Monitoring and Fatigue Analysis of Suspension Bridge Rocker Arms

Abstract

The South Channel Bridge was first opened in 1958 in Cornwall, Ontario and is a suspension bridge with a total length of 1061 metres and a roadway width of 8.2 metres. It links the Akwesasne Mohawk Territory to the United States and Canada across the St. Lawrence River. The stiffening truss of the bridge deck is connected to the towers by eight rocker arms. The rocker arms have pin-ended connections and therefore are expected to primarily be subjected to axial loads. One of the rocker arms developed fatigue cracks and fractured in 2015 causing the deck to descend 150 mm at the expansion joint. The entire rocker arm assembly was replaced, and by 2021 concerns about the other seven rocker arms led to the installation of a monitoring system consisting of 32 vibrating wire strain gauges. This thesis describes the collection and processing of the monitoring data. The data was used to develop a fatigue damage assessment for the rocker arms. Baseline data for fatigue damage was provided by an assessment of the rocker arm replaced in 2015. Because of the level of fatigue damage detected during the monitoring, the bridge owner decided to replace all the original rocker arms in 2022. This limited the monitoring data to six months. Nevertheless, the thesis provides important insights into the expected stresses in a properly operating rocker arm, some of the practical issues with monitoring a suspension bridge, and how a simple fatigue life analysis combined with monitoring data can provide bridge owners with a practical method to make decisions on maintenance and repair.