Separation and extraction of bridge dynamic strain data

Through comparing the measured data of dynamic strains due to loading and temperature by the stain gauge and temperature sensor at the same location, the information in the strain data was divided into three parts in the frequency domain by using the defined index named power spectral density (PSD)-ratio index. The three parts are dominated respectively by temperature varying, stresses, and noises and thus can be distinguished from the determined the separatrix frequencies. Also, a simple algorithm was developed to separate the three types of information and to extract the strain caused mainly by structural stresses. As an application of the proposed method, the effect of strain deformation and noises on the fatigue assessment was investigated based on the separated data. The results show that, the determined values of separatrix frequencies are valuable for the monitoring data from other bridges. The algorithm is a multiresolution and hierarchical method, which has been validated as a simple and effective method for data analyses, and is suitable for the compression and preprocessing of the great amount monitoring data and easy to be integrated into the structural health monitoring (SHM) soft system. The strain due to temperature varying attributes a little to the errors of fatigue assessment; however, the noises or random disturbance existed in the monitoring data have much responsibility for the errors, and the main reason is that the random disturbance shifts the real strain/stress amplitude picked up by real structural stress or strain.     

Fatigue stress monitoring and analyses for steel box girder of Runyang Suspension Bridge

The monitoring of fatigue stress of steel bridge is a key issue of bridge health monitoring and safety assessment. This paper aims to find out the strain history features of the girder components of Runyang Suspension Bridge (RYSB) under vehicle and environmental loading during its first year of service based on the strain-history data recorded by structural health monitoring system installed in the bridge. The fatigue stress spectrums of steel box girders under normal traffic load, heavy trucks and typhoon loads were studied as well as the correlation between varying strain and temperature based on real-time monitoring of temperature histories. The results show that, monitoring on local strain in health monitoring systems of RYSB can effectively provide the information on fatigue stresses. The range of the equivalent fatigue stress on the monitoring location of the bridge is currently low, mainly due to varying traffic loading and temperature. There exists significant correlation between varying temperature and mean value of fatigue stress. Effect of the passing of heavy trucks on bridge fatigue is quite significant since the value of the cumulative fatigue damage generated by heavy trucks is 10 to 100 times larger than that by normal traffic. Therefore, the effect on fatigue cumulative damage due to heavy trucks or overloaded vehicles needs to be monitored and paid much attention to.     

Fatigue stress monitoring and analyses for steel box girder of Runyang Suspension Bridge

The monitoring of fatigue stress of steel bridge is a key issue of bridge health monitoring and safety assessment. This paper aims to find out the strain history features of the girder components of Runyang Suspension Bridge (RYSB) under vehicle and environmental loading during its first year of service based on the strain-history data recorded by structural health monitoring system installed in the bridge. The fatigue stress spectrums of steel box girders under normal traffic load, heavy trucks and typhoon loads were studied as well as the correlation between varying strain and temperature based on real-time monitoring of temperature histories. The results show that, monitoring on local strain in health monitoring systems of RYSB can effectively provide the information on fatigue stresses. The range of the equivalent fatigue stress on the monitoring location of the bridge is currently low, mainly due to varying traffic loading and temperature. There exists significant correlation between varying temperature and mean value of fatigue stress. Effect of the passing of heavy trucks on bridge fatigue is quite significant since the value of the cumulative fatigue damage generated by heavy trucks is 10 to 100 times larger than that by normal traffic. Therefore, the effect on fatigue cumulative damage due to heavy trucks or overloaded vehicles needs to be monitored and paid much attention to. __________ Translated from Journal of Southeast University (Natural Science Edition), 2007, 37(2): 280–286 [译自: 东南大学学报(自然科学版)]     

Gage‐Free Stress Estimation of a Beam‐like Structure Based on Terrestrial Laser Scanning

Abstract: Terrestrial laser scanning (TLS) is a technique that remotely obtains the three‐dimensional (3D) coordinates of an object using laser pulses. It is advantageous when used to obtain the 3D coordinates of the overall shape as well as any particular area or point of a target object. In addition, using TLS for the stress monitoring of structures will not require the installation of a sensor on the target structure whose structural response will be assessed. Thus, TLS can resolve the limitations of conventional sensors based on strain monitoring. This article presents a computational model for the automatic estimation of the stresses of beam structures using TLS in association with a finite element method. The method is experimentally applied to the stress estimation of a simply supported steel beam subjected to a concentrated load. In this experimentation, the maximum and minimum errors between the estimated stresses using TLS and directly measured stresses from electrical strain gages are found to be 7.2% and 2.2%, respectively.     

A method for correcting field strain measurements to account for temperature effects

Measurements of deformation and strain are of critical importance for the structural health monitoring of geosynthetic reinforced soil (GRS) structures. For some commonly used wiring configurations and sensors, field strain measurements can exhibit apparent strains that are caused by changes in wire temperature or changes in gauge temperature, which do not correspond to changes in actual strain in the geosynthetic. Correcting for these effects is important for separating out real structural behavior from wiring and sensor issues, and is imperative for avoiding false trigger warnings in automated instrumentation reporting systems. The current paper consequently presents a method for correcting field strain measurements to account for temperature effects. Data collected from sensors embedded in a heavily-instrumented GRS abutment over a two-year monitoring period is presented. The distributed thermistor array embedded in the GRS abutment is shown to be particularly useful for understanding temperature effects on the measured foil strain gauge data. The presented approach and associated framework for data correction are useful for practicing engineers and other researchers, as the general concepts from this study can be applied to data collected from many instrumented geosynthetic field projects.     

A Computationally Efficient Algorithm for Real‐Time Tracking the Abrupt Stiffness Degradations of Structural Elements

Real‐time structural identification and damage detection are necessary for on‐line structural damage detection and optimal structural vibration control during severe loadings. Frequently, structural damage can be reflected in the stiffness degradation of structural elements. In this article, a time‐domain three‐stage algorithm with computational efficiency is proposed for real‐time tracking the onsets, locations, and extents of abrupt stiffness degradations of structural elements using measurements of structural acceleration responses. Structural dynamic parameters before damage are recursively estimated in stage I. Then, the time instants and possible locations of degraded structural elements are detected by tracking the errors between the measured data and the corresponding estimated values in stage II. Finally, the exact locations and extents of stiffness degradations of structural elements are determined by solving simple constrained optimization problems in stage III. Both numerical examples and an experimental test are used to validate the proposed algorithm for real‐time tracking the abrupt stiffness degradations of structural elements in linear or nonlinear structures using measurements of structural acceleration responses polluted by noises.     

Bewertung der Ermüdungsfestigkeit von Baustrukturen in Offshore‐Windenergie‐Anlagen

Fatigue assessment of support structures for offshore wind energy converters. The structure of wind energy converters secures a reliable energy production. Hence an adequate reliability of the structure and additionally permanent assessment and maintenance is required. Within this paper methods for the assessment of the important damage mechanism fatigue in combination with monitoring of the fatigue reliability utilizing damage indicators are proposed. These methods comprise structural system, material degradation and probabilistic models as well as monitoring data of an offshore wind energy converter prototype. The structural reliability of the structure as designed is calculated based on an overall dynamic analysis using a SN‐approach. Herewith fatigue sensitive components are identified. Damage indicators for monitoring the fatigue reliability based on damage equivalent strains are presented.     

温度对盐岩疲劳特性影响的试验研究

 对不同温度下的盐岩试件进行单轴压缩试验,发现盐岩单轴抗压强度随着温度的升高明显下降,峰值应变却随着温度的升高而逐渐增大。根据不同温度下盐岩的单轴抗压强度选取合适的上、下限应力值,对各温度条件下盐岩试件进行循环荷载试验,发现：盐岩在相同应力比循环荷载作用下,疲劳寿命随着温度的升高而增加;不同温度下盐岩的疲劳损伤发展模式都包括减速损伤阶段、匀速损伤阶段和加速损伤阶段;随着温度的升高匀速疲劳损伤阶段所占的比例增加;与轴向应变曲线相比,体积应变的3个损伤阶段更加明显。最后应用弹塑性材料损伤分析公式,对盐岩的疲劳损伤进行分析,发现盐岩损伤发展模式与盐岩试件的体积应变发展模式较为接近,因此选取体积应变来描述盐岩的疲劳损伤较为合理。     

深圳证券交易所营运中心抬升裙楼施工阶段应力监测与分析

深圳证券交易所营运中心的抬升裙楼采用超长巨型悬挑钢桁架结构体系,具有悬挑跨度大、受力复杂、临时支撑卸载难度大的特点。采用自主研发的基于有线应变数据采集与无线信号传输的应变监测系统,在施工阶段,尤其是在卸载过程中,对抬升裙楼关键构件进行实时应力监测。通过长期应变监测数据统计分析、双轴对称测点应力对比分析以及焊接对结构局部应力增量影响试验研究,对施工中的抬升裙楼进行实时安全评估。结果表明：卸载过程中裙楼结构受力稳步转换;龙骨焊接施工产生的焊接残余应力对焊接区域附近的结构局部应力增量有显著影响。     

组合EMD和PCA的多路径改正方法

多路径效应及高频随机噪声是目前影响GPS变形监测精度的主要因素。针对GPS变形监测中的多路径效应与高频随机噪声的周日重复性特征,提出了EMD(Empirical Mode Decomposition)与PCA(Principal Component Analysis)组合的GPS噪声改正方法。该方法先通过EMD对各天数据剔除高频随机噪声,再采用PCA对相关性较强的多路径效应进行提取和消除。实测数据分析表明,该组合方法能有效地削弱多路径效应及高频随机噪声,较单一滤波方法具有一定的优越性。     

Evaluation of a strain monitoring system for existing steel railway bridges

The old steel bridges that integrate the existing railway lines are structures built with materials that are no longer used and whose knowledge has being lost over the years, often presenting severe problems of deterioration and subjected to loading environments very different from those for which they were designed. In this context, adequate strain monitoring is a crucial tool in supporting the behavior characterization and safety assessment of these structures.This article presents and discusses the monitoring systems installed in the Trezói Bridge, within a research project aimed at developing and applying procedures for evaluation of the structural integrity of steel railway bridges. The field observations of the structural behavior were accomplished by using two different types of sensors: electric and fiber optic strain sensors. The electric monitoring system was designed and installed on the bridge to supply the experimental data for the research project, while the fiber optic monitoring system was firstly applied to evaluate the reliability of the former and to check its efficiency, and secondly to provide some redundancy of the measurements at critical locations. The obtained results are analyzed to characterize the bridge behavior and the capabilities and limitations of both types of sensors to acquire the relevant data for the bridge service condition and fatigue assessment are discussed, namely in what concerns the ability to accurately capture the static and dynamic components of the structural response and the frequency content of interest.     

现有铁路钢桥应变监测系统评估

现有铁路线中旧式钢桥所用的是目前已不再使用的材料,常常会出现老化问题,使用中常遇到与设计荷载完全不同的荷载情况。因此,应变监测是保障结构性能和进行安全评估的重要工具。介绍并讨论了安装在Trezói大桥的应变监测装置系统,用该系统评估铁路钢桥结构。采用两种不同的传感器(电子传感器和光纤传感器)进行结构性能的现场观测。所设计的电子监控系统安装在桥梁上,为研究项目提供试验数据,光纤光学显示器系统用于评估系统的可靠性以确保其安全,并对试验点的测量值进行筛选。对结果进行分析,描述桥梁特性和性能,讨论两种传感器在获得桥梁使用情况和疲劳评估相关数据方面的局限性,也即关注其精确采集结构的静态与动态响应及频率的能力。     

Statistical analysis of online strain response and its application in fatigue assessment of a long-span steel bridge

This paper covers reliability assessment of the fatigue life of a bridge-deck section based on the statistical analysis of the strain–time histories measured by the structural health monitoring system permanently installed on the long-span steel bridge under study. Through statistical analysis of online strain responses in the frequency domain using multiple linear regression, a representative block of daily cycles of strain history is obtained. It is further assumed that all cycles of online strain response during bridge service are repetitions of the representative block. The rain-flow counting method is then used to determine the stress spectrum of the representative block of daily cycles. The primary assessment of fatigue life at a given value of failure probability is undertaken for the sample component in a bridge-deck section by using the classification of details for welded bridge components and the associated statistical fatigue model provided by the British Standard BS5400. In order to evaluate bridge fatigue at any value of failure probability, a modified probability model is proposed based on BS5400. The fatigue life of the considered component in the bridge-deck section is then evaluated for some other values of probability of failure which are not included in BS5400 by use of the modified probability model. The analytical results show that the modified model is practical for reliable evaluation of the service life of existing bridges under random traffic loading.     

A Bayesian Probabilistic Approach for Acoustic Emission‐Based Rail Condition Assessment

The investigation described in this article aims at developing a Bayesian‐based approach for probabilistic assessment of rail health condition using acoustic emission monitoring data. It comprises the following three phases: (i) formulation of a frequency‐domain structural health index (SHI), via a linear transformation method, tailored to damage‐sensitive frequency bandwidth; (ii) establishment of data‐driven reference models, using Bayesian regression about the real and imaginary parts of the SHI derived with monitoring data from the intact rail; and (iii) quantitative evaluation of discrimination between the new observations representative of current rail health condition and the baseline model predictions in terms of Bayes factor. If the deviation of the new observations from the predictions is within an acceptable tolerance, no damage is flagged, and the new data are further used to update and refine the reference models. If the observations deviate substantially from the model predictions in a probabilistic sense, damage is signaled, damage severity is quantified, and damage location determined. The proposed approach is examined by using field monitoring data acquired from an instrumented railway turnout, and the coincidence between the assessment results and the actual health conditions demonstrates its effectiveness in damage detection, localization, and quantification.     

Wireless Monitoring of a Multispan Bridge Superstructure for Diagnostic Load Testing and System Identification

Abstract: This article focuses on the deployment of a wireless sensor system (WSS) developed at Clarkson University for structural monitoring purposes. The WSS is designed specifically for diagnostic bridge monitoring, providing independent conditioning for accelerometers, strain transducers, and temperature sensors in addition to high‐rate wireless data transmission and is capable of supporting large‐scale sensor arrays. A three‐span simply supported structure was subjected to diagnostic load testing as well as ambient vibration monitoring. A total of 90 wireless and several wired sensors, including accelerometers and strain transducers were used in the deployment. Strain measurements provided capacity and demand characteristics of the structure in the form of neutral axis locations, load distributions, and dynamic allowances which ultimately produced an inventory and operating load rating for the structure. Additionally, modal characteristics of the structure, including natural frequencies and mode shapes, were derived from measured accelerations and discussed briefly.     

斜拉桥钢桥面板细节疲劳分析

正交异性钢桥面板存在大量焊接疲劳薄弱坏节,活载反复作用下容易发生疲劳破坏,国内外针对钢桥面板的力学性能及细节疲劳研究主要考虑第二结构体系效应,通过混合有限元方法模拟斜拉桥第一结构体系效应对疲劳应力的影响,以全梁段板壳单元代替相应位置的鱼骨主梁,采用蒙特·卡洛方法模拟车流,加载于疲劳细节的应力影响面,选用Eurocode疲劳曲线验算各疲劳细节的累积损伤,结果表明受压梁段由于第一体系效应影响可不进行疲劳验算,而对受拉梁段钢桥面系进行疲劳评估时可以忽略全桥结构体系效应的影响.     

改进BP神经网络的钢结构应力缺失数据重构

钢结构健康监测过程中的应力数据缺失会干扰监测各环节的运行状态,无法保障施工阶段的安全,因此解决数据缺失问题至关重要。钢结构在施工阶段应力应变监测中,由于受到外界诸多复杂因素的影响,导致监测数据不准确、缺失以及局部应力数据的重构值与真实值偏差较大等问题。采用改进BP神经网络分别对300组和30组钢结构应力应变监测数据进行重构,并对改进BP神经网络的数据重构方法进行适用性分析。结果表明:相比线性回归法,改进BP神经网络法进行离散型缺失数据的重构平均误差降低0.7%,特别是对于局部缺失数据,改进BP神经网络法的重构精度更高,平均局部误差降低2.2%; 为达到较好的重构精度,使用改进BP神经网络对缺失数据重构时,数据的缺失率不宜超过20%; 改进BP神经网络法可为钢结构应力缺失数据重构以及结构健康监测提供技术支持,具有较好的实用性。     

光纤Bragg光栅传感技术在隧道监测中的应用

在阐述光纤Bragg光栅（FBG）传感技术的工作原理和优点的基础上,以昆明白泥井3#隧道为实例,提出FBG传感器在隧道内的铺设方案及温度补偿技术。经过对隧道断面的24h连续监测,讨论隧道内最大温差对Bragg反射中心波长变化的影响,并对隧道每一监测断面上温度传感系统所取得的Bragg波长值进行可靠性的估计,验证FBG监测系统的可靠性。对隧道进行8个月的定期监测,并对隧道断面的应变变化情况进行分析,分析结果表明：干湿季节交替是导致测点应变波动的重要因素,而雨季是致使测点应变变化增大的主要原因。目前隧道二次衬砌结构稳定;同时,研究结果表明,FBG可准确地测出隧道的应变分布,将其应用于隧道变形监测中是可行和有效的。     

Fatigue life estimates for a slender orthotropic steel deck

Fatigue cracks in several types of welded joints and geometrical details have been reported to occur in a large number of slender orthotropic decks of existing steel bridges in many parts of the world. Some of these cracks are initiated very early, a few years after the bridge is brought into service. That is why it is said that fatigue life estimation for the welded joints of orthotropic decks is not an easy designing task. This is so because the random variation of local stresses is sensitive to many factors, in particular to those involved in the dynamic interaction between the vehicle’s tyres, the pavement and the steel structure.This paper presents and discusses the main causes for the observed cracks and the outcome of the ultimate fatigue life estimates for typical welded and geometrical details of a slender orthotropic deck with trapezoidal cross-section longitudinal ribs. This is the deck of an existing long-span steel bridge, which has been strengthened by adding a reinforced concrete layer fixed with shear studs to the steel deck top plate. This was done to avoid continued and extensive repair work and to enhance the fatigue life expectancy of the steel deck structure. The fatigue life estimation is done with the aid of refined numerical modelling and in situ experimental strain measurements and also by taking into account all the built-in structural alterations, changes in volume of traffic and in vehicles loading which have occurred during this bridge’s 32 years of service life.     

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.