Seismic performance assessment of conventional steel and steel-concrete composite moment frames using CFST columns

The research reported in this paper focuses on the assessment of the seismic performance of conventional steel moment-resisting frames (MRFs) and steel-concrete composite moment-resisting frames employing circular Concrete-Filled Steel Tube (CFST) columns. Two comparable archetypes (i.e. one steel MRF, with steel columns and steel beams; and one composite MRF, with circular CFST columns and steel beams) are designed, and used as the basis for comparison between the seismic performance associated with each typology. Both structures are designed against earthquake loads following the recommendations of Eurocode 8. The comparison of the obtained design solutions allows concluding that the amount of steel associated with the main structural members is higher for the steel-only archetype, even though the composite MRF has the higher level of lateral stiffness. This aspect is particularly relevant when one considers that a minimum level of lateral stiffness (associated with the P-Δ inter-storey drift sensitivity coefficient, θ), is imposed by the European code, which may ultimately govern the design process. The two case-studies are then numerically modelled in OpenSees, and their seismic performance is assessed through fragility assessment for a number of relevant limit states, and, finally, earthquake-induced loss estimation. In general, the results obtained clearly indicate that the composite MRF with circular CFST columns exhibits better seismic performance than the equivalent steel-only archetype. This is noticeably shown in the comparison of the fragility curves associated with the collapse limit state, which tend to show substantially higher probabilities of exceedance, at similar levels of 1st-mode spectral acceleration, for the steel-only case. Furthermore, seismic losses at several seismic intensity levels of interest tend to be higher for the steel MRF.

Seismic Performance of Steel MRF Structures with Nonlinear Viscous Dampers from Real-Time Hybrid Simulations

Real-time hybrid earthquake simulations (RTHS) were performed on steel moment-resisting frame (MRF) structures with nonlinear viscous dampers. The test structures for the RTHS contain a moment-resisting frame (MRF), a frame with nonlinear viscous dampers (DBF), and a gravity load system with associated seismic mass and gravity loads. The MRFs have reduced beam section beam-to-column connections and are designed for 100%, 75%, and 60%, respectively, of the base shear strength required by ASCE 7-10. RTHS were performed to evaluate the seismic performance of these MRF structures. Two phases of RTHS were conducted: (Phase-1) the DBF is the experimental substructure in the laboratory; and (Phase-2) the DBF with the MRF is the experimental substructure. Results from the two phases of RTHS are evaluated. The evaluation shows that the RTHS provide a realistic and accurate simulation of the seismic response of the test structures. The evaluation also shows that steel MRF structures designed with reduced strength and with nonlinear viscous dampers can have excellent seismic performance.