Deflection Monitoring of the Forth Road Bridge by GPS
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Researchers at the IESSG at the University of Nottingham, in conjunction with colleagues from Brunel University, have carried out deflection monitoring work on structures, notably bridges, for a decade. Initial work was carried out on the Humber Bridge, London's Millennium Bridge and the Wilford Footbridge in Nottingham. These trials were carried out over a number of years, using a whole succession of GPS receivers. The initial trials showed that the use of carrier phase GPS could indeed allow sub centimetre movements to be detected, in addition to which, the frequencies of the movements could be calculated. Today, the authors are carrying out such work using state of the art dual frequency surveying grade code and carrier phase GPS receivers.
The Forth Road Bridge has an overall length of 2.5 km, a main span length of 1,005m, and was opened in 1964. Traffic has steadily increased over this bridge, from 4 million vehicles in 1964 to over 23 million in 2002. In addition, the heaviest commercial vehicles weighed 24 tonnes; the current limit is 44 tonnes. When the bridge opened, it brought to an end an 800 year history of ferryboat service across the river at Queensferry.
Such bridges experience traffic loading greater than that initially anticipated. The following paper details how GPS can be used to evaluate the performance of such a structure. On the 8 and 9 February 2005, a series of tests was conducted upon the Forth Road Bridge in Scotland. During the trials, 7 Survey grade GPS receivers were located upon the bridge, and a further two located as reference stations adjacent to the structure. In addition, a high accuracy Applanix INS, POS-RS, was also located upon the bridge; this is the subject of another paper. Of the 7 receivers on the bridge, four were located at the 1/8, 1/4, 1/2 and 3/4 span on the East side of the deck, whilst a fifth was located at the 1/2 span on the west side of the bridge. A further two receivers were located on top of the two towers at the south end of the bridge. All the receivers gathered data, almost non-stop, for a 48 hour period, at a rate of 10Hz or 20 Hz. Leica 530, 510 and GPS1200 receivers were used during the trials.
During the trials, a weather station was used to gather the wind speed and direction, as well as the temperature. This could then be used to evaluate the total force i.e. wind and traffic, experienced by the bridge.
During previous trials upon structures, the 10Hz GPS data has been densified with accelerometers capable of gathering data at up to 1,000 Hz. However, as this structure is so large, such high speed movements were not expected, and hence no accelerometers were used, and only the INS.
The Ordnance Survey of Great Britain has 74 active station GPS receivers located around the UK. These stations gather data at 1Hz, but then the data is made available at a 15s epoch rate to the public via their web site. In addition to this, the OS are currently establishing their own Network RTK system in the UK. During the trials, the GPS data from a number of OS stations located adjacent to Edinburgh were gathered for the IESSG in order to use these as a comparison with the bridge data processed relative to the reference stations next to the bridge.
During the trials, gusts of up to 60 mph were experienced, and the traffic loading was very heavy, especially at rush hour times. In addition, during the trials, a 100 tonne lorry passed over the bridge, and a series of trials were carried out with two 40 tonne lorries, equipped with DGPS to ascertain their locations, and having the bridge closed to other traffic. This is the most controlled of all the trials, as the wind loading is known from the weather station and the only traffic present on the bridge are the two 40 tonne lorries. The expected movements were calculated from the FEM, and the true results compared very well to these. Further to this, during the trials, IESSG staff took shifts to occupy the points, sitting in cars whilst the GPS receivers gathered the data to post-process in an On The Fly manner. During the data gathering exercise, it was evident that the bridge did move, and it was also possible to see a rippling effect on the bridge deck. On processing the data, movements of almost a metre were seen and the rippling effect was evident in the data as well.
The results are compared to finite element models (FEM) that exist of the bridge. The 3D coordinates available from the GPS results were transformed into frequencies of the structure's movements. These frequencies and magnitudes of the movements compared very well with the FEM.
The following paper details the trials, as well as the post processing techniques carried out on the single and dual frequency carrier phase data. The results are given for all the locations upon the bridge, showing how the bridge moves over a 46 hour period with a variety of loading. Further to this, detail is given on how the GPS results were compared to the FEM, and how such results can indeed be used for structural health monitoring.
- Reprinted with permission from The Institute of Navigation (http://ion.org/) and The Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation, (pp. 1016-1021). Fairfax, VA: The Institute of Navigation.
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