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CONTENTS
Volume 16, Number 2, August 2015
 

Abstract
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Key Words
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Address
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Abstract
Optimal sensor placement (OSP) is a critical issue in construction and implementation of a sophisticated structural health monitoring (SHM) system. The uncertainties in the identified structural parameters based on the measured data may dramatically reduce the reliability of the condition evaluation results. In this paper, the information entropy, which provides an uncertainty metric for the identified structural parameters, is adopted as the performance measure for a sensor configuration, and the OSP problem is formulated as the multi-objective optimization problem of extracting the Pareto optimal sensor configurations that simultaneously minimize the appropriately defined information entropy indices. The nondirective movement glowworm swarm optimization (NMGSO) algorithm (based on the basic glowworm swarm optimization (GSO) algorithm) is proposed for identifying the effective Pareto optimal sensor configurations. The one-dimensional binary coding system is introduced to code the glowworms instead of the real vector coding method. The Hamming distance is employed to describe the divergence of different glowworms. The luciferin level of the glowworm is defined as a function of the rank value (RV) and the crowding distance (CD), which are deduced by non-dominated sorting. In addition, nondirective movement is developed to relocate the glowworms. A numerical simulation of a long-span suspension bridge is performed to demonstrate the effectiveness of the NMGSO algorithm. The results indicate that the NMGSO algorithm is capable of capturing the Pareto optimal sensor configurations with high accuracy and efficiency.

Key Words
structural health monitoring; optimal sensor placement; glowworm swarm optimization algorithm; information entropy; multi-objective optimization

Address
Guang-Dong Zhou, Hong-Nan Li: College of Civil and Transportation Engineering, Hohai University, Nanjing 210098, China
Ting-Hua Yi: School of Civil Engineering, Dalian University of Technology, Dalian 116024, China
Huan Zhang: State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China

Abstract
Microwave remote sensing is probably the most recent experimental technique suitable to the non-contact measurement of deflections on large structures, in static or dynamic conditions. In the first part of the paper, the main techniques adopted in microwave remote sensing are described, so that advantages and potential issues of these techniques are presented and discussed. Subsequently, the paper addresses the application of the radar technology to the measurement of the vibration response on the stay cables of two cable-stayed bridges. The dynamic tests were performed in operational conditions (i.e. with the excitation being mainly provided by micro-tremors, wind and traffic) and the maximum deflections of the cables were generally lower than 5.0 mm. The investigation clearly highlights: (a) the safe and simple use of the radar on site and its effectiveness to simultaneously measure the dynamic response of all the stay cables of an array; (b) the negligible effects of the typical issues and uncertainties that might affect the radar measurements; (c) the accuracy of the results provided by the microwave remote sensing in terms of natural frequencies and tension forces of the stay cables; (d) the suitability of microwave interferometry to the repeated application within Structural Health Monitoring programmes.

Key Words
dynamic testing; microwave remote sensing; radar; stay cable; structural health monitoring

Address
Carmelo Gentile: Politecnico di Milano, Department of Architecture, Built environment and Construction engineering (ABC), P.za Leonardo da Vinci 32, 20133 Milan, Italy
Alessandro Cabboi: University of Cagliari, Department of Civil and Environmental Engineering and Architecture, Piazza d

Abstract
Bolted joint connection is the most commonly used connection element in structures and devices. The loosening due to external dynamic loads cannot be observed and measured easily and may cause catastrophic loss especially in an extreme requirement and/or environment. In this paper, an innovative Real-time Cross-Correlation Method (RCCM) for monitoring of the bolted joint loosening was proposed. We apply time reversal process on stress wave propagation to obtain correlation signal. The correlation signal\' s peak amplitude represents the cross-correlation between the loosening state and the baseline working state; therefore, it can detect the state of loosening. Since the bolt states are uncorrelated with noise, the peak amplitude will not be affected by noise and disturbance while it increases SNR level and increases the measured signals\' reliability. The correlation process is carried out online through physical wave propagation without any other post offline complicated analyses and calculations. We implemented the proposed RCCM on a single bolt/nut joint experimental device to quantitatively detect the loosening states successfully. After that we implemented the proposed method on a real large structure (reaction wall) with multiple bolted joint connections. Loosening indexes were built for both experiments to indicate the loosening states. Finally, we demonstrated the proposed method\'s great anti-noise and/or disturbance ability. In the instrumentation, we simply mounted Lead Zirconium Titanate (PZT) patches on the device/structure surface without any modifications of the bolted connection. The low-cost PZTs used as actuators and sensors for active sensing are easily extended to a sensing network for large scale bolted joint network monitoring.

Key Words
Real-time Cross Correlation Method (RCCM); bolted joint loosening detection; PZT; time reversal; stress wave propagation; active sensing

Address
Jiabiao Ruan, Gangbing Song: Department of Mechanical Engineering, University of Houston, Houston, TX, USA
Zhimin Zhang: Institute of Applied Physics, University of Electronic Science and Technology of China, Chengdu,
Sichuan, P.R. China
Tao Wang, Yourong Li: College of Mechanical Engineering and Automation, Wuhan University of Science and Technology, Wuhan,
Hubei, 430081, P.R. China

Abstract
In recent years the monitoring of structural behavior through acquisition of vibrational data has become common practice. In addition, recent advances in sensor development have made the collection of diverse dynamic information feasible. Other than the commonly collected acceleration information, Global Position System (GPS) receivers and non-contact, optical techniques have also allowed for the synchronous collection of highly accurate displacement data. The fusion of this heterogeneous information is crucial for the successful monitoring and control of structural systems especially when aiming at real-time estimation. This task is not a straightforward one as measurements are inevitably corrupted with some percentage of noise, often leading to imprecise estimation. Quite commonly, the presence of noise in acceleration signals results in drifting estimates of displacement states, as a result of numerical integration. In this study, a new approach based on a time domain identification method, namely the Unscented Kalman Filter (UKF), is proposed for correcting the \"drift effect\" in displacement or rotation estimates in an online manner, i.e., on the fly as data is attained. The method relies on the introduction of artificial white noise (WN) observations into the filter equations, which is shown to achieve an online correction of the drift issue, thus yielding highly accurate motion data. The proposed approach is demonstrated for two cases; firstly, the illustrative example of a single degree of freedom linear oscillator is examined, where availability of acceleration measurements is exclusively assumed. Secondly, a field inspired implementation is presented for the torsional identification of a tall tower structure, where acceleration measurements are obtained at a high sampling rate and non-collocated GPS displacement measurements are assumed available at a lower sampling rate. A multi-rate Kalman Filter is incorporated into the analysis in order to successfully fuse data sampled at different rates.

Key Words
Structural Health Monitoring (SHM); online system identification; heterogeneous data fusion; artificial white noise observations; Unscented Kalman Filter (UKF); multi-rate filter

Address
Eleni N. Chatzi: Institute of Structural Engineering, ETH Zurich, Wolfgang-Pauli-Strasse 15, CH-8093, Switzerland
Clemente Fuggini: Industrial Innovation Division, D\' Appolonia S.p.A., Via San Nazaro 19, 16145 Genova, Italy

Abstract
As commonly known, sensor errors and faulty signals may potentially lead structures in vibration to catastrophic failures. This paper presents a new approach to deal with sensor errors/faults in vibration control of structures by using the Fault detection and isolation (FDI) technique. To demonstrate the effectiveness of the approach, a space truss structure with semi-active devices such as Magneto-Rheological (MR) damper is used as an example. To address the problem, a Linear Matrix Inequality (LMI) based fixed-order H FDI filter is introduced and designed. Modeling errors are treated as uncertainties in the FDI filter design to verify the robustness of the proposed FDI filter. Furthermore, an innovative Fuzzy Fault Tolerant Controller (FFTC) has been developed for this space truss structure model to preserve the pre-specified performance in the presence of sensor errors or faults. Simulation results have demonstrated that the proposed FDI filter is capable of detecting and isolating sensor errors/faults and actuator faults e.g., accelerometers and MR dampers, and the proposed FFTC can maintain the structural vibration suppression in faulty conditions.

Key Words
space truss structure; sensor errors; fault detection and isolation; fault tolerant control

Address
Han Wang: Department of Mechanical Engineering, University of Houston, TX, USA
Luyu Li, Gangbing Song: Department of Mechanical Engineering, University of Houston, TX, USA; School of Civil Engineering, Dalian Institute of Technology, China
James B. Dabney MThomas L. Harman: School of Science and Computer Engineering, University of Houston - Clear Lake, TX, USA

Abstract
GPS and strong-motion sensors are broadly used for the monitoring of structural health and Earth surface motions, focusing on response of structures, earthquake characterization and rupture modeling. Several studies have shown the consistency of the two data sets within at certain frequency (e.g., 0.03 f 0.2Hz). The compatibility of Precise Point Positioning (PPP) GPS and strong-motion data was assessed by comparing their respective displacement waveforms for several frequency bands (f 0.4Hz). For this purpose, there are used GPS and strong-motion records of the Mw9.0 Tohoku 2011 earthquake at 23 very close spaced sites and conclude that the agreement between the two datasets depends on the frequency of the excitation, the direction of the excitation signal and the distance from the excitation source.

Key Words
GPS; strong-motion; Tohoku earthquake; filter; time lag; displacement; coherence; frequency bands; consistency

Address
Panos Psimoulis: Nottingham Geospatial Institute, The University of Nottingham, Nottingham NG7 2TU, UK; Geodesy and Geodynamics Lab., Geodesy and Photogrammetry Institute, ETH Zurich, Zurich 8093, Switzerland
Nicolas Houlié: Geodesy and Geodynamics Lab., Geodesy and Photogrammetry Institute, ETH Zurich, Zurich 8093, Switzerland; Seismology and Geodynamics, Institute of Geophysics, ETH Zurich, Zurich 8092, Switzerland
Michael Meindl, Markus Rothacher: Geodesy and Geodynamics Lab., Geodesy and Photogrammetry Institute, ETH Zurich,
Zurich 8093, Switzerland

Abstract
This paper aims to conduct the reliability-based assessment of the welded joint in the orthotropic steel bridge deck by use of a mesh-insensitive structural stress (MISS) method, which is an effective numerical procedure to determine the reliable stress distribution adjacent to the weld toe. Both the solid element model and the shell element model are first established to investigate the sensitivity of the element size and the element type in calculating the structural stress under different loading scenarios. In order to achieve realistic condition assessment of the welded joint, the probabilistic approach based on the structural reliability theory is adopted to derive the reliability index and the failure probability by taking into account the uncertainties inherent in the material properties and load conditions. The limit state function is formulated in terms of the structural resistance of the material and the load effect which is described by the structural stress obtained by the MISS method. The reliability index is computed by use of the first-order reliability method (FORM), and compared with a target reliability index to facilitate the safety assessment. The results achieved from this study reveal that the calculation of the structural stress using the MISS method is insensitive to the element size and the element type, and the obtained structural stress results serve as a reliable basis for structural reliability analysis.

Key Words
orthotropic steel bridge deck; welded joints; hot spot stress; finite element analysis; reliability index; failure probability

Address
X.W. Ye: Department of Civil Engineering, Zhejiang University, Hangzhou 310058, China
Ting-Hua Yi: School of Civil Engineering, Dalian University of Technology, Dalian 116023, China
C. Wen, Y.H. Su: School of Civil Engineering, Lanzhou University of Technology, Lanzhou 730050, China

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