A revolution in structural monitoring

As infrastructure monitoring becomes more crucial, so too does the availability of appropriate technology. High-frequency automated data collection is the way forward. This article provides an overview of tilt sensors, vibration sensors, crack meters and other technologies that are revolutionizing the field of asset monitoring.

In all of Western Europe, the infrastructure is facing a major challenge of renovating and replacing most civil structures. Post-war construction was done under time pressure and with limited resources, and as a result these constructions are reaching their end of life. Asset monitoring is a critical component in the maintenance and management of infrastructure. Monitoring provides time for solutions to be engineered, and resources to be gathered.

In this day and age, data is the key to automated insights and predictive maintenance. Using structural knowledge, the right set of data can tell an engineer whether a bridge is healthy, or at the point of collapse. Continuous monitoring provides a constant check on the critical components of a structure, and serves as an early warning system. An early warning system can function on different levels: as part of a direct fail system by monitoring for a crack in a girder, for example, or as part of predictive maintenance by monitoring for things like vibrations and tilt to recognize the subtle behavioural changes of an asset over time. In both cases, the ultimate goal of an early warning system is to guarantee the safety of the public, while extending the operational life of the assets in the infrastructure.

Total stations

Traditionally, monitoring of civil structures is done through manual inspections, as well as surveys using total stations. Total stations are a staple in the surveying industry. As these surveys are focused on deformations of determined points over a long period of time with large distances between measure points, they are fundamentally distinctive from laser scans or photogrammetric models.

Total stations face several challenges when applied for structural health monitoring. For a total station setup to work effectively, a clear line of sight is required. This is a challenge in many environments like tunnels, as well as underneath or inside large concrete bridges. Total stations as well as the (permanently installed) prisms need to be clear of dust, dirt, snow and ice in order to get a clear reading. This is especially problematic in tunnels, where prisms often get dirty and measurements become unusable.

When using total stations for monitoring the structural health of a structure in (semi) critical condition, the total stations are often permanently installed as an early warning system to obtain continuous data. The frequency of measuring depends on the amount of measure points per cycle, but is often set at approximately 1 cycle every 15 minutes. In most civil structure environments, total stations have a maximum measure distance of 200m due to obstructions and to uphold the required accuracy. The measurement accuracy is generally around 1mm, but due to the large amount of measurements done, this can be substantially improved mathematically.

Alternative solutions

To gain thorough insight into the structural health of civil structures and guarantee safety for users, two things play a crucial role: structural knowledge of the asset, and obtaining the right data for data-driven decision-making. Knowing the structural weaknesses of a bridge, for example, allows an engineer to place dedicated sensors and gather high-frequency and high-accuracy measurements, providing insights about the structural health. Some of today’s technologies, and their benefits versus traditional surveying, are listed below.

Tilt sensing: Tilt sensors measure the inclination relative to gravity. These sensors are often compact and robust, and can provide valuable information about the movements of a structure or its individual components. These sensors are highly useful when line-of-sight is obstructed, or the environment is dusty and total stations are not an ideal option. The sensors can be placed almost anywhere, and can be applied to monitor relative movements within a structure such as a bridge. Their real-time monitoring capability makes them ideal for early warning systems, specifically when paired with assisting systems like total stations or crack meters to validate any passed thresholds.

Tilt sensors can vary widely in their specifications, but high-precision models may offer accuracy within arc seconds (0.005° or better). These sensors, often accelerometers and gyroscopes, mostly use micro-electromechanical systems (MEMS) technology. MEMS provides continuous data with frequencies up to several kHz.

Crack meters: Crack meters are designed to monitor the opening and closing of cracks in structures. They are often used to detect movement across a crack and can be crucial in determining the structural health of concrete and masonry. Crack meters are available in both mechanical and electronic forms, with various measurement ranges and accuracies, measuring in either 1D or 3D. 1D crack meters can obtain far higher accuracies, working with technologies such as vibrating wires being able to measure up to 1/1,000mm with 100Hz. The choice of sampling frequency depends on the expected rate of crack propagation and the need for data resolution. Lower sampling rates provide the benefit of sensors working solely on battery power. However, when the goal is to gain accurate insight into the structures movement and behaviour (e.g. the reaction of a bridge when a large lorry passes over it), high frequency can show the vibrations in the structure and provide the extra insight the structural engineer may require.

Vibration sensors: Besides having the potential to result in structural damage, vibrations are key to understanding the dynamic behaviour of structures such as bridges or high-rise buildings. There are two main technologies used in vibration sensor technology: accelerometers and geophones. Accelerometers can detect accelerations from micrometres per second upwards, and ranges above 1,000Hz. Piezoelectric, capacitive and MEMS are the most common technologies used in modern high-precision vibration sensors. Geophones consist of a coil of wire suspended within a magnetic field. When ground vibrations occur, the coil moves relative to the magnetic field, inducing an electrical current in the wire. Geophones are intrinsically simple in design, and relatively cost effective compared to accelerometers. When detection of low frequencies is key, geophones may be a suitable sensor choice.

Fibre optic sensors: Fibre optic sensors are an extremely versatile tool that can be applied in many different use cases, but most commonly they are used to measure a combination of temperature and strain. Contrary to other monitoring systems, modern structures often have fibre optic sensors embedded within them. In addition to providing a unique sensing capability, this also protects the sensor fibre from the outside environment. Fibres can also be installed on existing structures, either cut and glued into the structure, or on top with a supporting frame. Fibre optic sensors are immune to electromagnetic interference, offer high sensitivity, require minimal maintenance, and can be interrogated at will. This means that while the fibre installation may be permanent, the surrounding hardware can be replaced and monitoring installations altered. Fibre optic sensors such as Fiber Bragg Grating (FBG) sensors provide strain measurements with high accuracy, often within the range of ±10 microstrain or better, which can translate to 1/1,000mm. In certain scenarios (see Figure 5), the measured frequency is more than 100Hz. In combination with other sources of data, such detailed information at various points in or along a structure can provide valuable insights into structural behaviour for engineers. 

Point clouds: Laser scans and photogrammetry are amongst the most common methods for obtaining point clouds. Laser scanners can achieve accuracies at levels ranging from a few millimetres to sub-millimetre, with the ability to capture hundreds of thousands and sometimes millions of points per second. This creates dense point clouds that accurately represent the surveyed structure. The range of these scanners can extend from a few metres up to several hundred metres, depending on the device. Common technologies include phase-based, time-of-flight, and more. Although point clouds can provide a highly detailed view of the surroundings, the measurement is fundamentally different from surveying. Besides advantages, there are currently some disadvantages associated with point clouds (see Table 2). However, with the continued growth of big data and the ongoing advancement of artificial intelligence (AI) and technology in general, the future is bright for point clouds in infrastructure surveys. Large amounts of information can be processed faster than ever, and with more accurate analysis. Tools like automated damage detection in concrete combined with photogrammetric point clouds can greatly improve the efficiency and effectiveness of human inspectors.

Conclusion

Traditional surveying with total stations will likely remain a staple in the industry as it is flexible, cost-friendly and accurate. When high-frequency data is required, however, the alternative options have to be considered. Although they are a fine solution in many cases, total stations are often a fish out of water when performing continuous measurements. Equipment gets dirty, and moving parts break or can even get stolen. When structural knowledge is combined with the aim of data collection, dedicated robust and cheap sensors can collect high-frequency measurements. As the amount of data itself grows, the gathered data becomes more valuable, and the technology to process it improves, these types of big data collection methods could help surveyors to get the best of both worlds if they focus on selecting the right tool for the right job and leveraging each approach where it suits best.

Further reading

The New NY Bridge project: https://www.newnybridge.com/crucial-calculations-project-surveyors-ensure-pinpoint-accuracy/

Geophones: https://www.guidelinegeo.com/help-articles/what-do-i-need-to-know-about-geophones/