Structural Control and Health Monitoring

doi: 10.1002/stc.1553pmid: N/A

Cha, Young‐Jin; Agrawal, Anil K.

doi: 10.1002/stc.486pmid: N/A

SUMMARY This paper presents novel decentralized output feedback control strategy for the active and semi‐active controls of the highway bridge benchmark phases I and II problems subjected to earthquake ground motions. The control force is calculated using two third‐order polynomial equations expressing a direct relationship between displacement and velocity of the control device and the control force. An advanced implicit redundant representation genetic algorithm is utilized to determine optimal coefficients of the two polynomial equations by minimizing the sum of three evaluation criteria for six prescribed earthquakes. The performance of the proposed controller is evaluated in terms of a set of prescribed 21 evaluation criteria and is compared with the other several control strategies published in the special issue of the benchmark problem. The results show that the proposed decentralized output feedback polynomial control strategy can achieve significant response reductions in the bridge system, and it is evidently superior to sample control strategies and other suggested active and semi‐active controls for the phase I problem and is quite competitive with respect to the sample and other semi‐active control approaches for the fully isolated phase II problem. Copyright © 2011 John Wiley & Sons, Ltd.

Richardson, Andy; Walsh, Kenneth K.; Abdullah, Makola M.

doi: 10.1002/stc.490pmid: N/A

SUMMARY Larger and more flexible structures are being constructed. As a result, there is an increasing need for more effective, efficient, and innovative displacement control strategies. One such method is the idea of connecting adjacent structures. This control method has the merit of generating sufficient control forces under low frequencies, a property that is necessary for minimizing the displacement response of high‐rise structures. In the present work, closed‐form equations are derived for solving the vibration control problem of connecting two adjacent structures. In this design method, the maximum absolute displacement transmissibility of each structure is reduced. The closed‐form solution is derived for a passive control method, where the structures are coupled via stiffness and damping elements connected in parallel. The presence of the stiffness element allows for the simultaneous tuning of both structures. The method is demonstrated for passive control of adjacent connected buildings subject to the El Centro ground motion. It is shown that the passive connection method is effective in reducing the response of both structures compared with the uncontrolled (unconnected) case. Copyright © 2011 John Wiley & Sons, Ltd.

Kao, Ching‐Yun; Loh, Chin‐Hsiung

doi: 10.1002/stc.492pmid: N/A

SUMMARY The objective of this paper is to develop methods for extracting trends from long‐term static deformation data of a dam and try to set an early warning threshold level on the basis of the results of analyses. The static deformation of a dam is mainly influenced by the water pressure (or water level) of the dam and the temperature distribution of the dam body. The relationship among the static deformation, the water level, and the temperature distribution of the dam body is complex and unknown; therefore, it can be approximated by static neural networks. Although the static deformation almost has no change during a very short time, it changes with time for long‐term continuous observation. Therefore, long‐term static deformation can be approximated dynamically using dynamic neural networks. Moreover, static deformation data is rich, but information is poor. Linear and nonlinear principal component analyses are particularly well suited to deal with this kind of problem. With these reasons, different approaches are applied to extract features of the long‐term daily based static deformations of the Fei‐Tsui arch dam (Taiwan). The methods include the static neural network, the dynamic neural network, principal component analysis, and nonlinear principal component analysis. Discussion of these methods is made. By using these methods, the residual deformation between the estimated and the recorded data are generated, and through statistical analysis, the threshold level of the static deformation of a dam can be determined on the basis of the normality assumption of the residual deformation. Copyright © 2011 John Wiley & Sons, Ltd.

Kampas, Georgios; Makris, Nicos

doi: 10.1002/stc.494pmid: N/A

SUMMARY This paper is concerned with a widely studied problem—that of the identification of the modal characteristics of freeway overcrossings and other bridges, which response is interacting with their approaching embankments and their foundation. The study implements a sophisticated parameter estimation method known as the prediction error method and examines in detail the sensitivity of the modal characteristics (frequency and damping) of the bridge when the input signals are taken at the free field, at the approaching embankments and pile caps, and on the abutments and the pile caps. The findings of this case study on the Meloland Road Overcrossing with the prediction error method are compared with the results from past system identification studies and the results from finite element analyses, which examined in depth the contribution of the approaching embankments in the bridge response. The study concludes that despite the appreciable energy dissipation capability of the approaching embankments the concrete bridge structure, while interacting mechanically with the embankments, remains small. Copyright © 2011 John Wiley & Sons, Ltd.

Chung, Lap‐Loi; Wu, Lai‐Yun; Yang, Chuang‐Sheng Walter; Lien, Kuan‐Hua; Lin, Mei‐Chun; Huang, Hsu‐Hui

doi: 10.1002/stc.496pmid: N/A

SUMMARY Optimal design for tuned mass dampers (TMDs) with linear or nonlinear viscous damping is formulated in order for design practitioners to directly compute the optimal parameters of a TMD in a damped structure subjected to wind excitations. The optimal TMD tuning frequency ratio and damping coefficient for a viscous TMD system installed in a damped structure under 10 white noise excitations are determined by using the time‐domain optimization procedure, which minimizes the structural response. By applying a sequence of curve‐fitting schemes to the obtained optimal values, design formulas for optimal TMDs are then derived. These are expressed as a function of the mass ratio and damping power‐law exponent of the TMD as well as the damping ratio of the structure. The feasibility of the proposed optimal design formulas is verified in terms of formulary accuracy and of comparisons with existing formulas from previous research works. In addition, one numerical example of the Taipei 101 building with a nonlinear TMD, which is redesigned according to the proposed optimal formulas, is illustrated in effort to describe the use of the formulas in the TMD design procedure and to investigate the effectiveness of the optimal TMD. The results indicate that the proposed optimal design formulas provide a convenient and effective approach for designing a viscous TMD installed in a wind‐excited damped structure. Copyright © 2011 John Wiley & Sons, Ltd.

Bozer, Ali; Altay, Gülay

doi: 10.1002/stc.497pmid: N/A

SUMMARY The optimization of tuned mass damper (TMD) involves the attachment of a damper to a TMD unit. Although this supplementary damping renders TMD to be operational under broad bandwidth inputs (i.e., wind excitation), it also considerably decreases the efficiency. In this paper, an alternative approach is suggested to eliminate the requirement for supplementary damping, hence to utilize TMD in full efficiency. For this purpose, combined structure/undamped TMD system is equipped with an active controller to track the response of an oscillator with natural frequency set to the operating frequency of the TMD unit. In this case, a proposed controller does not aim returning a system to its zero state; instead, it supports the TMD unit to suppress vibration effectively. This tracking type controller is formulized by both linear quadratic regulator, and H∞ control schemes, and control performances of each system are evaluated through numerical simulations. Copyright © 2011 John Wiley & Sons, Ltd.

Fallah, Arash Yeganeh; Taghikhany, Touraj

doi: 10.1002/stc.498pmid: N/A

SUMMARY Consistency of real‐time data transmission is an important factor in active and semi‐active control of structures. Recent advances in wireless sensors and their reliable data communication have attracted many attentions in structural control engineering. Wireless sensors have been also preferred to wire‐base types because of problems associated with installing extensive length of coaxial wires. In addition, embedded computing technologies in wireless sensors as local controller reduce size of transferring data and provide stable transmission. However, there are some issues in implementation of wireless connection like communication range and latency. This study presents the feasibility of developing a decentralized controller by using H2/LQG as robust‐optimum algorithm in cable‐stayed bridge. Because of large scale of structure, the order of model is reduced by using two different reduction methods in finite time and frequency. The controller is developed in centralized and decentralized solutions. The decentralized controller through developing a gradient‐based method is designed to efficiently solve system of closed‐loop H2 optimization problem. The structural responses in centralized and decentralized solutions are compared with each other under three different earthquake records considering time delay in data transmission. Results indicate that for large‐scale bridges with time‐delayed data communication issue, H2/LQG controller in decentralized solution gives maximum reduction of response indices in comparison with other strategies. Copyright © 2011 John Wiley & Sons, Ltd.

Rama Raju, K.; Meher Prasad, A.; Muthumani, K.; Gopalakrishnan, N.; Iyer, Nagesh R.; Lakshmanan, N.

doi: 10.1002/stc.499pmid: N/A

SUMMARY The supplemental passive and semi‐active dampers such as viscous fluid dampers and magnetorheological (MR) dampers normally placed in either Chevron or Toggle brace are increasingly used to provide enhanced seismic protection for new/retrofit existing buildings and bridges. The experimental nonlinear force–velocity relationships of MR dampers at different current inputs are fitted to fractional velocity power law. A three‐storey quarter length scale steel moment‐resisting frame model with two MR dampers fixed in upper toggle brace mechanism placed at ground floor level is designed and fabricated to study its seismic response characteristics. The natural frequencies and corresponding damping ratios of the model with MR damper at different current inputs are found. A procedure for modeling of MR dampers as nonlinear viscous fluid dampers is described. A methodology to find effective damping of the structural model is developed, using the formulations for nonlinear viscous fluid dampers given in literature. The methodology developed is used for finding the effective damping of structural model fitted with MR dampers in upper toggle bracing mechanism in different storeys of frame model. The model is subjected to two types of seismic excitations, and from studies of responses, it is found that the reduction in responses because of provision of MR dampers are to be quite significant. Copyright © 2011 John Wiley & Sons, Ltd.

Wang, Zuocai; Chen, Genda

doi: 10.1002/stc.502pmid: N/A

SUMMARY In this study, a moving‐window least squares fitting method is proposed for rapid identification of cracks and flexural rigidities in multispan bridges. First, the dynamic deflections of a continuous bridge were locally measured under a dynamic point load. Their integrations over time, referred to as ‘integration‐over‐time deflections’, were used to derive ‘integration‐over‐time slopes’. These virtually static measurements over a short segment of the bridge were then fitted into a cubic curve in the least squares sense. Finally, the coefficient of the square term of the fitted curve was used to determine both the magnitude and location of local flexibility because of cracking and/or changing in flexural rigidity of the bridge. For multispan continuous bridges, an iterative procedure was developed to ensure that the end moments of various spans are compatible with the identified cracks and rigidity changes. To illustrate the proposed method, prismatic girder bridges with multiple cracks of various depths or non‐prismatic girder bridges were analyzed. Sensitivity analysis was conducted on the effects of weighting factor, noise level, load type, window length, and bridge discretization. Numerical results demonstrated that the proposed method can accurately detect cracks and identify the change in flexural rigidity. The five‐point equally weighted algorithm is recommended for practical applications. The spacing of two discernible cracks is equal to the window length. The identified results are insensitive to noise because of integration of the dynamic measurements. Copyright © 2011 John Wiley & Sons, Ltd.