Seismic Behavior of Symmetric and Asymmetric Steel Structures with Rigid and Semirigid Diaphragms

The torsional behavior of braced frames can be significantly influenced by the sequence of brace buckling, hinge development in beams or columns, and in-plane deformation of the diaphragm, even for the case of symmetric configurations. To understand these effects, six four-story steel braced structures with different diaphragm in-plane stiffness and configurations of vertical force resisting systems are investigated in this paper. The analyses indicate that (1)the torsional coefficient used to define the levels of torsional irregularity in the current design provision can significantly underestimate the real torsional behavior in the inelastic range, (2)the ultimate strength of structures with rigid diaphragms is higher than that of structures with semirigid diaphragms, (3)asymmetric structures with extreme torsional irregularities and semirigid diaphragms evidenced the highest ductility demands, and (4)higher modes significantly affect behavior after the onset of inelasticity due to buckling of the braces. Two new measures of torsional behavior, a maximum rotation angle and a maximum interstory rotation angle, are proposed and found to give better insight than current measures based on the displacement used in the current design provision.