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PROCEEDINGS OF THE NINTH INTERNATIONAL CONFERENCE ON COMPUTATIONAL STRUCTURES TECHNOLOGY
Edited by: B.H.V. Topping and M. Papadrakakis
Determination of Bridge Natural Frequencies Using a Moving Vehicle Instrumented with Accelerometers and a Geographical Positioning System
A. González1, E. Covián2 and J. Madera2
1UCD School of Architecture, Civil Engineering and Landscape, University College Dublin, Ireland
A. González, E. Covián, J. Madera, "Determination of Bridge Natural Frequencies Using a Moving Vehicle Instrumented with Accelerometers and a Geographical Positioning System", in B.H.V. Topping, M. Papadrakakis, (Editors), "Proceedings of the Ninth International Conference on Computational Structures Technology", Civil-Comp Press, Stirlingshire, UK, Paper 281, 2008. doi:10.4203/ccp.88.281
Keywords: frequency, vehicle, bridge, acceleration, GPS, monitoring.
Tests in free or forced vibration are commonly used to accurately obtain bridge frequencies, but they are time consuming and labour expensive. This paper investigates the possibility of using a vehicle to obtain the bridge frequencies from the vehicle vibrations without the need for a bridge installation. These vehicle vibrations can be measured with accelerometers mounted onboard. The vehicle is simply driven over the bridge and the corresponding bridge frequencies are extracted from the spectrum of vehicle accelerations.
Preliminary studies by Yang et al.  showed it is theoretically feasible to extract bridge frequencies from the dynamic response of a passing vehicle modelled as a sprung mass over a simply supported beam. Lin and Yang  followed up on this research by carrying out a experimental test, that showed that the bridge frequency could be easily identified at vehicle speeds of less than 40 km/h on a six-span bridge (30 m per span). At speeds beyond this value the bridge frequency was blurred by high frequency components resulting from the cart structure and road roughness.
This paper uses a three-dimensional finite element vehicle-bridge interaction model to test this measuring technique in other scenarios. The accuracy of the predicted frequencies is tested for various speeds, road roughness, damping levels and traffic conditions to establish the theoretical limitations of the approach. A experimental test is also carried out in a main route near Oviedo, Northern Spain, where a vehicle instrumented with accelerometers and a geographical positioning system (GPS) is driven over a long-span bridge to obtain its frequency.
This paper investigated the feasibility of using an instrumented vehicle to monitor the natural frequencies of a bridge. It has been seen an accurate determination of the bridge frequency requires a sufficient degree of dynamic excitation of the bridge. So, for the theoretical bridge structural form under study, the first natural frequency of the bridge was unidentifiable and it has only been possible to extract the second natural frequency of the bridge for very low speeds. For the experimental test, it was not possible to extract the bridge frequency, which is attributed to the relatively high vehicle speed and the lack of heavy traffic on the bridge that would ensure the bridge deflection to be significant compared to the height of the road irregularities.
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