Structural monitoring

In today's world there is an on-going need to understand the health and welfare of large civil engineering structures such as Buildings, Bridges and Dams etc. The resulting data can be used to determine strategies to address identified problems or alternatively develop cost efficient maintenance strategies and assess their effectiveness. With a number of high profile problems over the last 5 years, there is an understandable growing interest in the field of structural monitoring. The Video Gauge has demonstrated that it is ideally suited to low cost and effective monitoring applications.

Some examples include:

• Bridges
• Dams and Levies
• Buildings
• Railway infrastructure
• Embankments
• Aircraft

Recent monitoring projects have shown that the Video Gauge is able to be set up very quickly (1 hour) when compared to conventional technology (3 days) and it is able to capture a considerable amount of usable data.

The Video Gauge offers continuous 24/7 monitoring of:

• Displacement
• Velocity
• Rotation
• Bending

The voltage input/output module provides a versatile interface between the Video Gauge and existing sensors, data-loggers and monitoring equipment. The output of other sensor can be fed into the Video Gauge where they are logged along side the Video Gauge’s own measurements. The module’s voltage outputs can be driven from any of the Video Gauge’s measurements and fed into existing logging or monitoring equipment.

Case studies

	Long range precision measurement Inaccessible areas of buildings, structures and equipment are just as likely to fail as more easily reachable ones. Measuring the performance of these can be difficult however, certainly with any degree of accuracy. Bridges, by their very nature throw up a number of challenges, whether they carry railway lines, or major trunk roads.
	Track displacment and tilt The key concern when assessing a railway line is whether it is fit for purpose – will it carry a train at the designated line speed safely? Of secondary importance are a number of other factors including ride comfort and assessing future maintenance requirements.
	Measuring voids As trains pass over any given location, they cause deformation of the rail. This is down to flexure of the rail, compression of the ballast, and voids under the sleepers. Of primary concern to the railway engineer is what the overall deflection of the rail is, in order to assess the potential tilt of the track and hence derailment risk. Whilst the Imetrum system can directly measure tilt of a 3m track section, it can also be used for a simple displacement measure.
	Monitoring buildings Most buildings are susceptible to small movements caused by environmental effects or nearby engineering works. Traditional techniques for measuring these involve attaching devices to the structure, which can be time consuming, costly or unsightly, as well as needing recalibrating at regular intervals.
	Rail bridges Because of the heavy, periodic loading caused by trains, the dynamic response of a Rail Bridge is often the most useful means of assessing its structural health. This is done on a daily basis by drivers as they report changes in ride comfort, and on a periodic basis by the track geometry vehicles. There are times however when a more localised and specific, or more accurate assessment is required.
	Road bridges Often, once monitoring requirements for a structure have been determined, the most difficult part of obtaining useable data is access to the structure (whether because of traffic loading, or difficulty accessing underneath the bridge). As a video based technology, Imetrum’s system can be set up safely out of the way, meaning no disruption to traffic and a safer working environment for those gathering the data.
	Accuracy of Video Gauge for displacement measurement Precise measurement of displacement is traditionally achieved using devices such as a Dial Test Indicator (DTI) or a linear variable differential transformer (LVDT). The aim of this experiment is to validate the Video Gauge as a method for measuring displacement by comparing it directly with a calibrated DTI.
	Measuring changes in wing shape in a wind tunnel Tests on a compliant wing that changed shape depending on the air flow over it required that the changes in shape be determined as the wind tunnel testing progressed. The changes had to be measured without influencing the air flow. At the same time the strains within the special core material that supported the wing skins had to be measured to check that it was behaving in the expected way (negative Poisson's ratio).
	Tracking water level in the test of a tuned liquid damper A tuned liquid damper is a simple way of reducing the response of a building to dynamic loads such as earthquake shocks. It consists of a tank of water with the depth to length ratio set so that the natural frequency of the water sloshing in the tank matches that of the structure. The Video Gauge was used to monitor the depth of the water with time as well as the pendulum displacements of the plate and the displacements of the support frame.
	Use of the Video Gauge in structural monitoring The deformations of long span bridges and similar structures need to be monitored to ensure their structural health. This can be done using an array of accelerometers but these suffer a baseline drift over time limiting confidence in their long-term outputs. A non-contacting system is to be preferred as this can eliminate this drift and allows a more direct measurement of movements in the structure.