Structural Health Monitoring

The number of structural health monitoring applications around the world is staggering. Looking at ‘tall’ buildings alone there are over one hundred thousand measuring more than 35 metres (115ft) or 12 stories in height. And there are well over a quarter of a million miles of railroad track around the world, a similar length of oil and gas pipeline and more than six hundred thousand bridges in the US alone. Many other similar applications in tunnels, dams, dikes, pipelines, oil and gas, mining and energy usage scenarios exist.

 

Bridge 1

 

A demonstration of the use of Weightless technology in a typical structural health monitoring application was recently made in Cambridge. For details of the demonstration please click here.

For BBC1 coverage of the event and Weightless technology, click here (Members only)

 

High-rise buildings, bridges, arenas, stadiums and domestic, commercial and industrial buildings are complex structures comprising multiple elements and components that are subject to myriad stress and fatigue points and which interact when exposed to external phenomena. Varying widely in construction materials, size, design, geometry, structural systems, and foundation characteristics there is a lot to go wrong. And frequently catastrophic implications when failures do occur.

And invisible to most when looking at a structure is a whole field of preventative maintenance known collectively as Structural Health Monitoring.

Structural Health Monitoring allows the rapid assessment of multiple aspects of the condition of infrastructure to increase safety and operational efficiency and to minimise failure and maintenance activities of complex buildings. Data derived from monitoring regimes allows infrastructure managers to improve the operation, maintenance, repair and the replacement of structural elements and components based on reliable, accurate and objective data.

Structural Health Monitoring allows reliable data on the actual condition of a structure to be captured and monitored, tracking its progress and detecting the appearance of new degradations. Concrete cracks and creeps and steel oxidises and cracks due to fatigue loading and localised failure points. Degradation of materials is caused by mechanical factors such as loading that is higher than the infrastructure was initially specified for and physical and/or chemical factors  such as the corrosion of steel, penetration of salts and chlorides in concrete and the freezing of concrete. A real-time snapshot of a building’s current state and evolution is enabled through the installation of permanent sensors that continuously monitor and report on relevant parameters.

 

Structural Health Monitoring Parameters

 

The actual parameters monitored in the infrastructure are application specific and dependent on multiple factors such as the type of structure, the location, the prevailing environmental conditions, the types of loads and stresses exerted on the structure and many more variables. Each of these give rise to the requirement for a diverse range of sensor types mounted in different ways in different parts of the structure and measuring different parameters. The single common factor is requirement to transmit data from the point of measurement - the sensor - to a central point where the it can be captured, stored, measured, processed, analysed and acted upon; connectivity.

 

Sensor types and sensed parameters

 

The types of parameters measured in structural health monitoring systems are diverse and include local strain, distributed strain, vibration, distributed soil stability, relative displacement, distributed temperature, leakage detection, rebar corrosion, load, pressure, tilt, rotation, differential settlement,  water pressure, water level, moisture ingress, chemical contamination, average strain, extension, rebar corrosion, local temperature, distributed temperature and many others. These all require specialist sensor types dedicated to the specific application and designed explicitly to be deployed in various structural environments, including hazardous areas.

Sensors

These sensors produce an electrical output in proportion to the parameter being measured and this is typically communicated to a storage and processing system using a physical connection -  electrical cabling. There are multiple downsides to this mode of connectivity which are addressed through the use of a appropriate and optimised wireless protocol to communicate between sensor and a centralised system hub.

 

Wired vs wireless connectivity

 

In a large structure there could be excessive lengths of cabling required to make the connection between the individual sensors and the central control system. These are subject to potential degradation and subsequent faults over time, potential interference over long cable runs, cost of deployment and maintenance, inconvenience, potential impracticality and in certain usage scenarios, an aesthetic implication. Wireless connectivity in structural health monitoring systems offers many key advantages over a physical cable but not without several significant problems.

 

Wireless protocols fall generally into two categories - short range, low cost and low power protocols such as Bluetooth, Wi-Fi and Zigbee or long range, high cost and high power consumption platforms such as 2G, 3G and 4G (LTE). Neither are particularly well suited to structural health monitoring systems where there are many sensors that must ideally be battery powered and transmit efficiently and reliably over a long distance and at low cost. Ideally what is required is a technology that marries the low cost and low power consumption of LAN and PAN technologies like Bluetooth with the long range capabilities of a cellular connection such as a 3G terminal.

 

Weightless is a new wireless Standard developed explicitly to support machine communications of exactly the type required for structural health monitoring systems. It supports the efficient communication of small amounts of data transmitted infrequently over a distance of several km and running on regular low cost primary cells for up to ten years. It typically operates in wireless spectrum with exceptionally good signal propagation characteristics which enables data to be reliably communicated through physical structures far more efficiently than Bluetooth, Wi-Fi, 3G, 4G etc technologies. It also permits signals to be transmitted over a significantly greater range - several kilometers - than LAN technologies and so does not require the use of data concentrators to enable connectivity with remote monitoring systems. It is low cost - a terminal typically costing less than USD$10. Network costs are low with license free TV white space spectrum and terminals need no SIM card. Maintenance costs are low through fit and forget battery powered terminals with a lifetime determined by the shelf life of the batteries - good quality alkaline primary cells (2 x regular AA cells) will power a Weightless terminal device for ten years under normal operating conditions.

 

Weightless technology offers compelling competitive advantages for machine to machine communications and is especially appropriate for large numbers of small, low cost sensors of the type demonstrated by these types of applications. With a cost of less than USD$10 per terminal device, great signal propagation characteristics, a range of several kilometres and a battery life measured in years the Weightless Standard enables the 95% of the Internet of Things market that alternative wireless technologies cannot commercially address and that are widely expected to require tens of billions of connections over the coming years.

Bridge Demonstration 2 

A demonstration of the use of Weightless technology in a typical structural health monitoring application was recently made in Cambridge. For details of the demonstration please click here.

 

The Weightless Special Interest Group has been established to manage the development of the Weightless Standard and to make the technology available to developers around the world. With close to 1000 Members around the world, a mature Specification and first silicon available, now is the time for developers of sensor interface equipment to engage with the technology.

 

Find out more about Weightless technology here.

Click here to join the Weightless community.

 

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