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Nondestructive Testing and Comparison of Approaches for Bridge Load Rating

Highway bridges in the United States are at risk due to age and deterioration. The average age of bridges in this country is more than 43 years old (AASHTO, 2008). According to Bridging the Gap, the five major problems for our nation's bridges are age and deterioration, congestion, soaring construction costs, maintaining bridge safety, and the need for new bridges (AASHTO, 2008). Based on the Federal Highway Administration, (FHWA) data 71,177 (11.8%) bridges out of 604,460 bridges were rated structurally deficient and 78,477 (13.0%) were rated functionally obsolete (FHWA, 2011). Computing bridge load ratings of these bridges is one of the tools that bridge owners can use to determine the maintenance needs of the bridges, including load posting for public safety and/or scheduling retrofit or replacement (Ellingwood et al., 2009). Therefore, having accurate load rating techniques and calculations are valuable tools for effective bridge management.

The Powder Mill Pond Bridge in Barre, Massachusetts was instrumented during construction and diagnostic load tests were performed. Load ratings were calculated using the Allowable Stress Design, Load Factor Design and Load Resistance Factor Design methods with four different approaches. Initially, the rating factors of the Powder Mill Pond Bridge were calculated by hand. The second approach used the bridge rating program Virtis 6.3, developed by the American Association of State Highway and Transportation Officials, and the results were compared with the hand calculations. The diagnostic load tests were used to modify the rating factors that were calculated by hand in the third approach. The diagnostic load test data was also used to calibrate a finite element model of the Powder Mill Pond Bridge. Finally, the fourth approach used a calibrated finite element model to determine load rating factors of Powder Mill Pond Bridge. The rating factors obtained using nondestructive test data and the finite element model were higher than conventional load rating factors since both methods take advantage of the true three dimensional system behavior of the structure. The comparison and subsequent explanation of the rating factors provide bridge owners insight into different approaches to calculate bridge load rating for maintenance and management decisions.

Ellingwood, B. R., Zureick, H. A., Wang, N. and O'Malley, C. (2009), "Condition Assessment of Existing Bridge Structures" Georgia Institute of Technology, August 1, 2009

American Association of State Highway and Transportation Officials (2008), "Bridging the Gap - Restoring and Rebuilding the Nation's Bridges" Federal Highway Administration (2011), "Our Nation's Highways: 2011"

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