Seismic behavior of novel precast steel-concrete castellated composite frames with joints strengthened by PVA-ECC

In this research, a kind of novel precast steel-concrete castellated composite frames with joints strengthened by PVA fiber reinforced engineered cementitious composites (PHCEFs) are systematically proposed. To clarify the seismic behavior of this kind of PHCEFs, according to the modified strength theory of steel-concrete castellated composite structures, 16 PHCEFs and 1 PHCCF (the joints filled by ordinary concrete) are designed with the main controlled parameters of axial compression ratio (n), steel strength of beams and columns (f_ysb/f_ysc), concrete strength in steel tubes of beams and columns (f_cb/f_cc), and types of materials filling joints (M). The three-dimensional refined finite element models (FEMs) are established by considering the nonlinear constitutive model of confined concrete and the slip effect between the interface, and the rationality of the FEMs is verified by comparison with existing experiments. The influence rules of different parameters on the seismic behavior of PHCEFs are systematically obtained. It is found that the increase of f_ysb and f_ysc can effectively improve the ultimate bearing capacity of PHCEFs and reduce the stiffness degradation rate. By adopting PVA fiber reinforced engineered cementitious composites (PVA-ECC) instead of ordinary concrete at joints, the energy dissipation capacity and ductility of the frames increase by 23.71% and 9.10% respectively, and the crack resistance of joints is significantly improved. As n increases from 0.3 to 0.5, the ductility of PHCEFs gradually decreases, but the ductility coefficient (μ) is always greater than 5.0. When n is greater than 0.5, μ decreases sharply to below 4.3. Based on the conversion relationship between the design axial compression ratio (n_d) and n, it is suggested that the limit values of n_d should be 0.65, 0.75, and 0.85, respectively, for three kinds of seismic grades. The plastic hinge formation mechanism of beams and columns in PHCEFs is rational, and there is no obvious damage at the joints, which meet the design criterion of “strong columns, weak beams, and strong joints”. Finally, the highly accurate restoring force models of PHCEFs are established, which can provide reliable theoretical support for the application of this type of steel-concrete castellated composite frames in actual engineering.