Seismic Performance of Square Nickel-Titanium Reinforced ECC Columns with Headed Couplers
Current bridge seismic design could be improved by moving towards performance based design to keep bridges operational and keep repair costs to a minimum by reducing damage to column plastic hinges and minimizing residual drifts. These objectives can be accomplished by using innovative materials such as Nickel-Titanium shape memory alloy bars (SMA) and engineered cementitious composites (ECC). This study focused on evaluating the performance of SMA/ECC in scaled bridge columns representing the piers of the SR-99 on-ramp structure that is scheduled for construction in Seattle, Washington. The column studies consisted of the design and construction of three 0.3-scale bridge column models that were tested under cyclic loading. Two of the columns had different lengths of SMA bars in the plastic hinge with ECC over the entire height. The third was a conventional steel reinforced concrete column and served as a reference. The average residual drift of the SMA/ECC columns was approximately 85% lower than the steel RC column residual drift. Shortening the length of SMA did not affect the self-centering capability of the column. The drift capacities of the SMA/ECC columns were at least 33% higher than that of the steel RC column. ECC was effective in minimizing damage to the plastic hinge. Damage in the SMA/ECC columns was limited to a single wide crack at the base that was repairable. The new method of connecting SMA bars with mild steel bars using up-set headed couplers was successful. These couplers can be more cost effective compared to threaded couplers because of the lower cost of making the heads and the fact that the entire bar section rather than a dog bone shape is utilized. Analytical studies conducted using OpenSees consisted of developing models for the three column models. The analytical models were validated by comparing the results with the experimental data. The models were then used in dynamic analysis to determine the performance of the columns under various earthquake motions. Dynamic analysis showed that SMA/ECC columns can dissipate more energy than steel RC columns due to their higher displacements. Parametric studies were conducted using Xtract software for moment curvature analysis on SMA/ECC beam sections. These results revealed that current ACI 318-11 analysis methods for flexural analysis of steel reinforced concrete are applicable to SMA/ECC sections with modification to the ultimate compressive strain for ECC and reinforcement limits.
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