Evaluating Fatigue Damage Using Strain Data

The US and many other countries around the world face the challenge of an aging infrastructure. In the US, there are 600,000 bridges in its inventory and a significant percentage of those bridges are nearing the end of their intended design life. Labor-intensive, visual inspections are used to identify possible safety problems in bridges and determine which ones can be safely kept in-service. This paper outlines ongoing research to improve inspection practices by providing the technology and methodology for real-time monitoring of steel bridges. A 76-year old, fracture-critical bridge was monitored with strain gages to determine the number and size of stress cycles in fatigue-sensitive members. From the cycle count, Palmgren-Miner’s rule can be used to determine an effective stress. The remaining fatigue life can then be calculated and compared to existing conditions and the age of the bridge. Because damage is expected to escalate over time, more frequent inspections may be needed when a bridge approaches its fatigue life. Thus, determining the remaining fatigue life of a bridge can be useful to bridge owners in prioritizing inspections. This paper will present the fatigue evaluation of the bridge and discuss the effectiveness of the recently-installed retrofit.

References
1. Downing, S. and D. Socie. “Simple Rainflow Counting Algorithms,” International Journal of Fatigue, 4 (1), 31-40, 1982. 2. ASTM E 1049, “Standard Practices for Cycle Counting in Fatigue Analysis,” American Society of Testing and Materials International, West Conshohocken, PA, 2005. 3. Fasl, J., T. Helwig, S. Wood, V. Samaras, A. Yousef, D. Potter, R. Lindenberg and K. Frank. “Development of Rapid, Reliable, and Economical Methods for Inspection and Monitoring of Highway Bridges,” IABMAS 2010, Philadelphia, PA, 11-15 July 2010. Taylor & Francis, 2010. 4. Miner, M. “Cumulative Damage in Fatigue,” Journal of Applied Mechanics, 12 (3), 159-164, 1945. 5. Swenson, K. and K. Frank. “The Application of Cumulative Damage Fatigue Theory to Highway Bridge Fatigue Design,” Research Report 306-2F, Center for Transportation Research, University of Texas at Austin, 1984. 6. Fasl, J., T. Helwig, S. Wood and K. Frank. “Using Strain Data to Estimate the Remaining Fatigue Life of a Fracture-Critical Bridge,” Transportation Research Research, Transportation Research Board, accepted March 15, 2012. 7. Connor, R. and J. Fisher. “Identifying Effective and Ineffective Retrofits for Distortion Fatigue Cracking in Steel Bridges Using Field Instrumentation,” Journal of Bridge Engineering, 11 (6), 745-752, 2006
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