Analyzing Prerepair and Postrepair Vibration Data from the Sarah Mildred Long Bridge after Ship Collision

The Sarah Mildred Long Bridge, an 854-m (2,804-ft) double-deck truss bridge in Portsmouth, New Hampshire, was struck by a 144-m (473-ft) cargo ship on April 1, 2013. After days of visual inspection and assessment, it was found that the main damage, significant bending of a vertical and a diagonal truss member, required replacement of the impacted members. According to the New Hampshire Department of Transportation, the repair cost $2.5 million. Before and after repairs, the authors collected vibration data on the bridge. This was a rare, valuable data set of a major structure at its damaged (prerepair) and healthy (postrepair) states. In this paper, the authors detail the data analyses and results and compare data collected before and after repairs at the Sarah Mildred Long Bridge. A finite-element model (FEM) of the damaged bridge span was created to compare the field data. By studying the frequency responses of the vibration data and the FEM, the authors were able to identify and explain noticeable changes between the bridge's healthy and damaged states.