It has been revealed by the symmetry limit analysis and the equilibrium path bifurcation analysis of multistory multibay weak-beam planar frames that there exists an unknown type of collapse behavior characterized by the growth of a bow-shaped overall deflection mode in the frame. A numerical response analysis, performed later, has shown that the bow-shaped overall deflection mode can also appear and grow in the frames subjected to cyclic ground motion. In that study, however, simple frame model was considered, so actual dynamic collapse behavior has not been clarified. In this paper, accurate dynamic response analyses are carried out for more realistic frame models which have a mass at every floor. It will be shown that there exists deformation concentration phenomena in the process of dynamic collapse, which is characterized by the growth of bow-shaped deflection mode and the concentration of this mode in the restricted lower region. A theoretical method will be proposed for predicting the height of the deformation concentration region.
