Health assessment of beams–experimental verification

A novel nondestructive structural health assessment procedure now under development at the University of Arizona and presented in a companion paper, is experimentally verified. The experimental verifications of the procedure to identify defect-free and defective fixed ended and simply supported beams are presented in this paper. In this approach, acceleration and rotational time histories are measured at pre-selected node points. They are then post-processed to remove several sources of error including noise, high frequency content, slope, and DC bias. The post-processed response information is then successively integrated to obtain the corresponding velocity and displacement time histories. Even when these sources of error were removed from the response information, the proposed method failed to identify the beams. Several factors including noise, data latency, scale factor and cross coupling error were investigated. Amplitude and phase errors in the accelerometer's measurements were found to be the root cause. Alternative approaches are proposed to mitigate them. Following the suggested procedures, defect-free and defective fixed ended and simply supported beams are correctly identified. The proposed NDE procedure is accurate and robust, and can identify defects at the local element level in the context of the finite element representation. The laboratory experiments clearly and conclusively verified the proposed algorithm, i.e. a beam can be identified without using input excitation information and using only noise-contaminated response information and established its application potential.     

System reliability of suspension bridges

Provisions for the design of existing suspension bridges often rely on a deterministic basis. Consequently, the reliability of these bridges cannot be assessed if current provisions are applied. In order to develop cost-effective design and maintenance strategies for suspension bridges a system reliability-based approach has to be used. This is accomplished by a probabilistic finite element geometrically nonlinear analysis approach. This study forms part of an investigation into the system reliability evaluation of geometrically nonlinear large span bridges recently undertaken at the University of Colorado. A brief review of reliability analysis of geometrically nonlinear elastic structures allows for the determination of its relevance to the assessment of suspension bridges. A probabilistic finite element geometrically nonlinear elastic code is used for system reliability evaluation of suspension structures. The allowable stress design procedures used by the Honshu Shikoku Bridge Authority for the design of suspension bridges are presented along with their application to the design of an existing bridge. This bridge is studied from a system reliability viewpoint to evaluate its reliability under different loading and damage scenarios. Such information calls attention to the fact that the reliability of cables, hanger ropes and girders are very different. Therefore, optimal maintenance decisions for suspension bridges designed according to allowable stress method are not consistent with those based on equal component reliability values.     

Evaluation of nonlinear static analysis methods and software tools for seismic analysis of highway bridges

The response of bridges when subjected to seismic excitation can be evaluated by a number of analysis methods. The traditional approach is to employ linear static or dynamic analyses coupled with appropriate modifiers to account for inelastic response while current design practice is moving towards an increased emphasis on nonlinear static analysis methods. In this study, a preliminary seismic response analysis of a two-span highway bridge was performed using linear dynamic analysis procedures to identify the potential for inelastic response. The bridge was subsequently analyzed in accordance with two nonlinear static analysis methods (capacity spectrum and inelastic demand spectrum methods) in an effort to evaluate the difference in global response predicted by the two methods and the effectiveness of various software programs for performing nonlinear static analysis. The results demonstrated that, for the two nonlinear static analysis methods used in this study, different predictions of nonlinear response occurred with neither method being regarded as producing correct results due to a number of simplifications inherent in the methods. Furthermore, the analysis revealed that some software programs are well suited to performing nonlinear static analysis, both from the point-of-view of performing comprehensive analyses and providing a graphical depiction of the evolution of damage (i.e. the behaviour of plastic hinges).     

Determination of the critical loading conditions for bridges under crossing trains

A procedure based on experimental and theoretical analyses to identify critical loading conditions on existing metallic railway bridges is presented. This method requires knowledge of the principal modal frequencies, and for this reason, a consolidated and simple procedure to study the bridge dynamics is herein explained. This consists of: preliminary studies; material and dynamic tests; and identification techniques to identify modal parameters and eventual non-linear behaviours. Generally the information collected can be used both to calibrate the bridge model and to obtain the refined frequency response function. In order to avoid high computational effort due to long time-history dynamic analyses by using the bridge model subjected to a series of train crossings, a new frequency domain approach for the identification of critical loading conditions is proposed. Evidence of the influence of the axle spacing and velocity of the vehicle on the dynamic magnification due to the train crossing is shown. The method is based on the construction of an excitation spectrum related to the train axle spacing and the velocity, given the weight of the vehicle. Comparison of the excitation spectrum with the frequency response function allows identification of the load patterns that bring the bridge to resonance conditions and might threaten bridge stability, bearing in mind continual changes in train technology.     

Parametric finite element investigation of the critical load capacity of elastomeric strip bearings

Elastomeric bearings are used in non-seismic bridge applications and for seismic isolation of structures. These bearings consist of a number of elastomeric (rubber) layers bonded to intermediate steel shim plates. For seismic isolation, the total thickness of rubber provides a low horizontal stiffness, whereas the close spacing of the intermediate shim plates provides a high vertical stiffness, relative to the horizontal, for a given bonded rubber area and shear modulus. During earthquake ground shaking, large lateral displacements will develop across the isolation interface and the individual bearings. The design of elastomeric bearings for seismic isolation requires that the stability of the individual bearings be demonstrated at the maximum bearing displacement. A component of the stability assessment is the determination of the critical load of the bearing at a given lateral displacement. Currently, the critical load is estimated using an approach whereby a ratio, that of the overlapping area between the top and bottom bearing endplates to the bonded rubber area, is used to reduce the critical load at zero lateral displacement, referred to herein as the overlapping area method. This study verifies the finite element method for predicting critical loads in elastomeric bearings, and uses the finite element method to investigate the dependency of the critical load on the bearing geometry. The results of the parametric study were also used to evaluate the predictive capabilities of the overlapping area procedure.     

Health monitoring of an instrumented SRMF building using earthquake data

This paper is a study of a six‐story welded steel moment‐resisting frame instrumented by California Strong Motion Instrumentation Program. Several earthquakes had shaken the building, and its response of strong motion data recorded during the earthquakes were made available. In particular, the Whittier Narrows (October 1987), Sierra Madre (June 1991), Northridge (January 1994) and Chino Hills (July 2008) earthquakes were considered in the studies reported here. The paper investigates the capability of input–output system identification methods, namely eigensystem realization algorithm with data correlation, and observer Kalman filter identification, in identifying dynamic characteristics of typical office building structures subjected to strong ground motion. It explores the possibility to extend the above system identification procedures to near‐real‐time estimation of the building response under future earthquakes in order to estimate the likelihood of brittle damage in welded beam to column connections. The modal identification results are compared with those obtained using the spectral analysis and the deterministic‐stochastic subspace identification method to verify the accuracy of the identified modal parameters. Copyright © 2013 John Wiley & Sons, Ltd.     

后张预应力钢筋混凝土桥梁结构健康监测基准的发展

建立长期结构健康监测基准和评估体系十分必要。通常,现场试验数据和有限元模型是建立基准的两个关键手段。首次引进伸缩支承的概念,建立包括温度变化的弯曲后张预应力钢筋混凝土桥梁基准现场数据库。数据库来源于未破坏试件的全年监测数据。通过有限元模型建立监测基准。对有限元模型修正的方法,包括材料性能的标定、弹性支承单元的使用、箱梁Mindlin板单元被最新的裂缝Mindlin板单元的置换以及桥的使用状态进行细化。通过有限元模型得到的基准结果与现场测试的基准结果非常吻合。所提出的结构健康监测基准能够进行实时破坏监测,建立监测技术以及进行状态评估。     

Structural health monitoring system of the long-span bridges in Turkey

Abstract

This study presents the general features of the structural health monitoring (SHM) system of the long-span cable-supported bridges in Turkey, namely the First Bosphorus Bridge, the Second Bosphorus Bridge (Fatih Sultan Mehmet Bridge), the newly constructed the Third Bosphorus Bridge (Yavuz Sultan Selim) and the Osman Gazi Bridge (Izmit Bay Bridge). Due to the critical function of the bridges in the transportation networks of Istanbul, structural health under extreme and operational load cases without interruption of their operation is essential. The SHM systems are already available and in use for the First and Second Bosphorus Bridges; however, the Yavuz Sultan Selim and the Osman Gazi Bridges, only have the preliminary SHM systems established. General preferences of the current and planned SHM systems of the bridges are given based on the monitoring objectives and requirements. Data collected from the SHM systems are also exhibited as an illustration for structural modal identification of the First Bosphorus Bridge. Based on the experiences from the current SHM systems, future works and planning are recommended to effectively design new SHM system and to efficiently update the current ones.     

Neural networks for prediction of deflection in composite bridges

Neural networks have been developed for prediction of deflections, at service load, in steel-concrete composite bridges incorporating flexibility of shear connectors, shear lag effect and cracking in concrete slabs. Three neural networks have been presented to cover simply supported bridges, two span continuous bridges and three span continuous bridges. The use of the neural networks requires a computational effort almost equal to that required for the simple beam analysis (neglecting flexibility of shear connectors, shear lag effect and cracking of concrete). The training and testing data for neural networks are generated using finite element software ABAQUS. The neural networks have been validated for number of bridges and the errors are found to be small. Closed form solutions are also proposed based on the developed neural networks. The networks/ closed form solutions can be used for rapid prediction of deflection for everyday design.     

施工监控在大跨径混凝土连续梁桥中的应用

结合大跨径预应力混凝土连续梁桥的特点,分析了影响该类桥梁施工监控的主要因素,介绍了施工监控方法及具体内容,并以具体工程为例,阐述了施工监控技术在该类桥梁施工中的实际应用,对今后同类桥梁工程具有指导作用。     

Dynamic identification and continuous dynamic monitoring of bridges: different applications along bridges life cycle

Abstract

This paper aims at stressing the major role that Dynamic Identification and Continuous Dynamic Monitoring can play in the observation and understanding of the structural behaviour of bridges along the different phases of their life cycle, based on the authors’ experience got at the Laboratory of Vibrations and Structural Monitoring (ViBest) of the University of Porto during the last 20 years. The paper comprehends two initial sections summarising some relevant aspects regarding the evolution observed in terms of excitation devices and instrumentation, as well as in the field of modal identification of large Civil structures. Subsequently, the importance of dynamic identification along bridges’ life cycle (design, construction, commissioning, and service or rehabilitation phases) is illustrated on the basis of relevant case studies, involving several outstanding structures. Lastly, the interest and potential of Continuous Dynamic Monitoring is also emphasised, describing several dynamic monitoring programmes implemented with different perspectives, such as: (i) the vibration serviceability safety checking in lively footbridges; (ii) the evaluation of dynamic effects of traffic loads in roadway and railway bridges and fatigue assessment; (iii) the vibration based damage detection; (iv) or the dynamic monitoring of wind effects on large bridges.     

结构健康监测和可靠性评估

对美国最大跨度桁架桥中的主要桁架构件以及整个结构系统的可靠性进行评估。根据构件和系统的可靠性指标,可以采用随机方法评估大跨桥的安全水平。然而,大多数旧的大跨桥是基于允许应力设计的,其可靠性不可能被保证。本研究的可靠性分析基于对恒荷载、活荷载和风载分布的评估。通过收集大量的输入和响应数据,对大桥进行长期结构健康监测。根据外部荷载影响的模式和大小,长期监测数据清楚揭示了不同结构的性能。案例显示,采用传统的分析方法难以确定由于温度引起的结构响应。为探讨温度对结构的影响以及在可靠性评估中考虑长期监测数据的作用,也对温度引起的响应进行分析。研究显示：温度导致的响应对整个系统的可靠性具有明显的影响。     

叉车车架有限元模态分析与结构改进

叉车是刚性悬挂车辆,车架作为整车的骨架,其动态特性直接关系到整车的振动性能.为改进和有效提高叉车的安全可靠性、驾驶平顺性和舒适性,应用ANSYS有限元方法对某型3t叉车车架进行模态分析,根据所得模态参数,结合叉车行驶路况和发动机激励频率的计算结果,对车架结构动态特性进行分析评价,在保持原始几何形状不变的前提下,提出最简约的车架模态修改方式,以避免车架对振动激励的放大响应或共振现象发生.     

Implications of subsoil-foundation modelling on the dynamic characteristics of a monitored bridge

Model updating based on system identification (SI) results is a well-established procedure to evaluate the reliability of a developed numerical model. In this inverse assessment problem, soil-foundation compliance is often not explicitly considered rigorously during design and/or purely numerical assessment. The present work aims to investigate the correlation between subsoil-foundation stiffness and modal characteristics of bridges, as a means to identify a threshold beyond which rigorous subsoil modelling is a prerequisite for reliable model updating. The second Kavala Ravine Bridge, in Greece, serves as the case study for this purpose for which a reasonably reliable finite element (FE) model is developed and updated based on ambient vibration measurements. Alternative soil profiles and subsequently redesigned foundation systems are then used to examine the effect that the correspondingly variable soil compliance would have on the natural frequencies of the bridge. It is shown that soil stiffness alone is not an adequate proxy to decide on the necessity for subsoil modelling, as the foundation stiffness (particularly in cases of softer soil profiles) tends to balance the dynamic properties of the holistic soil-foundation system. The soil-foundation stiffness is therefore the key parameter that dictates the need for refined modelling of soil–structure interaction in the framework of SI-based model updating.     

Seismic response of liquid-filled elevated tanks

The main focus of the current study is to evaluate the performance of elevated tanks under seismic loading. In this study, the finite element (FE) technique is used to investigate the seismic response of liquid-filled tanks. The fluid domain is modeled using displacement-based fluid elements. Both time history and modal analyses are performed on an elevated tank. Using the FE technique, impulsive and convective response components are obtained separately. Furthermore, the effect of tank wall flexibility and sloshing of the water free surface are accounted for in the FE analysis. In this study complexities associated with modeling of the conical shaped tanks are discussed. This study shows that the proposed finite element technique is capable of accounting for the fluid-structure interaction in liquid containing structures. Using this method, the study of liquid sloshing effects in tanks with complex geometries such as conical tanks is made possible. The results of this study show that the current practice predicts the response of elevated tanks with reasonable accuracy.     

Reliability assessment of cable-stayed bridges based on structural health monitoring techniques

This paper presents the reliability analysis approach of long-span cable-stayed bridges based on structural health monitoring (SHM) technology. First, the framework of structural reliability analysis is recognised based on SHM. The modelling approach of vehicle loads and environmental actions and the extreme value of responses based on SHM are proposed, and then models of vehicle and environmental actions and the extreme value of inner force are statistically obtained using the monitored data of a cable-stayed bridge. For the components without FBG strain sensors, the effects and models (extreme values) of dead load, unit temperature load, and wind load of the bridge can be calculated by the updated finite element model and monitored load models. The bearing capacity of a deteriorated structure can be obtained by the updated finite element model or durability analysis. The reliability index of the bridge's critical components (stiffening girder in this study) can be estimated by using a reliability analysis method, e.g. first order reliability method (FORM) based on the models of extreme value of response and ultimate capacity of the structure. Finally, the proposed approach is validated by a practical long-span cable-stayed bridge with the SHM system. In the example, reliability indices of the bridge's stiffening girder at the stage after repair and replacement after 18 years of operation, and the damaged stage are evaluated.     

Fatigue reliability assessment of retrofitted steel bridges integrating monitored data

This paper focuses on fatigue reliability assessment of retrofitting distortion-induced cracking in steel bridges integrating monitored data. The fatigue reliability assessment of the connection details is based on the approach used in the AASHTO standard design specifications with all necessary information from finite element modeling (FEM) and structural health monitoring (SHM). Both in-plane traffic loading and out-of-plane relative displacements are considered along with different connection boundary conditions. The primary cause of the observed fatigue cracks before retrofitting is identified as the out-of-plane relative displacements, while the potential fatigue cracking re-initiation after retrofitting depends on the boundary conditions and critical locations that can be identified from the validated FEM. When the identified critical locations are different from the SHM sensor locations, the original monitored data may be modified by using a spatial adjustment factor (SAF). The proposed approach is illustrated by using an actual bridge monitored by the Advanced Technology for Large Structural Systems (ATLSS) Center, a National Engineering Research Center at Lehigh University.     

Seismic effects of elevating bridges with steel pedestals in the southeastern United States

Steel pedestals have been used in the southeastern parts of the United States to elevate highway bridges and decrease the likelihood of vehicle collisions with bridge decks. However, the seismic performance of bridges elevated with steel pedestals is still unknown. To investigate the effect of elevating bridges with steel pedestals, this paper uses the results of previously conducted experimental tests to model the hysteretic behavior of three types of steel pedestals in a detailed 3D finite element model of a representative bridge. One short pedestal (with a height of 500 mm) and two tall pedestals (with a height of 850 mm) are studied. The structural responses of the studied bridge with the addition of steel pedestals are compared to the structural responses of the same bridge before elevation, where elastomeric bearings support the deck. This study considers five different locations in the southeastern United States and for each of them selects 20 artificial ground motions at two hazard levels of 2% and 10% probability of exceedance in 50 years. The selected artificial ground motions are applied to the representative bridge model in four cases: one case with elastomeric bearings and three cases with three studied pedestals. Because of the large amount of resulting data, a statistical effects model is employed. The statistical effects model is a statistical tool that uses the statistics of the data to investigate the effect of each studied parameter such as bearing type, bridge location and hazard level on the structural responses. Results show that elevating bridges with the studied steel pedestals decreases longitudinal and transverse deck displacements, longitudinal shear and moment in columns, cap beam moment and pounding force. In the transverse direction, elevating the bridge leads to an increase in the abutment force. Also, results show that the studied tall steel pedestals are more effective than the studied short steel pedestal in decreasing longitudinal shear and moment in columns and decreasing transverse deck displacements while offering a height advantage. A study of the stability of the pedestals in this paper shows that the three types of studied pedestals may become unstable in earthquakes, thereby serving as a means to help determine where the installation of steel pedestals would not seem detrimental.     

流变模型在隧道监控数据处理中的应用

以某工程为例,采用岩石流变学中广泛应用的开尔文模型和广义开尔文模型对隧道围岩变形数据进行了回归分析和预测。结果表明,开尔文模型拟合精度较差,而广义开尔文模型与实测数据较为吻合,误差较小,能够更加准确的反映隧道围岩变形量与时间之间的关系,这对于准确预测围岩变形量和确定二次衬砌的时间具有一定的参考价值。