Figure 19. Frequency responses of all periodic solution branches in Case D (Table 1). Refer to Table 2 for branch categorization. The first row (a,b) and the second row (c,d) are the results when the base accelerations are 11 m/s 2 and 16 m/s 2 , respectively. (a,b) Show sweep responses, stroboscopic points, and HBA solutions of Beam 1 and Beam 2, respectively. Unstable HBA solutions are suppressed in the figure for simplicity. In (c,d) , the graphs are enlarged to show that Branch D and Branch F are connected under the stronger base excitation. The solid line represents stable HBA solutions, and other lines indicate unstable HBA solutions of each branch as follows: double-dotted line—Branch A and Branch B, dot-dashed line—Branch D and Branch F, dotted line— Branch G. The triangle markers indicate the bifurcation points as follow: ns Neimark–Sacker bifurcation. The denotation, ‘unstable bridge’, in red color is discussed in the