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CONTENTS
Volume 3, Number 3, July 2007
 

Abstract
Nonlinear time-domain system identification (SI) algorithm is proposed to assess damage in a shear building by synchronously estimating time-varying stiffness and damping parameters using measured acceleration data. Mass properties have been assumed as the a priori known information. Viscous damping was utilized for the current research. To chase possible nonlinear dynamic behavior under severe vibration, an incremental governing equation of vibrational motion has been utilized. Stiffness and damping parameters are estimated at each time step by minimizing the response error between measured and computed acceleration increments at the measured degrees-of-freedom. To solve a nonlinear constrained optimization problem for optimal structural parameters, sensitivities of acceleration increment were formulated with respect to stiffness and damping parameters, respectively. Incremental state vectors of vibrational motion were computed numerically by Newmark-b method. No model is pre-defined in the proposed algorithm for recovering the nonlinear response. A time-window scheme together with Monte Carlo iterations was utilized to estimate parameters with noise polluted sparse measured acceleration. A moving average scheme was applied to estimate the time-varying trend of structural parameters in all the examples. To examine the proposed SI algorithm, simulation studies were carried out intensively with sample shear buildings under earthquake excitations. In addition, the algorithm was applied to assess damage with laboratory test data obtained from free vibration on a three-story shear building model.

Key Words
nonlinear time-domain SI; incomplete measurement; noise; time-window; moving average.

Address
Soobong Shin; Department of Civil Engineering, Inha University, Incheon, Korea
Seong Ho Oh; Department of Civil Engineering, Dong-A University, Busan, Korea

Abstract
In this paper, an improved GA-based damage detection algorithm using a set of combined modal features is proposed. Firstly, a new GA-based damage detection algorithm is formulated for beam-type structures. A schematic of the GA-based damage detection algorithm is designed and objective functions using several modal features are selected for the algorithm. Secondly, experimental modal tests are performed on free-free beams. Modal features such as natural frequency, mode shape, and modal strain energy are experimentally measured before and after damage in the test beams. Finally, damage detection exercises are performed on the test beam to evaluate the feasibility of the proposed method. Experimental results show that the damage detection is the most accurate when frequency changes combined with modal strain-energy changes are used as the modal features for the proposed method.

Key Words
genetic algorithm; damage detection; vibration-based; natural frequency; modal strain-energy; free-free beam.

Address
Jeong-Tae Kim and Jae-Hyung Park; Department of Ocean Engineering, Pukyong National University, Busan, Korea
Han-Sam Yoon; Research Center for Ocean Industrial Development, Pukyong National University, Busan, Korea
Jin-Hak Yi;Korea Ocean Research & Development Institute, Ansan, Korea

Abstract
The load carrying capacity of a bridge needs to be properly assessed to operate the bridge safely and maintain it efficiently. For the evaluation of load carrying capacity considering the current state of a bridge, static and quasi-static loading tests with weight-controlled heavy trucks have been conventionally utilized. In these tests, the deflection (or strain) of the structural members loaded by the controlled vehicles are measured and analyzed. Using the measured data, deflection (or strain) correction factor and impact correction factor are calculated. These correction factors are used in the enhancement of the load carrying capacity of a bridge, reflecting the real state of a bridge. However, full or partial control of the traffic during the tests and difficulties during the installment of displacement transducers or strain gauges may cause not only inconvenience to the traffic but also the increase of the logistics cost and time. To overcome these difficulties, an alternative method is proposed using an excited response part of full measured ambient acceleration data by ordinary traffic on a bridge without traffic control. Based on the modal properties extracted from the ambient vibration data, the initial finite element (FE) model of a bridge can be updated to represent the current real state of a bridge. Using the updated FE model, the deflection of a bridge akin to the real value can be easily obtained without measuring the real deflection. Impact factors are obtained from pseudo-deflection, which is obtained by double-integration of the acceleration data with removal of the linear components on the acceleration data. For validation, a series of tests were carried out on a steel plate-girder bridge of an expressway in Korea in four different seasons, and the evaluated load carrying capacities of the bridge by the proposed method are compared with the result obtained by the conventional load test method.

Key Words
load carrying capacity; ambient vibration test; modal identification; model updating; deflection correction factor; impact factor; steel plate girder bridge.

Address
Jin-Hak Yi; Coastal Engineering Research Department, Korea Ocean Research and Development Institute,
Sa-2-Dong, Ansan, Gyeonggi, 426-744, Korea
Soojin Cho, Ki-Young Koo, and Chung-Bang Yun; Dept. of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology,
Guseong-Dong, Yuseong-Gu, Daejeon, 305-701, Korea
Jeong-Tae Kim; Dept. of Ocean Engineering, Pukyong National University, Busan, 608-737, Korea
Chang-Geun Lee and Won-Tae Lee; Structural Division, Korea Highway Corporation, Hwasung, Gyeonggi, Korea

Abstract
Smart sensors densely distributed over structures can provide rich information for structural monitoring using their onboard wireless communication and computational capabilities. However, issues such as time synchronization error, data loss, and dealing with large amounts of harvested data have limited the implementation of full-fledged systems. Limited network resources (e.g. battery power, storage space, bandwidth, etc.) make these issues quite challenging. This paper first investigates the effects of time synchronization error and data loss, aiming to clarify requirements on synchronization accuracy and communication reliability in SHM applications. Coordinated computing is then examined as a way to manage large amounts of data.

Key Words
coordinated computing; data compression; distributed computing strategy; smart sensors; time synchronization; data loss; data aggregation.

Address
T. Nagayama; Department of Civil Engineering, University of Tokyo, Tokyo 113-8656, Japan
S. H. Sim; Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
Y. Miyamori; Kitami Institute of Technology, Hokkaido 090-8507, Japan
B. F. Spencer, Jr.; Department of Civil Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

Abstract
Structural control technologies have attracted great interest from the earthquake engineering community over the last few decades as an effective method of reducing undesired structural responses. Traditional structural control systems employ large quantities of cables to connect structural sensors, actuators, and controllers into one integrated system. To reduce the high-costs associated with labor-intensive installations, wireless communication can serve as an alternative real-time communication link between the nodes of a control system. A prototype wireless structural sensing and control system has been physically implemented and its performance verified in large-scale shake table tests. This paper introduces the design of this prototype system and investigates the feasibility of employing decentralized and partially decentralized control strategies to mitigate the challenge of communication latencies associated with wireless sensor networks. Closed-loop feedback control algorithms are embedded within the wireless sensor prototypes allowing them to serve as controllers in the control system. To validate the embedment of control algorithms, a 3-story half-scale steel structure is employed with magnetorheological (MR) dampers installed on each floor. Both numerical simulation and experimental results show that decentralized control solutions can be very effective in attaining the optimal performance of the wireless control system.

Key Words
structural control; wireless communication; embedded computing; decentralized control; velocity feedback control.

Address
Yang Wang; Department of Civil and Environmental Engineering, Stanford Univ., Stanford, CA 94305, USA
R. Andrew Swartz and Jerome P. Lynch; Department of Civil and Environmental Engineering, Univ. of Michigan, Ann Arbor, MI 48109, USA
Kincho H. Law; Department of Civil and Environmental Engineering, Stanford Univ., Stanford, CA 94305, USA
Kung-Chun Lu and Chin-Hsiung Loh; Department of Civil Engineering, National Taiwan Univ., Taipei, Taiwan

Abstract
A good understanding of normal modal variability of civil structures due to varying environmental conditions such as temperature and wind is important for reliable performance of vibration-based damage detection methods. This paper addresses the quantification of wind-induced modal variability of a cable-stayed bridge making use of one-year monitoring data. In order to discriminate the wind-induced modal variability from the temperature-induced modal variability, the one-year monitoring data are divided into two sets: the first set includes the data obtained under weak wind conditions (hourly-average wind speed less than 2 m/s) during all four seasons, and the second set includes the data obtained under both weak and strong (typhoon) wind conditions during the summer only. The measured modal frequencies and temperatures of the bridge obtained from the first set of data are used to formulate temperature-frequency correlation models by means of artificial neural network technique. Before the second set of data is utilized to quantify the wind-induced modal variability, the effect of temperature on the measured modal frequencies is first eliminated by normalizing these modal frequencies to a reference temperature with the use of the temperature-frequency correlation models. Then the wind-induced modal variability is quantitatively evaluated by correlating the normalized modal frequencies for each mode with the wind speed measurement data. It is revealed that in contrast to the dependence of modal frequencies on temperature, there is no explicit correlation between the modal frequencies and wind intensity. For most of the measured modes, the modal frequencies exhibit a slightly increasing trend with the increase of wind speed in statistical sense. The relative variation of the modal frequencies arising from wind effect (with the maximum hourly-average wind speed up to 17.6 m/s) is estimated to range from 1.61% to 7.87% for the measured 8 modes of the bridge, being notably less than the modal variability caused by temperature effect.

Key Words
cable-stayed bridge; structural health monitoring; modal variability; wind effect; temperature effect; correlation analysis.

Address
Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

Abstract
Parametric density concept is proposed for a long-range pipeline health monitoring. This concept is designed to obtain the attenuation of ultrasonic guided waves propagating in underwater pipelines without complicated calculation of attenuation dispersion curves. For the study, three different pipe materials such as aluminum, cast iron, and steel are considered, ten different transporting fluids are assumed, and four different geometric pipe dimensions are adopted. It is shown that the attenuation values based on the parametric density concept reasonably match with the attenuation values obtained from dispersion curves; hence, its efficiency is proved. With this concept, field engineers or inspectors associated with long-range pipeline health monitoring would take the advantage of easier capturing wave attenuation value, which is a critical variable to decide sensor location or sensors interval.

Key Words
pipelines monitoring; guided waves; attenuation; parametric density.

Address
Won-Bae Na; Department of Ocean Engineering, Pukyong National University, Busan, Korea
Han-Sam Yoon; Research Center for Ocean Industrial Development, Pukyong National University, Busan, Korea

Abstract
For structural health monitoring (SHM) of civil infrastructures, displacement is a good descriptor of the structural behavior under all the potential disturbances. However, it is not easy to measure displacement of civil infrastructures, since the conventional sensors need a reference point, and inaccessibility to the reference point is sometimes caused by the geographic conditions, such as a highway or river under a bridge, which makes installation of measuring devices time-consuming and costly, if not impossible. To resolve this issue, a vision-based real-time displacement measurement system using digital image processing techniques is developed. The effectiveness of the proposed system was verified by comparing the load carrying capacities of a steel-plate girder bridge obtained from the conventional sensor and the present system. Further, to simultaneously measure multiple points, a synchronized vision-based system is developed using master/slave system with wireless data communication. For the purpose of verification, the measured displacement by a synchronized vision-based system was compared with the data measured by conventional contact-type sensors, linear variable differential transformers (LVDT) from a laboratory test.

Key Words
displacement measurement; vision-based system; digital image processing; load carrying capacity; time synchronization; multi-point measurement.

Address
Jong Jae Lee; Department of Civil & Environmental Engineering, Korea Advanced Institute of Science and Technology, Guseong-dong, Yuseong-gu, Daejeon, Korea
Yoshio Fukuda and Masanobu Shinozuka; Department of Civil & Environmental Engineering, University of California Irvine, Irvine, CA, 92697, USA
Soojin Cho and Chung-Bang Yun; Department of Civil & Environmental Engineering, Korea Advanced Institute of Science and Technology, Guseong-dong, Yuseong-gu, Daejeon, Korea

Abstract
The effectiveness of the newly developed smart passive control system employing a magnetorheological (MR) damper and an electromagnetic induction (EMI) part for seismic protection of base isolated structures is numerically investigated. An EMI part in the system consists of a permanent magnet and a coil, which changes the kinetic energy of the deformation of an MR damper into the electric energy (i.e. the induced current) according to the Faraday

Key Words
MR damper; electromagnetic induction; smart passive control; base isolation system; seismic protection.

Address
Hyung-Jo Jung and Kang-Min Choi; Department of Civil and Environmental Engineering, KAIST, Daejeon 305-701, Korea
Kyu-Sik Park; Department of Civil and Environmental Engineering, University of Illinoise at Urbana Champaign, IL 61801, USA
Sang-Won Cho; The Boundary Layer Wind Tunnel Lab., The University of Western Ontario, Ontario, Canada


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