Now showing 1 - 10 of 14
  • Publication
    Bridge scour monitoring using accelerometers placed on bridge piers - a numerical investigation
    Bridge scour has been identified as one of the leading causes of bridge failures in recent years, and caused the collapse of the Malahide Viaduct, a mainline railway bridge on the TEN-T network, in North Dublin in 2009. Scour is the term given to the removal of supporting soils from around foundations as a result of increased hydraulic stresses, a process exacerbated in times of heavy flooding. Scour is notoriously difficult to predict due to its dependence on many uncertain hydraulic parameters and difficulties in determining the exact condition of underwater foundations. This is a particular problem for rail bridges which may have been built more than 150 years ago. Protecting structures against scour generally involves relatively expensive installations that may not prove economically viable for widespread usage. Therefore, monitoring of scour-critical infrastructure is an attractive solution for infrastructure managers. The most common method for scour monitoring is to undertake visual inspections using divers to obtain the depth of scour around a foundation. This can prove time consuming and be particularly dangerous in times of heavy flooding, when scour is most critical. This paper charts the development of a new method of scour monitoring that uses the bridge structure itself to indicate the severity of the scour problem. Scour causes a reduction in foundation stiffness by removing soil from around foundation elements. This can manifest itself as a change in the natural frequency of bridge piers, due to the increase in effective length and reduction in soil stiffness. This paper presents a numerical examination of the effect of scour on the frequency response of a range of bridge pier geometries. The purpose is to show the effectiveness of the technique at remotely monitoring scour using accelerometers placed above the waterline.
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  • Publication
    The effect of scour on the dynamic response of an offshore wind turbine
    (Civil Engineering Research Association of Ireland, 2014-08-29) ; ;
    Wind turbines are dynamically sensitive structures, with excitation forces arising from the rotor spinning at a specific angular velocity and the blade passing the turbine tower at a set frequency. It is critical that the structural design of the turbine is undertaken in such a way that the system natural frequency resides away from the resonant excitation bands. This will become increasingly important over the next decade as turbines evolve and the operational frequency bands change. The system natural frequency is governed by the structural properties of the turbine tower and the nacelle weight, combined with the stiffness of the soil-foundation elements. An accurate estimate of the soil stiffness is crucial to ensure a realistic model of the overall turbine behaviour. Over 75% of offshore wind turbines currently have monopile foundations, which are designed as a soft-stiff system, with the functional design frequency of the turbine structure falling between the upper and lower excitation frequency bands. The dynamic stability of the foundation is provided by the interaction between the monopile shaft and the adjacent soil strata. In this paper, the effect of scour on the frequency response of an offshore wind turbine is investigated numerically for a range of different soil densities. The turbine system is modelled using simple numerical modelling techniques. Euler-Bernoulli beam elements are used to model the tower. Altered versions of these elements are used to model the monopile, with an extra node and degree of freedom to allow the input of lateral soil stiffness into the model. The nacelle and rotor system is modelled as a lumped mass at the top of the turbine tower. In-situ CPT-based approaches are used for modelling the soil stiffness. The effect of scour is investigated for each design case.
      202
  • Publication
    Effect 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.
      293
  • Publication
    Dynamic soil-structure interaction modeling using stiffness derived from in-situ Cone Penetration Tests
    This paper presents the results of an experimental and numerical investigation into the natural frequency of a pile driven into dense sand. The experimental arrangement involves fitting accelerometers along the pile shaft and using a modal hammer to induce lateral vibration. The natural frequency is obtained by performing Fourier analysis on the acceleration signals. A numerical model is developed that models the pile as a beam supported by lateral springs. The natural frequency is obtained by performing an eigenvalue analysis in the numerical model. The spring stiffness is derived by first obtaining the G0 value for the sand at the installation location. This is achieved using the rigidity index, a correlation between the cone tip resistance qc value and the small-strain shear modulus G0. The G0 value is converted to lateral spring stiffness values using an equation derived analytically from the beam on an elastic foundation case. Good agreement is observed between the experimentally measured natural frequency and that which is calculated from the numerical model. This research paves the way for more accurate assessments of dynamic soil-structure interaction, and can be particularly useful in the design of structures that are dynamically sensitive such as wind turbines.
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  • Publication
    Monitoring of scour critical bridges using changes in the natural frequency of vibration of foundation piles - A field investigation
    The highly publicised failure of the Malahide Viaduct Railway Bridge in Ireland in 2009 was attributed to erosion of the supporting soils, commonly referred to as foundation scour. This is a widespread geotechnicalstructural problem that has been identified as the main cause of failure of bridges in the United States. Monitoring scour is of significant importance to ensure the continued safe operation of the ageing bridge asset network. Most of the current monitoring regimes rely on expensive underwater instrumentation that is often subject to damage during times of flooding, when scour risk is at its highest. In this paper, a novel scour monitoring approach based on dynamic measurement techniques is described. The investigation involves the mounting of accelerometers on the structure of interest, which may be used as a non-intrusive monitoring scheme above the waterline. A significant advantage of this method over traditional scour monitoring approaches is that the structure itself is used to monitor the scour damage.
      234
  • Publication
    Development of a Vehicle-Bridge-Soil Dynamic Interaction Model for Scour Damage Modelling
    (Hindawi Publishing Corporation, 2015) ; ;
    Damage 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.
      353Scopus© Citations 31
  • Publication
    A review of bridge scour monitoring techniques
    The high profile failure of the Malahide Viaduct, in Dublin, Ireland which is a part of the EU TEN-T network of critical transport links was caused by foundation scour. In a study of five hundred bridge failures that occurred in the United States between 1989 and 2000, flooding and scour were the cause of 53% of the recorded failures (Wardhana and Hadipriono 2003). Scour is a common soil-structure interaction problem. In light of current changes in climate, increased frequency of flooding, coupled with the increased magnitude of these flood events, will lead to a higher risk of bridge failure occurring. Monitoring scour is of paramount importance to ensure the continued safe operation of the aging bridge asset network. Most monitoring regimes are based on using expensive underwater instrumentation that can often be subject to damage during times of flooding, when scour risk is at its highest. This paper presents a critical review of existing scour monitoring equipment and methodologies with a particular focus on those that use the dynamic response of the structure to indicate the existence and severity of the scour phenomenon affecting the structure. A sensitivity study on a recently developed monitoring method is also undertaken.
    Scopus© Citations 208  550
  • Publication
    A comparison of initial stiffness formulations for small-strain soil-pile dynamic Winkler modelling
    Dynamic Soil-Structure Interaction (DSSI) is an area of much ongoing research and has wide and varied applications from seismic response analysis to offshore wind foundation response. DSSI covers a wide range of load regimes from small-strain vibrations to large-strain cyclic loading. One of the most common ways to model DSSI uses the Winkler model, which considers the soil as a series of mutually independent springs. The difficulty with modelling DSSI arises with the inelastic and nonlinear load–displacement response of soil with increasing strain, therefore modelling of large-strain DSSI relies on the specification of many interrelated parameters. The relative magnitude of these parameters can have a significant effect on the modelled response. In this paper, the specification of an initial stiffness coefficient to model the elastic (small-strain) response of a soil–pile system is investigated. The coefficient of subgrade reaction method can be used to generate spring stiffness moduli for Winkler type models. A number of subgrade reaction theories have been proposed and their application to the problem of static loading has been widely studied. However, relatively little research concerning the application of these models for small-strain dynamic loading has been undertaken. This paper describes a sensitivity study in which a number of subgrade reaction models were used to estimate the frequency response at small-strain levels for a range of pile geometries and ground conditions. A field investigation was undertaken on two piles with different slenderness ratios to estimate the frequency response and damping ratios. The experimental results were compared to predictions of damped natural frequency obtained from numerical models using the force input and measured damping ratio from each experiment. The ability of each subgrade reaction formulation to model the response at small-strain levels is evaluated.
    Scopus© Citations 58  488
  • Publication
    An investigation of the changes in the natural frequency of a pile affected by scour
    Scour 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. Vibration¿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.
    Scopus© Citations 130  522
  • Publication
    Design Tools Available For Monopile Engineering
    (European Wind Energy Association, 2014-03-13) ; ; ;
    Monopiles have been by far the most common support structure used for offshore turbines, with approximately 75% of existing wind farms founded on these large diameter steel tubes EWEA(2014). However, despite the widespread prevalence of monopiles across the wind sector, the design tools commonly used by industry have typically evolved from those developed by the oil and gas sector, which apply to significantly different design conditions. This paper introduces some of the design approaches available in practice and identifies some of the limitations of current offshore codes. Finite Element Methods (FEM) are suggested as a means of more accurately considering offshore soil behaviour, although the importance of accurate calibration of these models against real data (lab and/or field data) is stressed. Novel means of determining the in-situ frequency response are also discussed and the potential implications for monopile design at different sites. Finally, some design aspects of XL monopiles are considered that suggest monopiles may be pushed into ever increasing water depths.
      352