Strength Degradation of Existing Bridge Columns under Seismic Loading

The strength and degradation behavior of reinforced concrete bridge columns under seismic loading were investigated. Experimental tests were conducted on four reduced-scale column specimens that incorporated deficiencies selected to be representative of those present in existing bridges designed before 1971. The columns were fixed against rotation at both the top and bottom, resulting in a transfer of shear forces through the column even after the lower hinging region lost its flexural capacity. The specimens were subjected to increasing levels of cycled inelastic displacements under constant axial load. The focus of the study was to characterize the load and displacement capacities present in older columns for purposes of seismic assessment and retrofit design. Flexure-dominated failures occurred in three of the specimens. A rapid degradation in flexural strength was observed at the bottom hinging regions of the tested columns because of the presence of lap splices and poor confinement. Top hinging regions that did not have lap splices exhibited degradation in flexural capacities at higher displacement ductilities because of eventual longitudinal bar buckling. In the fourth specimen, flexural yielding was initially observed at both the top and bottom of the column, but this was followed by an eventual brittle shear failure. Various procedures for assessing flexural and shear behaviors were compared with the observed experimental results.