Unbonded Prestressed Columns for Earthquake Resistance
AuthorLarkin, Alexander Scott
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Modern structures are able to survive significant shaking caused by earthquakes. By implementing unbonded post-tensioned tendons in bridge columns, the damage caused by an earthquake can be significantly lower than that of a standard reinforced concrete bridge column, by reducing the amount of residual displacement. Reducing the residual displacement will reduce the amount of damage and allow for faster repairs and minimal closure time on the bridge. The objective of this research was to investigate new construction details for unbonded post-tensioned bridge columns that will reduce the amount of damage caused by an earthquake. Two 0.4-scale columns containing unbonded tendons were selected for testing. The two columns were the same except for the amount of longitudinal reinforcement crossing the joint between the column base and the footing. SAP2000 was used to model each column, showing a close correlation between the calculated and measured results. A parametric study was conducted on the specimens investigating various axial dead loads, initial post-tensioning force, tendon location, and an increase in the concrete strength. For the specimens to be true scale models, the amount of post-tensioning required in a full-scale column was taken into consideration. The large amount of prestress needed in a full-scale column requires separate tendons being spread out around the center of the column cross section. Greased and sheathed strands were incorporated for their additional corrosion protection. The introduction of the unbonded tendons showed a significant reduction in residual displacements, and the longitudinal reinforcement ratio had a large effect on the re-centering magnitude. A smaller longitudinal reinforcement ratio also produced a larger displacement ductility.