Mike Baker examines how field-proven sensor technology lies at the heart of structural health monitoring innovation
Structural health monitoring (SHM) has been employed for many years in civil infrastructure in various 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 off site each time a fresh set of data is required.
With many structures throughout the world in urgent need of strengthening, rehabilitation, or replacement, SHM has seen renewed focus. Major advances in communications, data transmission and computer processing have enabled SHM solutions to provide 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.
One of the core drivers 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 – eg 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 USA is either structurally deficient or functionally obsolete. In Canada, more than 40% 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.
Field-proven technologies lie at the heart of SHM 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 (to measure horizontal and vertical angular inclination; accelerometers (to measure acceleration and deceleration of dynamic systems; and load cells (transducers used to convert a force into an electrical signal and offer measurement of tension, compression and shear forces).
Improvements in electronics packaging and assembly methods have allowed the sensing devices employed in SHM solutions to become smaller, more cost effective and so sensitive that there is no longer a need to excite a structure 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.
Real-world case studies
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 SHM 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 SHM 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 SHM for example, brings a number of benefits, such as enabling cost-effective, condition-based 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.
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) developed by Sherborne Sensors 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.
Mike Baker is MD of Sherborne Sensors.