David Hester

Determining the presence of scour around bridge foundations using vehicle-induced vibrations

Bridge scour is the number one cause of failure in bridges located over waterways. Scour leads to rapid losses in foundation stiffness and can cause sudden collapse. Previous research on bridge health monitoring has used changes in natural frequency to identify damage in bridge beams. The possibility of using a similar approach to identifying scour is investigated in this paper. To assess if this approach is feasible, it is necessary to establish how scour affects the natural frequency of a bridge, and if it is possible to measure changes in frequency using the bridge dynamic response to a passing vehicle. To address these questions, a novel vehicle–bridge–soil interaction (VBSI) model was developed. By carrying out a modal study in this model, it is shown that for a wide range of possible soil states, there is a clear reduction in the natural frequency of the first mode of the bridge with scour. Moreover, it is shown that the response signals on the bridge from vehicular loading are sufficient to allow these changes in frequency to be detected.

Empirical mode decomposition of the acceleration response of a prismatic beam subject to a moving load to identify multiple damage locations

Empirical 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.

Development of a Vehicle-Bridge-Soil Dynamic Interaction Model for Scour Damage Modelling

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.

A bridge-monitoring tool based on bridge and vehicle accelerations

Previous research on damage detection based on the response of a structure to a moving load has reported decay in accuracy with increasing load speed. Using a 3D vehicle–bridge interaction model, this paper shows that the area under the filtered acceleration response of the bridge increases with increasing damage, even at highway load speeds. Once a datum reading is established, the area under subsequent readings can be monitored and compared with the baseline reading, if an increase is observed it may indicate the presence of damage. The sensitivity of the proposed approach to road roughness and noise is tested in several damage scenarios. The possibility of identifying damage in the bridge by analysing the acceleration response of the vehicle traversing it is also investigated. While vehicle acceleration is shown to be more sensitive to road roughness and noise and therefore less reliable than direct bridge measurements, damage is successfully identified in favourable scenarios.

Vision-Based Bridge Deformation Monitoring

Optics-based tracking of civil structures is not new, due to historical application in surveying, but automated applications capable of tracking at rates that capture dynamic effects are now a hot research topic in structural health monitoring. Recent innovations show promise of true non-contacting monitoring capability avoiding the need for physically attached sensor arrays. The paper reviews recent experience using the Imetrum Dynamic Station (DMS) commercial optics-based tracking system on Humber Bridge and Tamar Bridge, aiming to show both the potential and limitations. In particular the paper focuses on the challenges to field application of such a system resulting from camera instability, nature of the target (artificial or structural feature) and illumination. The paper ends with evaluation of a non-proprietary system using a consumer grade camera for cable vibration monitoring to emphasise the potential for lower cost systems where if performance specifications can be relaxed.

Impact of Road Profile when Detecting a Localised Damage from Bridge Acceleration Response to a Moving Vehicle

Previous 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 ‘damage’ 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.

Characterisation of transient actions induced by spectators on sport stadia

In a wake of Hillsborough disaster of 1989, all stadia hosting major sport championships in the UK were converted from terraced to all-seated. Driven by spectators’ demands for improving the quality of their experience and organisers’ interest in increasing the capacity of their venues, a debate has arisen recently about the possible introduction of safe standing areas. Some issues have been already highlighted, mainly related to security aspects and comfort. This study investigates how the introduction of safe standing areas and the expected increase of the density of a crowd could impact the dynamic loading induced by the spectators and the resulting structural response. To this end an experimental campaign has been conducted in a laboratory delving into the effects of common actions performed by seated and standing cheering spectators. The data on dynamic behaviour of a lively test structure in both conditions have been collected, simultaneously with data on behaviour of the spectators. The forces applied by the spectators have been inferred using inverse dynamics, by analysing the structural response. The results are presented in the context of human-to-structure interaction and human-to-human coordination.