Temperature and Traffic Load Effects on Modal Frequency for a Permanently Monitored Bridge

Abstract

Many of the damage detection techniques available compare measured parameters of a structure in a known, reference state to an unknown, possibly damaged state. In the case where the measured parameters are the modal characteristics of the structure, it has been shown that these can be sensitive to changes other than damage, such as temperature. Therefore it is useful to install permanent monitoring systems that can track normal changes in measured modal characteristics. The Ironworkers Memorial Second Narrows Crossing, in Vancouver, Canada, had a permanent monitoring system installed in 2011. The purpose of the system is to monitor the response of the bridge during seismic and other potentially damaging events, and to perform long term performance monitoring. During an assessment of the modal parameters it was observed that there is a regular fluctuation in the measured frequencies, so a detailed study on a 17-day segment of continuous data was performed. Analysis was performed on both temperature data and acceleration amplitude (to represent traffic on the bridge). The modal fluctuations were very regular, with maximum at night and minimums in the day, which correlated well with the maximum traffic in the day and minimums at night. The temperature fluctuations coincided with daily changes but had a more variable pattern, which was not directly correlated to changes in frequency. It is also observed that the traffic load on the bridge amplifies the response of the first vertical mode seven and ten times during daytime weekdays and daytime weekends, respectively whereas the first torsional mode of the structure amplifies 5.5 and 4 times in daytime weekdays and weekends, respectively.