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Redundancy in Long-Span Bridges for Risk Mitigation in a Multi-Hazard Environment
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2021-05-01
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[PDF-25.77 MB]
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Edition:Final Report
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Abstract:As a critical part of the infrastructure system, long-span bridges can be exposed to extreme conditions, such as floods, scours, and hurricanes, or severe dynamic loads, for example due to seismic shaking, blast, or impact. In spite of their importance to the resilience of adjacent communities, their redundancy and ability to mobilize alternative load paths when needed are not as well understood as for shorter span bridges. Traditional design approaches are unable to provide explicit measures of residual safety of such bridges, and the current redundancy evaluation approach suggested by the NCHRP Report 776, which was proposed based on short to medium-span bridges, is inappropriate for long-span bridges. Therefore, a new performance-based method was developed in this research to quantitatively evaluate the redundancy and robustness of long-span bridges, especially after abnormal loading events that result in member loss. Three long-span cable supported bridges (i.e., a cable-stayed bridge, a tied-arch bridge, and a suspension bridge) were selected, and detailed implicit and explicit finite element models were developed for them. The explicit models were employed to investigate the dynamic behavior of the bridges under various cable loss scenarios and critical live load distributions. The simulation results showed that only structural members in the vicinity of cable loss were primarily affected to a certain degree, while the overall performance of the bridges was affected only slightly. The progressive collapse behavior of the bridges was additionally studied by successively removing members until system failure occurred. The behavior of the bridges subjected to over loading was examined through pushdown analyses for both the intact and damaged states with single cable loss, and critical limit states were identified for each bridge. A new performance-based robustness evaluation method and a robustness index, well-suited for both short-span and long-span bridges, were presented and verified on short to medium-span bridges. Using the new method, the robustness of the three long-span bridges was quantitatively evaluated for the limit states identified through the pushdown analyses. The results showed that: (1) the effect of single cable loss on each bridge can be captured explicitly, demonstrating the applicability of the new method, especially for long-span bridges, and (2) despite the adverse effects of single cable loss, there was no significant reduction in the reliability and robustness of all three bridges, i.e., they are very robust against single cable loss scenarios.
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