Retrofitted Concrete Bridge Columns Under Shaketable Excitation

ACI Structural Journal, Jul/Aug 2005 by Laplace, Patrick N, Sanders, David H, Saiidi, M Saiid, Douglas, Bruce M, El-Azazy, Saad

Column 6FS2 (steel-jacketed)

Column 6FS2 was subjected to the large initial 1.05g PGA motion. Spalling of the footing surface at the base of the column occurred due to this motion. No bars or core damage were visible within the 2.54 cm (1 in.) gap at the base of the column. The 0.88g PGA motion caused no further damage in the column. The final aftershock of 1.4g PGA produced severe damage in the gap region at the base of the column. The longitudinal bars buckled in this region and core damage was visible. Severe spalling of the footing surface occurred. Figure 2 shows the damage within the gap region after the final motion.

Columns 6FS1 and 6FS2 observed damage comparisons

There was more visible damage for Column 6FS1 than for Column 6FS2 when subjected to the same 1.05g PGA motion. Column 6FS1 had visible longitudinal steel in the gap region and was considered failed after this motion. Column 6FS2 did not have the same damage until after the two aftershocks of 0.88g and 1.05g PGA motions. Correlation between visual damage levels and level of excitation may be more accurate based on the loading protocol that used the initial large amplitude motion versus the typical incremental procedure, because it better represents large initial earthquake damage in field conditions. The overall performance is approximately the same but damage paths are different.

Column 6FC (carbon fiber jacket)

Due to the carbon fiber shell coverage, visual damage was obscured to a significant extent, although circumferential cracking could, with some effort, be detected in the epoxy matrix. Minor cracking at the base of the column at the gap between the carbon fiber and footing surface occurred during the 0.35g PGA motion. The 0.88g PGA response was the first motion where minor circumferential cracking occurred in the carbon shell at the top of the lap splice. The cracking was barely visible in the epoxy matrix. Minor spalling in the gap region occurred after this motion. The footing surface spalled after the 1.05g PGA motion. The surface spalled more severely after the 1.23g PGA motion. Longitudinal bar buckling occurred during the 1.40g PGA motion. The column strength dropped significantly at this earthquake, and the test was stopped. Figure 3 shows the damage at the base of the column after the final motion.

MEASURED PERFORMANCE

Force-displacement envelopes

Figure 4 shows the measured force displacement curves for the flexural columns. There was a significant difference in the capacity envelopes between the two identical steel jacket columns. The steel jacket column subjected to the initial high-amplitude earthquake motion showed significantly higher capacity throughout the deformation than the steel jacket column subjected to the incremental motions. Both steeljacketed columns show a similarity in the deflection at which they began yielding and degrading in strength. Column 6FS1 showed a consistently lower strength than Column 6FS2. This may be due to more bars slipping in Column 6FS1 caused by more low-level motions as compared to Column 6FS2. Column 6FS1 also exhibited a smaller elastic stiffness than Column 6FS2. The lower stiffness of Column 6FS1 may also be due to the sensitivity of the lap splice to multiple lowlevel cyclic motions from the initial earthquakes, in effect degrading the bond between the lap splice.