It is proposed to collect structural data of a damaged bridge in the next 4-6 weeks before repair is completed. The Sarah Long Bridge, a 2804-foot double deck truss bridge, in Portsmouth, New Hampshire, was struck by a 473-foot cargo ship on April 1, 2013. After visual inspection and assessment, it was determined that the primary structural damage is the severely bending of two members of the 227-foot segment of the steel truss and they must be replaced. The bridge is closed to traffic till the repairs are completed. The New Hampshire Department of Transportation estimates the repair to cost $2.5 million and take 4-6 weeks. The proposed work fits RAPID's goal given that the collision is a rare, unanticipated event and the short time period available to collect perishable vibration data on the damaged bridge.
The project team will deploy wireless sensors on the bridge and record ambient vibrations before and after the repairs. Additionally, the damaged structural members of the bridge will be documented and analyzed. The proposed measurements will provide rare data sets of a major structure at its damaged (pre-repair) and healthy (post-repair) states, which will help understand structural behavior of a damaged truss. The vibration data of the bridge at its healthy and damaged states will be analyzed and compared to identify changes in natural frequencies. In addition, the bridge data will also be used to develop a finite element analytical model of the bridge truss. The analytical model will be used to understand structural vibration characteristics of the truss and can postulate how frequencies of the truss will change with other damaged members. This exercise will answer the question of change in vibration frequencies with different levels of damage; and ultimately help in structural health monitoring field.
The project objective was to study an in-service damaged bridge structure and provide a study case of structural health monitoring (SHM). On April 1, 2013, the Sarah Mildred Long Bridge—a 2804-foot double deck truss bridge connecting Portsmouth, New Hampshire and Kittery, Maine—was struck by a 473-foot cargo ship. After days of visual inspections and assessments, it was determined that the primary structural damage was the significant bending of two steel truss members on the Portsmouth side. These members were to be replaced at the cost of $2.5 million. The bridge was closed to traffic while the repairs were underway for approximately a month. The project team collected vibration data before and after the repairs; this provided a rare, valuable set of field data of a major structure at its healthy (post-repairs) and damaged (pre-repairs) states. Intellectual Merit: The intellectual merit of the project is to demonstrate that SHM systems can detect damage in real-world events. Comparing the data of the bridge’s healthy and damaged states, there were clear differences between the vibration responses. This finding was verified by a computer model of the bridge that was used to simulate the vibration response of the damaged and repaired structure. In other words, damage or a sudden change in structural characteristics could be detected quickly and systemically if a SHM system was installed in this bridge to collect structural data. Broader Impact: There is a great need to safeguard the nation's bridges as they are major components of our national critical infrastructure. As a significant number of bridges approach and exceed their anticipated lifespans, it is becoming very challenging to maintain them. The I-35W Mississippi River Bridge collapse in 2007 serves as a constant reminder of the impact of bridge failures. The results of this project allows engineers a rare opportunity to study a bridge in its healthy and damaged states. Critical assessment of the data collected through this RAPID project will lead to improved designs and monitoring of bridges and rapid structural assessments in case of major events (e.g., ship crash, wind storms).
