EARTHQUAKE SIMULATOR TESTS ON THE MITIGATION OF RESIDUAL DISPLACEMENTS OF REINFORCED CONCRETE BRIDGE COLUMNS

To minimize residual displacements in reinforced concrete columns, a design is proposed whereby a longitudinal post-tensioning tendon replaces some of usual longitudinal mild reinforcing bars. The seismic performance of such partially prestressed, reinforced concrete columns is investigated through a series of earthquake simulator tests. The effects of unbonding of longitudinal mild reinforcement and providing a steel jacket are also investigated. The partially prestressed, reinforced concrete columns studied perform remarkably well under strong ground excitations. Very small permanent deformations are observed after the tests, especially when the longitudinal mild reinforcement is unbonded and a steel jacket is provided. Introduction In recent years, reinforced concrete bridge columns with high ductility capacity are designed and constructed in regions of high seismicity to avoid collapse of the supported bridge during strong ground shaking (California Department of Transportation, 2001; Japan Road Association, 2002). While such conventionally designed reinforced concrete bridge columns are likely to ensure life safety, large residual displacements may exist following extreme earthquakes, necessitating long-term closure of highways while expensive repairs or even complete replacement is carried out. Thus, mitigation of post-earthquake residual displacements of bridge columns has become a major concern. A recent analytical study conducted by the authors (Sakai and Mahin, 2004a & 2004b) proposed a new method to reduce residual displacements by incorporating an unbonded prestressing tendon at the center of a lightly reinforced concrete column. The study demonstrates that (1) incorporating an unbonded prestressing strand at the center of a lightly reinforced concrete cross section can achieve restoring force characteristics similar to a conventionally designed column upon loading, but with substantially less residual displacement upon unloading; (2) such self-centering columns perform very well under uni-directional earthquake excitation; predicted residual displacements of the proposed columns are only about 10% of those of conventionally detailed columns while the peak responses are virtually identical; and (3) unbonding of longitudinal mild reinforcing bars enhances the origin-oriented tendency of the column’s hysteresis. Experimental studies have been conducted following the analytical study to assess the effectiveness of this approach. This paper presents a series of earthquake simulator tests 1 Research Engineer, Earthquake Engineering Research Team, Public Works Research Institute, Tsukuba, Japan 2 Graduate Student Researcher, Dept. of Civil and Envir. Engrg., Univ. of Calif., Berkeley, CA USA 3 Professor, Dept. of Civil and Envir. Engrg., Univ. of California, Berkeley, USA carried out to investigate the seismic behavior of the proposed columns. The effects of locally unbonding longitudinal mild reinforcement and providing a steel jacket in the plastic hinge region are also explored experimentally. Specimens Table 1 and Figures 1 and 2 show the specimens tested in this study. Figure 3 shows the test setup. A scaling factor of 4.5 is assumed for the specimens. The diameter of all specimens is 406 mm, and the height from the bottom of the column to the center of gravity of the top mass is 2.44 m, resulting in an effective aspect ratio of 6. The design concrete strength is 34.5 MPa.