Unbonded Prestressed Columns for Earthquake Resistance
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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 residual displacement. Reducing residual displacement will reduce the amount of damage and allows for faster repairs and minimal closure time of the bridge. The objective of this research was to investigate new construction details for unbonded post-tensioned bridge columns that will reduce damage caused by an earthquake. Two 0.4-scale columns containing unbonded tendons were selected for testing. The two columns were identical 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 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 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. The re-centering effect was not as dominant with an increase in longitudinal reinforcement. A smaller longitudinal reinforcement ratio also produced a larger displacement ductility.
Report No. CCEER-12-02