Reinforced concrete bridge columns located in regions of high seismicity are designed with large ductility capacity for adequate protection against collapse. This type of design tends to result in large permanent displacements. To maximize post-event operability and to minimize repair costs, new design strategies to reduce these residual displacements are necessary.
To minimize residual displacements in reinforced concrete columns, a new method is proposed whereby longitudinal prestressing strands replace some of the typical longitudinal mild reinforcing bars. The seismic performance of such columns with prestressing strands is investigated through a series of quasistatic analyses and dynamic analyses.
The results from quasistatic analyses for more than 250 columns with various configurations of strands demonstrate that (1) incorporating a single bundle of unbonded strands at the center of the cross section results in an 85% reduction of the quasistatic residual displacement; (2) post-yield stiffness can be controlled by varying the amount of strands incorporated into the columns; and (3) smaller amounts of longitudinal rebar are preferable for reducing the residual displacement; however, this results in smaller flexural strength and lower levels of energy dissipation. Additional quasistatic analyses suggest that unbonding of the longitudinal mild reinforcement accentuates this recentering tendency.
Based on design recommendations developed, a series of columns with different heights are designed. For the suite of near-fault ground motions considered, the maximum and residual displacement response spectra for post-tensioned columns and conventional reinforced concrete columns are generated. The spectra show that the post-tensioned columns exhibit maximum displacements similar to those for conventionally reinforced concrete designs, and residual displacements are reduced by more than 50% on average. Larger post-yield stiffness of post- tensioned columns results in smaller residual displacement. On the other hand, columns with smaller energy-dissipation capacity and smaller post-yield stiffness require approximately 10– 30% larger seismic demand than those of conventional reinforced concrete columns, although the residual displacements of the columns are still reduced from those of the conventional columns.
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