Displacement mode including torsion when subjected to major earthquake is clearly related to strength balance of each frame while elastic torsional vibration is characterized by stiffness balance. In the previous research, we presented a displacement mode prediction method especially focusing on single-story timber structure having large strength eccentricity. The method is improved by taking the effect of plasticity of transverse walls and bi-axial input motion into account. Finally, necessary additional strength for seismic design of torsionally coupled structure is calculated and is related to eccentricity index. The accuracy of the evaluation method is demonstrated via numerous time history analyses.
