Long-Term Dynamic Response of Bridges in Cold and EarthquakeProne Regions

Abstract

This thesis investigates the sensitivity of the long-term in-situ dynamic response of bridges to changing structural parameters and environmental conditions, with a focus on cold and earthquake-prone regions. The interpretation of data from structural health monitoring in an adverse environment can be a significant challenge due to changes in the bridge dynamic response that are not associated with deterioration or damage. The effect of seasonal freezing on bridge dynamic response was assessed using dynamic monitoring data collected over a year from the bridge in Alaska. Modal frequencies in the transverse direction increased significantly, and the corresponding mode shapes changed due to seasonal freezing. Elastic finite element models of the bridge showed that these changes were controlled by the stiffness of the foundation soil and the bearings. Assessment of a range of bridge typologies derived from the original bridge showed that the fundamental transverse period was significantly reduced in winter across all bridge typologies. The mode shapes of the bridges with shorter piers and few spans change considerably in winter. Significant redistribution of the bending moments and shear demands along the pier columns and piles was found in freezing conditions. These findings reinforce the need to account for seasonal temperature fluctuation in bridge seismic design to prevent potential damage modes that may be accentuated during freezing temperatures. The effect of earthquake excitation and repair on the transverse dynamic response was assessed using a dataset collected over four years from an overpass in New Zealand. A gradual increase in the natural frequency was observed during repair works, followed by an abrupt drop in frequency after moderateintensity earthquake excitation. The normalized modal amplitudes of the superstructure remained relatively stable, suggesting that they are not a good indicator of the change in overpass response. The variation in overpass response was shown to be controlled by changes in foundation stiffness and the strengthening of the connection between girders and abutments. Strengthening of the piers, which were the most damaged structural elements, had minimal impact on the overpass dynamic parameters.