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Fitness-for-Service Assessment and Root Cause Analysis of Cracked Suspension Bridge Roadway Girders

Critical elements of urban infrastructure, such as river crossings, are vital to the health, safety, and continuous operation of city life. Those that are subjected to frequent cyclic stresses, such as the members of a bridge, are particularly vulnerable to fatigue. As such, routine inspections are vital to maintain structural health. When cracks were observed in roadway floor beams near the expansion joints of a suspension bridge servicing both rail and vehicular traffic, it was important to identify the root cause in order to properly address the problem and mitigate further cracking. Because of the cracked floor beams’ proximity to the expansion joints, it was presumed that the joints were locked, causing stresses induced by vehicular traffic to be transmitted to the floor beams. A comprehensive fitness-for-service assessment was conducted including dynamic strain gage testing in order to ascertain the magnitude of the induced service strains and their frequency of occurrence. The collected data was used to determine the accumulated fatigue damage and remaining fatigue lives of the floor beam connections. A vibration assessment was also conducted in order to address concerns regarding the functionality of the modular expansion joint. Results revealed that the roadway expansion joints were in working order and that vehicular traffic directly above the cracked locations was not the primary cause of cracking. Rather, the largest fluctuations in strain were quasi-static and induced by trains on the adjacent rail track. Cracks were repaired and temporary stiffener plates were installed to mitigate cracking following the investigation.

1. AASHTO LRFD Bridge Design Specifications (5th edition), American Association of State Highway and Transportation Officials, Washington, DC. 2010. 2. British Standard BS 7608: 1993, Code of practice for fatigue design and assessment of steel structures, British Standards Institute, London, England.1993. 3. Barsom, J. and R.S. Vecchio. Fatigue of Welded Components, Welding Resarch Council Bulletin 422, June 1997
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