Structural Monitoring

Structural Health Monitoring, what is it?

Infrastructure, over their lifespan and operational period, undergo degradation due to the natural aging of their constituent materials and exposure to natural phenomena such as earthquakes, strong winds, and other extraordinary weather events. Because they are subject to degradation, they must undergo regular monitoring in order to maintain their original safety requirements.

The value of safety has taken on decisive importance in engineering culture over the last decades, which is why risk must be managed in all phases of a structure’s life cycle, from design to construction, and from construction to ongoing maintenance.

Structural monitoring, or Structural Health Monitoring, is the activity of controlling, verifying, and evaluating the structural behavior of a construction.

Today’s increasingly advanced technologies allow for the measurement of structural effects such as rotations, displacements, and cracks, as well as variations caused by natural events like wind, earthquakes, or ground settlement, as well as by human activity.

The advantage of continuous monitoring is precisely the ability to timely detect any defects or anomalies that may affect the behavior of an infrastructure and to intervene promptly in order to restore the necessary safety requirements and extend the lifespan of the structure itself.

Static and dynamic structural monitoring

Structural monitoring can be either static or dynamic.

Static monitoring measures displacements and rotations, while dynamic monitoring measures natural (wind, traffic) or induced vibrations and monitors the effects of these vibrations on the structures. The acquisition of information about the structural behavior of a construction subjected to natural or induced excitations is referred to as dynamic characterization.

In structural monitoring, both instruments that allow for measurements to be manually taken by a professional technician and sensors for automatic surveying are used. These sensors convert parameters like temperature, length, and inclination into electrical signals, which are then sent to a data acquisition unit (data logger) for storage and processing.


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What are the components of a monitoring system?

A monitoring system consists of sensors, a data acquisition and collection system, and software that processes the acquired and collected data to provide the professional with the necessary information.

The sensors are installed directly on the structure under examination and measure displacements, accelerations, and rotations. Some of these devices are capable of measuring other environmental parameters such as internal and external temperature, humidity, and wind speed.

The sensors are connected via cables or wirelessly to a data acquisition and collection system. The data provided by the sensors is stored and transmitted to a remote device, such as a PC, for analysis and interpretation using management software.

Structural inspection and visual inspection

Before proceeding with the installation of the monitoring system, it is necessary to establish the condition of the structure. Various techniques are employed for this purpose, including visual inspection and other non-destructive tests, which we will examine in detail later.

Visual inspection provides a general overview of the condition of the structure and its structural elements. This is achieved by verifying aspects such as coatings, concrete surfaces, the presence of fractures, deformation, exposure of reinforcement bars, moisture, dirt, and any potential leaks.

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Acoustic emission monitoring systems use sensors that allow for the detection of fractures and their evolution. The slow growth of a defect generates low acoustic emissions, while the rapid growth of a defect generates high acoustic emissions.

Non-Destructive Testing (NDT)

Among the non-destructive techniques employed for acquiring data on the quality and condition of a structure are:

  • SonReb Method: a combination of ultrasonic and sclerometric methods used to determine the in-situ concrete strength by correlating it with ultrasonic velocity and rebound index.
  • Ground Penetrating Radar (GPR) Surveys: This is a geophysical subsurface investigation technique that involves high-frequency electromagnetic pulses sent from a transmitting antenna. The time it takes for the pulses to return to the receiver after encountering any anomalies is measured.
  • Thermographic Surveys: These surveys allow for the measurement of the infrared radiation emitted by a structure, recording the surface temperature. This is a crucial data point for assessing the condition of materials, as different surface temperatures can indicate imperfections or critical issues with structural elements.
  • Pull-Out Tests: These tests allow for the measurement of concrete strength by inserting a anchor and measuring the force required to pull it out.
  • Ultrasonic Tomography: This technique enables the detection of any discontinuities present in materials using ultrasonic waves that pass through the object under examination. The waves bounce back when they encounter a void.
  • Magnetic Particle Testing: This method can be employed only on ferromagnetic materials and detects magnetic variations that occur in case of defects in the material under analysis.
  • Dye Penetrant Testing: This technique allows for the detection of defects by using a colored or fluorescent-contrasted liquid that makes surface or subsurface defects visible.

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Instruments for Structural Monitoring

The choice of the instrument used in structural monitoring operations depends on various factors such as environmental conditions, the level of accuracy and reliability of the measurement desired, and of course, the type of monitoring.

In recent years, due to the importance that monitoring activities have taken on in risk mitigation and optimal preservation of built structures, instruments and sensors have become increasingly precise and effective.

Electric displacement transducers are typically used in dynamic structural monitoring and installed near fractures or critical points. They allow for the recording of any displacements, movements, and rotations of load-bearing elements in a masonry structure, such as load-bearing walls or structural elements.

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Vibrating wire strain gauges, on the other hand, are sensors that allow for the measurement of deformations and can be mechanical, acoustic, or electrical. The choice should be made based on the most suitable type of strain gauge for environmental conditions, temperature, and the desired measurement accuracy.

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Measurement tools for dynamic monitoring: inclinometers

Inclinometers allow for the measurement of inclinations and rotations experienced by a structure under load. They are attached to the structure and connected to a data acquisition system that allows for observing the behavior of the structure at programmable time intervals.

Inclinometers are the most commonly used sensors for their effectiveness in the dynamic characterization of structures.


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Measurement instruments for static monitoring

Accelerometers, on the other hand, are instruments that allow for the measurement of vibrations or accelerations experienced by a structure under load, and can be of two types: piezoelectric or capacitive.

Piezoelectric accelerometers are used when it is necessary to measure vibrational phenomena between 1-1000 Hz; capacitive accelerometers are employed when dealing with low-frequency oscillations below 0.1 Hz

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