Pilate Moyo

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.

Application of quasi-distributed fibre Bragg grating sensors in reinforced concrete structures

In this paper we report on our results for the applications of fibre Bragg gratings (FBGs) in structural health monitoring. Multiplexed FBG-based strain sensors were fixed onto the reinforced bars (rebars) in concrete structures to determine the strain changes at different locations within the structures during loading and unloading tests. A similar set of FBG-based strain sensor arrays was also mounted onto the surface of the structures for the purpose of comparison. At the same time an FBG-based sensor array optimized for temperature measurement was also distributed alongside the strain sensors to obtain the temperature information, as well as to compensate for the temperature-induced wavelength shifts on those FBG strain sensors. The results obtained followed the same trend as that expected from concrete structures subjected to loading tests.

Application of Box-Jenkins Models for Assessing the Effect of Unusual Events Recorded by Structural Health Monitoring Systems

In-service civil infrastructure experience short-lived and transient changes in strain from time to time resulting for example from ground movements, development of cracks, heavy traffic, and accidents. With the advent of instrumented structural monitoring it is now possible to capture these events. In this paper, an approach based on Box-Jenkins transfer functions is proposed for assessing the effect of these events on structural behaviour and performance. The analysis is based on strain data recorded by a structural health monitoring system installed in a major bridge at construction stage.

Monitoring of Singapore-Malaysia second link during construction

The Singapore-Malaysia Second Link is a prestressed bod girder carrying a dual carriageway with three lanes on each carriage. The bridge serves as a relief to the existing crossing locate in northern Singapore. The bridge was compete in 1997 and opened to traffic in the same year. Owing to its importance a suite of instruments was installed in three segments of the bridge in order to monitor its short-term and long-term behavior and performance under construction loads, environmental loads, and vehicular loads. The paper present a description of the instrumentation, results of the monitoring during construction, and observations of the bridge response to loading during construction.

Structural performance assessment and fatigue analysis of a railway bridge

Increased economic activity in South Africa has put pressure on both rail and road transportation systems. In particular, there is a need to increase loading on the rail systems. The railway network consists of a number of steel bridges approaching the end of their design life. The fitness of these bridges to carry higher loads needs to be assessed. The present paper describes a systematic procedure for the condition assessment of a 100 year old wrought iron railway bridge. Experimental modal analysis techniques were used to calibrate the finite-element model. The calibrated model was used to predict the behaviour of the bridge under proposed loading regimes. Strain measurements were taken at selected locations and were used to assess the fatigue life of the bridge. This work demonstrates the effectiveness of combining experimental modal analysis with actual structural response from live loading for condition assessment and performance prediction.

Determination of load-strain characteristics of concrete slabs by using embedded fiber Bragg grating sensors

In this paper, we present our work on the fiber Bragg grating (FBG) sensors for structural health monitoring in 5m long concrete structures. Two sets of sensors were securely fastened onto the surfaces of the top and bottom reinforced bars respectively before concrete was poured in. Another set of the sensors was mounted onto the slab surface. These sensors were then monitored to observe the strain experienced at different locations within concrete slab. Loading and unloading cycle tests and failure test were performed on the completed structure. From the results obtained using the FBG sensors, we were able to correlate t he load-strain behavior of the slabs to the failure state as observed on the slab surface. These data are useful in determining the maximum allowable load before failure sets in. At the same time, we made comparisons of the data obtained using our FBG sensors with those obtained with electrical strain gauges. The two sets of data show a similar trend during the loading and unloading tests as well as during the failure tests.

Application of fiber-Bragg-grating-based strain sensors for civil infrastructure health monitoring

Over past few years, the concept of structural health monitoring has been emerging as a new area of research. Fiber Bragg grating (FBG) based sensor offers a new sensing approach with a number of advantages over conventional sensors. This new sensing technology is suitable for the harsh environment of construction industry due to its robustness, ruggedness and ease of installation. Two unique advantages of FBG based sensors are immunity to electromagnetic interference and multiplexing capability. This paper reports some of the results of a multi-disciplinary program on the FBG based sensors involving the School of Electrical and Electronic Engineering and the School of Civil and Environment Engineering at Nanyang Technological University, Singapore.

Identification of unusual events in multichannel bridge monitoring data using wavelet transform and outlier analysis

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. However, converting large amount 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 in Singapore 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 localizing 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.

Simultaneous monitoring of strain and temperature in concrete structures with embedded fiber Bragg gratings

Fiber Bragg grating (FBG)-based strain and temperature sensor array were embedded into the concrete structure in order to provide real-time information on its strain and temperature distribution. The sensors were wavelength- multiplexed along a single fiber. The temperature and strain sensors were specially designed and optimized for their measurands. The calibration experiments of those FBG sensors, and parameter monitoring during the structural curing processes were also presented in this paper. These fiber optic strain and temperature sensor show many advantages over the traditional electrical strain gauges and thermocouples.

Bridge health monitoring using wavelet analysis

Bridge health monitoring is defined as the continuous monitoring of a bridges state properties such as static and dynamic response in order to diagnose the onset of anomalous structural behavior. This involves measuring and evaluating the state properties and relating these to defined performance parameters. The process of measuring state properties, either continuously or periodically, produces large amounts of data. Careful analysis of dat would detect any sudden and gradual changes in the bridge, and thus enable damage detection, location, and severity. There is therefore a need for data analysis tools that enable bridge managers to identify as much information as possible on bridge behavior and performance at given tie. The ability of wavelet transforms to detect abrupt changes; gradual change beginnings and ends of events make them well suited for the analysis of bridge health monitoring data. This paper presents the application of wavelet analysis to identify events and changes in a bridge during its construction.