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Crack observation is important for evaluating the structural performance and safety of reinforced concrete (RC) structures. Most of the existing image-based crack detection methods are based on edge detection algorithms, which detect cracks that are wide enough to present dark areas in the obtained images. Cracks initiate as thin cracks, generally having width less than the width of a pixel in images; such cracks are generally undetectable by edge detection-based methods. An image analysis method is proposed to observe the development and distribution of thin cracks on RC surfaces; it also allows estimation of crack widths. Image matching based on optical flow and subpixel is employed to analyze slight concrete surface displacements. Camera calibration is included to eliminate perspective effects and lens distortion. Geometric transformation is adopted so that cameras do not need to be perpendicular to the observed surface or specified positions. Formulas are proposed to estimate the width of shear crack opening. The proposed method was then applied to a cyclic test of an RC structure. The crack widths and their development analyzed by the image analysis were verified with human inspection in the test. In addition, concrete surface cracks that appeared at a very early stage of the test could be observed by the proposed method before they could be detected by the naked eye. The results thus demonstrate that the proposed image analysis method offers an efficient way applicable not only for structural tests but also for crack-based structural-health-monitoring applications.

This paper proposes a passive vibration control device, series rolling-pendulum tuned mass damper (SRPTMD), with a “ball-in-ball” configuration. A conventional pendulum TMD (PTMD) generally requires a long cable length that usually exceeds one-story height for high-rise buildings. A rolling-pendulum TMD (RPTMD) is a mass that can roll on a base with a curvature instead of swaying with a cable, significantly reducing the requirement of vertical rooms. In addition, a ball-in-ball SRPTMD is equivalent to a system with two degrees of freedom in series. This study aimed to derive equations of motion of the primary building-SRPTMD system, conduct a parametric study for SRPTMD, and investigate the structural control performance of an SRPTMD. Results showed that an SRPTMD performed similarly to an RPTMD. One advantage of an SRPTMD is that the fundamental natural frequency of an SRPTMD can be altered to a certain extent by changing the radius ratio of the inner ball to the outer ball, whereas the natural frequency of an RPTMD can only be altered by changing the curvature of its base, which is far more difficult. Another advantage is that the two modal frequencies of an SRPTMD can be manipulated by selecting a specific set of radius ratios between the base, the outer ball, and the inner ball, which means that an SRPTMD has higher potential on multiple modes control.

Conventional tuned mass damper (TMD) is a popular and generally accepted vibration control device in the field of passive structural control. However, it was found that the control efficacy of a conventional TMD may significantly degrade when the TMD’s frequency does not tune to its desired value. In addition, the vibration energy of controlled structure transferred into the TMD is dissipated by viscous or friction damper and becomes waste heat. In this paper, a new type of TMD, called electromagnetic TMD inerter (EM-TMDI) is developed by replacing the viscous dampers with electromagnetic rotary transducers so that a more flexible viscous damping can be achieved and part of the energy originally dissipated by the dampers could be harvested. A flywheel with variable mass moment of inertia will be introduced into the transmission system of the TMD to adjust TMD’s frequency to mitigate the frequency detuning effect and to enhance the control efficacy of TMD system. The theoretical derivation is performed to generate the relationship between the DC motor and the transmission system of the EM-TMDI. Optimal design method considering the inerter of rotary transducers will be developed. This study first designed and manufactured a scale-down, double-deck EM-TMDI. A series of shaking table tests were conducted at NCREE Tainan laboratory to verify the capability of inerter to change TMD’s frequency.

Nuclear containment structure is one of the most important infrastructure systems ensuring the safety of a nuclear power plant. In this paper, the structural behavior of cylindrical concrete containment structure was investigated using two 1/13-scaled nuclear containment specimens subjected to reversed cyclic loadings. The presentation will first describe the experimental program, including the dimensions, the reinforcement detailing, the test setup, and the loading method. Second, the experimental results of the specimens are discussed including the cracking patterns, the total load versus displacement curves, and the failure modes. Third, the test results were compared to the analytical results predicted at the University of Houston using a 3D finite element program with the CSMM-based shell elements. The predicted results agree very well with the experimental data.

In this paper, an optimal H∞ control algorithm using direct velocity feedback is used for the design of control systems in reducing structural seismic responses. An optimum design strategy was developed for deciding two control parameters, γ and α, which are introduced in the control algorithm and play important roles in system stability and control performance. γ is a positive attenuation constant, which denotes a measure of control performance, and α is a control weighting factor, indicating the relative significance between control force requirement and response reduction. Analytical results show that decreases in γ or increases in α yield better control performance but require larger control forces. The selection range of γ and α for a controlled system yielding overdamped or unstable responses is found. To assure system stability and better performance than linear quadratic regulator (LQR) control, analytical expressions of the upper and lower bounds of γ and α are derived for direct velocity...

This paper analyzes the soil–structure interaction (SSI) effect on vibration control effectiveness of active tendon systems for an irregular building, modeled as a torsionally coupled (TC) structure, subjected to base excitations such as those induced by earthquakes. An H∞ direct output feedback control algorithm through minimizing the entropy, a performance index measuring the trade-off between H∞ optimality and H2 optimality, is implemented to reduce the seismic responses of TC structures. The control forces are calculated directly from the multiplication of the output measurements by a pre-calculated frequency-independent and time-invariant feedback gain matrix, which is obtained based on a fixed-base model. Numerical simulation results show that the required numbers of sensors, controllers and their installation locations depend highly on the degree of floor eccentricity. For a large two-way eccentric building, a one-way active tendon system placed in one of two frames farthest away from the center of resistance (C.R.) can reduce both translational and torsional responses. The SSI effect is governed by the slenderness ratio of superstructure and by the stiffness ratio of soil to superstructure. When the SSI effect is significant, the proposed control system can still reduce the structural responses, however, with less effectiveness than that of the assumed fixed-base model. Therefore, the TC and SSI effects should be considered in the design of active control devices, especially for high-rise buildings located on soft site.

In this paper, an optimal H∞ control algorithm was applied to the design of an active tendon system installed at the first story of a multi-story building to reduce its interstory drift due to earthquake excitations. To achieve optimal control performance and to guarantee the stability of the control system, an optimum strategy to select control parameters γ and α was developed. Analytical expressions of the upper and the lower bounds of γ and α were obtained for a single degree-of-freedom system with state feedback control. The selection ranges for both γ and α are graphically defined so that the controlled system is always stable and the control performance is better than by the conventional LQR control algorithm. Numerical results from a controlled three-story building under real earthquake excitations demonstrate that the peak first interstory drift can be significantly reduced with maximum control force around 10% of the building weight. An optimum design flow chart was provided. In addition, for a time-delayed structure, this study gave explicit formulae to calculate the critical values of γ and α. The system stability and control performance can thus be guaranteed even with time delay.

An optimal discrete-time direct output-feedback control algorithm is developed with the consideration of sampling period and appropriate time delay in its control force action. Optimal-delayed output-feedback gains are derived through a variation process and will be complementarily arranged with respect to the applied delay time by either altering their magnitude or phases to assure the efficacy and stability of the delayed direct digital control system. Parametric studies of single-degree-of-freedom (SDOF) systems with different feedback types demonstrate the relationships of the delayed gains and the modal properties corresponding to the sampling period used and the intentionally delay time added. The maximum allowable delay time defined at the onset of instability of controlled system can be the optimal delay time that reaches the optimal control performance if the delay time is considered in the delayed feedback gains. The inclusion of sampling period and delay time in the optimization process for discrete-time control illustrates the beneficial effect under the same delay time. Numerical simulation results of proposed direct digital control principles on a three-degree-of-freedom (3DOF) shaking table structure model show that the efficacy of the collocated control cases with optimal delay time to reduce the dynamic responses subjected to earthquake excitations is assured. Copyright © 2007 John Wiley & Sons, Ltd.

In this paper, an H ∞ control algorithm is developed to reduce the earthquake response of structures with consideration of control force execution time delay. To achieve optimal control performance and assure control system stability, the strategy to select two control parameters γ and α is extensively investigated. It was found that the decrease of γ or increase of α results in significant reduction of structural responses. To obtain the same control results, a larger α has to be selected for stiff structures than for flexible structures. In addition, in active control of a real structure, control force execution time delay cannot be avoided. Relatively small delay times not only can render the control ineffective, but also may cause system instability. In this paper, explicit formulas for the maximum allowable delay time and critical control parameters are derived for the design of a stable H ∞ control system. Some solutions are also proposed to lengthen the maximum allowable delay time.

An H∞ direct output feedback control algorithm through minimizing the entropy, a performance index measuring the tradeoff between H∞ optimality and H2 optimality, is developed in this paper to reduce the earthquake response of structures. To achieve optimal control performance and assure control system stability, the strategy to select both control parameters γ and α is extensively investigated considering the control force execution time delay. It is found that a lower bound of γ and an upper bound of α exist. The selection beyond these values will cause the control system instability. For a damped structure, analytical expressions of direct output feedback gains, controlled frequencies and damping ratios are derived. It can be proved that the conventional LQR control is a special case of the developed H∞ control. In real active control, control force execution time delay cannot be avoided. This paper gives explicit formulas of maximum allowable delay time and critical control parameters for the design o...

Active control systems have already been installed in real structures and are able to decrease the wind- and earthquake-induced responses, while the active mass damper (AMD) is one of the most popular types of such systems. In practice, an AMD is generally assembled in- situ along with the construction of a building. In such a case, the AMD and the building is coupled as an entire system. After the construction is completed, the dynamic properties of the AMD subsystem and the primary building itself are unknown and cannot be identified individually to verify their design demands. For this purpose, a methodology is developed to obtain the feedback gain of the AMD controller and the dynamic properties of the primary building based on the complex eigen-parameters of the coupled building-AMD system. By means of the theoretical derivation in state-space, the non-classical damping feature of the system is characterized. This methodology can be combined with any state-space based system identification technique as a procedure to achieve the goal on the basis of the acceleration measurements of the building-AMD system. Results from numerical verifications show that the procedure is capable of extracting parameters and is applicable for AMD implementation practices.

The more the development of super high-rise buildings, the faster the speed of elevator in order to shorten the riding time of elevator and the waiting time of passengers. With the increase of elevator speed, the horizontal vibration of passenger car becomes more significant resulting in the decrease of serviceability and safety of elevator, and the discomfort of passengers. The horizontal vibration is mainly generated from the elevator wheels running on rough and winding guide rails. In this paper, a four degree-of-freedom (DOF) elevator system was established to examine the characteristics of the excitations and to analyze the dynamic responses of the elevator. An active mass driver (AMD) was developed to reduce the horizontal acceleration of passenger car in the elevator based on H∞ direct output feedback control algorithm. The optimal control force is obtained from the multiplication of direct output measurements by a pre-calculated time-invariant gain matrix. To achieve optimal control performance, the strategy to select both control parameters γ and α was investigated extensively. Numerical verification results show that decrease in γ or increase in α yields better control performance with an acceptable magnitude of control force. The selective ranges of γ and α making a controlled system become overdamped or unstable were found. To assure system stability and control efficiency, the upper bound of α were derived and illustrated graphically. An optimum design flowchart was also proposed. Finally, a full-scaled high-speed elevator system was investigated to prove the applicability and control effectiveness of the proposed AMD system.Copyright © 2011 by ASME

In this paper, an H∞ direct output feedback control algorithm is employed to design the control system in reducing structural seismic responses. Control forces are obtained from the multiplication of direct output measurements by a pre-calculated time-invariant feedback gain matrix. To achieve optimal control performance, the strategy to select both control parameters γ and α is developed. Analytical results show that the decrease of γ or the increase of α makes better control performance, but requires larger control forces. It is also found that lower and upper bounds of γ and α exist. The selection beyond these values will cause control system instability. In this paper, analytical expressions of the upper and lower bounds of γ and α are derived. Structural seismic responses can thus be reduced effectively with appropriate selection of γ and α. Moreover, control force execution time delay cannot be avoided. Relatively small delay time not only can render the control ineffective, but also may cause system instability. In this study, explicit formulas to calculate maximum allowable delay time and critical control parameters are derived for the design of a stable control system. Some solutions are also proposed to increase the maximum allowable delay time. The desired control performance can be guaranteed even with time delay.Copyright © 2009 by ASME

In this paper, an H∞ direct output feedback control algorithm through minimizing the entropy, a performance index measuring the tradeoff between H∞ optimality and H2 optimality, is employed to design the control system in reducing structural responses due to dynamic loads such as earthquakes. The control forces are obtained from the multiplication of direct output measurements by a pre-calculated time-invariant feedback gain matrix. To achieve optimal control performance, the strategy to select both control parameters γ and α is extensively investigated. The decrease of γ or increase of α results in better control effectiveness, but larger control force requirement. For a single degree-of-freedom (SDOF) damped structure, exact solutions of output feedback gains and control parameters are derived. It can be proved analytically that the LQR control is a special case of the proposed H∞ control. Direct velocity feedback control is effective in reducing structural responses with very small number of sensors and controllers compared with the DOFs of the structure. In active control of a real structure, control force execution time delay cannot be avoided. Relatively small delay time not only can render the control ineffective, but also may cause system instability. In this study, explicit formulas to calculate maximum allowable delay time and critical control parameters are derived for the design of a stable control system. Some solutions are also proposed to increase the maximum allowable delay time.Copyright © 2005 by ASME