High Frequency Deflection Monitoring of Bridges by GPS

The use of GPS for the deflection and deformation monitoring of structures has been under investigation for a number of years. Previous work has shown that GPS not only measures the magnitude of the deflection of the structure, but also it is able to measure the frequency of the movement. Both sets of information are useful for structural engineers when assessing the condition of the structure as well as evaluating whether Finite Element (FE) models of such structures are indeed correct. GPS has the advantage of resulting in an absolute 3-D position, with a very precise corresponding time tag. However, until recently, the maximum data rate was typically 10-20 Hz, meaning that the maximum detectable frequency was about 5-10 Hz. GPS also has the disadvantage of multipath and cycle clips, and the height component’s accuracy is typically 2 – 3 times worse than that of plan. Previous work at the IESSG has included the integration of RTK GPS, gathering data at a rate of up to 10Hz, with that of data from an accelerometer, typically gathering data at up to 200 Hz. Accelerometers tend to drift over time, and can not detect low vibration frequencies, but the acceleration data can be double integrated resulting in changes in positions.
The integration of GPS and accelerometers can help to overcome each others’ shortfalls. This paper investigates the use of high rate carrier phase GPS receivers for deflection monitoring of structures. Such receivers include the Javad JNS100, capable of gathering data at up to 100 Hz. Static trials have been conducted to investigate the precision of such a receiver, as well as the potential applications of such a high data rate. Trials were carried out in a controlled environment and actual bridge monitoring, and comparisons made with a Leica SR510 receiver