Giu Lee

Structural Health Monitoring Systems for a Steel Structure Using an Ambient Vibration

This study aims at identifying the structural dynamic characteristics using an ambient vibration, and developing a health monitoring system which adopts damage detecting algorithms. One of the main problems for this system design is to measure long-time dynamic response signals and simultaneously estimate the structural dynamic properties. In order to be suitable for a long-time monitoring, we conduct an ambient dynamic testing for a 3-floor moment resistance steel structure and analyze structural dynamic characteristics using time domain estimation techniques. Also, a damage detecting test is performed to evaluate damage state by various detecting algorithms (modal correlation method (MAC & COMAC), eigen-parameter change method). Finally, this paper suggests the optimal algorithms for the identification of the structural damage locations and damage quantities with all such comparisons. The algorithms presented in this paper prove to be applicable in structural health monitoring of structures.

Field validation of a wireless sensor unit using self-anchored suspension bridge

This paper attempts to validate the effectiveness of wireless sensor unit by field experiments on a self-anchored suspension bridge. This wireless sensor unit was developed at Konyang Universitys SIS lab in Korea for real-time dynamic response measurement of structures. This sensing unit called SWMAS(Smart Wireless MEMS-based Sensor System) consists of a sensor system module, a control and processing module, and a wireless modem module. In order to evaluate whether SWMAS would be applicable to structural monitoring system, experiments were performed to a full-scaled structure, which was a self-anchored suspension bridge, a wire-based monitoring system placed inside. In the field experiments, the data from the ambient vibrations of the bridge were acquired in real-time using SWMAS. All data acquired were compared with those of wire-based monitoring system. As a result, the comparison proved that SWMAS would be effectively applicable to smart monitoring system.

Identifying dynamic characteristics of structures to estimate the performance of a smart wireless MA system

In this paper, a smart wireless MEMS-based accelerometer(MA) system has been designed and experimented for smart monitoring system of civil structures. In order to estimate the performance of a smart wireless MA system(SWMAS), dynamic characteristics of our model structure need to be identified. This system thus employed a high-performance AVR microcontroller, a wireless modem, and MA for multiplex communication capability and real time duplex communication. Various performance and experimental tests have been carried out to evaluate whether this system is suitable for monitoring system of civil structures. First, we examined its sensitivity, resolution, and noise, specifically to evaluate the performance of the smart wireless MA system. The results of experiments enabled us to estimate performance of the MA in SWMAS in comparison to the value of data sheet from MA. Second, characteristics of model structure were analyzed by the ambient vibration test based on the NExT combined with ERA. Finally, this analysis was compared to the one that was made by FE results, and the comparison proved that a smart wireless MA system was fitted in smart monitoring system effectively.

Designing a smart damping system to mitigate structure vibration: Part 2. Experimental approval of unified Lyapunov control algorithm

This paper proposes an effective vibration control algorithm using a smart damping system for vibration mitigation. The proposed algorithm, which extends Lyapunov stability theory into a unified system, enables to correct the errors in quantization by its increased stability. The validity of this design method is proved in the experiment on a control model of three-storied building structure. Smart damper is used for shear mode MR damper in the experiment, and its control effectiveness is evaluated. In order to make a more accurate control model experimentally, model updating is performed on the basis of the analysis of dynamic characteristics of structure and of the mathematical analysis of a lumped mass model, and then it employs a state space model redefined by structural property matrix. In this vibration control experiment, control effect under the influence of each different earthquake magnitude is evaluated to various performance index, and thus the algorithm presented here is proved to be valid.

An Analysis of the Characteristics of SPG Bridges Using NExT and ERA for Real-Time Monitoring

The paper analyzes the characteristics of a Steel Plate Girder (SPG) bridge structure using Natural Excitation Technique (NExT) and Eigensystem Realization Algorithm (ERA). This analysis is to estimate how the long measuring time which causes difficulty in a real-time monitoring can be reduced and so to accurately identify the physical characteristics of a structure. It also attempts to assess its applicability to a real bridge structure. To explain more in detail, we make Hankel Matrix from Cross Correlation, and then the characteristics of a structure are identified using ERA. The suggested algorithm, in this paper, is applied to the SPG bridges and the results are shown in the good correlation. And we estimate whether it is applicable in real-time measuring, and analyze some problems and suggest the alternatives to them. Introduction The dynamic characteristics of a structure contain useful information about its structural condition, and so vibration testing can be considered as a global non-destructive structural real-time monitoring technique [1][2]. In a real-time monitoring system, however, it is not easy to excite large structures such as bridges and buildings, which has thereby become an important issue. As input excitation devices for civil structures, people have used rotating mass vibrators, electro-hydraulic vibrators, and impact testing based on a suspended mass. However, those forced excitation devices are not only very expensive but also may cause some possible damage to the structure. In order to avoid such problems, an ambient vibration testing is suggested in which the input excitation signals are not forced or controlled. This ambient vibration testing is effectively and conveniently used to measure the vibration response while the structure is under any traffic loads, winds, and earthquakes. This paper proposes a structural real-time monitoring method for civil structures. Instead of using the input excitation devices mentioned above, this method employs the natural excitation technique (NExT) developed by James et al. [3][4]. This approach does not need to specify the excitation source, so it is applicable to any problems in general where excitation occurs all the time or excitation is not structurally measurable. The eigensystem realization algorithm (ERA) is then used to determine estimates of the structure modal parameters [5][6]. Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 2012-2017 doi:10.4028/www.scientific.net/KEM.270-273.2012

Real-time Long Term Measurement Using Integrated Framework for Ubiquitous Smart Monitoring

Ubiquitous monitoring combining internet technologies and wireless communication is one of the most promising technologies of infrastructure health monitoring against the natural of man-made hazards. In this paper, an integrated framework of the ubiquitous monitoring is developed for real-time long term measurement in internet environment. This framework develops a wireless sensor system based on Bluetooth technology and sends measured acceleration data to the host computer through TCP/IP protocol. And it is also designed to respond to the request of web user on real time basis. In order to verify this system, real time monitoring tests are carried out on a prototype self-anchored suspension bridge. Also, wireless measurement system is analyzed to estimate its sensing capacity and evaluate its performance for monitoring purpose. Based on the evaluation, this paper proposes the effective strategies for integrated framework in order to detect structural deficiencies and to design an early warning system.

Estimation of control capacity for squeeze mode MR damper with electro-magnetic system

This paper is focused on developing a Squeeze Mode MR damper with large control capacity for effective vibration mitigation in infrastructures such as long span bridges and skyscrapers, etc. In order to maximize the magnetic field required for control of MR fluid, this device is designed by separating electromagnet system from the main cylinder for large capability of control force without changing its total length. This developed MR damper is tested to estimate its maximum control capacity and dynamic range(defined as the ratio of the maximum force to the minimum force that MR devices provide), inputting various strength magnet fields. These experimental results make it possible to estimate its maximum control force by drawing a curve showing the relationship between generated forces and applied magnetic fields. In order to verify its performance as a semi-active control device, its dynamic range is calculated. Through all tests, the developed MR is proved to be an effective device for the response control of infrastructures.

Structural vibration control of civil structures with squeeze-mode smart damper: experimental comparisons

This paper experiments on each of two control algorithms which are adapted into a unified control algorithm, in order to decide which is better for semi-active control of civil structures using a squeeze mode smart damper. These algorithms are Lyapunov algorithm and a clipped optimal algorithm, both of which are superior in efficiency and reliability to any existing semi-active control algorithms. Such adaptation makes it easier to develop a control algorithm because it accommodates both continuous and discrete time signals at the same time, and to analyze the control characteristics in the case of broadly distributed natural frequency by securing enlarged stable regions. In order to prove its validity, we performed vibration control tests using a prototype steel plate girder bridge. Since the model is a reduced one, we also scaled EI Centro earthquake wave to the same scale as the reduced model bridge. Various performance indexes have been used to see which algorithm is most effective in control. Also, other experiments were performed to define the control characteristics which would enable us to see how all control conditions--displacement control, force control, and acceleration control--work with each control algorithm. Those experiments showed that each control algorithm works differently according to each different control condition. It is found that Lyapunov algorithm of the two is more effective for semi-active control in the unified control system. Therefore, it is necessary to design a control system according to structural conditions and circumstances.

Dynamic experiment and modeling of squeeze mode smart damper for semi-active control of civil structures

Magneto-rheological fluid is the fluid which is controllable with applied magnetic fields. This fluid is effective as a semiactive control device such as MR damper. In this paper, a new MR technology is developed with squeeze mode smart damper. And various dynamic tests are performed to identify the dynamic characteristics of this device. This squeeze mode smart damper can be used permanently, and can be freely allocated at the sub-region of large structures such as buildings and civil engineering infrastructures. Various dynamic tests are carried out to evaluate the performance of the squeeze mode smart damper in many loading conditions. Force-displacement and force-velocity hysteresis loops are also investigated for evaluation of its dynamic performance. In order to predict the dynamic performance of this device, two types of analytical models are compared with experimental results. A power model based on the damping and velocity, and a Bingham model are adopted in the viewpoint of practical usage. These results verify that the developed smart damper is effective in semi-active control of civil structures.

An Adoption of Energy Dissipation Ratio for Real-Time Monitoring

This research is proposed for structural real-time health monitoring of a large structure based on the identified dynamic properties from the ambient measurements. This method uses the energy transfer ratio(ETR). ETR index is estimated as the ratio of energy transfers to total energies in each mode for the non-proportional damping systems. This index is computed by the dynamic parameters, such as natural frequencies and modal damping ratios. The dynamic system identification method uses the natural excitation technique(NExT) with the eigen-system realization algorithm(ERA) for the estimation of the dynamic parameters. Based on this process, ETR can be extracted. This research is significant of the construction of baseline structure for the bridge damage identification, and proves the effectiveness of the proposed method for real time monitoring. Introduction Bridge structures usually vibrate in response to some ambient sources such as traffic, wind, and earthquake, and it is relatively easy to measure the response signals from such vibration. So, it would be useful to develop the techniques that would analyze the dynamic modal parameters by using only output signals, and those techniques contribute to the construction of a real-time monitoring system. There have been many researches on the development of structural heath monitoring system in order to maintain the stability of structures from the external loads such as wind, earthquake, or something that unexpectedly occurs [1, 2]. For example, James et. al. suggested the Natural excitation techniques (NExT) which uses ambient vibration to identify the dynamic characteristics of structures, and applying the techniques he solved the measuring problem of input signals[3]. Juang and Pappa also presented the eigen-system realization algorithm, an structural characteristics analysis algorithm, which is concerned with time-domain and so appropriate for the system where multi-input, multi-output, MIMO are used[4, 5]. As another example, Caicedo suggested that the combination of the ERA and the NExT be useful in structural health monitoring with his dynamic simulation of cable-stayed bridge and his experiment on the four-story high prototype building. However, he did not apply it to the structural real-time monitoring[6~8]. This study presents the techniques using energy dissipation ratio which monitor the structural health in real time. For monitoring, real-time data were acquired by the NExT and the ERA. The EDR is derived by using the damping and eigenvalues calculated from those data, and from the values an algorithm is developed to detect any structural damage. We also examined how the EDR changes in Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 977-983 doi:10.4028/www.scientific.net/KEM.270-273.977