Deflection Monitoring of the Forth Road Bridge by GPS

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 101 ION GNSS 18th International Technical Meeting of the Satellite Division, 13-16 September 2005, Long Beach, CA 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.