Analysis of Degrading Concrete Structures: The Morandi Bridge, A Case Study

Thesis (Master's)--University of Washington, 2021The concrete infrastructure of the United States is reaching its nominal design life. More than 65% of the highway bridges in the US are reinforced concrete or pre-stressed concrete construction and 20% of those bridges do not meet current mini-mum design standards. Damage in these structures often manifests as reinforcement corrosion, an insidious mechanism that may remain hidden until damage has progressed to an advanced state. Corrosion reduces reinforcement area, reduces bond strength, and induces concrete cracking. The deterioration process is exacerbated by live loads that increase over time. Cyclic live loads are especially damaging when paired with corrosion damage, reducing reinforcement fatigue life by magnitudes. The challenge of dealing with aging infrastructure must begin with the identification of at-risk structures. Failure of aging structures is often characterized by a lack of redundancy, defined as the ability of a structure to withstand the loss of a load-resisting component. In the case of load redistribution due to member failure, a structure may develop complex and unanticipated stress distributions. Proper tools are needed to analyze the complex stress-states that arise upon load-redistribution due to member failure. This study presents two analytical schemes for the analysis of combined corrosion and fatigue, based on experimental data from the literature. The methods can predict the fatigue life of corroded reinforcement if alternating stresses in the reinforcement are known. Additionally, this thesis reviews truss model theories for the prediction of a prestressed concrete member's response to combined loads, with a focus on the torsional response. The tools are implemented in a case study of the Morandi Bridge, which partially collapsed on August 14, 2018 in Genoa Italy, killing 43. Potential collapse mechanisms observed from surveillance footage are investigated with the aid of the corrosion, fatigue, and combined loading tools presented in this work. The results of the study suggest that failure of the Morandi Bridge could have originated in the cable stay due to the combined effect of corrosion and fatigue. After failure of a stay, the unbalanced structure would not have been able to redistribute the loads and the bridge's main deck would have failed in torsion