A major part of a company’s revenue is invested in its infrastructure in the form of road and railway transport systems, bridges, wind farms, dams, ports, power and airports. Such structures have a tendency to deteriorate with time as their structural strength starts dwindling. This is because they are subjected to extreme environmental conditions such as rain, snow, storm and extreme heat and that causes the construction materials including concrete and steel to degrade. In addition to this, these structures have to constantly bear physical loads caused by traffic, earthquakes and overloading, which further accelerates the aging process.
Hence, it is crucial to identify the structural damage in its initial stages to prevent any potential accidents or collapse. These structures require regular inspection, investigation, proper monitoring for the detection of damages, drawbacks or any kind of issues so that they can be timely repaired, refitted and restored.
Structural Health Monitoring is a well-known technique used in structural engineering to gather information about the current status of a structure by periodically or continuously measuring structural vibration response and detecting the damages in different sections of the structure. It is meant to help maintenance managers and decision-makers prioritize rehabilitation and replacement in the structure.
Monitoring Metrics to Consider
A number of parameters are measured in a typical Structural Health Monitoring Setup.
Acceleration is the most common parameter that represents vibration in a structure. It is used to define the frequencies and shapes of the different modes of vibration. These values are then compared with previous acceleration measurements to determine if the bridge has deteriorated or has been damaged.
In a structure, the strain in concrete and the steel is an important parameter to monitor. It provides the measure of change in the dimension of the structure which helps in the estimation of the stresses present at the particular location in the structure.
The overall linear movement of a structure either in relation to its original position or on a global scale is monitored through displacement measurement. It can be measured in one, two or three independent directions.
In a structure such as a bridge, the vehicles and external loads on particular areas of a bridge are measured. This data is then used to enforce weight restrictions and to define the expected range of typical traffic loads.
The angular change of components in a bridge is useful in determining distortion in bridge geometry. The rate at which the deflection of a flexural member changes with respect to length is monitored along with the angular changes with respect to a vertical plane.
Monitoring Approaches and their Limitations
Visual inspection has been the most-applied traditional monitoring technique that has several limitations. The static nature of the assessment results in the delay of response in maintenance decisions. As visual inspection is dependent on inspectors’ subjective assessment, there could be discrepancies in the measured values and the actual values. It generally involves repair and intervening only when some amount of damage has already occurred. These interventions often led to unplanned downtime of the structure or facility. From a life-cycle cost analysis perspective, this can be a more expensive option for the organization.
Perhaps the most significant shortcoming of visual inspection is the limited accessibility and the reliance on having a clear light of sight to conduct the assessment. Any internal problems that are not visible will not be identified. Moreover, the technique is both time-consuming and expensive.
Structural Health Monitoring can be of various types; temporary, continuous, periodic, acyclic or combinations of the before mentioned. There can be a requirement for a temporary controlled static test, which is a short-term measurement with well-defined loads on the structure. As employing a complete setup can be costly, it is only reasonable to apply continuous or dynamic monitoring only for structures that are either exceptionally important or are exposed to extreme events like typhoons, earthquakes.
Dynamic monitoring, on the other hand, doesn’t have control over the input excitation of the structure. The excitation is rather created by external forces such as wind, waves, human activity, traffic etc. This type of monitoring is usually continuously performed for a longer span of time and a lot of data is recorded and interpreted.
An ideal structural Health Monitoring technique follows a series of steps to detect anomalies on a structure. At first, the damage is detected in the structure, followed by locating its probable spot. Then, the damage is classified, whether it’s a crack or concrete damage. The extent of this damage is calculated, based on which the residual life of the structure is predicted.
These systems are generally tailored for each application with a careful selection of sensor and placement and the size and complexity of the structure and the desired functional characteristics. The system can rely on single or multiple sensor types. The electrical outputs from these sensors are digitised for further processing and stored and interpreted for diagnosis of the structure’s condition. A comprehensive SHM system is likely to require a significant initial investment, however, the maintenance and operation costs are going to be on the lesser side.
Until recently, most SHM systems relied on cables to connect sensors and a centralised power and data acquisition source. The drawback of using such an arrangement is the huge amount of hardware it requires for full-scale deployment.
Current Trends in Structural Health Monitoring
Latest Technologies including MEMS, Acoustic sensors, fiber optics, and latest developments in the Internet of Things (IoT), mobile networks (5G), and wireless connectivity have brought in a great revolution in the field of Structural Health Monitoring.
Over the past few years, wireless sensors have become a viable option to alleviate all the costs associated with cabled monitoring systems. The wireless systems can save the difficulties and cost of connecting many wires over large distances on long-span bridges. Such an SHM system typically comprises a number of onboard sensors including ports to external sensors, along with radio communication and computational processing options.
These systems are capable of dealing with the large chunks of data generated during the monitoring process. They are integrated with microprocessor technology to perform computation locally without requiring an additional system for that.
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