A Comprehensive Methodology to Estimate the Current Safety and to Predict the Remaining Useful Life of Suspension Bridge Cables

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In the US, various transportation systems, including long-span cable suspension bridges,have been allowed to deteriorate because of inadequate investment in maintenance and life extension technology, and particularly because of clearly insufficient support for research in the area of structural assessment and protection. In-depth inspections of the main cables in the suspension bridges in the New York metropolitan area have found a quite large number of broken wires inside the cables and at the anchorage, showing extensive deterioration and brittle fractures. These findings pose the difficult task of determining the actual strength of main suspension cables in existing bridges. The safety evaluation of suspension bridge cables has always been done considering as a safety factor the ratio between the estimated cable strength over the computed ultimate load. However, in view of the many uncertainties involved with the cable deterioration pattern and with wire corrosion mechanisms, and considering the relatively small proportion of wires that are inspected and/or tested, such a safety factor is useful only as a first approximation, but not in assessing the overall safety of the bridge. A better and moreappropriate measure of the overall safety of main suspension cables is the use of probabilistic indicators such as the reliability or the safety index. Without a well-defined methodology for estimating the residual strength of an existing cable, suspension bridge owner agencies do not have a clear idea of what to search for in a cable inspection and so they base their maintenance program mainly on previous experiences and on trial and error attempts. This grant proposes to go beyond the objectives of the NCHRP Project 10-57 by developing a comprehensive methodology for a rigorous safety appraisal of the cable system in existing suspension bridges. The ultimate objectives of this study will focus on 1) the estimation of the actual residual strength and of the actual safety of main cables in suspension bridges based on results of inspections and laboratory testing of cable wires, and on 2) the prediction of the remaining useful life of suspension bridge cables using the same results. In this way, the estimated overall safety of the cables of different bridges can be directly compared. A new cable model that accounts for limited and different ductility of the corroded bridge wires will also be developed, using a probabilistic framework to account for the random nature of the deterioration pattern. The determination of the statistical parameters to be used in the new prediction model will be done using a unique set of inspection and test data, collected over 30 years of bridge testing by the Carleton Laboratory at Columbia University, and on the results of current research projects on wire deterioration. This project will make a significant contribution to the bridge engineering community with the introduction of a more rigorous description of the overall safety and remaining useful life of main cables in existing suspension bridges.