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Structural Health Monitoring

Monitoring, a very common term used in - I think every field present today. But monitoring what? The answer is a "system". Why do we need to monitor? To check for the changes happening in the system as the day passes by and to remain prepared for the future changes. For example let us talk about health monitoring. What do you do when you feel sick? You go and see a doctor. What does doc do? He examines you and from the diagnosis s/he prescribes you some medicine. But is that it? No, he calls you again and again till s/he feels that your are good to go. When does he leave you? When s/he is certain that you are perfectly fine and there are no more negative changes happening in your body, till then s/he "Monitors" you.  He checks every other day or week or may be month, just to see the changes that are going through in your body. If s/he stops monitoring your health in such a way, then I am pretty sure it will take longer to get yourself some cure. So this is the importance of monitoring. 

In case of you, your body and doc., you were a living being who can fight against the negative changes, you adapt to the environment, so in most of the cases even if you don't see doc. for a long time, it is fine but still you need to see a doc. at some point of time. But the system about which we are going to discuss right now is not capable of adapting itself to the surrounding changes. For example steel structures get rusty if we do not take necessary steps and so losses it's stiffness. So who know if it is just an earthquake away from falling down. It cannot do anything on its own. So somebody needs to monitor the structure on a regular basis just to see what changes is our structure going through. In short we monitor its health. We are not the doctor the structure is our patient. 

Structural engineering is all about probabilistic analysis and design. Neither load not the material strength is all exact, we deal with variations in everything. So the important thing is to keep a track on the structural behavior which when subjected to extreme loading does not fail or do not loses its stiffness or go to a near collapse state at any point throughout its lifetime. We have learned from our past failures about the importance of keeping the track because structure can fail suddenly even in case of very small loading. Eg: Tacoma Narrows bridge.(I won't tell you anything, or else I will ruin one more blog.. :P)

Schematic of how the system works. (Data collection from recorders)

 Structural health monitoring system (Courtesy : MIT)

What do we monitor in a structure? When is it important to check the health of a structure? 

The following are the key things that we monitor in a structure:
1. Detecting the damage in the structure
2. Type of damage
3. Location of damage
4. Severity of damage

But stop right there and ask yourself this - "How to detect the damage in the structure?"

When you get the data from the recording instruments and you see the change in natural frequency of the system or you detect deflection exceeding its limits or you see that the rotations are exceeding at joints or you notice that the system is deforming without any dynamic loading and just under it's own weight, you can say that there is something wrong with the structure. All the cases describes the loss of stiffness of the structure in one direction or the other. It shows that whatever changes the structure is going through are not good and you need to take immediate steps to get it straight.

Now the important point is to detect the location of damage. In case of instrumentation for SHM, you don't have just one recording device, in fact they are located all over the building so wherever the damage is located can be found from the changes in recording history of one particular instrument. For example in a portal frame you attached the instrument at the connections, at the center of beam and at the center of two columns. Suppose the beam is damaged at the center, then in that case you will see sudden changes in the reading from the instrument attached at the beam. But if all the instruments are showing some change in the readings then there might be something wrong with the overall building itself. It might be possible that throughout the age of the building it might have lost its stiffness and so its natural frequency is changing.

The above picture represents how failure can be detected from the readings.

Monitoring in a building

It is always important to check the data collected from the building immediately after natural hazards like Tsunami and earthquakes. Such hazards can severely damage the building and can cause the building to loose a lot amount of stiffness. From the recordings one can also analyze how the shear wave traveled across the building, which was the first point of damage, how the damage spread across the building, which member failed first, which joint failed first, etc. If the building failed then from the data collected one can actually produce an animation of the complete failure of the building and can learn from such failure about many important factors that we were missing. Also in case of blasts inside building one can make sure about the safety of the building and so on.

Also, we all know how structural engineering evolved slowly from past failures and tons of experiences. But how to make sure whether designed load is the upper limit that the building can ever face? Well structural health monitoring can help us in that case too. From its collected data we can always reverse calculate the load that was acting and from this reverse calculation we can make sure that it is a controlled load. Also the frequency of the vehicles passing above the bridge can easily be calculated from such recordings.

Some axioms that you must know for structural health monitoring.
  • Axiom I: All materials have inherent flaws or defects;
  • Axiom II: The assessment of damage requires a comparison between two system states;
  • Axiom III: Identifying the existence and location of damage can be done in an unsupervised learning mode, but identifying the type of damage present and the damage severity can generally only be done in a supervised learning mode;
  • Axiom IVa: Sensors cannot measure damage. Feature extraction through signal processing and statistical classification is necessary to convert sensor data into damage information;
  • Axiom IVb: Without intelligent feature extraction, the more sensitive a measurement is to damage, the more sensitive it is to changing operational and environmental conditions;
  • Axiom V: The length- and time-scales associated with damage initiation and evolution dictate the required properties of the SHM sensing system;
  • Axiom VI: There is a trade-off between the sensitivity to damage of an algorithm and its noise rejection capability;
  • Axiom VII: The size of damage that can be detected from changes in system dynamics is inversely proportional to the frequency range of excitation.

If I go on then there will be a long list of application of structural health monitoring but it is up to the readers' interest as to which depth s/he wants to go into the field. But one thing is sure, this field is still growing is will prove very critical in the near future. So if you love this field then definitely go ahead and study for it. But before it, make sure that all your basics in structural engineering are clear.

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