A Unified Theory of Progressive Failure in Corroded Reinforced Concrete Bridge Piers Under Combined Axial, Shear, and Cyclic Lateral Loading

Reinforced concrete bridge piers that have experienced significant reinforcement corrosion represent a class of structurally compromised members whose failure behaviour under traffic-induced cyclic lateral loading combined with axial load is not captured by existing progressive failure theories, which are predominantly calibrated for uncorroded members. This study develops a unified theory of progressive failure for corroded reinforced concrete bridge piers under combined axial, shear, and cyclic lateral loading, motivated by the large number of bridge piers across Nigeria's major river crossings that are in an advanced state of corrosion deterioration. The theoretical development integrates a corrosion-modified mechanical model for reinforcement bar cross-sectional reduction and bond degradation, a concrete cover cracking and spalling propagation model derived from experimental data on corrosion-induced expansion, and a nonlinear fibre beam-column formulation incorporating the modified material models. The unified failure theory is calibrated against an experimental database of 64 corroded pier specimens tested under cyclic loading compiled from international literature and supplemented by 12 original specimens incorporating corrosion levels representative of field conditions at bridge piers along the Niger, Benue, and Cross Rivers in the North Central and South South geopolitical zones. Prediction accuracy of the unified theory for peak lateral load capacity, post-peak strength degradation rate, and energy dissipation per cycle was compared against four existing corroded member models. Results demonstrate that the unified theory reduces mean prediction error for peak capacity from 21 percent for the best existing model to 9 percent, and captures post-peak behaviour with significantly improved fidelity. Keywords: progressive failure theory, corroded bridge piers, cyclic loading, reinforced concrete, unified model.