Civil Engineering Reference
In-Depth Information
contactless monitoring (i.e. no critical problems in setting up the measurement system, especially in the case of bridges with
frequent train passages), possibility to perform a multipoint measurement with a single camera, quick and simple
measurement device set-up (no transducers nor cables on the structure), no need of access in critical areas, especially
below the bridge to fix transducers. The mentioned aspect are even more attractive in the case of bridges crossing rivers or
deep valleys.
An additional advantage in using a camera for displacement measurements is that every row or column in the pixel matrix
can be considered a sensor on its own: so the complete camera corresponds to a very high number of parallel sensors,
allowing for distributed sensing or giving the chance to exploit this redundancy to improve measurements reliability. Due to
this reason, cameras are usually referred to as 'dense' sensors. However, cameras show the common problems of relative
displacement transducers and have a restricted frequency bandwidth.
9.2 State of the Art
Thanks to the development of digital cameras, to the growth of computer processing capabilities and to the new image
processing libraries, the use of vision-based measuring systems to measure the vibration of targets has become popular in the
last years, with particular reference to the vibration monitoring of civil structures.
One of the issues in this kind of measurement is the type of target used for the measurement itself: in most the cases planar
black and white targets are being attached to the structure in order to improve the measurement technique reliability and to
reduce result uncertainty of the [
1
-
5
]. In other circumstances, the natural texture of the structure under investigation can be
used for the measurement [
6
-
8
]. The main advantages of the markeless case are that the preparation of the set-up is much
faster and that it is not necessary to have access to the structure to be monitored, no permissions, no target maintenance.
In [
1
] and [
9
] bridge vibration measurement is carried out using fit-to-the-purpose targets fixed to the structure; in this
case the target is constituted by one black circle in white background, while in [
2
] a planar target with four circles is used. In
[
3
] cross-shaped targets are used and the viewing system is equipped with an additional reference system, which decreases
the sensitivity of the camera basement to ambient vibrations. Two types of targets are used in [
4
]: ring-shaped and random
ones; in this case multiple target are measured contemporarily with a single camera, allowing multi-point measurements. In
some applications active targets are used; for example in [
5
] LEDs are used for suspension bridge vibration monitoring.
Markerless solutions are proposed in [
6
,
7
] and [
8
], to monitor cable vibrations in cable-stayed bridges also in the case of
power head transmission lines. In [
10
] vibration measurements obtained through image acquisition and processing are used
to develop a modal analysis of a simple structure.
9.3 Design of the Vision-Based Measurement System and Testing Layout
In vision-based bridge monitoring a compromise must be looked for between two needs: on one side a wide field of view can
theoretically allow to get the whole dynamic deformed shape, but this fights against the need to get a reasonable resolution in
terms of pixel/mm. If the will to get the complete bridge deformed shape pulls towards the need of having a wide view, there
is the serious risk that this choice impacts on resolution, so that the peak to peak vibration amplitude is confined within a few
pixels, thus worsening the signal-to-noise ratio. Due to this reason, in the past, most dynamic measurements have been
performed on small structure portions, trying to increase contrast, developing fit-to-the-purpose targets to be fixed to the
structure, being rather close to the target and assuming that motion is rigid, to rely on the redundancy offered by the grabbed
images to increase the reliability of results: in fact a single image allows to measure the motion of multiple targets, therefore
in case of rigid motion the average of the displacements estimated by different targets allows to obtain a more robust motion
estimation.
The steps faced in this paper are to which extent it is reasonably possible to move from 1D measurements to 2D dense
measurements along a bridge span by using cameras. Once fixed the limits, this method can be really attractive, as it works
with no sensors on the bridge, no power supply, in a simple approach manageable by whichever worker. This allows to get
more dense checks (since the measurement setup can be easily assembled and disassembled), dramatically changing all the
approaches to maintenance and testing.
The chosen test bed is a 50 m long steel trussed railway bridge crossing a river close to a village named Erba (Fig.
9.1a
).
Trains run on bridge at low speed during the whole day, approximately every 30 min. In this work, vision based measures
will be exploited in order to quantify the bridge sag during the trains pass-by: a movie of the structure during every train
