PSI - Issue 64

M. Esmaelian et al. / Procedia Structural Integrity 64 (2024) 2091–2100 M. Esmaelian/ Structural Integrity Procedia 00 (2024) 000 – 000

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expected to contribute to a better understanding of the seismic behavior and resilient future design of Fe-SMA prestressed segmental columns. 2. Self-Centering Technique for Existing RC Columns As discussed in the Introduction, self-centering behavior in bridge columns is critical for the post-seismic serviceability required for emergency response. Steel strands are impractical due to their cumbersome prestressing process and the large number of existing columns, making Fe-SMA bars a promising alternative. This section summarizes a retrofitting technique (originally presented in (Raza et al., 2023)) to achieve self-centering in existing RC columns using prestressed Fe-SMA bars. The retrofitting steps are shown in Figure 1. In the first step, to achieve self-centering behavior, the ratio of ED to added Fe-SMA bars should be reduced by cutting some of the steel bars at the foundation-column joint as the bonded bars reduce the self-centering capacity of the column. The next step is to remove the concrete cover of the existing column by hammering or water jetting as shown in Figure 1 (b). The next step is to install the longitudinal Fe-SMA bars by drilling holes in the foundation and loading block as shown in Figure 1 (c). After helical spirals have been added to prevent the longitudinal bars from buckling and the longitudinal Fe-SMA bars have been grouted into the holes, the Fe-SMA bars are activated by heating them to 160°C or higher using a gas flame or electrical resistive heating. Finally, the retrofit process is completed by shotcreting the column to cover the Fe-SMA bars and spirals. In addition, the idea of partial bonding (rather than full bonding) can be used to overcome the potential problem of corrosion in the newly added prestressed bars. This is achieved by using smooth-surfaced bars along the height of the column, with a threaded end region at either tip to provide anchorage. This technique prevents the bar from yielding along the column height, thus delaying early loss of PT force.

Figure 2 compares the residual drifts of existing RC columns and those retrofitted with the proposed technique. Retrofitted columns C1, C2, and C3, with an Ed/Fe-SMA ratio of 0.15, maintain residual drifts below the satisfactory limit of 1% up to a target drift ratio of 4%. The retrofitted column C4 with an ED/Fe-SMA ratio of 0.45 showed residual drifts of <1% up to a target drift ratio of 3%. This higher ratio in C4 results in larger residual drifts due to plastic deformation of the ED bars. Fe-SMA bars contribute to the self-centering moment until their initial prestress is lost, after which the Fe-SMA bars are expected to behave as ED bars, thus contributing to the energy dissipation capacity. The results showed that all the retrofitted columns had lower residual drifts than the conventional RC columns, demonstrating that the retrofitting technique could efficiently improve the self-centering behavior of existing RC columns. Figure 1 Steps of retrofitting technique for self-centering behavior: (a) existing column; (b) concrete cover removal; (c) installation of longitudinal Fe-SMA bars; (d) installation of spirals, activation and shotcreting

Figure 2 Comparison of residual drifts of retrofitted RC columns with conventional non-retrofitted columns (Raza et al., 2023)