Data-driven stochastic subspace identification of flutter derivatives of bridge decks

Most of the previous studies on flutter derivatives have used deterministic system identification techniques, in which the buffeting forces and the associated responses are considered as noises. In this paper, one of the most advanced stochastic system identification, the data-driven stochastic subspace identification technique (SSI-DATA) was proposed to extract the flutter derivatives of bridge decks from the buffeting test results. An advantage of the stochastic method is that it considers the buffeting forces and the responses as inputs rather than as noises. Numerical simulations and wind tunnel tests of a streamlined thin plate model conducted under a smooth flow by the free decay and the buffeting tests were used to validate the applicability of the SSI-DATA method. The results were compared with those from the widely used covariance-driven SSI method. Wind tunnel tests of a two-edge girder blunt type of Industrial-Ring-Road Bridge deck (IRR) were then conducted under both smooth and turbulent flows. The identified flutter derivatives of the thin plate model based on the SSI-DATA technique agree well with those obtained theoretically. The results from the thin plate and the IRR Bridge deck helped validate the reliability and applicability of the SSI-DATA technique to various experimental methods and wind flow conditions. The results for the two-edge girder blunt type section show that applying the SSI-DATA yields better results than those of the SSI-COV. The results also indicate that turbulence tends to delay the onset of flutter compared with the smooth flow case.     

Weighting ensemble least-square method for flutter derivatives of bridge decks

In this paper, a reliable but simple identification method, here called the weighting ensemble least-square method (WELS), has been developed to extract all eight flutter derivatives of bridge deck from free vibration records. For every wind speed, free vibration test of section model is generally repeated several times in order to obtain more reliable parameter estimates. In the WELS method, many free vibration records at the same wind speed are regarded as an ensemble. The common mode parameters are then identified simultaneously from the ensemble data. The parameter fit is obtained by a nonlinear least-square method. Weighting factors are proposed to make each experiment record with the same weight in total residual error analysis. The ensemble composed of many records can reduce the effect of the colored noise of few records on the convergence of least-square iteration process. The flutter derivatives of two section models are identified to indicate the reliability and effectiveness of the WELS method.     

Identification of flutter derivatives of bridge decks with free vibration technique

The flutter derivatives of bridge decks can be determined in a unique manner on condition that the complex modal parameters of the system at one reduced frequency are obtained. Based on the idea, a new method of identifying the flutter derivatives of bridge decks is proposed and it can overcome some shortcoming of the existing method and extend the applicability of the free vibration technique at high wind velocity. The identified results have agreements with the target ones of an ideal thin-plate section and those of a thin-plate section measured by the forced vibration technique. The proposed method is reliable and effective to extract the flutter derivatives from coupled free vibration.     

Parametric study on flutter derivatives of bridge decks

The method for identification of flutter derivatives of bridge decks developed by the authors is first briefly described in this paper. To investigate the effects of dynamic parameters of a bridge deck model on the flutter derivatives a test of the sectional Jiangyin Bridge deck's models with different dynamic parameters was carried out in a boundary layer wind tunnel using the present identification method. In both smooth and simulated turbulent flow conditions, a plate model and the sectional deck model of the Jiangyin Bridge were tested to further survey the effects of turbulence on flutter derivatives. The identified results tend to indicate that the effects of parameters of the model and turbulence on the flutter derivatives are negligible.     

Relationships among aerodynamic admittance functions, flutter derivatives and static coefficients for long-span bridges

Wind actions on long-span bridges are commonly considered as the superimposition of buffeting forces and self-excited forces, depending on the aerodynamic admittance functions and on the flutter derivatives, respectively. Since bridge deck sections are bluff bodies, the aerodynamic admittance functions and the flutter derivatives have to be determined experimentally by wind tunnel tests. This paper introduces a generalized quasi-static theory, defining new relationships among the flutter derivatives and the aerodynamic admittance functions. All the relationships are theoretically verified for the zero circular frequency; based upon experimental results, the validation of the relationships among the flutter derivatives is also provided for non-zero values of the frequency.     

桥梁断面颤振导数识别的随机搜索方法

建立了识别颤振导数的普适性更强的分析模型,该模型可适用于竖弯与扭转信号长度不同的情形;阐明总体最小二乘法加权理论,推导了对数衰减简谐函数平均值和标准差计算公式;提出随机搜索方法用于搜索信号特征的最佳拟合值;仿真算例表明,随机搜索方法是有效可行的,并且计算精度令人满意。融合随机方法,分别采用竖弯、扭转分项和总体最小二乘法对苏通大桥主梁断面颤振导数进行识别;识别结果表明,相对分项最小二乘而言,总体最小二乘并不能显著提高颤振导数的识别精度。     

全桥气弹模型颤振导数识别

将节段模型颤振导数识别方法用于全桥气弹模型,对其可行性和便利性进行详细论述;给出气弹模型模态质量的确定方法;采用随机子空间方法和随机搜索方法识别均匀流场和紊流场中苏通大桥气弹模型的18颤振导数,并和特征系统实现算法识别结果进行对比分析。研究结果表明:节段模型颤振导数识别方法是识别气弹模型颤振导数的有效和实用方法;模型模态质量可以方便地根据几何缩尺比由实桥对应值直接推算得到,且比通过模型试验实测精度更高;不同方法识别得到的均匀流场中苏通大桥气弹模型大部分颤振导数基本吻合;相对于紊流场中的气弹模型而言,均匀流场中的节段模型颤振导数识别精度更高。     

A new technique for identification of eighteen flutter derivatives using a three-degree-of-freedom section model

The prediction of flutter instability is of major concern for design of flexible structures. This necessitates the identification of aeroelastic parameters, known as flutter derivatives from wind tunnel experiments. The extraction of flutter derivatives becomes more challenging when the number of degrees of freedom (DOF) increases from two to three. Since the work in the field of identifying all 18 flutter derivatives has been limited, it has motivated the development of a new system identification method (iterative least squares method or ILS method) to efficiently extract the flutter derivatives using a section model suspended by a three-DOF elastic suspension system. The accuracy of a particular flutter derivative was determined by comparing the results obtained from all possible DOF combinations.     

Shrinkage and cracking studies of high performance concrete for bridge decks

Restrained shrinkage cracking is a critical issue that raises the concern of widespread use of high-performance concrete (HPC) in bridge deck. Present studies were undertaken to compare the different HPC and propose concept to use local field data to define a threshold for cracking potential. We developed 18 HPC mixtures, suitable for bridge decks in shrinkage-prone locations, using supplementary cementitious materials - fly ash, slag, silica fume, and metakaolin; and local aggregates with three different w/cm: 0.40, 0.35, and 0.30. Basic properties as well as shrinkage and cracking properties were evaluated. In addition to comparing among HPC performance, a correlation was made between commonly measured parameters such as strength, shrinkage, and modulus of elasticity with cracking onset obtained from ring tests. Finally, field data from no-crack and well performing bridges were used to define a threshold safe limit. This concept can be used for design of HPC mixtures to reduce cracking potential from materials point of view for any other locations.     

Time-domain and frequency-domain approaches to identification of bridge flutter derivatives

Flutter derivatives are essential for flutter analysis of long-span bridges, and they are generally identified from the vibration testing data of a sectional model suspended in a wind tunnel. Making use of the forced vibration testing data of three sectional models, namely, a thin-plate model, a nearly streamlined model, and a bluff-body model, a comparative study was made to identify the flutter derivatives of each model by using a time-domain method and a frequency-domain method. It was shown that all the flutter derivatives of the thin-plate model identified with the frequency-domain method and time-domain method, respectively, agree very well. Moreover, some of the flutter derivatives of each of the other two models identified with the two methods deviate to some extent. More precisely, the frequency-domain method usually results in smooth curves of the flutter derivatives. The formulation of time-domain method makes the identification results of flutter derivatives relatively sensitive to the signal phase lag between vibration state vector and aerodynamic forces and also prone to be disturbed by noise and nonlinearity.     

连续梁桥模态参数识别的随机子空间方法

简要介绍了协方差随机子空间方法,在此基础上,采用一等截面连续梁桥作为算例,通过ANSYS程序进行建模分析,对该连续梁桥进行识别,讨论了系统阶次的选择对识别结果的影响。     

桥梁风洞试验模态参数识别的随机子空间方法

模态参数识别是桥梁风洞试验中的一项重要内容,发展了基于参考点、无需输出协方差估计的识别振动系统模态参数的随机子空间方法,其识别精度和可靠性通过数值算例验证。对状态空间理论中的稳定图作了全面的阐释。根据苏通大桥和苏拉马都大桥主梁三自由度节段模型风洞试验采集到的位移信号,采用随机子空间方法识别了侧弯、竖弯和扭转模态频率和阻尼比,并且与随机搜索方法识别结果进行对比。分析结果表明,随机子空间方法和随机搜索方法识别结果非常吻合,本文发展的随机子空间方法是识别风洞试验桥梁模态参数的一种有效和实用方法。     

Forced motion and free motion aeroelastic tests on a new concept dynamometric section model of the Messina suspension bridge

The work presents a new original experimental rig to more deeply investigate the aerodynamic behaviour of long span suspension bridges. Set-up and model are designed to grant an accurate study on main aeroelastic phenomena in bridge engineering. More in details, experimental set-up and a 1:60 scale sectional model of Messina bridge deck are presented. The complete rig—composed by dynamometric model, suspension set, experimental set-up and active turbulence generator—is designed in order to execute both forced and free motion tests, allowing to change the average position in terms of angle of attack and yaw angle and to investigate flutter derivatives, admittance functions and vortex-induced vibrations.     

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.     

EMD-based random decrement technique for modal parameter identification of an existing railway bridge

Vibrational measurement data are often nonstationary and modal parameter identification based on these data is of practical value for structural health monitoring and condition assessment. The empirical mode decomposition (EMD) is a most recent tool for analysis of nonstationary signals. An EMD-based random decrement (RD) technique is presented to identify modal parameters from monitoring vibrational data. The nonstationary measurement data are first decomposed into a series of quasi-stationary intrinsic mode functions (IMFs) by EMD. The RD technique is then applied to the selected IMFs to obtain the free-decay response. The modal frequencies and damping ratios are finally identified from the free-decay response by minimizing the error between the measured free-decay responses and the predicted responses from a parametric model. The present method is applied to extract the modal parameters of the Nanjing Yangtze River Bridge from the measured responses. The identification result is compared to those from finite element analysis as well as from the experimental result identified with the peak-picking (PP) method. In addition, the modal frequencies of the bridge loaded with heavy trains are also identified and compared to the ‘empty’ bridge. The EMD-based random decrement (RD) technique provides an effective and promising tool for modal parameter identification for large bridges and other structures.     

Necessity of the bridge health monitoring system to mitigate natural and man-made disasters

In Korea, there has been a wide consensus about the necessity of bridge monitoring systems. This is primarily due to a chain of accidental bridge collapses over the past decade. The first generation bridge monitoring system originated from an instrumentation system introduced upon the reconstruction of the New-Haengju Bridge after its sudden collapse in 1995. After that, on-line instrumentation systems were installed in existing cable-supported bridges including Namhae, Jindo and Dolsan Bridges. These systems can be considered the second generation bridge health monitoring systems (BHMSs). These were then followed by the development of the third generation bridge monitoring systems installed and currently in service on the Seohae and Yeongjong Bridges. This study tracks the background and advent of Korean bridge monitoring systems and their developmental history up to the present time. It also investigates current associated studies in progress and the limitations of contemporary BHMS technologies, and suggests possible remedial solutions for a follow-up bridge monitoring system.     

FRP板多跨桥的荷载分布与动力响应

采用FRP板的桥日渐普及,需要掌握FRP板桥的性能。FRP板桥的性质(如质量、刚度)与传统的混凝土板桥明显不同。因此,在目前的研究中采用详细的有限元分析方法来研究FRP板桥的荷载分布及其动力响应。基于三维轮载-桥耦合模型,研究了钢和多跨混凝土桥中桥梁与轮载的相互作用。考虑时域内板的粗糙度对交通产生的竖向作用,得到了桥的动力响应。比较了FRP板桥与混凝土板桥的荷载分布与动力响应。此外,尚存在一些争论,如是否研究板和梁的共同作用,对FRP板桥是否采用简单的非组合设计。建模时采用全组合和部分组合的FRP板桥对这些问题进行了讨论。     

Optimal joint deployment of flow and pressure sensors for leak identification in water distribution networks

ABSTRACT

A multi-objective optimization methodology is proposed herein for accurate identification of leakage in water distribution networks (WDNs) using pressure and flow sensors. We first model leakage at potential nodes using the EPANET software, and then divide WDN into near-homogenous zones using k-means clustering algorithm based on geographic distribution of nodes. Finally, flow and pressure sensors locations are optimized using the NSGA-II algorithm to identify the leakage zone accurately. Novelty of the proposed approach lies in sequential optimization of flow and pressure sensors placement, which helps improve the accuracy of leakage zone identification in WDNs. The objective functions of this study are: 1) maximizing accuracy of identified leakage zone and 2) minimizing number of sensors (and hence operational costs). Simulation results of the Mesopolis WDN corroborate the efficiency and effectiveness of the proposed approach.     

Daily concentrations of trace metals in aerosols in Beijing, China, determined by using inductively coupled plasma mass spectrometry equipped with laser ablation analysis, and source identification of aerosols

This paper describes the daily concentrations of trace metals and ionic constituents in the aerosol of Beijing, China from March 2001 to August 2003. Daily PM10 concentrations were also measured from September 2001 to August 2003. The daily average PM10 concentration at Beijing, China from September 2001 to August 2003 was 171+/-117 microg m(-3) (n = 673), which is 5-fold higher than at Yokohama, Japan. Trace metal concentrations were analyzed by using inductively coupled plasma mass spectrometry equipped with a laser ablation sample introduction (LA/ICP-MS), which is a rapid and simultaneous method for multi-element analysis. The daily average metal concentrations in TSP in Beijing from March 2001 to August 2003 were: Al: 3.5+/-2.4 (n = 727), Ti: 0.47+/-0.35 (n = 720), V: 0.013+/-0.010 (n = 716), Cr: 0.019+/-0.015 (n = 618), Mn: 0.24+/-0.16 (n = 730), Fe: 5.5+/-3.9 (n = 728), Co: 0.0046+/-0.0055 (n = 629), Ni: 0.022+/-0.024 (n = 680), Cu: 0.11+/-0.11 (n = 660), Zn: 0.77+/-0.60 (n = 726), As: 0.048+/-0.047 (n = 731), Se: 0.010+/-0.010 (n = 550), Cd: 0.0068+/-0.0082 (n = 709), Sb: 0.033+/-0.036 (n = 687), and Pb: 0.43+/-0.50 (n = 728) (unit, microg m(-3)). All the metal concentrations in TSP in Beijing, China were 1.7-21.8 times higher than those in TSP in the center of Tokyo, Japan. Notably, As concentrations in TSP in Beijing were 20-fold higher than those in Tokyo. Source identification of aerosols in Beijing was carried out by using the chemical mass balance (CMB) receptor model, with the daily concentration of metals in the aerosol. The major primary sources of the aerosol of Beijing were considered to be soil dust and coal combustion. Vehicle exhaust contribution tended to increase.     

Wearable Technologies,Portable Architectures and the Vicissitudes of the Space Between

The omnipresence of the iPod and mobile phone has ensured portable and wearable technologies' highly privileged position in contemporary society. They are at the top of the pile where conspicuous consumption is concerned - you only have to visit an Apple store on a Saturday afternoon to witness the degree to which this highly sought-after gadgetry has become subject to consumer frenzy and speculation. Here, Despina Papadopoulos reviews the particular social and cultural impact of wearable devices. She also welcomes in a new generation of interactive designers who are investigating the human and emotional potential of emergent technologies. Copyright © 2007 John Wiley & Sons, Ltd.