• Structural Monitoring and Maintenance
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• 제7권4호
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• pp.283-294
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• 2020

In this paper, an adaptive MEMD based modal identification technique for linear time-invariant systems is proposed employing multiple vibration measurements. Traditional empirical mode decomposition (EMD) suffers from mode-mixing during sifting operations to identify intrinsic mode functions (IMF). MEMD performs better in this context as it considers multi-channel data and projects them into a n-dimensional hypercube to evaluate the IMFs. Using this technique, modal parameters of the structural system are identified. It is observed that MEMD has superior performance compared to its traditional counterpart. However, it still suffers from mild mode-mixing in higher modes where the energy contents are low. To avoid this problem, an adaptive filtering scheme is proposed to decompose the interfering modes. The Proposed modified scheme is then applied to vibrations of a reinforced concrete road bridge. Results presented in this study show that the proposed MEMD based approach coupled with the filtering technique can effectively identify the parameters of the dominant modes present in the structural response with a significant level of accuracy.

• Smart Structures and Systems
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• 제15권3호
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• pp.717-733
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• 2015

Modal parameters of a structure are commonly used quantities for system identification and damage detection. With a limited number of studies on the statistics assessment of modal parameters, this paper presents procedures to properly account for the uncertainties present in the process of extracting modal parameters. Particularly, this paper focuses on how to deal with the measurement error in an ambient vibration test and the modeling error resulting from a modal parameter extraction process. A bootstrap approach is adopted, when an ensemble of a limited number of noised time-history response recordings is available. To estimate the modeling error associated with the extraction process, a model prediction expansion approach is adopted where the modeling error is considered as an "adjustment" to the prediction obtained from the extraction process. The proposed procedures can be further incorporated into the probabilistic analysis of applications where the modal parameters are used. This study considers the effects of the measurement and modeling errors and can provide guidance in allocating resources to improve the estimation accuracy of the modal data. As an illustration, the proposed procedures are applied to extract the modal data of a damaged beam, and the extracted modal data are used to detect potential damage locations using a damage detection method. It is shown that the variability in the modal parameters can be considered to be quite low due to the measurement and modeling errors; however, this low variability has a significant impact on the damage detection results for the studied beam.

• Smart Structures and Systems
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• 제17권3호
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• pp.363-389
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• 2016

Deficient modes that cannot be always identified from different sets of measurement data may exist in the application of operational modal analysis such as the stochastic subspace identification techniques in large-scale civil structures. Based on a recent work using the long-term ambient vibration measurements from an instrumented cable-stayed bridge under different wind excitation conditions, a benchmark problem is launched by taking the same bridge as a test bed to further intensify the exploration of mode identifiability. For systematically assessing this benchmark problem, a recently developed SSI algorithm based on an alternative stabilization diagram and a hierarchical sifting process is extended and applied in this research to investigate several sets of known and blind monitoring data. The evaluation of delicately selected cases clearly distinguishes the effect of traffic excitation on the identifiability of the targeted deficient mode from the effect of wind excitation. An additional upper limit for the vertical acceleration amplitude at deck, mainly induced by the passing traffic, is subsequently suggested to supplement the previously determined lower limit for the wind speed. Careful inspection on the shape vector of the deficient mode under different excitation conditions leads to the postulation that this mode is actually induced by the motion of the central tower. The analysis incorporating the tower measurements solidly verifies this postulation by yielding the prevailing components at the tower locations in the extended mode shape vector. Moreover, it is also confirmed that this mode can be stably identified under all the circumstances with the addition of tower measurements. An important lesson learned from this discovery is that the problem of mode identifiability usually comes from the lack of proper measurements at the right locations.

• Smart Structures and Systems
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• 제15권3호
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• pp.665-682
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• 2015

Structural identification or St-Id is 'the parametric correlation of structural response characteristics predicted by a mathematical model with analogous characteristics derived from experimental measurements'. This paper describes a St-Id exercise on Humber Bridge that adopted a novel two-stage approach to first calibrate and then validate a mathematical model. This model was then used to predict effects of wind and temperature loads on global static deformation that would be practically impossible to observe. The first stage of the process was an ambient vibration survey in 2008 that used operational modal analysis to estimate a set of modes classified as vertical, torsional or lateral. In the more recent second stage a finite element model (FEM) was developed with an appropriate level of refinement to provide a corresponding set of modal properties. A series of manual adjustments to modal parameters such as cable tension and bearing stiffness resulted in a FEM that produced excellent correspondence for vertical and torsional modes, along with correspondence for the lower frequency lateral modes. In the third stage traffic, wind and temperature data along with deformation measurements from a sparse structural health monitoring system installed in 2011 were compared with equivalent predictions from the partially validated FEM. The match of static response between FEM and SHM data proved good enough for the FEM to be used to predict the un-measurable global deformed shape of the bridge due to vehicle and temperature effects but the FEM had limited capability to reproduce static effects of wind. In addition the FEM was used to show internal forces due to a heavy vehicle to to estimate the worst-case bearing movements under extreme combinations of wind, traffic and temperature loads. The paper shows that in this case, but with limitations, such a two-stage FEM calibration/validation process can be an effective tool for performance prognosis.

• Smart Structures and Systems
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• 제25권1호
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• pp.111-122
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• 2020

Researchers up to date have introduced several Structural Health Monitoring (SHM) techniques with varying advantages and drawbacks for each. Satellite positioning systems (GPS, GLONASS and GALILEO) based techniques proved to be promising, especially for high natural period structures. Particularly, the GPS has proved sufficient performance and reasonable accuracy in tracking real time dynamic displacements of flexible structures independent of atmospheric conditions, temperature variations and visibility of the monitored object. Tall structures are particularly sensitive to oscillations produced by different sources of dynamic actions; such as typhoons. Wind forces induce in the structure both longitudinal and perpendicular displacements with respect to the wind direction, resulting in torsional effects, which are usually more complex to be detected. To efficiently track the horizontal rotations of the in-plane sections of such flexible structures, two main issues have to be considered: a suitable sensor topology (i.e., location, installation, and combination of sensors), and the methodology used to process the data recorded by sensors. This paper reports the contributions of the measurements recorded from dual frequency GPS receivers and uni-axial accelerometers in a full-scale experimental campaign. The Canton tower in Guangzhou-China is the case study of this research, which is instrumented with a long-term structural health monitoring system deploying both accelerometers and GPS receivers. The elaboration of combining the obtained rather long records provided by these two types of sensors in detecting the torsional behavior of the tower under ambient vibration condition and during strong wind events is discussed in this paper. Results confirmed the reliability of GPS receivers in obtaining the dynamic characteristics of the system, and its ability to capture the torsional response of the tower when used alone or when they are combined with accelerometers integrated data.

• Smart Structures and Systems
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• 제4권2호
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• pp.221-232
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• 2008

This paper represents the final results of a research program sponsored by the European Commission through project WIND-CHIME ($\underline{W}$ide Range Non-$\underline{IN}$trusive $\underline{D}$evices toward $\underline{C}$onservation of $\underline{HI}$storical Monuments in the $\underline{ME}$diterranean Area), in which the possibility of using advanced seismic protection technologies to preserve historical monuments in the Mediterranean area is investigated. In the current research, the dynamic characteristics of two outstanding Mamluk-Style minarets, which similar minarets were reported to experience extensive damage during Dahshur 1992 earthquake, are investigated. The first minaret is the Qusun minaret (1337 A.D, 736 Hijri Date (H.D)) located in El-Suyuti cemetery on the southern side of the Salah El-Din citadel. The minaret is currently separated from the surrounding building and is directly resting on the ground (no vaults underneath). The total height of the minaret is 40.28 meters with a base rectangular shaft of about 5.42 ${\times}$ 5.20 m. The second minaret is the southern minaret of Al-Sultaniya (1340 A.D, 739 H.D). It is located about 30.0 meters from Qusun minaret, and it is now standing alone but it seems that it used to be attached to a huge unidentified structure. The style of the minaret and its size attribute it to the first half of the fourteenth century. The minaret total height is 36.69 meters and has a 4.48 ${\times}$ 4.48 m rectangular base. Field investigations were conducted to obtain: (a) geometrical description of the minarets, (b) material properties of the minarets' stones, and (c) soil conditions at the minarets' location. Ambient vibration tests were performed to determine the modal parameters of the minarets such as natural frequencies and mode shapes. A $1/16^{th}$ scale model of Qusun minaret was constructed at Cairo University Concrete Research Laboratory and tested under free vibration with and without SMA wire dampers. The contribution of SMA wire dampers to the structural damping coefficient was evaluated under different vertical loads and vibration amplitudes. Experimental results were used along with the field investigation data to develop a realistic 3-D finite element model that can be used for seismic risk evaluation of the minarets. Examining the updated finite element models under different seismic excitations indicated the vulnerability of such structures to earthquakes with medium to high a/v ratio. The use of SMA wire dampers was found feasible for reducing the seismic risk for this type of structures.

• 한국구조물진단유지관리공학회 논문집
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• 제9권4호
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• pp.227-234
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• 2005

본 논문에서는 토목구조물의 스마트 모니터링 시스템을 위한 MEMS 형식의 가속도 센서를 부착한 스마트 무선 센서 장치를 설계하고 제작하였다. 그리고 다양한 성능 실험을 통하여 장치의 성능을 평가하였다. 첫째 장치에 부착한 가속도 센서의 민감도와 분해능, 잡음을 평가하기 위한 실험을 실시하였다. 실험의 결과는 센서의 데이터 쉬트의 값과 비교하여 센서의 성능을 평가 하였다. 두 번째로는 무선 센서 장치를 이용하여 상시 가진을 받는 모형구조물의 동특성을 NExT와 ERA 알고리즘을 사용하여 분석하였다. 이와 같이 분석된 동적 특성은 유한요소 해석 결과와 상호 비교하여 그 유용성을 입증하였고, 스마트 모니터링 시스템에 무선 센서 장치가 효과적으로 적용될 수 있는 가능성을 제시하였다.

• Structural Monitoring and Maintenance
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• 제1권2호
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• pp.183-195
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• 2014

Scour is one of the leading causes of overwater bridge failures worldwide. While monitoring systems have already been implemented or are still being developed, they suffer from limitations such as high costs, inaccuracies, and low reliability, among others. Also, most sensors only measure scour depth at one location and near the pier. Thus, the objective is to design a simple, low cost, scour hole topography monitoring system that could better characterize the entire depth, shape, and size of bridge scour holes. The design is based on burying a robust, waterproofed, piezoelectric sensor strip in the streambed. When scour erodes sediments to expose the sensor, flowing water excites it to cause the generation of time-varying voltage signals. An algorithm then takes the time-domain data and maps it to the frequency-domain for identifying the sensor's resonant frequency, which is used for calculating the exposed sensor length or scour depth. Here, three different sets of tests were conducted to validate this new technique. First, a single sensor was tested in ambient air, and its exposed length was varied. Upon verifying the sensing concept, a waterproofed prototype was buried in soil and tested in a tank filled with water. Sensor performance was characterized as soil was manually eroded away, which simulated various scour depths. The results confirmed that sensor resonant frequencies decreased with increasing scour depths. Finally, a network of 11 sensors was configured to form a distributed monitoring system in the lab. Their exposed lengths were adjusted to simulate scour hole formation and evolution. Results showed promise that the proposed sensing system could be scaled up and used for bridge scour topography monitoring.

• 한국정보통신학회논문지
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• 제11권8호
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• pp.1478-1485
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• 2007

본 논문에서는 풍향 풍속 계측모듈 및 DSP 센서인터페이스 회로 보드를 포함하는 기상계측 시스템을 제안한다. 이 DSP 시스템은 풍향풍속모듈, 대기압센서, 대기 온도 센서의 정보를 받아들이고, 빠르게 처리하여 PC 모니터링 시스템에 전달한다. 특히 풍향 풍속 모듈과 DSP 하드웨어는 직접 설계하여 적용한다. 풍향 풍속 모듈은 바람에 관한 벡터적 정보를 얻기 위해 4개의 박막형 RTD(Resistive Temperature Detectors) 저항센서를 히팅 코일에 의해 일정하게 가열된 원기둥 모양의 지지 표면에 벡터적으로 배치하는 구조를 채택한다. 이 구조를 채택한 계측 모듈은 진동, 습기, 부식 등에 강인하면서 정확한 계측을 가능케 한다. 센서 신호처리 회로는 TI사의 고속 DSP인 TMS320F2812 사용한다. 적용된 풍향 풍속 모듈을 통해 얻어진 데이터와 DSP 인터페이스 회로보드의 빠른 데이터 처리를 통해 저렴한 기상계측시스템을 구성 할 수 있었다.

• 한국구조물진단유지관리공학회 논문집
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• 제20권3호
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• pp.10-17
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• 2016

본 연구는 단경간 교량의 정적하중입력/변위출력관계를 이용한 새로운 교량 유한요소모델 개선 방법을 제안하였고, 실내 모형교량 실험을 통해 검증하였다. 기존의 유한요소모델개선기법은 실험으로부터 얻어진 모드계수와 유한요소모델로부터 예측된 모드계수가 유사해지도록 유한요소모델을 개선하는데, 이 과정에서 구조계의 질량행렬에 대한 가정을 필요로 한다. 제안된 기법은 질량행렬을 가정하지 않고, 오히려 질량행렬 추정을 가능하게 하는 장점을 가진다. 제안된 기법은 두 단계로 구성된다. 첫째, 정적 하중입력-변위응답으로부터 강성행렬을 개선하고, 둘째, 실측된 고유진동수를 이용하여 질량행렬을 개선한다. 실험검증을 위하여 실내 모형교량을 제작하였고, 제안된 기법을 이용하여 모형교량의 탄성계수를 추정하였으며, Universal Testing Machine으로 부터 얻어진 탄성계수와 비교하였다. 또한 기존의 유한요소모델개선기법으로 추정된 탄성계수와 비교하였다. 실험의 결과들로부터 제안된 기법이 합리적으로 탄성계수와 질량밀도를 추정하는 것이 관찰되었고, 기존의 유한요소모델개선기법은 고차모드를 사용했을 때 상대적으로 큰 오차를 주는 것이 관찰되었다. 추가적으로 유한요소모델링 오차에 대하여 토의하였다.