What is Structural Health Monitoring - SHM ?
Sensors power next-generation Structural Health Monitoring
Structural Health Monitoring (SHM) is an emerging field that provides information on demand about any significant change or damage occurring in a structure. It has been employed for many years in civil infrastructure in carious forms, ranging from visual observation and assessment of structural condition, to technology-led approaches involving deployment of an array of sensors that can include accelerometers, inclinometers and strain measurement devices on site. These sensors can be deployed on a permanent basis or moved on and offsite each time a fresh set of data is required.
Conventional forms of inspection and monitoring are only as good as their ability to uncover potential issues in a timely manner. One of the major difficulties with SHM instruments for example, is managing the huge volumes of data that sensor arrays generate. Meanwhile, visual inspections and evaluations are insufficient for determining the structural adequacy of bridges or buildings.
With many civil structures throughout the world in urgent need of strengthening, rehabilitation, or replacement, SHM has seen renewed focus. There have been major advances in communications, data transmission and computer processing, which have enabled SHM solutions providing the ability to acquire vast volumes of data in relatively short periods of time and transfer it via high-speed fibre-optic or wireless connections to a central database. Subsequent analysis and modelling of this data can provide critical intelligence for maintenance and management strategies, as well as improved design.
Shoring-up civil structures
The immediacy and sensitivity of SHM enables it to serve a variety of applications. It can allow for short-term verification of new or innovative designs, as well as the early detection of problems and subsequent avoidance of catastrophic failures. When implemented as part of a maintenance strategy, it can assist with the effective allocation of resources, reducing both service disruptions and maintenance costs.
One of the core drivers however, is the growing requirement for refurbishment of critical transport infrastructure. Many owners and operators need timely information to ensure continued safe and economic operation of ageing infrastructure, while the construction and engineering industry faces a mounting challenge to shore-up supporting civil structures. Deterioration can be due to multiple factors, including the corrosion of steel reinforcement and consequent breakdown of concrete, or the fact that some structures may be sound, but have become functionally obsolete - e.g. a bridge that is no
longer able to support growing traffic volumes, vehicle sizes and weights.
According to the American Society of Civil Engineers (ASCE), one in four bridges in the US is either structurally deficient or functionally obsolete. In Canada, more than 40 per cent of operational bridges were built over 30 years ago and have been impacted by the adverse climate and extensive use of de-icing salts. And in the UK, an increasing number of bridges and other structures need to be strengthened to comply with legal minimum requirements specified by European Community legislation. Efforts to reinforce the resilience of key infrastructure to extreme weather events are also ongoing.
Sensors in the loop
The aim of SHM is many fold, including monitoring the in-situ behaviour of a structure accurately and efficiently, to assess its performance under various service loads, to detect damage or deterioration, and to determine its health or condition in a timely manner.
Although a single definition has yet to be universally agreed, SHIM describes the confluence of structural monitoring and damage detection, with the physical diagnostic tool being the integration of various sensing devices and ancillary systems. The latter can include data acquisition and processing, communications and networking, and damage detection and modelling software powered by sophisticated algorithms.
Field-proven technologies lie at the heart of SHIM innovation. For the past few decades, closed loop sensors have proven to be highly robust, reliable, repeatable and accurate in a variety of applications where extremely precise measurements are required. Such devices include:
Inclinometers - measure horizontal and vertical angular inclination to very high levels of precision, and output the data in analogue or digital form. In SHIM applications, inclinometers are employed to monitor movement over time of bridges, buildings and other large structures. In addition, customised products can offer specific performance specifications to meet exacting requirements.
Accelerometers - measure acceleration and deceleration of dynamic systems. Low `g' range accelerometers are used within SHIM to monitor accelerations induced into bridges and other structures to check design calculations and long-term critical safety. Accelerometers can also be used in the development phase of projects to ensure design calculations correlate with actual measurements in the application.
Load cells - transducers used to convert a force into an electrical signal and offer measurement of tension, compression and shear forces. Load cells are available in many physical shapes and forms to suit particular applications and types of loading. The majority of today's designs employ precision strain gauges as the primary sensing element, whether foil or semiconductor, and feature low deflection and high frequency response characteristics. SHIM applications for load cells include bridge lifting/weighing, vehicle/ crane load monitoring, and earthquake force monitoring.
Bridging old and new
Improvements in electronics packaging and assembly methods have allowed the sensing devices employed in SHIM solutions to become smaller, more cost effective, and so sensitive that there is no longer a need to excite a structure in order to gain vital information about its integrity. By placing the right number of sensors in the appropriate positions on a bridge for example, analysts now have the raw data required via ambient sources such as wind gust loads, foot falls, and traffic flows.
Moreover, advanced algorithms have been developed that allow asset owners and managing authorities to acquire both short and long-term structural integrity assessments that prove essential in taking decisions regarding repairs and upgrades, strengthening projects, financing, insurance, and dispute resolution.
A long-span suspension bridge currently under construction in Asia employs a sensor network that includes Sherborne Sensors' precision servo inclinometers and accelerometers. This sensor network enables the identification of structural problems at an early stage, prolonging the life of the structure, identifying areas of concern, and improving public safety.
SHM's benefits have also been clearly demonstrated at a remote steel bridge in the heart of Brazil's Amazon basin. Supporting freight trains carrying 10% of the world's iron ore each year, the bridge had been rolling back and forth whenever an ore carrying heavy-laden train was crossing. A horizontal crack had also appeared in one of the supporting concrete girders, with train drivers returning to the mines reporting increasingly violent vibrations as they crossed - despite their cars being empty.
A sensors-based SHIM solution was brought in to monitor the bridge over a period of time and, using its data collector devices and advanced analysis techniques, discovered that the crack in the concrete was not the cause. Rather, it was the frequency of the movement of the returning trains coupled with that of the bridge. The solution was simply to reduce the speed of the trains by 20km per hour when they crossed the bridge un-laden, and the vibration was eliminated, without the need for costly engineering works to the bridge.
Using conventional methods, a displacement sensor would have been placed over the crack to measure how it responded to ambient vibration over time. But such a device would not have told the bridge owners why the crack had come about, and whether it had anything to do with the movement in the structure.
In this scenario, an SHIM solution takes raw vibration data from field-proven and trusted sensors, and turns it into valuable information enabling analysts to provide a holistic diagnosis of a structure. This ensures asset owners and management authorities are fully-equipped with the knowledge to establish the most appropriate strategy for modifying a structural system to repair current weaknesses, minimise further issues and thus prolong the life of the asset.
As more capable sensors are deployed, the opportunity exists for engineers to find even more efficient and effective ways to acquire data, analyse the vast volumes being stored, identify areas for improvement and most importantly, act on the information provided. Automated SHIM for example, brings a number of benefits, such as enabling cost-effective, conditionbased maintenance as opposed to conventional schedule-based approaches.
Current commercial monitoring systems suffer from various technological and economic limitations that prevent their widespread adoption. In particular, the fixed wiring used to route from system sensors to the centralised data hub represent one of the greatest limitations since they are physically vulnerable and expensive from an installation and subsequent maintenance standpoint. The introduction of wireless sensor networks in particular is attracting significant interest.
A wireless sensor network consists of `nodes', which can range from a few to several hundred sensors,
with each node connected to one or several sensors. This model provides a practical solution for bridging information systems and the physical world. One of the major potential benefits is that often a large number of individual wireless sensors can be monitored using a single display device, or with a wide variety of fixed base stations and hand-held readers that are already available.
Wireless solutions are shown to reduce installation costs and sensor installation times dramatically. They also increase safety levels because they can often be configured remotely or prior to installation, and exchanged easily for calibration and maintenance. Conversely, the more permanent a sensor installation, the more costly the maintenance requirement tends to be. In addition, a solution that combines both wireless data transmission and battery operation, together with low power consumption is preferable.
The Wireless Tilt System (WTS) for example, is designed to provide structural engineers with a complete measurement solution able to record and log data remotely without the cost and complexity of traditional wired methods. The engineer simply fits the low power inclinometers to strategic points on a given structure or component thus helping to determine range of motion, as well as any structural weaknesses and whether maintenance is required. This simple and cost-effective solution is extremely beneficial, especially when multiple readings must be obtained.
Building business intelligence
Although implementing change in the civil engineering and construction industry takes time, new approaches to SHIM can deliver immediate benefits to asset owners, financiers, and public authorities in reducing the risk of litigation, improving public safety, and the sustainability of critical civil transport infrastructure. Using the latest SHIM solutions, structural performance detection and monitoring can be performed continuously, on a periodic basis, or in direct response to an event that may have affected the structure.
A variety of innovative structural integrity assessment solutions are being developed that provide the vital information that analysts use to compare the dissipation of vibrations with either the predicted behaviour of the structure given its design and materials, or with baseline measurements captured earlier. Customised servo accelerometers for example, are central to the data collector devices used to capture these baseline measurements and enable users to establish whether a structure transfers loads as designed.
When placed either singly or in an array on bridges or other structures for a period, data collector devices record a structure's three-dimensional movement in extreme detail. Further successful applications include road deck frequency and mode shape determination; seismic structural monitoring; vertical, lateral and rotational acceleration measurements of decks, cables and bridge towers; and integration with GPS systems to improve deflection frequency response. However, determining the most appropriate sensor technology for the application, and also the interpretation of the data, is where the knowledge and experience of a specialist supplier of sensor technology comes to the fore.
From an article by Mike Baker
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