Civil Engineering Reference
In-Depth Information
Edge (tape)
Pattern (tape)
Pattern (rivet)
2
1
Dis. Edge (tape) mean = 0.022 mm
Dis. Edge (tape) st.dev = 0.357 mm
0
-1
Dis. Patt (tape) mean = 0.154 mm
Dis. Patt (tape) std.dev = 0.354 mm
-2
Dis. Patt (rivet) mean = -0.237 mm
Dis. Patt (rivet) std.dev = 0.823 mm
7
8
9
10
11
12
13
14
15
time [s]
Fig. 9.12
Five patterns test, target-less analyses: discrepancies with respect to the interferometer data
9.5 Concluding Remarks
Static and dynamic monitoring of civil structures represents a crucial step in serviceability and health assessment. Vision-
based measuring techniques are in these years gaining more and more importance, due to their advantages in terms of density
of measured points and static and dynamic capabilities. In theory, nowadays image processing algorithms allow to
appreciate displacement fields with an accuracy that can reach a few thousandths of pixel, but in real applications the
performances strongly decreases due to external uncertainty contributions, such as lighting conditions, sensor noise, camera
vibration and scaling factor estimation. As a consequence their application results in a compromise between field of view
and accuracy. In this work, the relation between measurement uncertainty and spatial resolution is studied in a real
environment. Two different state of the art image processing algorithms, edge detection and pattern matching, are exploited
to estimate the vertical sag of a railway bridge subjected to train transit. The collected data are compared with a low
uncertainty reference, a laser interferometer transducer, in order to quantify the vision-based measuring uncertainty.
At first, the displacement is measured in correspondence of fit-to-the-purpose targets mounted on the structure: the root
mean square of the discrepancy between camera-based measurements and reference transducer shows an approximately
linearly increasing trend with respect to the setup scaling factor. The RMS of the discrepancy has been decomposed into its
two components: the standard deviation, representing the random component of the discrepancy and characterized by a
deterministic trend if plotted against the scaling factor, and the mean discrepancy (bias), that, on the contrary, does not show
any evident trend. Furthermore, it has been proven that the measuring uncertainty estimated in static condition (no loading of
the structure) is able to correctly quantify the measuring system uncertainty, suggesting an easy way to estimate such a
parameter in an on-the-field application.
In the last part of the paper the displacement of the bridge deck is measured without using the targets but relying only on
the natural texture of the bridge. In this conditions the measurement reliability is strongly affected by the structure texture
contrast, however, in favourable conditions, small differences in the measuring performances are found with respect to the
measurement with target mounted on the bridge. The markerless approach can still be considered attractive in cases where
either the presence of the operator on the structure has to be minimized or the surface presents inherent textures comparable
to the tested one.
References
1. Sang-Chan K, Hak Kyeong K, Chae-Gue L, Sang-Bong K (2006) A vision system for identifying structural vibration in civil engineering
constructions. In: SICE-ICASE international joint conference 2006, Bexco, 18-21 Oct 2006
2. Yoshio F, Feng MQ, Masanobu S (2009) Cost-effective vision-based system for monitoring dynamic response of civil engineering structures.
Struct Control Health Monit. doi:
10.1002/stc.360
3. Olaszek P (1999) Investigation of the dynamic characteristic of bridge structures using a computer vision method. Measurement 25:227-236
4. Giorgio B, Alfredo C, Paolo M, Emanuele Z, Maurizio F (2012) Cameras as displacement sensors to get the dynamic motion of a bridge:
performance evaluation against traditional approaches. 6th international conference on bridge maintenance, safety and management (IABMAS
2012), Lake Como, 8-12 July 2012
5. Mazen Wahbeh A, Caffrey JP, Masri SF (2003) A vision-based approach for the direct measurement of displacements in vibrating systems.
Smart Mater Struct 12:785-794
