Long span bridges

The deformations of these iconic and economically critical structures need to be monitored to assess their structural health and ongoing load capacity. Leading Structural Engineers have argued that deflection measurements are of primary interest in bridge assessment, including FEA validation, however these have been hard to obtain. Accelerometers suffer a baseline drift over time as they are unable to pick out low frequency movements, and GPS struggles to achieve acceptable vertical resolution, limiting confidence in their outputs. Laser based systems are gaining ground, but struggle for measurement frequency if more than a couple of points need to be monitored.

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The data shown here of the vertical displacement of the first Severn Bridge, show that multi-point dynamic deflections can comfortably be measured from the bridge pier, without the addition of targets to the deck. Complex movements can be observed, such as the displacement caused by a large vehicle, or wind induced vibration. The approach can be applied to any structure subject to complex loadings, such as an aircraft fuselage or wind turbine blade, and other civil engineering structures.

During construction and shortly after opening, the Imetrum system was used to characterise cable stay vibrations and deck displacements on the second Severn Crossing. The graph is of data taken from monitoring deck displacements. Over short periods the Video Gauge and accelerometer readings are in good agreement, but the agreement breaks down over longer periods as the accelerometer readings drift.

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Advantages

The use of the Video Gauge eliminates the problem of baseline drift with conventional double integrated extensometer readings. it also permits a reconfiguration of which parts of the structure are to be monitored with minimal effort, allowing both cable and deck displacements to be characterised. The video based system allows for a quick and easy setup for short term work, and can also be installed as part of a longer term monitoring regime, as was done on both Humber and Severn bridges.