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Structural health monitoring (SHM) involves the observation and analysis of a system over time using periodically sampled response measurements to monitor changes to the material and geometric properties of
engineering structure Structural engineering is a sub-discipline of civil engineering in which structural engineers are trained to design the 'bones and muscles' that create the form and shape of man-made structures. Structural engineers also must understand and cal ...
s such as
bridge A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or rail) without blocking the way underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually somethi ...
s and buildings. 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. 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. The SHM process involves selecting the excitation methods, the sensor types, number and locations, and the data acquisition/storage/transmittal hardware commonly called health and usage monitoring systems. Measurements may be taken to either directly detect any degradation or damage that may occur to a system or indirectly by measuring the size and frequency of loads experienced to allow the state of the system to be predicted. To directly monitor the state of a system it is necessary to identify features in the acquired data that allows one to distinguish between the undamaged and damaged structure. One of the most common feature extraction methods is based on correlating measured system response quantities, such a vibration amplitude or frequency, with observations of the degraded system. Damage accumulation testing, during which significant structural components of the system under study are degraded by subjecting them to realistic loading conditions, can also be used to identify appropriate features. This process may involve induced-damage testing,
fatigue testing Fatigue testing is a specialised form of mechanical testing that is performed by applying cyclic loading to a ''coupon'' or structure. These tests are used either to generate fatigue life and crack growth data, identify critical locations or de ...
, corrosion growth, or temperature cycling to accumulate certain types of damage in an accelerated fashion.


Introduction

Qualitative and non-continuous methods have long been used to evaluate structures for their capacity to serve their intended purpose. Since the beginning of the 19th century, railroad wheel-tappers have used the sound of a hammer striking the train wheel to evaluate if damage was present. In rotating machinery, vibration monitoring has been used for decades as a performance evaluation technique. Two techniques in the field of SHM are wave propagation based techniques and vibration based techniques. Broadly the literature for vibration based SHM can be divided into two aspects, the first wherein models are proposed for the damage to determine the dynamic characteristics, also known as the direct problem, and the second, wherein the dynamic characteristics are used to determine damage characteristics, also known as the inverse problem. Several fundamental axioms, or general principles, have emerged: * 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. SHM System's elements typically include: *
Structure A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...
*
Sensor A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
s *
Data acquisition Data acquisition is the process of sampling signals that measure real-world physical conditions and converting the resulting samples into digital numeric values that can be manipulated by a computer. Data acquisition systems, abbreviated by the acro ...
systems * Data transfer and storage mechanism *
Data management Data management comprises all disciplines related to handling data as a valuable resource. Concept The concept of data management arose in the 1980s as technology moved from sequential processing (first punched cards, then magnetic tape) to r ...
* Data interpretation and diagnosis: # System Identification # Structural model update # Structural condition assessment # Prediction of remaining
service life A product's service life is its period of use in service. Several related terms describe more precisely a product's life, from the point of manufacture, storage, and distribution, and eventual use. Service life has been defined as "a product li ...
An example of this technology is embedding
sensor A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
s in structures like
bridge A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or rail) without blocking the way underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually somethi ...
s and
aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines ...
. These sensors provide real time monitoring of various structural changes like
stress Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
and
strain Strain may refer to: Science and technology * Strain (biology), variants of plants, viruses or bacteria; or an inbred animal used for experimental purposes * Strain (chemistry), a chemical stress of a molecule * Strain (injury), an injury to a mu ...
. In the case of civil engineering structures, the data provided by the sensors is usually transmitted to a remote data acquisition centres. With the aid of modern technology, real time control of structures (Active Structural Control) based on the information of sensors is possible


Health assessment of engineered structures of bridges, buildings and other related infrastructures

Commonly known as Structural Health Assessment (SHA) or SHM, this concept is widely applied to various forms of infrastructures, especially as countries all over the world enter into an even greater period of construction of various infrastructures ranging from bridges to skyscrapers. Especially so when damages to structures are concerned, it is important to note that there are stages of increasing difficulty that require the knowledge of previous stages, namely: # Detecting the existence of the damage on the structure # Locating the damage # Identifying the types of damage # Quantifying the severity of the damage It is necessary to employ signal processing and statistical classification to convert sensor data on the infrastructural health status into damage info for assessment.


Operational evaluation

Operational evaluation attempts to answer four questions regarding the implementation of a damage identification capability: : i) What are the life-safety and/or economic justification for performing the SHM? : ii) How is damage defined for the system being investigated and, for multiple damage possibilities, which cases are of the most concern? : iii) What are the conditions, both operational and environmental, under which the system to be monitored functions? : iv) What are the limitations on acquiring data in the operational environment? Operational evaluation begins to set the limitations on what will be monitored and how the monitoring will be accomplished. This evaluation starts to tailor the damage identification process to features that are unique to the system being monitored and tries to take advantage of unique features of the damage that is to be detected.


Data acquisition, normalization and cleansing

The data acquisition portion of the SHM process involves selecting the excitation methods, the sensor types, number and locations, and the data acquisition/storage/transmittal hardware. Again, this process will be application specific. Economic considerations will play a major role in making these decisions. The intervals at which data should be collected is another consideration that must be addressed. Because data can be measured under varying conditions, the ability to normalize the data becomes very important to the damage identification process. As it applies to SHM, data normalization is the process of separating changes in sensor reading caused by damage from those caused by varying operational and environmental conditions. One of the most common procedures is to normalize the measured responses by the measured inputs. When environmental or operational variability is an issue, the need can arise to normalize the data in some temporal fashion to facilitate the comparison of data measured at similar times of an environmental or operational cycle. Sources of variability in the data acquisition process and with the system being monitored need to be identified and minimized to the extent possible. In general, not all sources of variability can be eliminated. Therefore, it is necessary to make the appropriate measurements such that these sources can be statistically quantified. Variability can arise from changing environmental and test conditions, changes in the data reduction process, and unit-to-unit inconsistencies. Data cleansing is the process of selectively choosing data to pass on to or reject from the feature selection process. The data cleansing process is usually based on knowledge gained by individuals directly involved with the data acquisition. As an example, an inspection of the test setup may reveal that a sensor was loosely mounted and, hence, based on the judgment of the individuals performing the measurement, this set of data or the data from that particular sensor may be selectively deleted from the feature selection process. Signal processing techniques such as filtering and re-sampling can also be thought of as data cleansing procedures. Finally, the data acquisition, normalization, and cleansing portion of SHM process should not be static. Insight gained from the feature selection process and the statistical model development process will provide information regarding changes that can improve the data acquisition process.


Feature extraction and data compression

The area of the SHM process that receives the most attention in the technical literature is the identification of data features that allows one to distinguish between the undamaged and damaged structure. Inherent in this feature selection process is the condensation of the data. The best features for damage identification are, again, application specific. One of the most common feature extraction methods is based on correlating measured system response quantities, such a vibration amplitude or frequency, with the first-hand observations of the degrading system. Another method of developing features for damage identification is to apply engineered flaws, similar to ones expected in actual operating conditions, to systems and develop an initial understanding of the parameters that are sensitive to the expected damage. The flawed system can also be used to validate that the diagnostic measurements are sensitive enough to distinguish between features identified from the undamaged and damaged system. The use of analytical tools such as experimentally-validated finite element models can be a great asset in this process. In many cases the analytical tools are used to perform numerical experiments where the flaws are introduced through computer simulation. Damage accumulation testing, during which significant structural components of the system under study are degraded by subjecting them to realistic loading conditions, can also be used to identify appropriate features. This process may involve induced-damage testing, fatigue testing, corrosion growth, or temperature cycling to accumulate certain types of damage in an accelerated fashion. Insight into the appropriate features can be gained from several types of analytical and experimental studies as described above and is usually the result of information obtained from some combination of these studies. The operational implementation and diagnostic measurement technologies needed to perform SHM produce more data than traditional uses of structural dynamics information. A condensation of the data is advantageous and necessary when comparisons of many feature sets obtained over the lifetime of the structure are envisioned. Also, because data will be acquired from a structure over an extended period of time and in an operational environment, robust data reduction techniques must be developed to retain feature sensitivity to the structural changes of interest in the presence of environmental and operational variability. To further aid in the extraction and recording of quality data needed to perform SHM, the statistical significance of the features should be characterized and used in the condensation process.


Statistical model development

The portion of the SHM process that has received the least attention in the technical literature is the development of statistical models for discrimination between features from the undamaged and damaged structures. Statistical model development is concerned with the implementation of the algorithms that operate on the extracted features to quantify the damage state of the structure. The algorithms used in statistical model development usually fall into three categories. When data are available from both the undamaged and damaged structure, the statistical pattern recognition algorithms fall into the general classification category, commonly referred to as supervised learning. Group classification and regression analysis are categories of supervised learning algorithms. Unsupervised learning refers to algorithms that are applied to data not containing examples from the damaged structure. Outlier or novelty detection is the primary class of algorithms applied in unsupervised learning applications. All of the algorithms analyze statistical distributions of the measured or derived features to enhance the damage identification process.


Specific structures


Bridges

Health monitoring of large bridges can be performed by simultaneous measurement of loads on the bridge and effects of these loads. It typically includes monitoring of: * Wind and weather * Traffic * Prestressing and stay cables * Deck * Pylons * Ground Provided with this knowledge, the engineer can: * Estimate the loads and their effects * Estimate the state of fatigue or other limit state * Forecast the probable evolution of the bridge's health The state of Oregon in the United States, Department of Transportation Bridge Engineering Department has developed and implemented a Structural Health Monitoring (SHM) program as referenced in this technical paper by Steven Lovejoy, Senior Engineer. References are available that provide an introduction to the application of fiber optic sensors to Structural Health Monitoring on bridges.


Examples

The following projects are currently known as some of the biggest on-going bridge monitoring * Bridges in Hong Kong. The ''Wind and Structural Health Monitoring System'' is a sophisticated bridge monitoring system, costing US$1.3 million, used by the Hong Kong Highways Department to ensure road user comfort and safety of the Tsing Ma,
Ting Kau Ting Kau is an area in west Tsuen Wan District, New Territories, Hong Kong. Ting Kau Village () is a village near the shore. Ting Kau is famous for the Ting Kau Bridge, spanning the Rambler Channel, from Ting Kau to Tsing Yi Island. Administr ...
,
Kap Shui Mun Kap Shui Mun () or Throat Gates (historically spelled Capsuimoon) is the channel between Lantau Island and Ma Wan in Hong Kong. It is part of major sea route along the coast of South China, from Victoria Harbour to the Pearl River. It joins nor ...
and Stonecutters bridges. The sensory system consists of approximately 900 sensors and their relevant interfacing units. With more than 350
sensors A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
on the Tsing Ma bridge, 350 on Ting Kau and 200 on Kap Shui Mun, the structural behaviour of the bridges is measured 24 hours a day, seven days a week. The sensors include
accelerometer An accelerometer is a tool that measures proper acceleration. Proper acceleration is the acceleration (the rate of change of velocity) of a body in its own instantaneous rest frame; this is different from coordinate acceleration, which is accele ...
s,
strain gauge A strain gauge (also spelled strain gage) is a device used to measure strain on an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports ...
s, displacement transducers, level sensing stations,
anemometer In meteorology, an anemometer () is a device that measures wind speed and direction. It is a common instrument used in weather stations. The earliest known description of an anemometer was by Italian architect and author Leon Battista Alberti ...
s, temperature sensors, dynamic weight-in-motion sensors and GPS receivers. Ogaja, Clement, Li, Xiaojing and Rizos, Chris. "Advances in structural monitoring with Global Positioning System technology: 1997–2006" , vol. 1, no. 3, 2007, pp. 171-179. doi: https://doi.org/10.1515/jag.2007.019 They measure everything from
tarmac Tarmac may refer to: Engineered surfaces * Tarmacadam, a mainly historical tar-based material for macadamising road surfaces, patented in 1902 * Asphalt concrete, a macadamising material using asphalt instead of tar which has largely superseded ta ...
temperature and strains in structural members to wind speed and the
deflection Deflection or deflexion may refer to: Board games * Deflection (chess), a tactic that forces an opposing chess piece to leave a square * Khet (game), formerly ''Deflexion'', an Egyptian-themed chess-like game using lasers Mechanics * Deflection ...
and
rotation Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...
of the kilometres of
cable Cable may refer to: Mechanical * Nautical cable, an assembly of three or more ropes woven against the weave of the ropes, rendering it virtually waterproof * Wire rope, a type of rope that consists of several strands of metal wire laid into a hel ...
s and any movement of the bridge decks and towers. * The Rio–Antirrio bridge, Greece: has more than 100 sensors monitoring the structure and the traffic in real time. * Millau Viaduc, France: has one of the largest systems with fiber optics in the world which is considered state of the art. * The Huey P Long bridge, USA: has over 800 static and dynamic strain gauges designed to measure axial and bending load effects. * The Fatih Sultan Mehmet Bridge, Turkey: also known as the Second Bosphorus Bridge. It has been monitored using an innovative wireless sensor network with normal traffic condition. * Masjid al-Haram#Third Saudi expansion,
Mecca Mecca (; officially Makkah al-Mukarramah, commonly shortened to Makkah ()) is a city and administrative center of the Mecca Province of Saudi Arabia, and the Holiest sites in Islam, holiest city in Islam. It is inland from Jeddah on the Red ...
,
Saudi Arabia Saudi Arabia, officially the Kingdom of Saudi Arabia (KSA), is a country in Western Asia. It covers the bulk of the Arabian Peninsula, and has a land area of about , making it the fifth-largest country in Asia, the second-largest in the A ...
: has more than 600 sensors ( Concrete pressure cell, Embedment type strain gauge, Sister bar strain gauge, etc.) installed at foundation and concrete columns. This project is under construction. * The
Sydney Harbour Bridge The Sydney Harbour Bridge is a steel through arch bridge in Sydney, spanning Port Jackson, Sydney Harbour from the Sydney central business district, central business district (CBD) to the North Shore (Sydney), North Shore. The view of the bridg ...
in Australia is currently implementing a monitoring system involving over 2,400 sensors. Asset managers and bridge inspectors have mobile and web browser decision support tools based on analysis of sensor data. * The
Queensferry Crossing The Queensferry Crossing (formerly the Forth Replacement Crossing) is a road bridge in Scotland. It was built alongside the existing Forth Road Bridge and carries the M90 motorway across the Firth of Forth between Edinburgh, at South Queensfe ...
, currently under construction across the Firth of Forth, will have a monitoring system including more than 2,000 sensors upon its completion. Asset managers will have access to data for all sensors from a web-based data management interface, including automated data analysis. * The Penang Second Bridge in Penang, Malaysia has completed the implementation and it's monitoring the bridge element with 3,000 sensors. For the safety of bridge users and as protection of such an investment, the firm responsible for the bridge wanted a structural health monitoring system. The system is used for disaster control, structural health management and data analysis. There were many considerations before implementation which included: force (wind, earthquake, temperature, vehicles); weather (air temperature, wind, humidity and precipitation); and response (strain, acceleration, cable tension, displacement and tilt). * The
Lakhta Center The Lakhta Center () is an 87-story skyscraper built in the northwestern neighbourhood of Lakhta, Saint Petersburg, Lakhta in Saint Petersburg, Russia. Standing tall, it is the List of tallest buildings in Russia, tallest building in Russia, t ...
, Russia: has more than 3000 sensors and more than 8000 parameters monitoring the structure in real time.


See also

*
Deformation monitoring Deformation monitoring (also referred to as deformation survey) is the systematic measurement and tracking of the alteration in the shape or dimensions of an object as a result of stresses induced by applied loads. Deformation monitoring is a major ...
* Civionics * ''
Structural Health Monitoring Structural health monitoring (SHM) involves the observation and analysis of a system over time using periodically sampled response measurements to monitor changes to the material and geometric properties of engineering structures such as bridges an ...
'', a peer-reviewed journal devoted to the subject * Value of structural health information


References


External links


NDT.net Open Access Database contains EWSHM proceedings and much more SHM articlesInternational Society for Structural Health Monitoring of Intelligent Infrastructure (ISHMII) SHM at low cost for earthquake zones


Journals


SHM Proceedings (NDT.net)Journal of Structural Health Monitoring (sagepub)Journal of Intelligent Material Systems & Structures (sagepub)Structural Control and Health Monitoring (John Wiley & Sons, Ltd.)Journal of Civil Structural Health Monitoring (Springer)Smart Materials and Structures (IOP)
*Smart Materials Bulletin (science direct) {{DEFAULTSORT:Structural Health Monitoring Structural engineering Maintenance Infrastructure asset management