Experimental and numerical study on seismic performance of RC/ECC hybrid frame structures supported by foundations with different elevations

The frame structures supported by foundations with different elevations (FSSFDEs) are particularly vulnerable during strong earthquakes. This paper proposes applying engineered cementitious composites (ECC), which exhibit tensile strain-hardening and multi-cracking properties, at weak parts such as upper embedded columns and joints to form RC/ECC hybrid FSSFDEs for enhancing their collapse resistance and post-earthquake recoverability. The performance tests and numerical simulations of ECC materials with different mixing ratios were compared to identify the optimal ECC ratio suitable for FSSFDEs with standard concrete strength. A five-story RC/ECC hybrid FSSFDE model was designed using the preferred ECC material, followed by a static low-cycle reciprocating test. The results were compared with those from a traditional RC model to evaluate the impact of ECC on crack resistance, damage modes, and collapse resistance. Additionally, finite element analysis was conducted to further investigate the seismic performance of RC/ECC hybrid FSSFDE models with various parameters. The findings indicate that ECC significantly reduces damage to upper embedded columns and joints, enhances maintainability, and preserves the integrity of the concrete. Furthermore, ECC improves the compressive flexural capacities of longitudinal reinforcement in columns, leading to a notable increase in collapse resistance. The damage modes, ductility, stiffness degradation behavior, and energy dissipation capacity of the RC/ECC hybrid specimen are better than those of the conventional RC specimen. Finite element parametric analysis reveals that utilizing ECC shifts damage from upper embedded members to the stories beneath the upper embedding end, facilitating more uniform and controllable development of structural plastic hinges. As the number of spans beneath the upper embedding end increases, the damage to upper embedded columns intensifies. Conversely, as the number of stories beneath the upper embedding end increases, damage transfers to the grounded side, exacerbating the damage to the upper embedded columns. Therefore, it is recommended to minimize the number of spans beneath the upper embedding end when the number of stories under the upper embedding end is high.