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CONTENTS
Volume 17, Number 3, March 2004
 

Abstract
A review of current procedures being used in engineering practice to analyze the response of structure/foundation systems subjected separately to different types of dynamic excitation, such as earthquake, sea-wave action, wind, or moving wheel loads, is presented. Separate formulations are given for analyzing systems in the time and frequency domains. Both deterministic and stochastic forms of excitation are treated. A distinction is made between demand and capacity analyses.

Key Words
dynamic analysis; structure/foundation system; time-domain; frequency-domain; nonlinear analysis; performance evaluation.

Address
International Civil Engineering Consultants, Inc., U.S.A.

Abstract
In this paper, load monitoring technologies using U-shape Magnetoelastic (EM or ME) sensors have been exploited systemically for the first time. The steel rod to be tested is the Japan 7 mm piano steel rod. The load dependence of the magnetic properties of the piano steel rod was manifested. Two experimental designs of U-shape magnetoelastic sensors were introduced, one with double pick-up concentric coils wound on the rod to be tested, the other with pick-up coil on one yoke foot. The former design is used to derive the correlation of the relative permeability with elastic tension, while the latter is aimed to reflect the stress induced magnetic flux variation along the magnetic circuit. Magnetostatic simulations provide interpretations for the yoke foot sensing technology. Tests with double pick-up coils indicate that under proper working points (primary voltages), the relative permeability varies linearly with the axial load for the Japan 7 mm piano steel rod. Tests with pick-up coil on the yoke foot show that the integrated sensing voltage changes quadratically with the load, and error is more acceptable when the working point is high enough.

Key Words
magnetoelastic sensors; concentric coils; magnetoelastic simulation; permeability; magnetic flux.

Address
Department of Civil and Materials Engineering, The University of Illinois at Chicago, Chicago, IL 60607, U.S.A.

Abstract
Magnetostrictive materials are routinely employed as actuator and sensor elements in a wide variety of noise and vibration control problems. In infrastructural applications, other technologies such as hydraulic actuation, piezoelectric materials and more recently, magnetorheological fluids, are being favored for actuation and sensing purposes. These technologies have reached a degree of technical maturity and in some cases, cost effectiveness, which justify their broad use in infrastructural applications. Advanced civil structures present new challenges in the areas of condition monitoring and repair, reliability, and high-authority actuation which motivate the need to explore new methods and materials recently developed in the areas of materials science and transducer design. This paper provides an overview of a class of materials that because of the large force, displacement, and energy conversion effciency that it can provide is being considered in a growing number of quasistatic and dynamic applications. Since magnetostriction involves a bidirectional energy exchange between magnetic and elastic states, magnetostrictive materials provide mechanisms both for actuation and sensing. This paper provides an overview of materials, methods and applications with the goal to inspire novel solutions based on magnetostrictive materials for the design and control of advanced infrastructural systems.

Key Words
magnetostrictive materials; actuators; sensors; magnetic activation; transducer design.

Address
Department of Mechanical Engineering, The Ohio State University, Suite 224, 650 Ackerman Road, Columbus, OH 43202, USA

Abstract
A new sensor system is proposed for measuring damage indexes. The damage index is a physical value that is well correlated to a critical damage in a device or a structure. The mechanism proposed here utilizes elastic buckling of a thin wire and does not require any external power supply for memorizing the index. The mechanisms to detect peak strain, peak displacement, peak acceleration and cumulative deformation as examples of damage indexes are presented. Furthermore, passive and active wireless data retrieval mechanisms using electromagnetic induction are proposed. The passive wireless system is achieved by forming a closed LC circuit to oscillate at its natural frequency. The active wireless sensor can transmit the data much further than the passive system at the sacrifice of slightly complicated electric circuit for the sensor. For wireless data retrieval, no wire is needed for the sensor to supply electrical power. For the active system, electrical power is supplied to the sensor by radio waves emitted from the retrieval system. Thus, external power supply is only needed for the retrieval system when the retrieval becomes necessary. Theoretical and experimental studies to show excellent performance of the proposed sensor are presented. Finally, a prototype damage index sensor installed into a 7 storey base-isolated building is explained.

Key Words
damage index; structural health monitoring; mechanical memory; wireless.

Address
Department of System Design Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan

Abstract
In order to extend the lifetime of buildings and civil infrastructure, patch type fibrous composite retrofitting materials are widely used. Retrofitted concrete columns and beams gain stiffness and strength, but lose toughness and show brittle failure. Usually, the cracks in concrete structures are visible to the naked eye and the status of the structure in the life cycle is estimated through visual inspections. After retrofitting of the structure, crack visibility is blocked by retrofitted composite materials. Therefore, structural monitoring after retrofitting is indispensable and self diagnosis method with optical fiber sensors is very useful. In this paper, we try to detect the peel out effect and find the strain difference between the main structure and retrofitting patch material when they separate from each other. In the experiment, two fiber optic Bragg grating sensors are applied to the main concrete structure and the patching material separately at the same position. The sensors show coincident behaviors at the initial loading, but different behaviors after a certain load. The test results show the possibility of optical fiber sensor monitoring of beam structures retrofitted by the composite patches.

Key Words
fiber optic sensors; carbon sheet; repair; self diagnosis; delamination and self monitoring.

Address
Ki-Soo Kim; Graduate School of Venture, Hoseo University, Mt.29-1, Sechul-Ri, Baebang-Myun, Asan City, Chung-nam, 336-795, South Korea
Sechul-Ri; Baebang-Myun, Asan City, Chung-nam, 336-795, South Korea
Young-Goo Kang and Hong Kim; Graduate School of Venture, Hoseo University, Mt.29-1, Sechul-Ri, Baebang-Myun, Asan City, Chung-nam, 336-795, South Korea
Chul Chung; ICES Co. Ltd., Graduate School of Venture, Hoseo University, Mt.29-1, Sechul-Ri, Baebang-Myun, Asan City, Chung-nam, 336-795, South Korea
Ho-Joon Lee; Department of Information and Telecommunication, Hoseo University, Mt.29-1, Sechul-Ri, Baebang-Myun, Asan City, Chung-nam, 336-795, South Korea

Abstract
Nonlinear dynamic analysis of a reinforced concrete (RC) frame under earthquake loading is performed in this paper on the basis of a hysteretic moment-curvature relation. Unlike previous analytical moment-curvature relations which take into account the flexural deformation only with the perfect-bond assumption, by introducing an equivalent flexural stiffness, the proposed relation considers the rigid-body-motion due to anchorage slip at the fixed end, which accounts for more than 50% of the total deformation. The advantage of the proposed relation, compared with both the layered section approach and the multi-component model, may be the ease of its application to a complex structure composed of many elements and on the reduction in calculation time and memory space. Describing the structural response more exactly becomes possible through the use of curved unloading and reloading branches inferred from the stress-strain relation of steel and consideration of the pinching effect caused by axial force. Finally, the applicability of the proposed model to the nonlinear dynamic analysis of RC structures is established through correlation studies between analytical and experimental results.

Key Words
RC frame; earthquake loading; anchorage slip; pinching effect; Bauschinger effect; moment-curvature relationship.

Address
Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 373-1 Gusong-dong, Yuseong-gu, Daejeon 305-701, Korea

Abstract
This paper deals with the development of h-version adaptive mesh refinement and recovery strategy using variable-node elements and its application to various engineering field problems with 2D quadrilateral and 3D hexahedral models. The variable-node elements which have variable mid-side nodes on edges or faces are effectively used in overcoming some problems in connecting the different layer patterns of the transition zone between the refined and coarse mesh. A modified recovery technique of gradients adequate for variable-node elements and proper selection of error norms for each engineering field problems are proposed. In the region in which the error is greater than the permissible refinement error, the mesh is locally refined by subdivision. Reversely, in some parts of the domain having the error smaller than the permissible recovery error, the mesh is locally recovered (coarsened) by combination. Hierarchical structures (e.g. quadtrees and octrees) and element-based storage structures are composed to perform this adaptive process of refinement and recovery. Some numerical examples of a 3D heat conduction analysis of the concrete with hydration heat and a 2D flow analysis of vortex shedding show effectiveness and validity of the proposed scheme.

Key Words
adaptive FEA; mesh refinement/recovery; variable-node element; quadrilateral element; hexahedral element.

Address
Chang-Koon Choi; Department of Civil and Environmental Engineering, KAIST, Daejeon 305-701, Korea
Eun-Jin Lee; DPRI, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
Won-Jin Yu; Samsung Engineering & Construction, Sungnam-Si, 463-721, Korea

Abstract
There exists a clear need to monitor the performance of civil structures over their operational lives. Current commercial monitoring systems suffer from various technological and economic limitations that prevent their widespread adoption. The wires used to route measurements from system sensors to the centralized data server represent one of the greatest limitations since they are physically vulnerable and expensive from an installation and maintenance standpoint. In lieu of cables, the introduction of low-cost wireless communications is proposed. The result is the design of a prototype wireless sensing unit that can serve as the fundamental building block of wireless modular monitoring systems (WiMMS). An additional feature of the wireless sensing unit is the incorporation of computational power in the form of state-of-art microcontrollers. The prototype unit is validated with a series of laboratory and field tests. The Alamosa Canyon Bridge is employed to serve as a full-scale benchmark structure to validate the performance of the wireless sensing unit in the field. A traditional cable-based monitoring system is installed in parallel with the wireless sensing units for performance comparison.

Key Words
wireless monitoring; wireless sensing unit; structural health monitoring.

Address
Jerome Peter Lynch; Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI 48109, USA
Kincho H. Law, Anne S. Kiremidjian, and Ed Carryer; The John A. Blume Earthquake Engineering Center, Stanford University, Stanford, CA 94305, USA
Charles R. Farrar, Hoon Sohn, David W. Allen, Brett Nadler and Jeannette R. Wait; Los Alamos National Laboratory, Los Alamos, NM 87545, USA

Abstract
In this paper, a Holder exponent, a measure of the degree to which a signal is differentiable, is presented to detect the presence of a discontinuity and when the discontinuity occurs in a dynamic signal. This discontinuity detection has potential applications to structural health monitoring because discontinuities are often introduced into dynamic response data as a result of certain types of damage. Wavelet transforms are incorporated with the Holder exponent to capture the time varying nature of discontinuities, and a classification procedure is developed to quantify when changes in the Holder exponent are significant. The proposed Holder exponent analysis is applied to various experimental signals to reveal underlying damage causing events from the signals. Signals being analyzed include acceleration response of a mechanical system with a rattling internal part, acceleration signals of a three-story building model with a loosing bolt, and strain records of an in-situ bridge during construction. The experimental results presented in this paper demonstrate that the Holder exponent can be an effective tool for identifying certain types of events that introduce discontinuities into the measured dynamic response data.

Key Words
holder exponent; structural health monitoring; discontinuity detection; wavelet transform.

Address
Weapon Response Group, Engineering Sciences and Applications Division,
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

Abstract
The ambient vibration measurement is a kind of output data-only dynamic testing where the traffics and winds are used as agents responsible for natural or environmental excitation. Therefore an experimental modal analysis procedure for ambient vibration testing will need to base itself on output-only data. The modal analysis involving output-only measurements presents a challenge that requires the use of special modal identification technique, which can deal with very small magnitude of ambient vibration contaminated by noise. Two complementary modal analysis methods are implemented. They are rather simple peak picking (PP) method in frequency domain and more advanced stochastic subspace identification (SSI) method in time domain. This paper presents the application of ambient vibration testing and experimental modal analysis on large civil engineering structures. A 15 storey reinforced concrete shear core building and a concrete filled steel tubular arch bridge have been chosen as two case studies. The results have shown that both techniques can identify the frequencies effectively. The stochastic subspace identification technique can detect frequencies that may possibly be missed by the peak picking method and gives a more reasonable mode shapes in most cases.

Key Words
modal analysis; parameter identification; ambient vibration; spectra; stochastic subspace identification; engineering structures.

Address
Department of Civil Engineering, Fuzhou University, Fuzhou, Fujian Province 350002, P.R. China

Abstract
In this paper, several modal identification techniques for output-only structural systems are extensively investigated. The methods considered are the power spectral method, the frequency domain decomposition method, the Ibrahim time domain method, the eigensystem realization algorithm, and the stochastic subspace identification method. Generally, the power spectral method is most widely used in practical area, however, the other methods may give better estimates particularly for the cases with closed modes and/or with large measurement noise. Example analyses were carried out on typical structural systems under three different loading cases, and the identification performances were examined throught the comparisons between the estimates by various methods.

Key Words
modal identification; output-only information; power spectral method; frequency domain decomposition; Ibrahim time domain; eigensystem realization algorithm; stochastic subspace identification.

Address
Jin-Hak Yi; Smart Infra-Structure Technology Center, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea
Chung-Bang Yun; Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea

Abstract
One of the uncertain damage parameters to jeopardize the safety of existing PSC bridges is the loss of the prestress force. A substantial prestress-loss can lead to severe problems in the serviceability and safety of the PSC bridges. In this paper, a nondestructive method to detect prestress-loss in beam-type PSC bridges using a few natural frequencies is presented. An analytical model is formulated to estimate changes in natural frequencies of the PSC bridges under various prestress forces. Also, an inverse-solution algorithm is proposed to detect the prestress-loss by measuring the changes in natural frequencies. The feasibility of the proposed approach is evaluated using PSC beams for which a few natural frequencies were experimentally measured for a set of prestress-loss cases. Numerical models of two-span continuous PSC beams are also examined to verify that the proposed algorithm works on more complicated cases.

Key Words
damage identification; structural safety; prestressed concrete bridge; prestress-loss; modal test; natural frequency.

Address
Jeong-Tae Kim; Smart Structure Engineering Laboratory, Department of Ocean Engineering, Pukyong National University, Busan 608-737, Korea
Chung-Bang Yun; Department of Civil and Environmental Engineering, KAIST, Daejon 305-701, Korea
Yeon-Sun Ryu and Hyun-Man Cho; Department of Ocean Engineering, Pukyong National University, Busan 608-737, Korea

Abstract
This paper presents an efficient algorithm for the estimation of damage location and severity in bridge structures using Probabilistic Neural Network (PNN). Generally, the Back Propagation Neural Network (BPNN)-based damage detection methods need a lot of training patterns for neural network learning process and the optimum architecture of a BPNN is selected by trial and error. In this paper, the PNN instead of the conventional BPNN is used as a pattern classifier. The modal properties of damaged structure are somewhat different from those of undamaged one. The basic idea of proposed algorithm is that the PNN classifies a test pattern which consists of the modal characteristics from damaged structure, how close it is to each training pattern which is composed of the modal characteristics from various structural damage cases. In this algorithm, two PNNs are sequentially used. The first PNN estimates the damage location using mode shape and the results of the first PNN are put into the second PNN for the damage severity estimation using natural frequency. The proposed damage assessment algorithm using the PNN is applied to a cable-stayed bridge to verify its applicability.

Key Words
probabilistic neural network; damage identification; cable-stayed bridge.

Address
Department of Civil & Environmental Engineering, Hanyang University, Ansan, Kyunggi 425-791, South Korea

Abstract
Semiactive control systems have received considerable attention for protecting structures against natural hazards such as strong earthquakes and high winds, because they not only offer the reliability of passive control systems but also maintain the versatility and adaptability of fully active control systems. Among the many semiactive control devices, magnetorheological (MR) fluid dampers comprise one particularly promising class. In the field of civil engineering, much research and development on MR fluid damper-based control systems has been conducted since this unique semiactive device was first introduced to civil engineering applications in mid 1990s. In 2001, MR fluid dampers were applied to the full-scale in-service civil engineering structures for the first time. This state-of-the-art paper includes a detailed literature review of dynamic models of MR fluid dampers for describing their complex dynamic behavior and control algorithms considering the characteristics of MR fluid dampers. This extensive review provides references to semiactive control systems using MR fluid dampers. The MR fluid damper-based semiactive control systems are shown to have the potential for mitigating the responses of full-scale civil engineering structures under natural hazards.

Key Words
semiactive control; MR fluid damper; dynamic models; control algorithms; full-scale applications.

Address
H.-J. Jung; Department of Civil and Environmental Engineering, Sejong University, Seoul 143-747, Korea
B. F. Spencer, Jr.; Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
Y. Q. Ni; Department of Civil and Structural Engineering, Hong Kong Polytechnic University, Hong Hum, Kowloon, Hong Kong
I.-W. Lee; Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea

Abstract
Consequence-Based Engineering (CBE) is a new paradigm proposed by the Mid-America Earthquake Center (MAE) to guide evaluation and rehabilitation of building structures and networks in areas of low probability - high consequence earthquakes such as the central region of the U.S. The principal objective of CBE is to minimize consequences by prescribing appropriate intervention procedures for a broad range of structures and systems, in consultation with key decision makers. One possible intervention option for rehabilitating unreinforced masonry (URM) buildings, widely used for essential facilities in Mid-America, is passive energy dissipation (PED). After the CBE process is described, its application in the rehabilitation of vulnerable URM building construction in Mid-America is illustrated through the use of PED devices attached to flexible timber floor diaphragms. It is shown that PED

Key Words
consequence-based engineering; risk assessment; fragility reduction; response modification; passive energy dissipation; multi-criteria decision making; parametric analysis; meta-modeling.

Address
Leonardo Due?s-Osorio, Joonam Park, Peeranan Towashiraporn, Barry J. Goodno, and David Frost; School of Civil and Environmental Engineering, Georgia Institute of Technology, 790 Atlantic Drive, Atlanta GA, 30332-0355, USA
James I. Craig; School of Aerospace Engineering, Georgia Institute of Technology, 270 FerstDrive, Atlanta GA, 30332-0150, USA
Ann Bostrom; School of Public Policy, Georgia Institute of Technology, 685 Cherry Street, Atlanta GA, 30332-0345, USA

Abstract
In this paper, the design, fabrication and characterization of a piezoelectric friction damper are presented. It was sized with the proposed practical procedure to minimize the story drift and floor acceleration of an existing 1/4-scale, three-story frame structure under both near-fault and far-field earthquakes. The design operation friction force in kip was numerically determined to range from 2.2 to 3.3 times the value of the peak ground acceleration in g (gravitational acceleration). Experimental results indicated that the load-displacement loop of the damper is nearly rectangular in shape and independent of the excitation frequency. The coefficient of friction of the damper is approximately 0.85 when the clamping force on the damper is above 400 lbs. It was found that the friction force variation of the damper generated by piezoelectric actuators with 1000 Volts is approximately 90% of the expected value. The properties of the damper are insensitive to its ambient temperature and remain almost the same after being tested for more than 12,000 cycles.

Key Words
piezoelectric actuators; friction damper; semi-active control; adaptive clamping force; variable friction damper; durability; seismic performance.

Address
Department of Civil, Architectural, and Environmental Engineering, University of Missouri-Rolla, 328 Butler-Carlton Hall, 1870 Miner Circle, Rolla, MO 65409-0030, USA

Abstract
Control systems are used to limit structural lateral deflections during large external loads such as winds and earthquakes. Most recently, the semi-active control approach has grown in popularity due to inexpensive control devices that consume little power. As a result, recently designed control systems have employed many semi-active control devices for the control of a structure. In the future, it is envisioned that structural control systems will be large-scale systems defined by high actuation and sensor densities. Decentralized control approaches have been used to control large-scale systems that are too complex for a traditional centralized approach, such as linear quadratic regulation (LQR). This paper describes the derivation of energy market-based control (EMBC), a decentralized approach that models the structural control system as a competitive marketplace. The interaction of free-market buyers and sellers result in an optimal allocation of limited control system resources such as control energy. The Kajima-Shizuoka Building and a 20-story benchmark structure are selected as illustrative examples to be used for comparison of the EMBC and centralized LQR approaches.

Key Words
energy market-based control; structural control; decentralized control; market-based control; semi-active dampers; large-scale systems.

Address
Jerome Peter Lynch; Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI 48109, USA
Kincho H. Law; Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA

Abstract
A hybrid control system is presented for seismic-resistant building structures with and without soil-structure interaction (SSI). The hybrid control is a damper-actuator-bracing control system composed of passive and active controllers. An intelligent algorithm is developed for the hybrid system, in which the passive damper is designed for minor and moderate earthquakes and the active control is designed to activate when the structural response is greater than a given threshold quantity. Thus, the external energy for active controller can be optimally utilized. In the control of a multistory building, the controller placement is determined by evaluating the optimal location index (OLI) calculated from six earthquake sources. In the study, the soil-structure interaction is considered both in frequency domain and time domain analyses. It is found that the interaction can significantly affect the control effectiveness. In the hybrid control algorithm with intelligent strategy, the working stages of passive and active controllers can be different for a building with and without considering SSI. Thus SSI is essential to be included in predicting the response history of a controlled structure.

Key Words
hybrid control; soil-structure interaction; intelligent control; shallow foundation; optimal location.

Address
X.Z. Zhang and F.Y. Cheng; Civil Engineering Department, University of Missouri-Rolla, Rolla, Missouri 65409, USA
M.L. Lou; Institute of Structural Engineering & Disaster Reduction, Tongji University, Shanghai, 200092, China

Abstract
Peak response is a more suitable index than mean response in the light of structural safety. In this study, a controller optimization method is proposed to restrict peak responses of building structures subject to earthquake excitations, which are modeled as partially stationary stochastic process. The constraints are given with specified failure probabilities of peak responses. LQR is chosen to assure stability in numerical process of optimization. Optimization problem is formulated with weightings on controlled outputs as design variables and gradients of objective and constraint functions are derived. Full state feedback controllers designed by the proposed method satisfy various design objectives and output feedback controllers using LQG also yield similar results without significant performance deterioration.

Key Words
stochastic process; crossing rate; failure probability; optimization; linear quadratic regulator.

Address
Ji-Hun Park; MIDASIT Co., Ltd., Seoul, Korea
Kyung-Won Min; Department of Architectural Engineering, Dankook University, Seoul, Korea
Hong-Gun Park; Department of Architecture, Seoul National University, Korea


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