- Hester, David

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# Hester, David

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- PublicationAn investigation of the changes in the natural frequency of a pile affected by scourScour around bridge foundations is one of the leading causes of bridge failure. Up until recently, the monitoring of this phenomenon was primarily based around using underwater instrumentation to monitor the progression of scour holes as they develop around foundation systems. VibrationÂ¿based damage detection techniques have been used to detect damage in bridge beams. The application of these vibration based methods to the detection of scour has come to the fore in research in recent years. This paper examines the effect that scour has on the frequency response of a driven pile foundation system, similar to those used to support road and rail bridges. The effect of scour on the vibration characteristics of the pile is examined using laboratory and field testing. It is clear that there is a very clear reduction in the natural frequency of the pile as the severity of scour increases. It is shown that by combining state-of-the-art geotechnical techniques with relatively simple finite element modelling approaches, it is possible to accurately predict the natural frequency of the pile for a given scour depth. Therefore, the paper proposes a method that would allow the estimation of scour depth for a given observed pile frequency.
540ScopusÂ© Citations 132 - PublicationA study of the influence of slag alkali level on the alkali-silica reactivity of slag concreteGround granulated blast furnace slag (ggbs), can reduce the alkali load in concrete, despite its relatively high alkali content. Most research has been devoted to the efficacy of slag with an alkali content of less than 1.0% and this is reflected in guidance documents. A comparative assessment was made of the effect, if any, of the alkali level of ggbs on potential alkali-silica reactivity. Expansion tests were performed on a matrix of concrete mixes using Irish normal Portland cement, two slags of differing alkali content, three aggregates and alkali loads of 5 and 6 kg Na2Oeq./m(3). A replacement level of 50% was used throughout. No significant difference in behaviour was apparent, irrespective of aggregate type or alkali load, indicating that the alkali level of the slag is not a contributory factor at the 50% replacement level.
ScopusÂ© Citations 40 1161 - PublicationDevelopment of a Vehicle-Bridge-Soil Dynamic Interaction Model for Scour Damage ModellingDamage detection in bridges using vibration-based methods is an area of growing research interest. Improved assessment methodologies combined with state-of-the-art sensor technology are rapidly making these approaches applicable for real-world structures. Applying these techniques to the detection and monitoring of scour around bridge foundations has remained challenging; however this area has gained attraction in recent years. Several authors have investigated a range of methods but there is still significant work required to achieve a rounded and widely applicable methodology to detect and monitor scour. This paper presents a novel Vehicle-Bridge-Soil Dynamic Interaction (VBSDI) model which can be used to simulate the effect of scour on an integral bridge. The model outputs dynamic signals which can be analysed to determine modal parameters and the variation of these parameters with respect to scour can be examined. The key novelty of this model is that it is the first numerical model for simulating scour that combines a realistic vehicle loading model with a robust foundation soil response model.This paper provides a description of the model development and explains the mathematical theory underlying the model. Finally a case study application of the model using typical bridge, soil, and vehicle properties is provided.
358ScopusÂ© Citations 31 - PublicationThe use of wavelets on the response of a beam to a calibrated vehicle for damage detectionThe monitoring of the dynamic properties of a structure as a tool to detect structural damage has been prevalent for some time. The fundamental theory behind this branch of non-destructive testing is that, if a crack was present in a structure it results in a localised loss in stiffness, which leads to a change in the modal properties of the structure. However, while it is reasonably easy to diagnose that a structure has suffered damage, the more challenging problem is to identify its location and severity. In recent years the wavelet transform has been used to locate discontinuities in measurements that could be associated to structural damage. This paper highlights the potential for using the wavelet technique to analyse the response of a beam to the passage of a moving load.
117 - PublicationEmpirical mode decomposition of the acceleration response of a prismatic beam subject to a moving load to Identify multiple damage locationsEmpirical Mode Decomposition (EMD) is a technique that converts the measured signal into a number of basic functions known as intrinsic mode functions. The EMD-based damage detection algorithm relies on the principle that a sudden loss of stiffness in a structural member will cause a discontinuity in the measured response that can be detected through a distinctive spike in the filtered intrinsic mode function. Recent studies have shown that applying EMD to the acceleration response, due to the crossing of a constant load over a beam finite element model, can be used to detect a single damaged location. In this paper, the technique is further tested using the response of a discretized finite element beam with multiple damaged sections modeled as localized losses of stiffness. The ability of the algorithm to detect more than one damaged section is analysed for a variety of scenarios including a range of bridge lengths, speeds of the moving load and noise levels. The use of a moving average filter on the acceleration response, prior to applying EMD, is shown to improve the sensitivity to damage. The influence of the number of measurement points and their distance to the damaged sections on the accuracy of the predicted damage is also discussed.
314ScopusÂ© Citations 43 - PublicationImpact of Road Profile when Detecting a Localised Damage from Bridge Acceleration Response to a Moving VehiclePrevious work by the authors have shown that the acceleration response of a damaged beam subject to a constant moving load can be assumed to be made up of three components: 'dynamic', 'static' and 'damage'. Therefore, appropriate filtering of the acceleration signal can be used to highlight the â€˜damageâ€™ component and quantify its severity. This paper builds on these findings to examine if the same approach can be used to identify damage in the more realistic case of a bridge loaded by a sprung vehicle travelling on a road profile. The consideration of a road profile has the effect of exciting the vehicle modes of vibration which will corrupt the spectrum of bridge accelerations with road/vehicle frequencies. Some of these vehicle frequencies may be lower than the first frequency of the bridge and close to the frequency of the 'damage' component. In the latter, the vehicle frequencies are difficult to remove without also filtering part of the 'damage' component out. As a result, the approach is shown to perform best for low vehicle speeds.
434ScopusÂ© Citations 6 - PublicationA wavelet-based damage detection algorithm based on bridge acceleration response to a vehiclePrevious research based on theoretical simulations has shown the potential of the wavelet transform to detect damage in a beam by analysing the time-deflection response due to a constant moving load. However, its application to identify damage from the response of a bridge to a vehicle raises a number of questions. Firstly, it may be difficult to record the difference in the deflection signal between a healthy and a slightly damaged structure to the required level of accuracy and high scanning frequencies in the field. Secondly, the bridge is going to have a road profile and it will be loaded by a sprung vehicle and time-varying forces rather than a constant load. Therefore, an algorithm based on a plot of wavelet coefficients versus time to detect damage (a singularity in the plot) appears to be very sensitive to noise. This paper addresses these questions by: (a) using the acceleration signal, instead of the deflection signal, (b) employing a vehicle-bridge finite element interaction model, and (c) developing a novel wavelet-based approach using wavelet energy content at each bridge section, which proves to be more sensitive to damage than a wavelet coefficient line plot at a given scale as employed by others.
1885ScopusÂ© Citations 188 - PublicationEmpirical mode decomposition of the acceleration response of a prismatic beam subject to a moving load to identify multiple damage locations(2011-06)
; ; Empirical Mode Decomposition (EMD) is a technique that converts the measured signal into a number of basic functions known as Intrinsic Mode Functions (IMFs). The EMD-based damage detection algorithm relies on the principle that a sudden loss of stiffness in a structural member will cause a discontinuity in the measured response that can be detected through a distinctive spike in the filtered IMF. Recent studies have shown that applying EMD to the acceleration response, due to the crossing of a constant load over a beam finite element model, can be used to detect a single damaged location. In this paper, the technique is further tested using simulations of a beam with multiple damaged sections. The use of a moving average filter on the acceleration response, prior to applying EMD, is also investigated. A bridge deck is modelled as a series of discretized beam elements where a loss of stiffness is introduced at some random locations. The ability of the EMD algorithm to detect more than one damaged section is analysed for a variety of scenarios including a range of bridge lengths, speeds of the moving load and noise levels. The influence of the number of measurement points and their distance to the damaged locations on the accuracy of the predicted damage is also discussed.286 - PublicationAn Investigation into the Acceleration Response of a Damaged Beam-Type Structure to a Moving ForceIn recent years there have been a growing number of publications on procedures for damage detection in beams from analysing their dynamic response to the passage of a moving force. Most of this research demonstrates their effectiveness by showing that a singularity that did not appear in the healthy structure is present in the response of the damaged structure. This paper elucidates from first principles how the acceleration response can be assumed to consist of 'static' and 'dynamic' components, and where the beam has experienced a localised loss in stiffness, an additional 'damage' component. The combination of these components establishes how the damage singularity will appear in the total response. For a given damage severity, the amplitude of the 'damage' component will depend on how close the damage location is to the sensor, and its frequency content will increase with higher velocities of the moving force. The latter has implications for damage detection because if the frequency content of the 'damage' component includes bridge and/or vehicle frequencies, it becomes more difficult to identify damage. The paper illustrates how a thorough understanding of the relationship between the 'static' and 'damage' components contributes to establish if damage has occurred and to provide an estimation of its location and severity. The findings are corroborated using accelerations from a planar finite element simulation model where the effects of force velocity and bridge span are examined.
549ScopusÂ© Citations 66 - PublicationEffect of vehicle velocity on exciting the lateral dynamic response of two-span integral bridges(Civil Engineering Research Association of Ireland, 2016-08-30)
; ; Vibration-based Structural Health Monitoring (SHM) is an area of ongoing research and has received much attention from researchers in recent years. Online damage detection methods for bridges rely on placing sensors on the structure to detect anomalies in measured parameters such as acceleration, frequency or displacement among others. Changes in these parameters can be used to infer the presence of damage such as cracking in bridge beams, foundation scour etc. These methods mostly rely on using the signals arising on a bridge from ambient traffic or environmental loading. For foundation scour detection purposes, the lateral response of a bridge is of particular interest in that this has been shown to be particularly sensitive to the scour phenomenon. Vehicle-Bridge Interaction (VBI) effects can have a significant influence on the condition of output vibrations from a bridge element. In this paper, the effect of vehicle travelling velocity on the lateral response of a typical highway two-span integral bridge is investigated. It is shown that depending on the velocity of the vehicle relative to the oscillatory period of the bridge it traverses, the bridge's dynamic response is either amplified or diminished by varying degrees.This phenomenon could influence the accuracy of a particular damage detection method relying on output system vibrations to infer damage.303