System identification of suspension bridge from ambient vibration response

The paper addresses and evaluates the application of system identification to a suspension bridge using ambient vibration response. To obtain dynamic characteristics of the bridge, two output-only time-domain system identification methods are employed namely, the Random Decrement Method combined with the Ibrahim Time Domain (ITD) method and the Natural Excitation Technique (NExT) combined with the Eigensystem Realization Algorithm (ERA). Accuracy and efficiency of both methods are investigated, and compared with the results from a Finite Element Model. The results of system identification demonstrate that using both methods, ambient vibration measurement can provide reliable information on dynamic characteristics of the bridge. The NExT-ERA technique, however, is more practical and efficient especially when applied to voluminous data from multi-channel measurement. The results from three days of measurements indicate the wind-velocity dependency of natural frequency and damping ratio particularly for low-order modes. The sources of these dependencies appear to be the effect of aerodynamic forces alongside the girder, and friction force from the bearing near the towers.