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Prototype of an optical sensor for structural dynamic and also quasi-static behaviour of buildings, to be used inside (based on FBGs)

During earthquakes, the distances between structural elements of buildings change (sustaining pillars, walls, stairs, ceiling and floor, etc.). These distances change even because of so-called "settlements" of buildings (especially new, but not only), leading to quasi-static (i.e. slow) distance drifts. In order to monitor such dynamic or slow distance changes, we choose to connect a quiet short fibre (1m) incorporating a fibre-Bragg-grating (FBG) on the two sides of a construction joint between two building blocks in the University "Politehnica" of Bucharest, at the entrance in the Physics Dept.

We expect width changes of this joint (either dynamical, during an earthquake, or slow, due to buildings settlemets), so the fibre stretching changes (it is pre-stretched, so it can sense joint width decreasing). The induced strain leads to a change in the pitch of the FBG which in turn leads to a change of the reflected wavelength when the FBG is illuminated along its axis with an incoherent light source, e.g. a superluminescent diode (SLD). From the study of scientific literature the dynamic range of strain in the FBG is between 1 microstrain and 5 milistrain.

The task was to find a topology of the displacement sensor, for a given length of the fibre including the FBG sensing element, for which the dynamic range of the horizontal displacement is of +-20mm, falls within the dynamic range of the strain for the optical fibre.

The configuration used by us (original) was to fix the fibre diagonally, under a small angle with the vertical, which was found of about 6.3 degrees. The fibre length for this configuration was of about 1 m (i.e. not too long).

Moreover, since FBGs are temperature sensitive too, we used a second FBG identical with the strain-sensing one, in series with it (i.e. wavelength multiplexed), but non-stretched, as a temperature reference. So, the wavelength difference between the two reflected lines (by the two FBGs) is sensitive to the construction joint width only, and completely insensitive to temperature (the IInd originality element of our sensor).

The measured resolution of the sensor (via calibration experiments) resulted better than 10 micrometers, with a linearity of +0.2% to -0.35%. In the absence of a significantly strong earthquake, we performed simulated dynamic tests, demonstrating the sensor ability to show dynamic movements.

This kind of sensor is very useful for structural monitoring of buildings, either new or old. Large-scale use of such sensors could help to setting up a database on various buildings behaviour, especially during earthquakes, which in turn could lead even to civil engineering design and construction technologies changes.

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University "Politehnica" of Bucharest (UPB)
Spl. Independentei 313
060042 Bucharest
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