Academic approaches in structural health monitoring (SHM) usually focus on fine detail or on aspects of the technology such as sensors and data collection, and areas that may be less useful to operators than information about the level of performance of their structures. The steps in the process of SHM such as data management, data mining, conversion to knowledge of structural behaviour and integrity are frequently absent, and even the most operationally successful SHM systems may lack the component where deep understanding on the nature of the structure performance is obtained. This paper presents experience gained in a number of SHM exercises where static and dynamic response data have been interpreted, with or without the aid of calibrated structural models, in order to characterise the mechanisms at work and the experiences of the structure.
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.
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.
In recent years, fiber optic sensors have been used for structural health monitoring because of the advantages they have over conventional strain gauges, such as multiplexing capability and immunity to electromagnetic interference (EMI). In addition, they have ability to monitor multiple parameters simultaneously, which is of significant benefit to researchers. However, this diverse sensitivity can cause "cross coupling" leading to imprecision in measurement. In this paper, we report our results on the development and testing of a temperature compensated fiber Bragg grating strain sensor. Results show that the proposed sensor structure can be used to effectively compensate for temperature variations in strain measurement.
This paper reports our work on the applications of fiber Bragg grating-based strain sensors for the vibration tests and mode analysis on concrete structures. The arrayed fiber grating strain sensors, which were wavelength-division-multiplexed along the fibers, were attached onto the reinforced bars (rebars) before concrete was poured in to form a 5.5m long, 0.3m wide, 0.15m deep reinforced concrete beam. The embedded sensors will provide quasi-distributed real-time dynamic strain information along the length of the beam. For verification with the FBG strain sensors, some electrical accelerometers were also placed on the top surface of the concrete beam. All the data from FBG sensors and electrical accelerometers were recorded and analyzed by a computer. In the experiments, a hammer and an electrical shaker were used to excite the structure. The experimental results obtained with the FBG sensors show good consistency with the theoretical analysis.
Development in sensing technologies and data acquisition systems is making it possible for engineers to acquire the actual state of strains and stresses in structures at construction stage and during their service life. The knowledge of actual internal forces in a structure is key to identification of parameters such as stiffness and support conditions enabling the structural performance to be accurately estimated. This paper reports on the theoretical and experimental work carried out using fibre Bragg gating (FBG) strain sensors, embedded into and attached onto the concrete beam, to obtain the strain distribution of the concrete beam, therefore deduce the information of the applied load.
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 is defined as the continuous monitoring of a bridge's 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.
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.
Initially developed for applications in the aerospace industry, fiber-optic Bragg grating sensors (FBG) have attracted attention in the civil engineering community. The interest in FBG sensors has been motivated by the potential advantages they can offer over existing sensing technologies. They are, immune to electromagnetic interference, small in size and can be easier to install than traditional electrical resistance strain gauges. They can also be multiplexed, that is, a single fiber may have more than one change. Although field test of FBG sensors have been reported in literature, there is a dearth of information on their installation procedures, their precision in quantifying strains of concrete structures, and robustness requirements for embedment in concrete structures. In particular the harsh environment during the construction of concrete structures is a great challenge in the installation of these fragile sensors. The paper reports on our experiences with FBG sensors in concrete structures. FBG sensor have been sued to quantify strain, temperature and to capture vibration signals. Th result of these studies indicate that, if properly installed, FBG sensors can survive the sever conditions associated with the embedment process and yield accurate measurements of strains and vibration response, so it is possible to benefit from their potential advantages.
In concrete curing process, the temperature and the volume of the structure, and the inner pressure/stress exerted on the rebars will change in a fashion that depends on the composition of the concrete mixture. The temperature, strain and pressure changes at various points of a 3m long concrete structure with a mixture of cement, water and aggregate were monitored over a period of several days. Fiber Bragg grating based sensors were used to monitor these parameters in vivo. These sensors were designed and optimized to measure the temperature, strain and pressure respectively.
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.
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