Seismic Performance of Retrofitted Reinforced Concrete Bridge Pier

ACI Structural Journal, Nov/Dec 2005 by Schoettler, Matthew J, Restrepo, José I, Seible, Frieder, Matsuda, Ed

Extrapolation of test results

Test results from Phase I are extrapolated to the actual pier because foundation flexibility was incorporated. The analysis from the "ductility relationships" section applies. The maximum displacement ductility obtained during stable hysteretic loops of the force-displacement relationship was μ^sub Δ,L^ = 1.10. This value can be extrapolated to μ^sub Δ,S^ = 1.12 using Eq. (1). Beyond this displacement ductility, Eq. (1) cannot be applied as the response was dominated by a nonlinear elastic rocking mode where the concept of displacement ductility is not valid.

After the completion of Phase I, the column was prepared for Phase II of the test. The preparations included recentering the column, grouting the 51 mm gap between the pile cap and base-block and placing tie-down rods through the pile cap.

TEST RESULTS-PHASE II

Overall response

The mechanism of response for Phase II of testing was a plastic hinge at the column base (refer to Fig. 10 for the lateral force versus lateral displacement response). A joint shear failure did not occur, and joint integrity was maintained despite substantial cracking in the footing.

The maximum longitudinal displacement was 363 mm, corresponding to a drift of 7.9% at μ^sub Δ,L^ = 4.0. Drift is calculated in Phase II as the lateral displacement at the point of loading divided by the height from the retrofitted pile cap surface to the point of loading, 4.57 m. At this ductility level, multiple ruptures of spiral reinforcement occurred and buckling of column longitudinal bars was observed. The residual drift after this loading cycle was 5.7% (Fig. 3(b) and 10(a)). The specimen performed well with only minor post-yield stiffness degradation up to a ductility level of μ^sub Δ,L^ = 4.0 when the test was stopped. A minor amount of pile cap uplift was recorded, less than 8 mm, during Phase II of testing. Displacements due to pile cap uplift were assessed during the data reduction, and the results shown were adjusted to eliminate displacements due to pile cap rotation.

Column response

Column yield displacement was calculated as 92.5 mm from stiffness obtained at ±0.75H^sub y,L^ (H^sub y,L^ is the yield force obtained from analytical pushover analysis incorporating column flexure, shear, and fixed-end rotation using measured reinforcement material properties and targeted concrete strength). The column had little strength degradation after cycles of μ^sub Δ,L^ = ±,4.0 despite major spalling at the column base. The onset of significant spalling occurred at the near field pulse of μ^sub Δ,L^ = 3.0. Longitudinal bar buckling occurred while attempting a displacement of μ^sub Δ,L^ = 4.0. Fracturing of transverse reinforcement occurred at this load, and fractures continued on the reverse cycle.

Column longitudinal strains exceeded the yield strain at a displacement ductility of 1.0 in all bars instrumented with gauges. The bidirectional loading protocol of this test had a significant impact on the longitudinal bar strains as the four instrumented bars exhibited similar strain readings during the loading cycles.