Thanh-Canh Huynh

Impedance-Based Cable Force Monitoring in Tendon-Anchorage Using Portable PZT-Interface Technique

In this paper, a portable PZT interface for tension force monitoring in the cable-anchorage subsystem is developed. Firstly, the theoretical background of the impedance-based method is presented. A few damage evaluation approaches are outlined to quantify the variation of impedance signatures. Secondly, a portable PZT interface is designed to monitor impedance signatures from the cable-anchorage subsystem. One degree-of-freedom analytical model of the PZT interface is established to explain how to represent the loss of cable force from the change in the electromechanical impedance of the PZT interface as well as reducing the sensitive frequency band by implementing the interface device. Finally, the applicability of the proposed PZT-interface technique is experimentally evaluated for cable force-loss monitoring in a lab-scaled test structure.

Vibration Characteristics of Gravity-Type Caisson Breakwater Structure with Water-Level Variation

Vibration characteristics of gravity-type caisson breakwater structures which have water-level variations are experimentally examined by using wave load excitations. To achieve the objective, the following approaches are implemented. Firstly, vibration analysis methods are selected to examine the dynamic characteristics of the lab-scale caisson. Secondly, vibration test on a lab-scale caisson which is installed in a two-dimensional wave flume is performed under several excitation sources and water levels. Thirdly, the compatibility of the wave-induced vibration responses is evaluated by comparison with the impact vibration responses, and the appropriate vibration analysis method is selected. Finally, the water-level effects on the caisson breakwater are examined based on the modal parameters which are experimentally measured for the 2D wave flume tests.

Effect of Temperature Variation on Vibration Monitoring of Prestressed Concrete Girders

The effect of temperature variation on vibration monitoring of prestressed concrete (PSC) girders is experimentally analyzed. Firstly, vibration features such as autoregressive (AR) coefficient, correlation coefficient of power spectral density (CC of PSD), natural frequency, and mode shape are selected to estimate the effect of temperature variation on vibration characteristics of PSC girders. Secondly, vibration experiments on a lab-scale PSC girder are performed under the condition of temperature variation. Finally, the vibration features with respect to the temperature variation are analyzed to estimate the effect of temperature in vibration characteristics of the PSC girder.

Feasibility Verification of Mountable PZT-Interface for Impedance Monitoring in Tendon-Anchorage

This study has been motivated to numerically evaluate the performance of the mountable PZT-interface for impedance monitoring in tendon-anchorage. Firstly, electromechanical impedance monitoring and feature classification methods are outlined. Secondly, a structural model of tendon-anchorage subsystem with mountable PZT-interface is designed for impedance monitoring. Finally, the feasibility of the mountable PZT-interface is numerically examined. A finite element (FE) model is designed for the lab-scaled tendon-anchorage. The FE model of the PZT-interface is tuned as its impedance signatures meet the experimental test results at the same frequency ranges and also with identical patterns. Equivalent model properties of the FE model corresponding to prestress forces inflicted on the lab-tested structure are identified from the fine-tuning practice.

Wave-Induced Vibration Monitoring for Stability Assessment of Harbor Caisson

Up-to-date structural health monitoring (SHM) studies have been focused mostly on in-land structures such as bridge and building. Only a few research efforts have been made for harbor structures such as caisson-type breakwater. For stability assessment of harbor caisson structures, it is very essential to monitor vibration responses with limited accessibility, to analyze the vibration features, and to specify the sensitive motions with respect to damage in the caisson-foundation’s interface. In this paper, a wireless sensing system for SHM of harbor caisson structures is presented. To achieve the objective, the following approaches were implemented. First, a wave-induced vibration sensing system was designed for global structural health monitoring. Second, global SHM methods which are suitable for damage monitoring of caisson structures were selected to alarm the occurrence of unwanted behaviors. Third, a SHM scheme was designed for the target structure by implementing the selected SHM methods. Operation logics of the SHM methods were programmed based on the concept of the wireless sensor network. Finally, the performance of the proposed system was globally evaluated for a field harbor caisson structure for which a series of tasks were experimentally performed by the wireless sensing system.

Evaluation of Vibration Characteristics of an Existing Harbor Caisson Structure Using Tugboat Impact Tests and Modal Analysis

Ambient and forced vibration tests are proposed to evaluate dynamic characteristics of a caisson-type breakwater, including natural frequencies and modal damping ratios. The feasibility of a numerical analysis model with fluid-structure-soil interaction effects, which play an important role in evaluating structural performance and safety, was investigated by comparing the numerical results with experimental results. The Oryukdo breakwater in Busan, Republic of Korea, was employed as the target structure. This breakwater was once heightened by installing additional parapet structures of about 4 m tall to improve the harbor tranquility in 2005. Vibration tests were carried out in 2000 (before heightening) and in 2011 (after heightening). Most caissons were tested in the first test, while only one caisson was tested in 2011. It was found that natural frequencies were reduced by 1.7%–4.3% after heightening, and similar results were observed from the numerical analysis. It was also found that forced vibration tests can yield more reasonable results than ambient vibration tests. Even though there is some discrepancy between experimental and numerical results, numerical analysis can be carried out to analyze dynamic characteristics and evaluate structural performance and safety.

Structural Damage Monitoring of Harbor Caissons with Interlocking Condition

The objective of this study is to monitor the health status of harbor caissons which have potential foundation damage. To obtain the objective, the following approaches are performed. Firstly, a structural damage monitoring(SDM) method is designed for interlocked multiple-caisson structures. The SDM method utilizes the change in modal strain energy to monitor the foundation damage in a target caisson unit. Secondly, a finite element model of a caisson system which consists of three caisson units is established to verify the feasibility of the proposed method. In the finite element simulation, the caisson units are constrained each other by shear-key connections. The health status of the caisson system against various levels of foundation damage is monitored by measuring relative modal displacements between the adjacent caissons.

FOS-Based Prestress Force Monitoring and Temperature Effect Estimation in Unbonded Tendons of PSC Girders

AbstractIn this study, the effect of temperature variation on prestress force monitoring by fiber Bragg grating (FBG) sensors embedded in prestressing tendons of prestressed concrete (PSC) girders is estimated. First, a fiber optic sensor (FOS)-based prestress force monitoring method is proposed for PSC girders with unbonded tendons. A temperature-effect estimation method is modeled to theoretically estimate the change of prestress force due to the temperature variation. Second, lab-scale experiments are performed on a PSC girder with a FBG sensor-embedded smart tendon. A series of temperature-variation and prestress-loss events are simulated for the PSC girder. Third, the feasibility of the FOS-based monitoring method is experimentally evaluated for the prestress-loss cases under constant temperature. Finally, the effect of temperature variation on the FBG sensor-embedded tendon is evaluated by the temperature-effect estimation method.

Impedance monitoring at tendon-anchorage via mountable PZT interface and temperature-effect compensation

In this study, the pre-stress force in pre-stressed concrete (PSC) girders is monitored via mountable PZT interface under varying temperature. Firstly, an impedance-based technique using mountable PZT interface is proposed for pre-stress-loss monitoring in tendon-anchorage systems. A cross correlation-based temperature-effect compensation algorithm using an effective frequency shift (EFS) of impedance signatures is visited. Secondly, lab-scale experiments are performed on a PSC girder instrumented with a mountable PZT interface at tendon-anchorage. A series of temperature variation and pre-stress-loss events are simulated for the lab-scale PSC girder. Thirdly, the feasibility of the mountable PZT interface for pre-stress-loss monitoring in tendon-anchorage is experimentally verified under constant temperature conditions. Finally, the PZT interface device is examined for pre-stress-loss monitoring under temperature changes to validate its applicability. The temperature effect on impedance signatures is compensated by minimizing cross-correlation deviation between impedance patterns of the mountable PZT interface.

Bolt-loosening identification of bolt connections by vision image-based technique

In this study, an algorithm using image processing techniques is proposed to identify bolt-loosening in bolted connections of steel structures. Its basic concept is to identify rotation angles of nuts from a pictured image, and is mainly consisted of the following 3 steps: (1) taking a picture for a bolt joint, (2) segmenting the images for each nut by image processing techniques, and (3) identifying rotation angle of each nut and detecting bolt-loosening. By using the concept, an algorithm is designed for continuous monitoring and inspection of the bolt connections. As a key imageprocessing technique, Hough transform is used to identify rotation angles of nuts, and then bolt-loosening is detected by comparing the angles before and after bolt-loosening. Then the applicability of the proposed algorithm is evaluated by experimental tests for two lab-scaled models. A bolted joint model which consists of a splice plate and 8 sets of bolts and nuts with 2×4 array is used to simulate inspection of bridge connections, and a model which is consisted of a ring flange and 32 sets of bolt and nut is used to simulate continuous monitoring of bolted connections in wind turbine towers.