Collapse behavior of weak column type steel moment resisting frames built with square hollow section columns subjected to bi-directional horizontal ground motion

Abstract

Steel moment-resisting frames (MRFs) built with square hollow section (SHS) columns can form weak-column mechanism under severe earthquakes even if it is able to form overall sway mechanism at a design level earthquake as designed; the reason for this is the vulnerability of the SHS columns to the damage caused by resisting bi-directional horizontal seismic forces. Clarifying the ultimate earthquake resistance of MRFs built with SHS columns considering the effect of a horizontal bi-directional seismic force, if it forms a weak-column mechanism, is an important issue for seismic design of steel MRFs.Before conducting the bi-directional inelastic response analysis, the hysteretic model of the SHS columns including the deteriorating range governed by local buckling subjected to bi-directional horizontal forces is verified by tests on the SHS columns. Using this model, progress of damage in SHS columns due to the orthogonal deformation can be evaluated.Subsequently, a series of response analyses of multi-story weak column type steel MRFs built with SHS columns subjected to bi-directional horizontal ground motion is conducted. From the analytical results it is clarified that, once stiffness of the collapsed story becomes negative, displacement significantly progresses toward the direction which collapse started. As a result, MRFs that form a weak column type mechanism have a smaller safety margin after reaching maximum strength.