Figure 9. Change in RMS voltage of Branch D (Table 2) of Beam 2 in Case C (Table 1) according to the base acceleration. The base acceleration is increased from 5 m/s 2 , used in Fig. 7, to 11 m/s 2 . The red, blue, green lines are obtained when the solution truncation order, H , is set to be 1, 3, and 5 (Eqs. (6), (7)), respectively. For the third- and fifth-order truncated solutions, the excitation frequency is 11 Hz. For the first-order truncated solution, the excitation frequency is 24.9 Hz. For these excitation frequencies, the HBA solutions are stable (refer to Fig. 7b,d).