Operational Modal Parameter Identification from Power Spectrum Density Transmissibility

Abstract: Operational modal analysis subjected to ambient or natural excitation under operational conditions has recently drawn great attention. In this article, the power spectrum density transmissibility (PSDT) is proposed to extract the operational modal parameters of a structure. It is proven that the PSDT is independent of the applied excitations and transferring outputs at the system poles. As a result, the modal frequencies and mode shapes can be extracted by combing the PSDTs with different transferring outputs instead of different load conditions where the outputs from only one load condition are needed. A five‐story shear building subjected to a set of uncorrelated forces at different floors is adopted to verify the property of PSDTs and illustrate the accuracy of the proposed method. Furthermore, a concrete‐filled steel tubular half‐through arch bridge tested in the field under operational conditions is used as a real case study. The identification results obtained from currently developed method have been compared with those extracted from peak‐picking method, stochastic subspace identification, and finite element analysis. It is demonstrated that the operational modal parameters identified by the current technique agree well with other independent methods. The real application to the field operational vibration measurements of a full‐sized bridge has shown that the proposed PSDTs are capable of identifying the operational modal parameters (natural frequencies and mode shapes) of a structure.     

Response surface‐based finite‐element model updating of a historic masonry minaret for operational modal analysis

This study aimed to use the response surface (RS) method for finite element (FE) model updating, using operational modal analysis (OMA). The RS method was utilized to achieve better agreement between the numerical and field‐measured structure response. The OMA technique for the field study was utilized to obtain modal parameters of the selected historic masonry minaret. The natural frequencies and mode shapes were experimentally determined by the enhanced frequency domain decomposition (EFDD) method. The optimum results between the experimental and numerical analyses were found by using the optimization method. The central composite design was used to construct the design of experiments, and the genetic aggregation approach was performed to generate the RS models. After obtaining the RS models, an attempt was made to converge the natural frequency values corresponding to the five‐mode shapes with the frequency values identified by the experimental analysis. ANSYS software was used to perform 3D finite element (FE) modeling of the historic masonry minaret and to numerically identify the natural frequencies and mode shapes of the minaret. The results of the experimental, initial, and updated FE model were compared with each other. Significant differences can be seen when comparing the experimental and analytical results with the initial conditions.     

基于模态应变能的结构损伤识别研究

文中应用有限元分析软件ANSYS10.0建立一个平面刚架结构模型。通过有限元模态分析获得未损伤结构与损伤结构的固有频率和振型。应用单元模态应变能法对模型进行损伤定位。结果证明这种方法对于平面刚架的损伤识别是有效的。     

某多层砌体教学楼环境振动试验及其模态分析

本文进行某多层砌体结构教学楼环境振动试验,利用基于传递率参数识别方法进行教学楼动力参数识别,并与该楼MI-DAS三维有限元模型理论计算结果进行比较,得出一些有价值的结论。     

下承式钢桁架桥动力特性有限元分析

采用有限元分析软件MIDAS/CIVIL 2010对南昌某下承式钢桁架桥进行离散,建立空间有限元分析模型,并对其进行动力特性分析,得到振型及自振频率等数值,寻求桥梁整体结构的变形规律,并了解结构的实际受力情况和变化规律,是今后的优化设计,谐响应分析,瞬态分析及谱分析等动力学分析的重要起点。     

某大跨度钢管拱桁架屋盖结构抗震性能分析

以某餐饮公司餐饮中心的钢屋盖拱桁架结构为研究对象,首先用设计程序3D3S 8.0对结构进行初步的选型与设计,然后用有限元分析软件ANSYS研究其动力特性和抗震性能,得到了该结构的自振频率、振型特点,地震作用下的位移分布、内力等.结果表明:设计出的钢管拱桁架屋盖结构,不论是结构形式、截面类型还是截面尺寸,在地震作用下均具...     

Vibration-based operational modal analysis of the Mikron historic arch bridge after restoration

This research presents finite element modelling, vibration-based operational modal analysis, and finite element model updating of a restored historic arch bridge. Mikron historic bridge, constructed on F?rt?na River in Rize, Turkey, is the subject of this case study. The General Directorate for Highways of Turkey repaired the bridge's main structural elements, arches, sidewalls, and filler material in 1998. To construct a 3D finite element model of the bridge, ANSYS finite element software estimated the analytical dynamic characteristics. Induced ambient vibrations such as human walking and wind excited the model bridge to allow measurement of the bridge's responses. Enhanced frequency domain decomposition in frequency domain and stochastic subspace identification in time domain methodologies extracted experimental dynamic characteristics. A comparison of the analytical and experimental results showed significant agreement between mode shapes, but some differences in natural frequencies appeared. Consequently, updating the finite element model of the bridge by changing boundary conditions minimised the differences between analytical and experimental natural frequencies. After the finite element model updating process, the differences between natural frequencies declined from 7% to 2%.     

Dynamic investigation of a concrete footbridge using finite element modelling and modal analysis

In this article, the application of a manual updating method for finite element (FE) model updating of a concrete footbridge using modal analysis approach is described in detail. An FE model was developed using DIANA (FEM software package) to estimate the response of structure under free-vibration analysis. Afterwards, ambient vibration test (AVT) was conducted to extract the dynamic properties. The fundamental mode shapes of the structure were successfully identified applying ARTeMIS (modal analysis computer program). The mode shape pairs of initial FE model and a complete set of test results were employed for manual updating. A parametric study was carried out to specify the most sensitive parameters of the model. For this purpose, boundary conditions, mass density and Young's modulus of elasticity were examined as uncertain parameters. Attempts to calibrate the primary FE model revealed that the spring constants of supports were the most effective parameters for updating process. The FE model was calibrated considering three main criteria consisting of combination of natural frequencies/mode shapes and modal assurance criteria (MAC)/mode shapes. The calibration strategy performed in the present study, including parametric study on uncertain parameters of initial FE model, parameter and target response selection and MAC calculation based on modified formulation, has been discussed. The updated FE model and the measured mode shape counterparts exhibited very good correlation.     

基于灵敏度分析和优化方法的异形钢结构烟囱模型修正

为了给异形钢结构烟囱的抗风、抗震计算及健康监测提供可靠的有限元计算模型,采用钢结构烟囱风洞试验模型的模态参数作为基准,将灵敏度分析和优化方法与大型有限元分析软件相结合修正烟囱结构模型的有限元模型。首先对烟囱结构参数化建模,然后通过灵敏度分析选择合适的优化变量,最后应用一阶优化方法修正烟囱结构的有限元模型。结构构件间连接的过分简化常会带来较大误差,因此烟囱结构的部分连接节点采用半刚性计算模型。结果表明,经过修正后的有限元模型动力计算结果与试验结果非常接近。     

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.     

Earthquake analysis of reinorced concrete minarets using ambient vibration test results

This paper describes a Turkish style reinforced concrete minaret, its finite element model, modal testing, finite element model updating and earthquake behaviour, before and after model updating. The minaret of a mosque located in Trabzon, Turkey is selected as an application. A three‐dimensional (3D) model of the minaret and its modal analysis is performed to obtain analytical frequencies and mode shapes using ANSYS finite element program. The ambient vibration tests are conducted on the minaret under natural excitations such as wind effects and human movement. The output‐only modal parameter identification is carried out by Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification methods in Operational Modal Analysis software and in doing so, dynamic characteristics (natural frequencies, mode shapes and damping ratios) are determined. A 3D finite element model of the minaret is updated to minimize the differences between analytical and experimental modal properties by changing some uncertain modelling parameters such as material properties and boundary conditions. The earthquake behaviour of the minaret is investigated using 1992 Erzincan earthquake before and after finite element model updating. Maximum differences in the natural frequencies are reduced from 21% to 8%, and good agreement is found between analytical and experimental natural frequencies. In addition to this, it is realized that finite element model updating is effective on the earthquake behaviour of the minaret. Copyright © 2008 John Wiley & Sons, Ltd.     

叉车护顶架振动模态分析

采用有限元方法对某中小吨位叉车悬置结构的护顶架整体进行模态分析,提取出护顶架的前15阶同有频率和振型.经与发动机工作运转时的激励频率对比得出如下结论:该型叉车不论是在怠速还是在正常行驶状态,护顶架整体不会产生共振现象;而发动机盖板在高速行驶状态易发生共振,需进行调整.此外,仪表盘也应加强以提高局部刚度.     

Quantification and localisation of damage in beam-like structures by using artificial neural networks with experimental validation

This paper presents a damage detection algorithm using a combination of global (changes in natural frequencies) and local (curvature mode shapes) vibration-based analysis data as input in artificial neural networks (ANNs) for location and severity prediction of damage in beam-like structures. A finite element analysis tool has been used to obtain the dynamic characteristics of intact and damaged cantilever steel beams for the first three natural modes. Different damage scenarios have been introduced by reducing the local thickness of the selected elements at different locations along finite element model (FEM) of the beam structure. The necessary features for damage detection have been selected by performing sensitivity analyses and different input–output sets have been introduced to various ANNs. In order to check the robustness of the input used in the analysis and to simulate the experimental uncertainties, artificial random noise has been generated numerically and added to noise-free data during the training of the ANNs. In the experimental analysis, two steel beams with eight distributed surface-bonded electrical strain gauges and an accelerometer mounted at the tip have been used to obtain modal parameters such as resonant frequencies and strain mode shapes. Finally, trained feed-forward backpropagation ANNs have been tested using the data obtained from the experimental damage case for quantification and localisation of the damage.     

Machine learning based novelty detection using modal analysis

Among many structural assessment methods, the change of modal characteristics is considered a well‐accepted damage detection method. However, the presence of environmental or operational variations may pollute the baseline and prevent a dependable assessment of the change. In recent years, the use of machine learning algorithms gained interest within structural health community, especially due to their ability and success in the elimination of ambient uncertainty. This paper proposes an end‐to‐end architecture to detect damage reliably by employing machine learning algorithms. The proposed approach streamlines (a) collection of structural response data, (b) modal analysis using system identification, (c) learning model, and (d) novelty detection. The proposed system aims to extract latent features of accessible modal parameters such as natural frequencies and mode shapes measured at undamaged target structure under temperature uncertainty and to reconstruct a new representation of these features that is similar to the original using well‐established machine learning methods for damage detection. The deviation between measured and reconstructed parameters, also known as novelty index, is the essential information for detecting critical changes in the system. The approach is evaluated by analyzing the structural response data obtained from finite element models and experimental structures. For the machine learning component of the approach, both principal component analysis (PCA) and autoencoder (AE) are examined. While mode shapes are known to be a well‐researched damage indicator in the literature, to our best knowledge, this research is the first time that unsupervised machine learning is applied using PCA and AE to utilize mode shapes in addition to natural frequencies for effective damage detection. The detection performance of this pipeline is compared to a similar approach where its learning model does not utilize mode shapes. The results demonstrate that the effectiveness of the damage detection under temperature variability improves significantly when mode shapes are used in the training of learning algorithm. Especially for small damages, the proposed algorithm performs better in discriminating system changes.     

汽轮发电机组框架式基础结构振动特性试验研究及分析

本文对某火力发电厂300MW汽轮发电机组框架式基础进行缩尺比为1/8的模型振动试验,通过模型振动试验,根据相似关系换算得到原型结构的自振频率、振型和振动线位移。在模型试验测试中,分别进行了无机组设备质量工况、有机组设备质量工况和扰力点动刚度测试。对测试结果进行分析得知,机组设备质量的施加使得结构的各阶自振频率降低。使用TGFP、SAP2000和ANSYS软件建立模拟轴系影响的计算模型,进行结构的模态和稳态分析,得到了基础结构的自振频率、振型和振动线位移。计算的扰力点幅频曲线与试验结果有较好的吻合。     

藏式宫殿建筑门厅结构动力特性识别与有限元模型修正

门厅结构是藏式宫殿建筑地垄结构中唯一的木构架承重结构。由于木构架的残损,可导致其刚度和承载能力均有所降低。为研究某藏式宫殿门厅结构的动力特性及安全性能,通过环境激励下的结构动力特性现场测试识别了门厅结构的东西向和南北向振动的前3阶模态参数。按照实际尺寸建立门厅结构的精细化有限元模型,通过模型设计参数灵敏度分析,确定边界弹簧刚度参数和顺纹承压木构件材性参数作为模型修正参数,基于实测结果采用参数型模型修正方法,使修正后模型的各阶模态频率计算值与实测值的误差小于5%,振型残差小于0.3,振型模态置信准则(modal assurance criterion,MAC)值大于0.9,表明修正后的模型可以较好地反映门厅结构的真实动力特性,可用于后续藏式宫殿门厅结构的状态评估。     

Experimental and analytical system identification of Eynel arch type steel highway bridge

This paper describes a full scale arch type steel highway bridges, its finite element modelling and Operational Modal Analysis. Eynel Highway Bridge which has arch type structural system with a total length of 336 m and located in the Ayvac?k county of Samsun, Turkey is selected as a case study. The bridge connects the villages which are separated with Suat U?urlu Dam Lake. The three dimensional finite element model is constructed using project drawings and an analytical modal analysis is then performed to generate natural frequencies and mode shapes in the three-orthogonal directions. The ambient vibration tests on the bridge deck under natural excitation such as traffic, human walking and wind loads are conducted using Operational Modal Analysis. Sensitive seismic accelerometers are used to collect signals obtained from the experimental tests. To obtain experimental dynamic characteristics, two output-only system identification methods are employed namely, Enhanced Frequency Domain Decomposition method in the frequency domain and Stochastic Subspace Identification method in time domain. The correlation between the finite element model and experimental results is studied. Good agreement is found between dynamic characteristics in the all measurement test setups performed on the bridge deck. It is demonstrated that the ambient vibration measurements using Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification methods are enough to identify the most significant modes of steel highway bridges. It is seen that there are some differences between analytical and experimental natural frequencies and experimental natural frequencies are generally bigger than the others.     

网架模型结构模态分析的试验研究

实验模态分析是综合运用线性振动理论、动态测试技术、数字信号处理和参数识别等手段,进行系统识别的过程。模态分析是有效的结构检测和安全评估的方法之一,它是根据测量模态参数(固有频率、阻尼比、振型、模态刚度、模态质量)相对于正常值产生的变化,并通过相关分析与识别来判断结构安全程度的一种先进方法。本文通过对网架模型结构进行试验模态分析,研究了试验模态技术在实际运用中的试验方法,识别系统的模态参数并与有限元分析进行比较,有限元结果与实测频率有一定的不同,但前四阶模态频率的差值百分比均在9%以内,说明实测的结果与有限元分析是比较吻合的。     

Modal Identification for High‐Rise Building Structures Using Orthogonality of Filtered Response Vectors

The modal parameters of civil structures (natural frequency, mode shape, and mode damping ratio) are used for structural health monitoring (SHM), damage detection, and updating the finite element model. Long‐term measurement has been necessary to conduct operational modal analysis (OMA) under various loading conditions, requiring hundreds of thousands of discrete data points for estimating the modal parameters. This article proposes an efficient output‐only OMA technique in the form of filtered response vector (frv)‐based modal identification, which does not need complex signal processing and matrix operations such as singular value decomposition (SVD) and lower upper (LU) factorization, thus overcoming the main drawback of the existing OMA technique. The developed OMA technique also simplifies parameters such as window or averaging, which should be designed for signal processing by the OMA operator, under well‐separated frequencies and loading conditions excited by white noise. Using a simulation model and a 4‐story steel frame specimen, the accuracy and applicability were verified by comparing the dynamic properties obtained by the proposed technique and traditional frequency‐domain decomposition (FDD). In addition, the applicability and efficiency of the method were verified by applying the developed OMA to measured data, obtained through a field test on a 55‐story, 214‐m‐tall high‐rise building.     

CW6140型车床床身模态分析

针对CW6140型车床床身结构建立有限元分析模型,对其进行模态分析,得到车床床身在无阻尼自由振动条件下的固有频率和模态振型,为车床在实际工作状态中避免发生共振以及后续的结构优化工作提供了重要的依据。