Local buckling behaviour of high-strength steel tubular columns subjected to one-sided cyclic loading and implications in seismic design of steel MRFs

After yielding, steel structures progressively exhibit an asymmetrical hysteretic behaviour under earthquakes biased in one direction (ratcheting effect). This response may be favourable to structural components because of the absence of symmetrical cyclic loading reversals. To evaluate the inelastic behaviour of components to asymmetrical cyclic loading, this study develops a set of one-sided cyclic loading histories for columns on the basis of a parametric study on the asymmetrical hysteretic behaviour of steel moment-resisting-frames (MRFs). Experimental and computational results indicate that steel tubular columns subjected to one-sided cyclic loading may exhibit 15% to 71% higher ductility compared with that under symmetrical cyclic loading showing adequate resistance to local buckling initiation and sufficient ductility afterwards. These reduced ductility demands may permit the use of high-strength steels in energy dissipative zones, such as high yield-to-tensile strength ratio (Y/T) steels, resulting in reduced material consumption and more economical seismic design of buildings. The above findings are utilised in the seismic design of a MRF employing high Y/T steel, which enjoys almost the same lateral strength as a corresponding conventional steel MRF but nearly 25% smaller cross-sectional area in columns. Nonlinear time-history analyses revealed that the high Y/T MRF behaved elastically under frequent events, whilst the ground floor columns of the conventional MRF yielded. Under rare seismic events, the average story drift ductility demand for the high Y/T MRF was only 1.77, while for the conventional MRF was 2.57. The former MRF successfully resisted the earthquake loads without experiencing local buckling in columns.