Detecting bridge dynamics with GPS and triaxial accelerometers

GPS and triaxial accelerometers have been used in field tests to record the response of the Wilford Bridge, a suspension footbridge over the River Trent in Nottingham, to forced vibration excited by more than 30 people with a total weight of 2353 kg, as well as subsequent decayed free vibration and ambient vibration caused by casual pedestrian traffic and weak wind loading. A peak-picking approach based on the bandpass filtering technique and Fast Fourier Transform (FFT) is employed to extract dominant local vibration frequencies and relevant vibration amplitudes of the bridge. It is found that the maximum frequency discrepancies between ambient and forced vibrations and that for ambient excitation against decayed vibration are 2.5% and 3.0%, respectively. The maximum frequency difference between different excitation manners is less than 2%. This provides evidence that precise structural natural frequencies of the bridge can be estimated from either the responding measurements of decayed free vibration or ambient vibration. These estimated frequencies, using GPS and accelerometer measurements, match well with the predictions from a dedicated Finite Element (FE) model created for the bridge. This paper concludes that GPS is a viable tool for both structural deflection monitoring and natural frequency detection and the measurements from a triaxial accelerometer can be used to validate the estimated dynamics from the GPS measurements and improve the overall monitoring system performance.