James M. W. Brownjohn

Structural health monitoring of civil infrastructure

Structural health monitoring (SHM) is a term increasingly used in the last decade to describe a range of systems implemented on full-scale civil infrastructures and whose purposes are to assist and inform operators about continued ‘fitness for purpose’ of structures under gradual or sudden changes to their state, to learn about either or both of the load and response mechanisms. Arguably, various forms of SHM have been employed in civil infrastructure for at least half a century, but it is only in the last decade or two that computer-based systems are being designed for the purpose of assisting owners/operators of ageing infrastructure with timely information for their continued safe and economic operation. This paper describes the motivations for and recent history of SHM applications to various forms of civil infrastructure and provides case studies on specific types of structure. It ends with a discussion of the present state-of-the-art and future developments in terms of instrumentation, data acquisition, communication systems and data mining and presentation procedures for diagnosis of infrastructural ‘health’.

Civil structure condition assessment by FE model updating: methodology and case studies

Development of methodology for accurate and reliable condition assessment of civil structures has become increasingly important. In particular, the finite element (FE) model updating method has been successfully used for condition assessment of bridges. However, the success of applications of the method depends on the analytical conceptualization of complex bridge structures, a well-designed and controlled modal test and an integration of analytical and experimental arts. This paper describes the sensitivity-analysis-based FE model updating method and its application to structure condition assessment with particular reference to bridges, including specific considerations for FE modeling for updating and the model updating procedure for successful condition assessment. Finally, the accuracy analysis of damage assessment by model updating was investigated through a case study.

Application of time series analysis for bridge monitoring

Despite recent considerable advances in structural health monitoring (SHM) of civil infrastructure, converting large amounts of data from SHM systems into usable information and knowledge remains a great challenge. This paper addresses the problem through the analysis of time histories of static strain data recorded by an SHM system installed in a major bridge structure and operating continuously for a long time. The reported study formulates a vector seasonal autoregressive integrated moving average (ARIMA) model for the recorded strain signals. The coefficients of the ARIMA model are allowed to vary with time and are identified using an adaptive Kalman filter. The proposed method has been used for analysis of the signals recorded during the construction and service life of the bridge. By observing various changes in the ARIMA model coefficients, unusual events as well as structural change or damage sustained by the structure can be revealed.

Measurements of static and dynamic displacement from visual monitoring of the Humber Bridge

Abstract A visual tracking system has been employed in the measurement of deck displacements at the centre of the 1410 m span of the Humber Bridge in the UK. The transputer-based system was developed for applications such as the monitoring or testing of large physical structures, where the actual displacements may not easily be otherwise determined. The system employs parallel processing techniques to track predictively the motion of multiple, independent objects at video frame rate. Results obtained using the system in a monitoring programme of the bridge are presented and discussed.

Humber bridge full scale measure campaigns 1990-1991

Abstract The Humber Bridge was the subject of an extensive monitoring campaign during 1990 and 1991 during which a large body of experimental data describing the loading and response processes was obtained. Some 70 instruments were used to measure wind speed and direction together with displacement, acceleration, rotation and temperature of the structure. The aim of the exercise was to validate mathematical modelling of the response of long span bridges to wind and to provide a picture of the relationships between the loading and response parameters. A full description of the instrumentation and monitoring arrangements is given together with some example results.

Identification of unusual events in multi-channel bridge monitoring data

Continuously operating instrumented structural health monitoring (SHM) systems are becoming a practical alternative to replace visual inspection for assessment of condition and soundness of civil infrastructure such as bridges. However, converting large amounts of data from an SHM system into usable information is a great challenge to which special signal processing techniques must be applied. This study is devoted to identification of abrupt, anomalous and potentially onerous events in the time histories of static, hourly sampled strains recorded by a multi-sensor SHM system installed in a major bridge structure and operating continuously for a long time. Such events may result, among other causes, from sudden settlement of foundation, ground movement, excessive traffic load or failure of post-tensioning cables. A method of outlier detection in multivariate data has been applied to the problem of finding and localising sudden events in the strain data. For sharp discrimination of abrupt strain changes from slowly varying ones wavelet transform has been used. The proposed method has been successfully tested using known events recorded during construction of the bridge, and later effectively used for detection of anomalous post-construction events.

Stochastic model of near-periodic vertical loads due to humans walking

Abstract A mathematical model has been developed to generate realistic synthetic vertical force signals induced by people walking. This model is both stochastic and narrow-band, which are the two essential features of walking loading not addressed adequately in the existing design guidelines for pedestrian structures, such as footbridges, long-span floors and staircases. The key reasons for this are (1) the lack of a comprehensive database of walking forces in the form of continuously recorded time series that can be used for development of statistically reliable characterisation of these forces for application in the civil engineering context, and (2) the lack of an adequate modelling strategy which can account for their narrow-band nature. This paper addresses both issues by establishing a large database of measured walking time series recorded by an instrumented treadmill, while the modelling strategy was motivated by the existing numerical generator of electrocardiogram (ECG) signals and speech recognition techniques. Hence, the new approach presented in this paper can serve as a framework for a more thorough and realistic treatment of vertical forces induced by people walking that could be adopted in the design practice.

Dynamic performance of a curved cable-stayed bridge

The dynamic behaviour of a 100 m span curved cable-stayed bridge constructed in Singapore has been studied by full-scale testing and analytical models. Comparison of test results and free vibration analyses show that modelling of the deck end fixity is an important factor, while correct modelling of stay cables and stress-stiffening effects is not so important except for identifying cable modes with weak superstructure interaction which may appear in the measured response. Estimates of stay-cable tension obtained from inducing cable vibrations may be inaccurate unless the anchorage stiffness and mass distribution are well known. Despite its limitations the ambient vibration test technique is most appropriate for a structure of this size.

Correlating dynamic characteristics from field measurements and numerical analysis of a high-rise building

Using the concept of lumped masses and rigid floor slabs, several mathematical models were built using a popular PC-based finite element program to model a tall building with a frame-core wall structural system. These models were analysed to obtain the first nine mode shapes and their natural frequencies which were compared with those from field measurements, using numerical correlation indicators. The comparison shows several factors that can have a significant effect on the analysis results. Firstly, outriggers connecting the outer framed tube system to the inner core walled tube system have a significant effect on fundamental translational mode behaviour. Secondly, detailed modelling of the core considering major and minor openings as well as internal thin walls has the strongest influence on torsional behaviour, whose measurements were shown to be an important aspect of the dynamic behaviour for the structure studied. Fine tuning of an analytical model requires not just considering variation in values of structural parameters but also attention to fine detail. Copyright

Energy dissipation from vibrating floor slabs due to human-structure interaction

Lightweight pre-cast flooring systems using post-tensioning to increase strength but not stiffness are increasingly popular, and vibration serviceability problems tend to govern design of such floors where human occupants are increasingly concerned with vibrations. At the same time as inducing response, stationary human observers can also participate in the response as mitigating influence and it is clear that a human behaves as a highly effective damper, even when seated.