1.1 Advances in Structural Health Monitoring Technology
The structural health monitoring process involves the observation and evaluation of a structure over time using periodically sampled measurements from a sensing system. Structural health monitoring is a popular and growing research field, providing a powerful tool for damage assessment and performance evaluation of engineering structures.
1.1.1 Structural Health in Civil Engineering
Civil infrastructure comprises bridges, buildings, towers, pipelines, tunnels, dams and other types of structures. Their continued safe and economical operation largely depends on proper maintenance and management. In order to evaluate optimal management strategies for existing civil infrastructure, accurate assessment of present and future safety is important and necessary (Ettouney and Alampalli 2012). Maintaining safe and reliable civil infrastructure for daily use is critical to the wellâbeing of the society. Thus, structural health can be stated as its current capacity for providing intended level of service in a safe and costâeffective manner against the expected hazards during its service life.
Despite the necessary design methodology initially used, civil engineering structures deteriorate with time. This deterioration is due to various reasons, including failure caused by cyclic traffic loads, effects of environmental factors (e.g. steel corrosion, concrete carbonation) and aging in the construction materials. Also, the deterioration can be caused by infrequent extreme events such as earthquakes, hurricanes and floods. Therefore, structural health will be affected by operational and environmental factors, including normal load conditions, current and future environments and expected hazards during the lifetime. All these factors are variables with uncertainties, so it is difficult to define the structural health in terms of its age and usage and its level of safety to resist severe natural actions. In order to reliably assess structural health and maintain structural safety, continued inâservice monitoring of the structure is essential.
Catastrophic structural failures, such as sudden collapse of the Iâ35 highway bridge (NTSB 2008), have highlighted problems associated with aging critical civil infrastructure. Severe natural disasters such as earthquakes and typhoons result in demands for quick condition assessment of civil structures (Brownjohn et al. 2011). Currently, the condition assessment of existing civil infrastructure such as bridges largely depends on visual inspection. This subjective and inaccurate condition assessment methodology has been identified as the most critical technical barrier to effective infrastructure management. For example, condition of bridges is typically expressed in terms of subjective indices on the basis of visual inspection alone. Thus, it is difficult to accurately evaluate structural condition from the inaccurate visual inspection data, even when this may be conducted by experts (Aktan et al. 1998). These issues have driven the research and development on the continuous observation and interpretation of fullâscale performance of civil engineering structures during their service life.
Health monitoring applications based on advanced sensors and realâtime monitoring for civil infrastructure offer great potential for informed and effective infrastructure management. Health monitoring is necessary for civil engineering structures since they may exhibit premature deterioration, structural damage and performance problems, or they may even have aged beyond their expected design life. Health monitoring can be utilised for tracking the responses of a structure along with inputs, if possible, over a sufficient duration to determine anomalies, to detect deterioration and to assess damage for decision making. Damage assessment methods using measured vibration modal data, such as natural frequencies and mode shapes, show promise for the health evaluation of engineering structures (Bicanic and Chen 1997, Chen 1998). Health monitoring can assess the performance of civil structures in a proactive manner using measured data and data interpretation algorithms, in order to correctly evaluate the current condition and to predict the remaining service life.
1.1.2 Aims of Structural Health Monitoring
Structural health monitoring is defined as the process of implementing a damage identification and health evaluation strategy for engineering structures. SHM uses sensing systems and associated hardware and software facilities to monitor the structural performance and operational environments of engineering structures. SHM involves the observation of a structure over time, using periodically sampled structural response and operational environment measurements from an array of sensors and then the evaluation of the current state and future performance of the structure. For longâterm SHM, the output of this process is periodically updated information regarding the capability of the structure to perform its intended function, by considering the inevitable aging and degradation resulting from operational environments (Farrar et al. 2003). Furthermore, SHM is adopted for rapid condition assessment to provide prompt and reliable information regarding the integrity of the structure after extreme events, such as an earthquake or blast loading.
SHM aims to identify structural damage and evaluate the health of the structure using monitored data. Damage is defined here as changes to the material and/or geometric properties of a structure, which affects the current state and future performance of the structure. The objectives of an SHM strategy can be outlined as the following five levels (Farrar et al. 2009).
- Level I: Damage detection, giving a qualitative indication that damage might be present in the structure
- Level II: Damage localisation, giving information about the probable position of damage
- Level III: Damage classification, giving information about the type of damage
- Level IV: Damage assessment, giving an estimate of the extent of damage
- Level V: Damage prognosis, giving information about the safety of the structure, e.g. estimate of remaining useful life
The level in the order given above represents increasing knowledge of the damage state. A higher level usually requires information available about all lower levels. The first two levels, damage detection and localisation, can be generally achieved using vibration based damage detection methods from structural dynamic response measurements. To identify the type of damage, data from structures with the specific types of damage must be available for correlation with the measured data. Analytical models are usually needed to achieve the fourth and fifth levels, damage assessment and prognosis. In general, these two levels may not be achieved without first identifying the type of damage present. Estimates of the future loading, together with predictive deterioration models, are necessary to accomplish the final level for damage prognosis.
SHM strategies offer useful information for optimising maintenance planning of engineering structures in service. To ensure a reliable operation and to schedule maintenance and repair work in a costâeffective manner, it is necessary to continuously monitor and assess the structural performance and to have an accurate estimation of the remaining useful life. Thus, the SHM strategy integrated with lifecycle management is necessary to calibrate structural assessment and predictions, to enable optimal operation and maintenance of engineering structures and, eventually, to operate the structures beyond their original design life.
1.1.3 Development of SHM Methods
Structural damage identification based on changes in the dynamic response of the structure has been practised in a qualitative manner for a long time. The beginnings of this damage detection method as an area of interest to engineers can be traced back as far as the time when tapâtesting (e.g. on train wheels) for fault detection became common. This field, however, did not really become established in research communities until the 1980s, when much interest was generated in the structural condition of offshore platforms, and later in the health of aerospace structures. Recently, the development of quantifiable SHM methods has been closely linked with the evolution and cost reductions of digital computing hardware and sensing systems. In conjunction with these developments, SHM has received considerable attention in the technical literature. The details of literature surveys on SHM development can be found in comprehensive reviews by Doebling et al. (1996) and Sohn et al. (2004).
The civil engineering community has investigated vibration based damage identificatio...
