NEWSLETTER

The use of kinematic GPS for monitoring structures, most notably bridges, has been an area of research at the Institute of Engineering Surveying and Space Geodesy (IESSG), the University of Nottingham, since 1994. This research has developed into a collaborative effort by researchers at the IESSG, led by Dr G W Roberts and at the Department of Mechanical Engineering, Brunel University, led by C J Brown. The researchers have used, amongst others, the Humber Bridge and the London Millennium Bridge to evaluate the use of GPS for such monitoring.

Originally, the technique had been particularly useful for large span structures, or for structures where access may be difficult. For example, the deformation of the 1.4km Humber Bridge had been determined by other conventional methods with much less reliability. Now however, it is recognised that with increased accuracy and frequency of measurement, the simultaneous evaluation of dynamic response and structural deformation can be useful data in assessing the performance of structures. Changes may be a significant indicator in monitoring structural condition.

Technology advance means that GPS data capture rates of 10 Hz are available. 20 Hz will soon be possible, and GPS processing gives data with no drift characteristics. Current research is looking at improving the quality and reliability of the GPS data, through mitigating multipath and cycle slips. The team believes even better data accuracy can be obtained, and if funding becomes available intend to investigate a new approach to signal capture.

The resulting 3D positional data can be interpreted at a precision of less than a millimetre. Of course GPS has the advantage of determining displacement/movement in a generalised 3D coordinate system, enabling transformation to any local coordinate system as required.

GPS positional data is gathered at precisely known time intervals. It is therefore also ideal for the determination of both velocity and acceleration. Dynamic response is fairly easily determined, and excellent agreement was obtained between measured frequencies and those predicted by finite element methods in the work on the Humber Bridge. A number of different data processing techniques have been applied to the data subsequently gathered on existing structures, and the research is currently evaluating strategies for the development of systems to give near real-time measurement of deformations and frequency response.

Current experience suggests quite small total displacements are measurable (say 6mm); frequencies not greater than about 3Hz are also achievable. The integration of higher data capture rates allied to improved accuracy will lead to greater potential application of GPS methods to structures whose natural frequencies are higher and whose deformations are less. The team confidently look forward to these achievements over the coming year.

The team are grateful to IESSG and Brunel University for funding basic research in the area. Thanks are also due to Roger Evans at Humber Bridge Board, and Leica UK Ltd for their encouragement and support.

For further information, contact:

Gethin Roberts, IESSG, The University of Nottingham
(e-mail Gethin.Roberts@nottingham.ac.uk, tel: 0115 951 3933)