跨越输电塔结构风振系数研究

在输电塔结构设计中,风振系数的取值非常重要。本文结合500kV崖门大跨越输电塔工程实例,通过输电塔结构时程分析,得到结构在风荷载作用下风振响应,统计了输电塔结构的风振系数取值,与多种现行规范进行比较,对大跨越输电塔结构抗风设计给出了建议。     

Aerodynamic damping properties of two transmission towers estimated by combining several identification methods

Based on full-scale measurement data, we report on several aerodynamic damping properties of two transmission towers under conditions of strong winds. In estimating the damping ratio from data that contain many components of close frequencies of tower and conductor vibrations, we cannot estimate the damping ratio of each component if we use an estimation method available for only a single-degree-of-freedom system. Therefore, we combined several identification methods and are introducing a new method of estimating damping properties. This method is applicable to the response record of a multi-degree-of-freedom system such as the coupled structure of a transmission tower-and-conductors. Using the new estimation procedure, we were able to extract the component of every vibration mode of the towers from a measured time history and estimate the accurate damping ratios individually; and, our results revealed that the wind speed dependency of the aerodynamic damping property of the coupled tower-and-conductors system had wide differing characteristics depending on the vibration mode. Moreover, using a three-dimensional nonlinear analysis, we discuss, in detail, the effects of the progressive deformation of a transmission-conductor-plane due to an increase in wind speed on the development of the aerodynamic damping of the coupled tower-and-conductors system.     

输电塔腐蚀安全评定的有限元分析

钢结构输电塔的腐蚀,导致钢结构输电塔承载性能发生变化,对输电线路的运行造成安全隐患。根据广西电网输电塔现场检测结果,运用有限元分析方法,对在役锈蚀钢结构输电塔结构性能进行分析,结合在役输电线路杆塔可靠性鉴定方法,给出在役锈蚀钢结构输电塔安全评定方法。     

Estimation of damping ratio of TV towers based on ambient vibration monitoring

Damping ratio is an important component in the dynamic analysis and plays a key role in design of supertall buildings. It is necessary to have reliable field measurement records for estimation of structural dynamic characteristics. The main objective of this paper is to estimate eigen mode damping ratio of TV towers as a kind of supertall buildings with the height of more than 250 m. In this regard, field measurements were performed on Milad TV tower, located in Tehran, which has a height of 435 m by ambient vibration monitoring. Damping ratio of this tower is determined using random decrement technique and compared with available data of a few other TV towers. For proposing eigen mode damping ratio of TV towers, it is shown that Rayleigh damping coefficients could not be calibrated properly for tall slender TV towers. A damping estimation chart of TV tower that is a function of natural frequency and structural aspect ratio is proposed, and the chart validity has been checked with field measurement results of other TV towers. The chart shows that structural damping for wind analysis proposed in design codes and standards is overestimated for tall slender TV towers. Copyright © 2011 John Wiley & Sons, Ltd.     

The Stretching Method for Vibration‐Based Structural Health Monitoring of Civil Structures

This article newly proposes the application of the stretching method, that is used in geophysics for detecting variations in the velocity with which waves propagate in the earth's crust from seismic noise recordings, in the context of vibration‐based Structural Health Monitoring (SHM) of civil structures. The result is a computationally efficient long‐term vibration‐based SHM tool, that follows the current trend of using a very limited number of sensors permanently installed on site to measure operational structural responses for the purpose of damage detection. In the SHM setting, the proposed method aims at a direct identification of small permanent shifts in the natural frequencies of the structure in a changing environment, which is achieved by maximizing the correlation coefficient between a reference waveform, computed in a training reference period in which the structure is assumed to be undamaged, and a stretched version of the same waveform evaluated at the current time. The comparison is performed in the frequency domain and the waveform of interest is obtained from cross‐correlations of the ambient vibration measurements. More specifically, in the case of multiple sensors, the waveform can be either the cross‐power spectral density of the signals recorded by a pair of sensors, or the largest singular value of the spectral matrix of the measurements. It follows that the method can be regarded as an extension of the classic Frequency Domain Decomposition (FDD). A key feature of the proposed stretching method is mitigating the effects of environmental fluctuations by time domain averaging of cross‐correlations over a proper period of time, before taking their Fourier transform to estimate the spectral densities. Such a time domain averaging is carried out in a relatively long period of time for estimating the reference waveform, whereas it is carried out in a shorter time for estimating the current waveform. The main features of the proposed methodology are its very low sensitivity to environmental fluctuations, resulting in a quite short training period length, and its low computational cost, which could be compatible with a direct integration within smart sensors with embedded electronics. The performance of the method is illustrated in the case study of an Italian historical monumental bell tower that has been monitored by the authors for more than 1 year.     

考虑塔-线耦合作用的输电塔体系风振系数研究

考虑塔-线耦合作用的体系风振响应以及对应情况下的输电塔等效风荷载的风振系数取值是输电塔抗风设计的基础,结合某220kV输电线路一塔两线实例,通过气弹性风洞试验和有限元数值模拟的方式,研究输电塔在考虑导、地线耦合作用下的风振响应规律,计算其对应的输电塔等效风荷载的风振系数,并与我国现行规范中的相关取值进行对比,结果表明:横担和输电线的存在使得塔身中上部的风振系数明显增大,在进行输电塔设计时需考虑其影响;建议采用《建筑结构荷载规范》(GB 50009—2012)和《高耸结构设计规范》(GB50135—2006)来计算风振系数时,对横担位置进行修正或单独考虑,而《架空输电线路荷载规范》(DLT5551—2018)更适用于输电塔类结构的风振系数的计算。     

Numerical analysis and full‐scale experiment on K‐joint deformations in the crank arms of lattice transmission towers

After completion of the construction process including wire stringing of 1,000 kV ultrahigh voltage (UHV) cat‐head electric transmission towers, the outward horizontal K‐joint deformation of one side crank arm may usually exceed 70 mm. Joint slip effects were ignored in the initial structural design of transmission towers, which induced that the calculated K‐joint displacements are much lower and the real deformation cannot meet with the requirement of the acceptant code. First, the shortcomings of the traditional unit load method used to calculate the displacements of transmission towers connected by bolt joints were analyzed. Second, a coefficient for describing the axial force state of bolted joints was proposed. The connection numbers for different types of members in the crank arms of a UHV cat‐head transmission tower were determined. The traditional unit load method was enhanced by importing the force state coefficient and specifying the detailed connection numbers. The K‐joint displacements calculated by the enhanced formula were compared with the experimental results by full‐scale tower experiment. It shows that the enhanced unit load method is suitable for calculating K‐joint deformations in the crank arms of UHV cat‐head transmission towers. Lastly, contribution weight coefficients of different members for K‐joint displacements of the UHV cat‐head transmission tower were calculated by the enhanced unit load method. For bolt joints connected with the tower members with high contribution weight coefficients, the manufacturing accuracy of bolt holes should be improved, and the bolt clearance should be decreased, which can significantly reduce the K‐joint deformation of UHV cat‐head transmission towers.     

Recording duration of ambient vibration monitoring for system identification of tall telecommunication towers

It is necessary to have reliable field measurement records for estimation of dynamic characteristics. Increasing duration of the record is one of the common methods for reduction of such errors in the results. Dynamic characteristics of telecommunication towers in higher modes are important for designers, but these parameters could be obtained from ambient vibration monitoring during typhoons in which duration of vibration is not long enough for reducing measurement errors. This paper suggests a procedure for obtaining the duration of recording time for more reliable determination of structural characteristics of telecommunication towers. It is suggested to record the vibration of towers in the form of a set of long‐term and short‐term duration recordings for lower and higher modes, respectively. This procedure was verified with field measurement of Milad telecommunication tower. The results show that natural frequencies determined by this procedure are in good agreement with finite element model analysis. Copyright © 2013 John Wiley & Sons, Ltd.     

江阴高压输电线路脉动实测及分析

采用环境随机激振方法对江阴500kV输电线路的一座拉线塔和自立塔分别进行了实测。得到了拉线塔横向、纵向及扭转向的一阶频率、振形和阻尼比。并对自立塔和拉线塔的实测结果进行了比较。     

大跨越输电塔有限元刚度模拟实例研究

大跨越输电塔是超高压输电网络的重要组成部分,具有塔体高、档距大及柔度大等特点,对地震、风以及导线覆冰等环境荷载反应灵敏。建立能够反映大跨越输电塔动态特性的有限元模型,对其进行结构健康监测与安全评估,并准确模拟其刚度是至关重要的。本文以500kV淮蚌线淮河大跨越输电塔为例,探讨了大跨度输电塔有限元建模中的刚度模拟。采用不同方案建立该塔的有限元模型,包括梁单元模型和梁杆混合模型,对两种模型进行了模态分析,获得了计算模态的频率及振型。而后,以实测模态数据为依据,对各方案进行了比较分析,确定梁杆混合的方案为优选方案。     

Reliability-Based Optimal Design of Electrical Transmission Towers Using Multi-Objective Genetic Algorithms

Abstract:   A hybrid methodology for performing reliability-based structural optimization of three-dimensional trusses is presented. This hybrid methodology links the search and optimization capabilities of multi-objective genetic algorithms (MOGA) with structural performance information provided by finite element reliability analysis. To highlight the strengths of the proposed methodology, a practical example is presented that concerns optimizing the topology, geometry, and member sizes of electrical transmission towers. The weight and reliability index of a tower are defined as the two objectives used by MOGA to perform Pareto ranking of tower designs. The truss deformation and the member stresses are compared to threshold values to assess the reliability of each tower under wind loading. Importance sampling is used for the reliability analysis. Both the wind pressure and the wind direction are considered as random variables in the analysis. The research results presented demonstrate the benefit of implementing MOGA optimization as an integral part of a reliability-based optimization procedure for three-dimensional trusses.     

Stationarity‐Based Approach for the Selection of Lag Length in Cointegration Analysis Used for Structural Damage Detection

Cointegration has been recently brought to structural health monitoring (SHM) as a new methodology for dealing with the problem of environmental and/or operational variability in monitored structures. However, it is well known that the choice of lag length in cointegration analysis has a strong influence on damage detection results. The article presents a new approach for optimal lag length selection in cointegration analysis used for structural damage detection. This new method is based on stationarity analysis of data representing undamaged condition. The proposed method is validated using Lamb wave data under the effects of temperature variations and vibroacoustic data obtained from nonlinear vibroacoustic modulation experiments with different low‐frequency vibration (or modal) excitations. The results demonstrate the effectiveness of the method for structural damage detection based on SHM data heavily affected by environmental or operational conditions.     

转角输电塔脉动实测与分析

高压输电塔-线体系是一种复杂的空间塔线耦联振动体系,对其精确计算十分困难,位于转角处的输电塔受力性能更为复杂。通过对一呼高为24m的干字形转角塔进行现场环境脉动测试,分析出该转角塔前几阶频率、振型、阻尼比等动力特性参数,得出转角塔的主要振动形式为扭转振动及纵横向的耦合振动。由于转角塔横担上下结构刚度差别较大,地线支架部分鞭梢效应明显。     

悬索桥主塔结构的有限元模拟方法研究：1．初始模型

在有限元模型误差来源分析的基础之上,以润扬悬索桥主塔结构环境振动测试结果为基准,提出了分层次分阶段的修正主塔结构有限元模型的方法。首先依据设计图纸建立最初的有限元模型,经过模型阶次误差分析和结构误差分析之后,确定了主塔各构件单元划分的数目和梁柱节点刚性区域的模拟方法。在此基础之上,文献[1]将进一步对本文所建立的初始有限元模型进行参数误差的修正,从而最终建立润扬悬索桥主塔结构的基准有限元模型。