Study on seismic performance of tapered castellated steel member portal frame

Low-cycle reciprocating loading test is applied to a tapered castellated steel member portal frame to study its seismic performance. Moreover, 52 finite element models are utilized to analyze the effects of the opening ratio of the beam web, the opening ratio of the column web, and the taper ratio of the beam on the failure mode, hysteresis curves, ductility, and energy-dissipating capacity of tapered castellated steel member portal frame. The results show that tapered castellated steel member portal frame can realize the failure mode of the plastic hinge at the beam end. The ductility coefficient of the specimen is between 4.78 and 5.49, and the maximum equivalent viscous damping coefficient is 0.146, indicating that the structure offers good ductility and energy-dissipating capacity. When the opening ratio of the castellated beam web is 60–70%, and the taper ratio of the castellated beam is less than 1.4, the structure exhibits good seismic performance. As castellated beams and columns have different stiffness values due to different web opening ratios, this study proposes an equivalent beam–column linear stiffness ratio to satisfy the seismic performance design requirement of “strong columns and weak beams” for tapered castellated steel member portal frame. A method is also devised to calculate the equivalent stiffness of tapered castellated members. Comparing the predictions of this method with experimental results reveals that its maximum error is 7.66%, indicating its high precision. It is recommended that the equivalent beam–column linear stiffness ratio of tapered castellated beam–column portal frames should be higher than 2.735. On this basis, an opening ratio of the castellated column web ranging from 50% to 70% has a negligible effect on the seismic performance of the structure.