Civil Engineering Reference
In-Depth Information
Chapter 9
Static and Dynamic Monitoring of Bridges by Means
of Vision-Based Measuring System
Giorgio Busca, Alfredo Cigada, Paolo Mazzoleni, Marco Tarabini, and Emanuele Zappa
Abstract
Civil structure static and dynamic monitoring is a key activity for both safety and maintenance purposes. In this
paper the use of cameras to monitor a bridge response to train transits is being considered and results are presented and
compared with a reference measurement, provided by a laser interferometer. A camera is a non contact measurement device,
having mainly two advantages: first of all the effort required to prepare the set-up is very low, because the camera is fixed in
the proximity of the structure and the targets to be mounted on the bridge (if required) do not need any cable connection for
both power supply and signals. Secondly cameras can measure the displacements of multiple targets in the field of view. The
leading idea of the paper is to use image acquisition and processing (Pattern Matching and Edge Detection) not only to
measure the displacement of a few targets but also to grab images from a wide structure portion in order to recover
displacements of a large number of points in the field of view. The extreme final solution would be having wide area
measurements with no targets, to make measurements really easy, with evident advantages, but also with some drawbacks to
be fully comprehended.
Keywords
Vision • Bridges • Dynamics • Monitoring • Uncertainty
9.1
Introduction
Static and dynamic testing of structures is at the same time a fundamental and critical operation, mainly due to the fact that
this type of testing is required for both safety and maintenance purposes. In the case of bridges, the need to periodically
assess the structure stability and runability makes this need relevant. Together with the regional railway company of
Lombardy (Ferrovie Nord Milano), it has been decided to consider a 50 m long steel trussed bridge, as a laboratory bridge, to
be used for testing vision-based motion monitoring techniques to get the best bridge response estimation and to gain
awareness of the limits related to this approach. The considered structure is an old steel trussed bridge crossing a river; it
allows good access from both sides, therefore responding to the requirements of being a sort of 'laboratory' bridge in which
any change in the measurement parameters are possible to obtain without too difficult operation. Trains travel on the bridge
at a rather low speed, and also dynamic phenomena involving the whole bridge are confined to the low frequency range. The
main goal of this work is to define the performances of vision-based displacement measurement techniques in terms of
sensitivity, resolution, uncertainty, in the harsh field environment, which might strongly deviate from the behaviour defined
during laboratory testing.
In this work vision approaches have been applied together with more traditional measurements: among these a single-
point laser interferometer has been adopted as the reference, at mid span, to measure the effects of train pass by in terms of
structural static and dynamic displacements (also the bandwidth definition is among the aims. The known advantages of
camera-based measurement devices, that makes them attractive for the application of bridge monitoring, include: remote
