Monitoring of Hakucho Suspension Bridge by ambient vibration measurement

Monitoring of a 720m span suspension bridge which is located at windy and seismically active area in northern part of Japan, opened in 1998 is introduced. The purpose of the monitoring is to understand the actual dynamic behavior and loading conditions, and to develop a health monitoring scheme using ambient vibration measurement which is readily available under service conditions. Data processing scheme based on Ibrahim's time domain method is constructed for identification of natural frequencies, mode shapes and modal damping ratios. Because measurement of the input force is not available for ambient vibration measurement, statistical treatment of measured data is required to obtain reliable results. This scheme is applied to analyze the actual ambient vibration measurement of the bridge under various environmental conditions. In the analysis, amplitude dependent softening of bearing supports which is not expected at the design calculation is identified. This preliminary analysis shows that the current monitoring system is directly applicable for identification of damage at the bearing supports which is one of the hot spots for fatigue and seismic damage, and also has the potential applicability to the health monitoring of the entire bridge.