1. An Introduction to Structural Health Monitoring
FEM1:
An Introduction to Structural Health Monitoring

2. Purpose
To acquaint you with the fundamentals of Structural Health Monitoring (SHM) with specific application to bridges.
To develop a vocabulary of terms used in SHM and gain insight into the components and systems commonly used in SHM applications.
To identify the ways in which SHM systems are being used in contemporary bridge design and maintenance.

3. Expected Learning Objectives
At the completion of this module you will be able to:
Identify and distinguish the components of SHM systems.
Relate the components of a viable SHM system in a schematic drawing.
Describe the benefits of implementing SHM.

4. Your Assignment
After you have read and reviewed the content provided, you will be required to:
Take an online examination to determine if you have achieved a satisfactory mastery of the stated learning objectives.
Submit an online response to a discussion question and list any questions you might have concerning anything you might not understand about the material.
Your online discussion question response will be discussed in an interactive manner in a classroom setting on the date specified in your Master Schedule.

5. Definitions/Descriptions of Structural Health Monitoring (1)
A “Glossary of Uncommon Terms” (Bisby, 2004) is provided to assist your reading in this field of civil engineering.
This list is not necessarily comprehensive, so you may run across terms you don’t understand and are not included in the Glossary. If that’s the case, see if you can find the term by searching on the Internet.
If so, list the term and the definition you found on the discussion question submission form.
If not, simply list the term under “Questions” on the online response form.

6. Definitions/Descriptions of Structural Health Monitoring (2)
Let’s start with the various ways in which practicing professionals define and/or describe SHM.
While the words may be different, you should be able to identify features that are common to all the definitions. Think about that as you review the definitions.

7. Definitions/Descriptions of Structural Health Monitoring (3)
“SHM refers to the broad concept of assessing the on-going, in-service performance of structures using a variety of measurement techniques.” (Bisby, 2004)

8. Definitions/Descriptions of Structural Health Monitoring (4)
“Structural Health Monitoring (SHM) aims to give, at every moment during the life of a structure, a diagnosis of the “state” of the constituent materials, of the different parts, and of the full assembly of these parts constituting the structure as a whole.”

9. Definitions/Descriptions of Structural Health Monitoring (5)
“Thanks to the time dimension of monitoring, which makes it possible to consider the full history database of the structure, and with the help of Usage Monitoring, it can provide a prognosis (evolution of damage, residual life, etc.).”

10. Definitions/Descriptions of Structural Health Monitoring (6)
Considering the diagnosis function only, SHM “is a new and improved way to make a non-destructive evaluation (NDE).”

11. Definitions/Descriptions of Structural Health Monitoring (7)
Actually SHM is much more than a non-destructive evaluation. “It involves the integration of sensors, possible smart materials, data transmission, computational power, and processing ability inside the structures.”

12. Definitions/Descriptions of Structural Health Monitoring (8)
SHM makes it possible “to reconsider the design of the structure and the full management of the structure itself and of the structure considered as part of a wider system.”

13. Definitions/Descriptions of Structural Health Monitoring (9)
“The process of implementing a damage identification strategy for aerospace, civil and mechanical engineering infrastructure is referred to as structural health monitoring (SHM).” (Farrar and Worden, 2007)

14. Definitions/Descriptions of Structural Health Monitoring (10)
SHM “can be defined as a non-destructive in-situ structural evaluation method that uses several types of sensors which are attached to, or embedded in, a structure.” The data collected “can be used to assess safety, integrity, strength or performance of the structure and identify damage at its onset.” (Bisby, 2004)

15. Definitions/Descriptions of Structural Health Monitoring (11)
“The SHM process involves the observation of a system over time using periodically sampled dynamic response measurements from an array of sensors, the extraction of damage-sensitive features from these measurements, and the statistical analysis of these features to determine the current state of system health.”

16. Definitions/Descriptions of Structural Health Monitoring (12)
“For long term SHM, the output of this process is periodically updated information regarding the ability of the structure to perform its intended function in light of the inevitable aging and degradation resulting from operational environments.”

17. Definitions/Descriptions of Structural Health Monitoring (13)
“After extreme events, such as earthquakes or blast loading, SHM is used for rapid condition screening and aims to provide, in near real time, reliable information regarding the integrity of the structure.[1]”

18. Definitions/Descriptions of Structural Health Monitoring (14)
The Illinois SHM Project describes SHM as “The ability to continuously monitor the integrity of civil infrastructure in real time” as a means “to reduce maintenance and inspection costs, while providing for increased safety to the public.” They cite that it is particularly imperative for evacuation routes and facilities be evaluated for safety after natural disasters.

19. Definitions/Descriptions of Structural Health Monitoring (15)
These definitions/descriptions have introduced a couple words which should be defined.
Damage
Sensor

20. Definitions/Descriptions of Structural Health Monitoring (16)
Damage
“Defined as changes to the material and/or geometric properties of a structural system, including changes to the boundary conditions and system connectivity, which adversely affect the system’s performance.”

21. Definitions/Descriptions of Structural Health Monitoring (17)
Sensor
“A device whose purpose is to detect events or changes in its environment, and then provide a corresponding output. A sensor is a type of transducer; sensors may provide various types of output, but typically use electrical or optical signals.”

22. Questions to Think About
1. What are the common features of the definitions/descriptions?
2. Thinking about your Mechanics of Materials Laboratory course, what kind of sensors could you (or did you) use to measure:
(a) load?
(b) deflection?
(c) strain?
(d) temperature?

23. Purposes of Structural Health Monitoring (1)
“Determine actual behavior and compare to assumptions used in the analysis.”
“Determine actual loading and compare to design loading.”
“Evaluate durability of structure to environmental, operational, and load conditions.”

24. Purposes of Structural Health Monitoring (2)
“Assess secondary design issues such as temperature, joints, movements, and vibrations.”
“Identify deterioration or damage of the structure and its components.”
“Evaluate safety and reliability of structure at any stage of its service life.”
“Assess the impact of maintenance or rehabilitation on future performance.”

25. Purposes of Structural Health Monitoring (3)
While the list on the previous slides is a reasonably exhaustive listing, it does not necessarily identify all the possible purposes of SHM.

26. Purposes of Structural Health Monitoring (4)
One particular unique application involved the widening of the Huey P. Long Bridge during the period, (Bassam and Schulze, 2011).
The specific issue was the transfer of construction loads from new structural members to old bridge truss members during construction.
The concern was the possibility of overloading the old truss elements.
To address this concern, a system was designed to monitor the changes in stresses and deformations continuously at all stages of the project.

27. Fundamental Axioms of SHM (1)
Worden, et. al. (2007) asserted that the progress in the development of SHM over the previous 20 years had matured to the point that several fundamental principles or axioms had emerged.

28. Fundamental Axioms of SHM (2)
Axiom I: All materials have inherent flaws and 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.

29. Fundamental Axioms of SHM (3)
Axiom IV: Sensors cannot measure damage. Feature extraction through signal processing and statistical classification is necessary to convert sensor data into damage information.
Axiom V: The length- and time- scales associated with damage initiation and evolution dictate the required properties of the SHM sensing system.

30. Fundamental Axioms of SHM (4)
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 the system dynamics is inversely proportional to the frequency range of excitation.

31. Schematic of Structural Health Monitoring Systems (1)
Now that you have been introduced to qualitative descriptions of SHM systems and the range of their purposes, let’s take a look at a schematic of a typical system and discuss the functions of the various components of the system.
Recognize that this is a generic system presented for illustrative purposes.
Actual systems will likely vary in their specific design but will generally consist of the components (subsystems) and functionalities herein discussed.

32. Schematic of Structural Health Monitoring Systems: Sensors (2)
Acquisition of data is achieved by an array of sensors selected to acquire the data needed to assess and, ultimately, analyze the performance of the bridge consistent with the purpose of the SHM system.
Functionally, sensors can be used to measure the response of the bridge to its primary force- traffic load and secondary forces such as temperature and wind.
Thus, the sensor array generally encompasses sensors to measure both bridge behavior and the external environment (e.g., temperature and wind speed and direction).

33. Schematic of Structural Health Monitoring Systems: Data Acquisition Unit (DAU) (3)
This onsite unit receives the inputs of the sensor array; converts the input signals (either analog or digital) to engineering parameters such as strain, load of deflection; and stores the data temporarily.
Since the sensors must communicate with the DAU, either a wired or wireless connection is needed between the sensors and the DAU.
Note: The sensors combined with the DAU may be referred to as the Data Acquisition System (DAS).

34. Schematic of Structural Health Monitoring Systems: Data Transmission System (DTS) (4)
The onsite DAU generally has limited storage capacity for data, so data are either continuously or intermittently transmitted to a central computer with both data processing and data storage capabilities.
Alternatively, the data could be transmitted to data storage unit or server which would provide long term secure storage of the data for access by a data processing unit.
As with the sensor to DAU connection, either a wired or wireless connections are used.
The wired connection is generally achieved through the use of existing telephone lines.

35. Schematic of Structural Health Monitoring Systems: Data Processing Unit (DPU) (5)
The function of this unit is to process (eliminate or minimize “noise”) and organize the data into a format amenable for analysis and diagnosis.

36. Schematic of Structural Health Monitoring Systems: Data Processing Unit (DPU) (6)
Noise: can consist of spurious readings.
These are data points that could be anywhere, and are sometimes ridiculously far from where the real data should have been.
With small data sets, these are often pruned by hand.
With large sensed datasets, these need to be automatically dealt with.
In high dimensions they are often impossible to physically “see.”

37. Schematic of Structural Health Monitoring Systems: Data Processing Unit (DPU) (7)
Measurement error:
This is a small amount of error that can occur on all data points and in every dimension.
It can occur because a sensor is not accurate enough, a rounding or approximation error occurs before the data reaches you, or a truncation error in a conversion to an abstract data type.
This type of data noise may have little effect on the structure you are trying to measure, but can violate noiseless assumptions and should be understood.

38. Schematic of Structural Health Monitoring Systems: Data Processing Unit (DPU) (8)
Background data:
These data are from something other than what you are trying to measure.
This could be unlabeled data (like unrated movies on Netflix) or people who don’t have a disease you are trying to monitor.
The problem is that it sometimes gets mixed in, and is then indistinguishable from the actual data of the phenomenon you are trying to monitor.

39. Schematic of Structural Health Monitoring Systems: Data Storage Unit (DSU) (9)
This unit is generally a server which can be used to receive and store data directly from the DAU or the DPU or both.
Further, the DSU can also be used to store the results of the analysis and diagnosis of the processed data.

40. Schematic of Structural Health Monitoring Systems: Diagnosis Process (DP) (10)
This is the process whereby the processed data are analyzed to:
detect damage.
compare bridge behavior with design and analysis predictions.
assess the bridge for any of the purposes cited.
This is a function or a component of the SHM system which involves knowledgeable professionals and whereas it makes use of computers, it is not a device.

41. Schematic of Structural Health Monitoring Systems: Decision Making Process (DMP) (11)
This function attempts to answer the question, what do the results of the Diagnosis Process mean in terms of the safety of the bridge or the current level of damage to the bridge or its actual vs. predicted behavior or any number of questions for which the SHM system was designed to answer.
As you might expect, this function must involve knowledgeable and experienced analysis, design and, possibly, construction professionals in collaboration with SHM specialists.

42. Schematic of Structural Health Monitoring Systems: Decision Making Process (DMP) (12)
The interrelation and interaction between these units and functions can be found on page 5 of the ISIS Canada document.
While this represents a reasonably generic and common system, other configurations and interactions are possible depending on the specific purpose of the SHM system.

43. Questions to Think About
Before reading the next section, take your eyes off the screen, take a piece of paper and respond to the following question.
List the possible advantages/benefits of SHM.
Now compare your responses with those provided below. How did you do?

44. Possible Benefits/Advantages of Structural Health Monitoring Systems (1)
Development of a better understanding of actual structural behavior because….
Analysis and design of structural elements and systems are based on idealized theoretical models.
Adherence to provisions of structural design codes (e.g., ACI for concrete, PCI for pre-stressed concrete, and AISC for steel).
Results of testing of coupon samples of structural materials.
Use of assumed or prescribed loads.

45. Possible Benefits/Advantages of Structural Health Monitoring Systems (2)
SHM allows collection of information that allow us to measure the behavior/performance of actual structures under actual loading conditions.
So the information collected from SHM system installations can be used to improve future analysis and design practices.

46. Possible Benefits/Advantages of Structural Health Monitoring Systems (3)
Earlier Detection of Damage
Early detection of damage allows remedial action to be taken before the level of damage reaches a critical state.
Thus, timelier and less costly repairs can be undertaken.
Should the level of damage detected suggest the possibility of near-term collapse of the bridge, action can be taken to remove the structure from service.

47. Possible Benefits/Advantages of Structural Health Monitoring Systems (4)
Continuous monitoring eliminates the need for and cost of periodic inspections which require manpower and bridge closures.
Assurances of a structure’s strength and serviceability
Reduction in down time:
Improved maintenance and management strategies
Encourages use of innovative materials and structural systems

48. Some Final Thoughts (1)
Traditionally, in the design of a bridge system, the scope of responsibility of the structural engineer was limited to the analysis and design functions.
Unless a problem arose during the construction of the bridge, the structural engineer rarely dealt with the bridge again unless a problem or damage was detected during the annual inspection of the bridge.
Basically, no attempt was made to compare the actual performance of the bridge with its predicted performance.

49. Some Final Thoughts (2)
With the advent of SHM, the scope of responsibility of the structural engineer has been extended effectively to the management of the bridge; not all bridges but those judged warrant SHM.
That is, utilizing the results of the SHM to ensure the safety and serviceability (i.e, satisfactory performance) of the bridge.

50. Some Final Thoughts (3)
Among other possibilities, performance encompasses evaluation of (Newhook and Mufti, xxxx):
Actual vs. predicted performance
Actual vs. assumed loading
Durability of bridge and bridge components to ambient conditions- load, environment, operation (in the case of a bascule bridge)
Effects of secondary factors such as temperature, vibrations, connections, supports, movements
Deterioration or damage of bridge- its components or as a system
Safety and serviceability at any stage of its service life
Impact of rehabilitation and maintenance on performance

51. Next Steps
1. Take the on-line mastery exam- must achieve a score of 80%
2. Submit the Discussion Question Response Form provided for FEM1. Make sure you download a copy of your response and bring to the scheduled class discussion.

52. Your assignment…
(1) Answer the following question. Save a copy of your response for discussion in a subsequent class period.
How does the application of SHM potentially extend and expand the professional responsibilities of the structural engineer?