Unveiling the sources of discrepancies in fiber-model predictions of residual displacement for RC columns: Insights from pseudo-dynamic tests

Residual displacement is a critical indicator for evaluating the post-earthquake functionality and repairability of reinforced concrete (RC) structures. Although fiber beam models have been widely used to simulate the seismic responses of RC structures, both existing studies and the findings of this study have shown varying discrepancies in their predictions of residual displacement: conventional fiber models generally underestimate residual displacement, whereas models incorporating the SITC (Stuff in the cracks) material tend to overestimate it. However, the sources of these discrepancies remain unclear, mainly because shake-table tests usually provide limited access to real-time hysteretic responses, while quasi-static tests cannot fully characterize the evolution of dynamic residual displacement under seismic excitation. To address this issue, pseudo-dynamic tests were conducted on two 1/5-scale RC columns, producing different levels of residual displacement. The tests aimed to compare the discrepancies between numerical simulations and experimental results in terms of hysteretic behavior and residual displacement, and to identify their underlying causes. The comparison results indicate that the numerical hysteretic curves obtained from conventional fiber models exhibit an abrupt turn from a low-stiffness state to a high-stiffness state during reverse loading, whereas the measured hysteretic responses show a smooth transition at this stage. The presence of the low-stiffness branch makes the numerical response more likely to unload toward the vicinity of zero displacement after the peak displacement. In addition, the SITC material generates additional strain during unloading, which introduces extra plastic deformation after the peak displacement in the numerical model and consequently leads to an overestimation of residual displacement. Based on the material-level mechanism analysis, a modified uniaxial concrete constitutive model is proposed and implemented in OpenSees. Comparisons with the experimental results demonstrate that the proposed model outperforms widely used fiber models in predicting dynamic residual displacement.