Figure 11. Parameter study on the frequency bandwidth of Branch F (Table 2) with respect to beam length ratio when the base acceleration is 10 m/s 2 . The solid line represents the frequency bandwidth of the stable HBA solution. The dot-dashed line indicates the frequency bandwidth of the unstable region between two stable regions (refer to Fig. 10a,b). According to the parameter study results, four zones can be categorized. When the beam length ratio belongs to Zone A, a disconnected region exists. However, in Zone B, the stable solution branch is not interrupted by instability. When the beam length ratio reaches Zone C, the stable bandwidth decreases rapidly. The range after another sudden bandwidth drop is denoted by Zone