Ki-Young Koo

Automated Impedance-based Structural Health Monitoring Incorporating Effective Frequency Shift for Compensating Temperature Effects:

This study presents an impedance-based structural health monitoring (SHM) technique considering temperature effects. The temperature variation results in significant impedance variations, particularly a frequency shift in the impedance, which may lead to erroneous diagnostic results of real structures, such as civil, mechanical, and aerospace structures. In order to minimize the effect of the temperature variation on the impedance measurements, a previously proposed temperature compensation technique based on the cross-correlation between the reference-impedance data and a concurrent impedance data is revisited. In this study, cross-correlation coefficient (CC) after an effective frequency shift (EFS), which is defined as the frequency shift causing two impedance data to have the maximum correlation, is utilized. To promote a practical use of the proposed SHM strategy, an automated continuous monitoring framework using MATLAB® is developed and incorporated with the current hardware system. Validation of the proposed technique is carried out on a lab-sized steel truss bridge member under a temperature varying environment. It has been found that the CC values have shown significant fluctuations due to the temperature variation, even after applying the EFS method. Therefore, an outlier analysis providing the optimal decision limits under the inevitable variations has been carried out for more systematic damage detection. It has been found that the threshold level shall be properly selected considering the daily temperature range and the minimum target damage level for detection. It has been demonstrated that the proposed strategy combining the EFS and the outlier analysis can be effectively used in the automated continuous SHM of critical structural members under temperature variations.

Smart structure technologies for civil infrastructures in Korea: recent research and applications

In this paper, recent research and application activities on smart structure technologies for civil infrastructures in Korea are briefly introduced. The development of structural health monitoring systems and effective retrofit/maintenance methodologies for infrastructures has become active in Korea since the mid-1990s, as the number of the deteriorated infrastructures, mostly built on the rapidly industrialised period of the 1970s, has increased very rapidly. Discussions are made on smart sensors and non-destructive technologies, monitoring and damage assessment methods for civil infrastructures, smart monitoring of geo-systems, structural control and centre-based research on smart structure technologies.

Damage detection of shear buildings using deflections obtained by modal flexibility

This paper presents a vibration-based damage detection method for shear buildings using the damage-induced deflections estimated by modal flexibility from ambient vibration measurements. This study intends to provide a basis for the damage detection problem of more complex building structures by investigating a rather idealized structure, a shear building. From analytical investigations on the damage-induced inter-story deflection (DI-ID) of shear buildings, it was shown that the DI-ID occurs only at damaged floors and not at intact floors as long as a proper load is applied to the building. To simplify the damage detection procedure a new load concept, the positive shear inspection load (PSIL), was proposed, defined as a load producing positive shear forces at all floors. It was shown that an excessive DI-ID under a PSIL indicates the existence of damage to the building and that the floors with excessive DI-ID are the damaged floors. A simple damage index Zi is proposed based on outlier analysis to account for measurement noise. Numerical and experimental studies on a 5-story shear building were carried out for two damage scenarios with 10% column EI reductions. The proposed method was found to identify the damage locations successfully for the multiple damage case as well as the single damage case, with no false-positive or false-negative detections. For the purpose of comparison, damage detection was also conducted using the damage index method and the mode shape curvature method. The mode shape curvature method was found to be the most sensitive metric, but it showed false-positive detections on an undamaged floor. The damage index method had no false-positive detections, but it was found to miss some damage in the multiple damage case.

Temperature Analysis of a Long-Span Suspension Bridge Based on Field Monitoring and Numerical Simulation

AbstractStructural temperature is an important form of loading for bridges, particularly for long-span steel structures. In this study, the temperature distribution of the Humber Bridge in the United Kingdom is investigated using numerical simulation and field measurements. A two-dimensional fine finite-element model of a typical section of the box girder of this long-span suspension bridge is constructed. The time-dependent thermal boundary conditions are determined using field meteorological measurements with external surface heat-convection coefficients varying according to differing local wind speeds they experience. Preanalysis is adopted to determine the initial thermal condition of the model, then transient heat-transfer analysis is performed and the time-dependent temperature distribution of the bridge is obtained, leading to numerical temperature data at different locations in different times that are in good agreement with the measured counterparts. The vertical and transversal temperature diffe...

Damage prognosis by means of modal residual force and static deflections obtained by modal flexibility based on the diagonalization method

In this paper, two effective damage detection methods for localizing and quantifying structural damage in shear frames are presented. Both of them are based on the diagonalization of the stiffness matrix of shear frames. The first method is devoted to estimating structural damage by the modification of modal residual force while the second method is based on the computation of the static displacements under a unique static force by using only a few modes. The most important feature of the presented methods is their simplicity in the computation of damage. The proposed algorithms have been numerically applied to two shear frames to show that they can successfully detect and quantify damage in these structures. Also, the efficiency of the presented methods has been demonstrated through damage simulations when the modal data have been contaminated with noise. Finally, the applicability of the presented methods has been demonstrated by studying a five-story shear frame on a shaking table. All of the obtained results emphasize the robustness and good performance of the presented methods in the damage diagnosis of shear frames. (Some figures may appear in colour only in the online journal)

Operational deformations in long-span bridges

Long-span bridges deform quasi-statically and dynamically under a range of operational conditions including wind, traffic and thermal loads, in varying patterns, at different timescales and with different amplitudes. While external loads and internal forces can only rarely be measured, there are well-developed technologies for measuring deformations and their time and space derivatives. Performance data can be checked against design limits and used for validating conceptual and numerical models which can in turn be used to estimate the external loads and internal forces. Changes in performance patterns and load–response relationships can also be used directly as a diagnostic tool, but excessive deformations themselves are also a concern in terms of serviceability. This paper describes application of a range of measurement technologies, focusing on response to extreme loads, for suspension bridges over the River Tamar (with 335 m main span) and Humber (with 1410 m man span). The effects of vehicular, thermal and wind loads on these very different structures are compared, showing that apart from rare extreme traffic and wind loads, temporal and spatial temperature variations dominate quasi-static response. Observations of deformation data and sensor performance for the two bridges are used to highlight limitations and redundancies in the instrumentation.

Impedance-based structural health monitoring considering temperature effects

This paper presents an impedance-based structural health monitoring (SHM) technique considering temperature effects. The temperature variation results in a significant impedance variation, particularly a frequency shift in the impedance, which may lead to erroneous diagnostic results of real structures such as civil, mechanical, and aerospace structures. A new damage detection strategy has been proposed based on the correlation coefficient (CC) between the reference impedance data and a concurrent impedance data with an effective frequency shift which is defined as the shift causing the maximum correlation. The proposed technique was applied to a lab-sized steel truss bridge member under the temperature varying environment. It has been found, however, the CC values are still suffering from the significant fluctuation due to the temperature variation. Therefore, an outlier analysis providing the optimal decision boundary has been carried out for damage detection. From an experimental study, it has been demonstrated that a narrow cut inflicted artificially to the steel structure was successfully detected using the proposed SHM strategy.

Effect of Solar Radiation on Suspension Bridge Performance

AbstractObservations of a U.K. suspension bridge show that thermal expansion and contraction cycles do not follow simple linear relationships with a single temperature value and that time lag and temperature distribution can be significant factors. This investigation explores these effects by simulating the transient thermal and quasistatic response of the Tamar Bridge with separate finite-element models of the bridge and suspension cables. Thermal loads are determined by calculated solar radiation intensities and temperature data from the bridge monitoring system. Because cloud cover plays an important role in the levels of solar radiation, cloud coverage was estimated indirectly using monitored temperature differences between the top and bottom of the suspended structure. The results demonstrate that peak temperatures of the suspended structure and cables occur at different times. The lag is caused by differing material properties and the surfaces’ ability to absorb and lose heat. Transient phenomena ma...

Suspension bridge response due to extreme vehicle loads

A 269 tonne trailer travelled across the Tamar Suspension Bridge in October 2010, and the authors monitored the response of the structure to the load. The following investigation documents the deflection of towers and the deck during the vehicles passage, as well as the change in cable tensions. This was achieved by studying monitored data from the bridge collected by accelerometers and strain gauges attached to the stay cables, as well as two robotic total stations that measured the deflection of the mid-span and the sway of the tower saddle. These results were subsequently compared to the response predicted by a finite element (FE) model of the bridge, indicating an accurate match. The FE model was also used to simulate the variation of the dynamic response of the structure, which suggests that the natural frequencies vary depending on the vehicles location to each mode shapes anti-nodes.

A flexibility-based method via the iterated improved reduction system and the cuckoo optimization algorithm for damage quantification with limited sensors

In this paper, a novel and effective damage diagnosis algorithm is proposed to localize and quantify structural damage using incomplete modal data, considering the existence of some limitations in the number of attached sensors on structures. The damage detection problem is formulated as an optimization problem by computing static displacements in the reduced model of a structure subjected to a unique static load. The static responses are computed through the flexibility matrix of the damaged structure obtained based on the incomplete modal data of the structure. In the algorithm, an iterated improved reduction system method is applied to prepare an accurate reduced model of a structure. The optimization problem is solved via a new evolutionary optimization algorithm called the cuckoo optimization algorithm. The efficiency and robustness of the presented method are demonstrated through three numerical examples. Moreover, the efficiency of the method is verified by an experimental study of a five-story shear building structure on a shaking table considering only two sensors. The obtained damage identification results for the numerical and experimental studies show the suitable and stable performance of the proposed damage identification method for structures with limited sensors.