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Volume 11, Number 4, April 2013

Effective monitoring, reliable data analysis, and rational data interpretations are challenges for engineers who are specialized in bridge health monitoring. This paper demonstrates how to use the Global Positioning System (GPS) and accelerometer data to accurately extract static and quasi-static displacements of the bridge induced by ambient effects. To eliminate the disadvantages of the two separate units, based on the characteristics of the bias terms derived from the GPS and accelerometer respectively, a wavelet based multi-step filtering method by combining the merits of the continuous wavelet transform (CWT) with the discrete stationary wavelet transform (SWT) is proposed so as to address the GPS deformation monitoring application more efficiently. The field measurements are carried out on an existing suspension bridge under the normal operation without any traffic interference. Experimental results showed that the frequencies and absolute displacements of the bridge can be accurate extracted by the proposed method. The integration of GPS and accelerometer can be used as a reliable tool to characterize the dynamic behavior of large structures such as suspension bridges undergoing environmental loads.

Key Words
suspension bridge; deformation monitoring; global positioning system; wavelet transform

Ting-Hua Yi :School of Civil Engineering, Faculty of Infrastructure Engineering, Dalian University of Technology,
Dalian 116023, China, State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China, State Key Laboratory of Subtropical Building Science, South China University of Technology,
Guangzhou 510641, China
Hong-Nan Li: School of Civil Engineering, Faculty of Infrastructure Engineering, Dalian University of Technology,
Dalian 116023, China
Ming Gu : State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China

replace the conventional NDE techniques with the smart material based SHM techniques, employing smart materials such as piezoelectric materials. For instance, the electromechanical impedance (EMI) technique employing piezo-impedance (lead zirconate titanate, PZT) transducer is known for its sensitivity in detecting local damage. For practical applications, various external factors such as fluctuations of temperature and loading, affecting the effectiveness of the EMI technique ought to be understood and compensated. This paper aims at investigating the damage monitoring capability of EMI technique in the presence of axial stress with fixed boundary condition. A compensation technique using effective frequency shift (EFS) by cross-correlation analysis was incorporated to compensate the effect of loading and boundary stiffening. Experimental tests were conducted by inducing damages on lab-sized aluminium beams in the presence of tensile and compressive forces. Two types of damages, crack propagation and bolts loosening were simulated. With EFS for compensation, both cross-correlation coefficient (CC) index and reduction in peak frequency were found to be efficient in characterizing damages in the presence of varying axial loading.

Key Words
electromechanical impedance (EMI); piezoceramics (PZT); axial load; structural health monitoring (SHM); smart materials; effective frequency shift (EFS); cross-correlation coefficient (CC)

Yee Yan Lim : Civil Engineering Program, School of Engineering & Information Technology, University Malaysia Sabah, Locked Bag No. 2073, 88999 Kota Kinabalu, Sabah, Malaysia
Chee Kiong Soh : Division of Structures and Mechanics, School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 63979

Monitoring systems currently applied to concrete bridges include strain gauges, inclinometers, accelerometers and displacement transducers. In general, vertical displacements are one of the parameters that more often need to be assessed because their information reflects the overall response of the bridge span. However, the implementation of systems to continuously and directly observe vertical displacements is known to be difficult. On the other hand, strain gauges and inclinometers are easier to install, but their measurements provide no more than indirect information regarding the bridge deflection. In this context, taking advantage of the information collected through strain gauges and inclinometers, and the processing capabilities of current computers, a procedure to evaluate bridge girder deflections based on polynomial functions is presented. The procedure has been implemented in an existing software system – MENSUSMONITOR –, improving the flexibility in the data handling and enabling faster data processing by means of real time visualization capabilities. Benefiting from these features, a comprehensive analysis aiming at assessing the suitability of polynomial functions as an approximate solution for deflection curves, is presented. The effect of boundary conditions and the influence of the order of the polynomial functions on the accuracy of results are discussed. Some recommendations for further instrumentation plans are provided based on the results of the present analysis. This work is supported throughout by monitoring data collected from a laboratory beam model and two full-scale bridges.

Key Words
bridge monitoring; deflection evaluation; strain gauges; inclinometers; polynomial fitting

Helder Sousa, Filipe Cavadas, Abel Henriques and Joaquim Figueiras : 1LABEST, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal
Joao Bento : Department of Civil Engineering and Architecture, Instituto Superior Tecnico, 1049-001 Lisboa, Portugal

This paper deals with the damage assessment for isolators of base-isolated building systems considering the torsion-coupling (TC) effect by establishing damage indices. The damage indices can indicate the reduction in lateral stiffness of the isolator story as explicit formulas in terms of modal parameters. In addition, the damage location, expressed in terms of the estimated damage index and eccentricities before and after damage, is also presented. Numerical analysis shows that the proposed algorithms are applicable for general base-isolated multi-story TC buildings. A procedure from the analysis of seismic response to the implementation of damage indices is demonstrated by using a numerical case. A system identification technique is employed to extract modal parameters from seismic responses of a building. Results show that the proposed indices are capable of detecting the occurrence of damage and preliminarily estimating the location of damaged isolator.

Key Words
base isolation; torsion-coupling effect; damage detection; damage locating

Jer-Fu Wang : 1921 Earthquake Museum of Taiwan, National Museum of Natural Science, Taichung City, Taiwan 413, R.O.C.
Ming-Chih Huang : Air Force Institute of Technology, Kaohsiung City, Taiwan 820, R.O.C.
Chi-Chang Lin: Department of Civil Engineering, National Chung Hsing University, Taichung City, Taiwan 402, R.O.C.
Tzu-Kang : Department of Civil Engineering, National Chiao Tung University, Hsinchu City, Taiwan 300, R.O.C.

Due to its easy operation and wide applicability, the ambient vibration method is commonly adopted to determine the cable force by first identifying the cable frequencies from the vibration signals. With given vibration length and flexural rigidity, an analytical or empirical formula is then used with these cable frequencies to calculate the cable force. It is, however, usually difficult to decide the two required parameters, especially the vibration length due to uncertain boundary constraints. To tackle this problem, a new concept of combining the modal frequencies and mode shape ratios is fully explored in this study for developing an accurate method merely based on ambient vibration measurements. A simply supported beam model with an axial tension is adopted and the effective vibration length of cable is then independently determined based on the mode shape ratios identified from the synchronized measurements. With the effective vibration length obtained and the identified modal frequencies, the cable force and flexural rigidity can then be solved using simple linear regression techniques. The feasibility and accuracy of the proposed method is extensively verified with demonstrative numerical examples and actual applications to different cable-stayed bridges. Furthermore, several important issues in engineering practice such as the number of sensors and selection of modes are also thoroughly investigated.

Key Words
stay cable force; ambient vibration method; effective vibration length; multiple measurements; mode shape ratio; modal frequency; flexural rigidity

Chien-Chou Chen, Wen-Hwa Wu, Chin-Hui Huang and Gwolong Lai : Department of Construction Engineering, National Yunlin University of Science and Technology, Yunlin 640,

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