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

From longitudinal system stiffness obtained at ±0.75H^sub y,L^, the yield displacement was calculated as Δ^sub yL^ = 88 mm, based on the yield force H^sub y,L^ obtained from an analytical pushover analysis. The analytical pushover analysis included foundation flexibility, column flexure and shear, and fixed-end rotation of the column base. The analysis used measured reinforcing material properties and targeted concrete strength, but assumed a cracked column that resulted in a minor discrepancy of initial stiffness between the predicted response and test results observed in Fig. 8. This yield displacement corresponds to a drift of δ^sub y,L^ = 1.6%. Following the force-controlled loading stage, two cycles of μ^sub Δ,L^ = ±1.0 were completed in the displacement controlled stage. Pile fractures were noted from loud bangs during the second cycle of μ^sub Δ,L^ = 1.0.

According to the proposed loading protocol, a near field pulse simulation towards μ^sub Δ,L^ = 3.0 was then initiated. During this loading step, multiple loud fractures could be heard from the piles in tension. Early in this displacement cycle, a sudden decrease in lateral force capacity resulted from tensile failures of the exterior piles. This decreased capacity in conjunction with an increase in actuator load rate resulted in a fictitiously high reading of lateral force. Instrumentation erroneously associated the increase in load rate as an increase in lateral force. The lateral load increase is noted in Fig. 8(a) as the fictitious increase in stiffness near μ^sub Δ,L^ = 1.0. After μ^sub Δ,L^ = 1.25, the response of the bent became dominated by a nonlinear elastic rocking behavior. The loading stage was stopped after the edge of the pile cap came into contact with the base block, which represented a displacement of Δ^sub u,L^ = 236 mm (Fig. 8(a)).

The permanent offset of the structure after the near field pulse simulation toward μ^sub Δ,L^ = 3.0 was Δ^sub res,L^ = 150 mm, and is noted on Fig. 8(a) as "residual displacement." This residual displacement indicates the displaced state of the structure when no longitudinal load was applied. The residual drift was calculated as δ^sub res,L^ = 2.7% and resulted almost entirely from a residual pile cap rotation. Column bending and fixed-end rotation of the column base comprised only 3% of this residual displacement.

An examination of Fig. 8(a) shows a capacity increase of the test unit between displacements of 203 and 236 mm. The increased capacity at this point is fictitious and resulted from contact of the pile cap and base block. This capacity increase should be ignored because it was not part of the system response. The same effect is noted on the reverse cycle of loading when the opposite side of the pile cap came in contact with the base block at a displacement of -94 mm and a longitudinal force of -187 kN.

Column response

During Phase I of testing, the column and retrofitted pile cap sustained flexural cracking, but no strength degradation. Column cracks extended nearly the full height of the column, but maximum crack widths were limited to 0.3 mm at the base of the column. The extent of flexural cracking up nearly the full column height is likely due to termination of longitudinal reinforcement.