Dynamic Deflections of a Stiff Footbridge Using 100-Hz GNSS and Accelerometer Data

The authors investigated the possibility of expanding the application of global navigation satellite systems (GNSS) into the monitoring of stiff civil-engineering structures from the current limit of dominant frequencies of 3-4 Hz to a new limit of 6-7 Hz. On the basis of experience from previous supervised learning experiments, the authors analyzed 100-Hz GNSS data collected with an optimal phase-locked loop and collocated accelerometer data, both describing the attenuation of forced excitations of a timber pedestrian bridge. Computed vertical apparent deflections were masked by noise, but it was still possible to accurately identify the natural frequency of the bridge, equal to 6.5 Hz, and weak fusion with accelerometer data led to accurate oscillation waveforms. The quality of the latter was assessed by structural constraints and by comparison of waveforms of acceleration and of damping ratios derived from GNSS and from accelerometers. These results made it possible to (1) assess the quality of 100-Hz data in monitoring real structures, (2) propose methods for deriving highly dynamic deflections on the order of a few millimeters, and (3) expand the application of GNSS into the measurement of three-dimensional dynamic deflections of stiffer structures, i.e., with natural frequencies of up to 6-7 Hz and vertical deflections of a few millimeters only-an important contribution to structural-health monitoring and structural identification. (C) 2015 American Society of Civil Engineers.