Now showing 1 - 10 of 19
  • Publication
    Modal analysis of a bridge using short-duration accelerations
    The application of unmanned aerial vehicle technology to bridge structural health monitoring has become a hot research topic due to its low cost, safety and high energy efficiency. However, flight duration and battery life are substantial technical limitations. Is a short data burst sufficient for damage detection? This paper intends to answer this question by developing a novel approach based on frequency domain decomposition to obtain the mode shapes from a short data burst. Then, the modal assurance criterion is used as an indicator of the differences between the estimated mode shapes from the short data burst and the exact eigenvectors from finite element analysis. Here, the short data burst is obtained from the simulated acceleration response of a bridge beam model due to the crossing of two quarter-cars. A new damage indicator based on the modal assurance criterion profile along the beam is proposed to locate and quantify damage.
  • Publication
    Finite Element Updating using Cross-Entropy Combined with Random-Field Theory
    (Civil-Comp Press, 2014-09) ; ;
    In this paper, the possibility of introducing random field theory into the cross-entropy algorithm is studied. Cross-entropy algorithm is an optimization process that Walsh and González (2009) use to estimate the stiffness distribution of a structure given a set of displacements. Although this method has been successfully tested, many lines of improvement are still opened. Random field theory is incorporated into the algorithm in an attempt to account for spatial variability of stiffness throughout the structure. For this purpose, a correlation function, variable in space, is defined and, as a result, a modification of the algorithm is proposed. The modified algorithm is then tested using numerical simulations in a scenario consisting of a simply supported beam.
  • Publication
    Estimation of structural parameters using static loading tests
    Static damage detection techniques make use of the observed change in the structure's static response (i.e., displacement, strain) to identify, locate and quantify damage. For a given load, the static response is related to the boundary conditions and distribution of structural stiffness throughout the structure. As damage can be characterised as a local reduction in structural stiffness, analysis of the structure's static response has been used extensively to locate and quantify damage. In recent years many new static damage detection techniques have been proposed, with most of the research being focused on the development of finite element based algorithms to estimate the stiffness of the structure. This paper reviews these techniques and discusses their potential when using modern surveying techniques.
  • Publication
    The use of accelerometers in UAVs for bridge health monitoring
    (Seoul National University, 2019-05-26) ; ;
    Unmanned Aerial Vehicles (UAVs) technology has gained considerable popularity in bridge structural health monitoring for its strengths, such as low cost, safety and high energy efficiency. This paper envisions a scenario in which accelerometers are mounted onto UAVs, which then are able to gather acceleration signals by self-attaching to the bridge. However, battery life is an issue in UAVs with the subsequent limitation in the duration of the measurements. Therefore, this paper carries out a simulation on mode shape extraction from a short data burst by utilising an output only technique, the so-called frequency domain decomposition (FDD). Modal assurance criterion (MAC) is used as a statistical indicator to check differences between the estimated mode shapes and the eigenvectors from finite element analysis. The short acceleration response is generated using a planar vehicle-bridge interaction system where the moving load is modelled as two quarter-cars and the bridge is modelled as a simply supported beam. The impact of signal noise, vehicle speed and signal duration on the accuracy of the estimated mode shapes is investigated. FDD is shown to achieve high values of MAC even for short data bursts. Damping ratio is identified as a significant source of MAC discrepancy in the extraction of mode shapes. The stiffness loss due to a crack is introduced in the beam to evaluate how damage affects the mode shape compared to operational effects. How the MAC values vary with crack location and damage severity is discussed for the first three mode shapes.
  • Publication
    Accuracy of instantaneous frequencies predicted by the Hilbert-Huang transform for a bridge subjected to a moving vehicle
    This paper investigates the accuracy of the Hilbert-Huang transform (HHT) in capturing the time-varying frequencies of a short-span bridge traversed by a vehicle travelling at a constant speed. The bridge and vehicle are modelled as a simply supported beam and a quarter-car, respectively. The HHT uses empirical mode decomposition to divide the original signal into mono-component signals, called intrinsic mode functions (IMFs), where the Hilbert transform can extract instantaneous frequencies (IFs). Each IMF is associated with a dominant frequency band, although mode mixing is possible. In order to improve the predicted frequencies, several filters are applied before and after performing the HHT with a threefold purpose: (i) to remove the static component, (ii) to isolate the first mode of vibration, and (iii) to obtain meaningful and denoised IFs. The influences of a localized stiffness loss in the bridge, different vehicle speeds, and three signal-to-noise ratios on the results are discussed.
  • Publication
    Academic Advising in Civil Engineering: design and evaluation of a hybrid model
    A project to formalise and expand Academic Advising has been implemented at the UCD Civil Engineering School. The goals of this project were twofold: on the one hand, it aimed at training faculty members in Academic Advising roles and providing them with the necessary resources. On the other hand, the project sought to expand student interaction, in particular by engaging students informally in order to build a rapport between them and the academic advisors that we expect will bring long term benefits. The resulting model combines elements of both the prescriptive, e.g., formal training, informative talks on key topics, and developmental approaches, e.g., coffee mornings for students and faculty members. The evaluation of the project was carried out through questionnaires and focus groups. It highlighted very positive feedback from the students, who find these new lines of communication with the academic staff to be useful and productive.
  • Publication
    Extraction of dynamic features from short acceleration data bursts: a review
    (Canadian Society for Civil Engineering, 2018-08-03) ; ;
    It is well known that structural damage can lead to changes in dynamic features such as frequencies, mode shapes, damping, vibration intensity, etc. Signal processing tools available to extract these features include Wavelet analysis, Fourier and Hilbert-Huang transforms. Acceleration data is typically used as input to these tools, given that it is a type of response with a relatively high dynamic component (i.e., oscillations in the response due to inertial forces of the structure) in relation to the static component (i.e., derived from time-varying static deflections as a result of time/spatial-varying loads). Almost all traditional signal processing approaches require access to long-time data sets. For instance, long periods of acceleration and multiple measurement points allow engineers to accurately define the mode shapes of a structure. In this paper, a scenario is envisioned where drones are used to charge sensors placed on bridges as well as to acquire the data recorded by the sensors for processing. The novelty is the challenge of monitoring structural condition in the context of acquiring limited quantities of data. The latter requires being able to deal with a very significant impact of edge effects and the loss of resolution due to the short duration of the signal. This paper reviews attempts to obtain bridge dynamic features overcoming these limitations, i.e., via multi-stage measurements as in the case of the Short Time Frequency Domain Decomposition method.
  • Publication
    Impact of short-duration acceleration records on the ability of signal processing techniques to derive accurate bridge frequencies
    This paper envisions a scenario in which unmanned aerial vehicles gather data from low-cost and flexible wireless sensor networks, i.e., accelerometers. However, flight duration, coupled with limited sensor battery time, is a substantial technical limitation. In order to assess the impact of these constraints on bridge monitoring, this paper analyses the extraction of bridge dynamic features from short-duration acceleration records. The short acceleration record is simulated using the theoretical response of a simply supported beam subjected to a moving load. Estimated frequencies are obtained in free vibration and compared with the natural frequencies calculated from formula. Given that short records limit the resolution in the frequency domain, the error in the prediction of frequencies will typically decrease as the duration of the signal increases. Signal processing techniques for extracting dynamic features include the Fast Fourier Transform, Frequency Domain Decomposition, Continuous Wavelet Transform and Hilbert-Huang Transform. This paper carries out an assessment of the accuracy of these signal processing techniques in extracting frequencies as a function of the duration of the measurements. Edge effects and loss of resolution are shown to remain key issues to be addressed when the duration of the signal is too short.
  • Publication
    Numerical analysis of techniques to extract bridge dynamic features from short records of acceleration
    The use of drones in Structural Health Monitoring (SHM) to charge sensors mounted on a bridge and download their data has gathered interest over the last years. This approach presents the advantage of avoiding the need for long cables running over the bridge or for permanent access to electric power on site. Nonetheless, limitations exist regarding the amount of data that can be transmitted through this method. In contrast to traditional approaches to SHM relying on long records to assess the condition of a structure, the scenario envisioned here deals with short amounts of data. In this paper, specific methodologies for extraction of dynamic features from short data bursts of acceleration signal are tested through numerical simulations. The bridge is modelled as a simply supported finite element beam model that is excited by a series of moving concentrated forces, which represent a random traffic load. Initial conditions are varied allowing for scenarios in which the acceleration record may start once the vehicle is already on the bridge, finish before its exit or combine periods of free and forced vibration. The theoretical acceleration response is obtained for healthy and damage conditions of the bridge, and then corrupted with noise. Focus is placed on how effective these techniques are in overcoming the shortcuts derived from noise and from the short duration of the signal. Therefore, techniques to mitigate common problems such as mode-mixing and edge effects are investigated.
  • Publication
    Evaluation of the True Behaviour of the End Supports in the Carbajal de La Legua Old Bridge
    Bridges are prone to bearing deterioration due to environmental causes or traffic overloading. For example, a bridge originally designed as simply supported may be behaving differently due to bearing aging and deterioration. The latter can lead to damage, not only to the bearings, but also to the rest of the structure. In this paper, load testing is used to evaluate the true behavior of the boundary conditions of an existing bridge and how they affect the overall structural response. For this purpose, a 16 m span concrete bridge is subjected to the forces applied by a truck loaded with quarry aggregate. Static measurements are obtained for several positions of the vehicle while dynamic ones are also obtained as the truck crosses the bridge at speed. These data is used to tune a 3D finite element model until achieving an accurate resemblance with the measurements. Boundary conditions are idealized with linear springs, which allow to consider all possible scenarios at both ends. Finally, spring constants that best fit the measurements are used to measure the impact of the boundary conditions on the distribution of internal forces in the structure.