Structural health monitoring alarms are a crucial tool for ensuring the safety and integrity of structures such as bridges and buildings. They monitor sensor data to detect potential issues and alert the relevant parties.
There are different types of alarms that can be used in different situations:
- Limit-exceeding alarms – These alarms are triggered when a sensor reading exceeds a pre-defined threshold, indicating that the structure is approaching or exceeding its design limits and requires further investigation.
- Corrosion alarms – These alarms are triggered when humidity levels or other corrosion factors surpass a threshold, potentially indicating a corrosion issue.
- Mechanical alarms – These alarms are triggered when sensors detect a potential mechanical issue, such as monitoring power consumption to detect a malfunction.
- Temperature alarms – These alarms are triggered when a sensor reading exceeds a pre-defined threshold, indicating that the structure is experiencing excessive temperature changes.
- Vibration alarms – These alarms are triggered when a sensor reading exceeds a pre-defined threshold, indicating that the structure is experiencing excessive vibration.
- Seismic alarms – These alarms are triggered when a sensor reading exceeds a pre-defined threshold, indicating that the structure is experiencing seismic activity.
- Multi-factor alarms – These alarms take multiple factors into account, such as a combination of sensor readings and environmental factors, to detect potential issues.
These are just a few examples of the types of alarms that can be used in structural health monitoring, and different systems may have different types of alarms depending on the specific requirements and risks of the structure being monitored. The goal of these alarms is to detect potential issues early on, allowing for timely intervention and preventative maintenance, which ultimately ensures the safety and longevity of the structure.
The alarms can take on various forms, such as simple threshold values for displacement or temperature, or more complex calculations taking into account multiple factors and the rate of change. In certain cases, it is not only the sensor values that are important, but also the circumstances surrounding them. For example, if a bridge equipped with accelerometers and strain gauges begin to register larger values from smaller vehicles, it could indicate that something has changed in the structure. Even though the measured accelerations are not larger than earlier measurements, coupled with the vehicle knowledge they show a difference in the structural behaviour.
It is important to note that the alarms are not necessarily meant to take immediate action, such as closing a bridge or evacuating a building, but rather to alert engineers of potential issues that require further investigation. As a result, there are two types of alarms;
- Imminent failure alarms – These alarms are triggered when a structure is at risk of immediate failure, such as in the case of a bridge not being safe for trains to cross, and require immediate action to prevent a catastrophe.
- Investigation alarms – These alarms are triggered when a change in the structure requires further investigation but is not an indication of imminent catastrophic failure. This could be the humidity being high enough inside a main cable of a suspension bridge to the point where corrosion can start building up.
In either case, it is crucial for structural engineers to be able to easily investigate the reason for the alarm and determine its validity, taking into account any unforeseen circumstances, erroneous values or errors in the system.