Study of Lateral Displacements and the Natural Frequency of a Pedestrian Bridge Using Low-Cost Cameras

Abstract

Vision-based techniques are frequently used to compute the dynamic deflections of bridges but they are rather computationally complicated and require demanding instrumentation. In this article, we show that it is possible to reconstruct the 2-D kinematics of flexible bridges using a simplified algorithm to analyze common video imagery. The only requirements are that the movement of the control points is clearly visible on the images and that next to each control point, there exist vertical and horizontal bridge elements defining the image scale. We applied this technique during controlled, forced excitations of a timber bridge that was stiff in the vertical but very flexible in the lateral axis because of cumulated damage. We used videos from low-cost cameras, in which the changes of the pixel coordinates of several control points during excitation events and their attenuation were clear. These videos were obtained during two annual structural health monitoring surveys using numerous sensors (Global Navigation Satellite Systems (GNSS), robotic total station (RTS), accelerometers), and hence the output of the video analysis was fully controlled. Because of various errors, the transformation of the video image coordinates into bridge coordinates yielded spurious deflections along the main axis of the bridge, which were used to control the uncertainty of our results. We found that the computed lateral deflections (i) were statistically significant, (ii) satisfied structural constraints, and (iii) were consistent with structural estimates derived from other sensors. Additionally, they provided accurate estimates of the natural frequency and the damping factor of the bridge. This approach can be applied in other cases of monitoring of flexible structures if the requirements for planar deformation, pixel resolution and scale definition are satisfied.

GNSS (GPS) Monitoring of Dynamic Deflections of Bridges: Structural Constraints and Metrological Limitations

“…A last argument: Dynamic deflections of short-span (20-30 m long) bridges above the threshold of a few cm are visible by naked eye, can be easily video-recorded from a nearby position (for an example see [31]), produce a sense of discomfort, and occasionally of fear to pedestrians and car drivers; they can also be analyzed using a combination of photogrammetric and typical video-processing techniques [32]. In fact, in favorable conditions (good visibility and proximity to the bridge) such techniques can identify and model even micro-vibrations [33][34][35].…”

“…Third, different sensors should be combined, for example GNSS and accelerometers, hopefully collocated with RTS (see Figure 2); or at least some control of GNSS data using RTS and other techniques permitting deflection measurements such as radar interferometers [22,23] and optical techniques [2,[32][33][34][35] during certain surveys is recommended. In our days, some of these studies can be based even on sensors embedded in smartphones; for example, smartphone accelerometers can easily permit to recognize a strong excitation with high deflections from a weak excitation with smaller deflections (e.g.…”

Vision-Based Vibration Monitoring of Structures and Infrastructures: An Overview of Recent Applications

“…Attention in this review article is given to the analysis of recent results obtained in vibration (displacement time histories) monitoring of civil engineering structures and infrastructures in the field, as documented in refereed journal articles published in the last four years [183][184][185][186][187][188][189][190][191][192][193][194][195][196][197][198][199][200]. The results presented are subdivided into six structural groups: steel bridges, steel footbridges, steel structures for sport stadiums, reinforced concrete structures, masonry structures, and timber footbridge.…”

“…Fradelos et al [200] illustrated the field vision-based monitoring of the Kanellopoulos timber arch footbridge (Patras, Greece), 30 m long and 2.9 m wide, made of glulam wood and metallic elements. The omission of X-bracing below the deck and poor construction of the metal X-bracing at its roof made the footbridge prone to lateral oscillations.…”

Research on an Optimized Evaluation Method of the Bearing Capacity of Reinforced Concrete Beam Based on the Bayesian Theory

“…The research results show that only monitoring the structural dynamic strain can accurately predict the dynamic deflection and acceleration of the bridge in the short-term performance evaluation, so as to reduce the difficulty of short-term monitoring of steel plate girder bridges. Fradelos et al [ 9 ] used approximate analysis image technology to collect the vibration signal of an arch bridge with a low-cost camera, and analyzed the natural frequency and damping ratio of the structure according to the dynamic deflection signal. The research showed that it was possible to reconstruct the two-dimensional kinematics of the flexible bridge by using the image taken by the non-precision camera, but the movement track of the control point is required to be clearly visible on the image, and there should be vertical and horizontal bridge elements defining the image scale beside each control point, which will undoubtedly increase the difficulty of bridge health monitoring and evaluation.…”