“…Denoting with ω i , i = 1, 2, the natural frequencies of the two linear vibration modes of the system, a large range of forcing frequencies is considered, which includes the fundamental parametric resonances (ω e = ω i ) of the two modes and their principal subharmonic parametric resonances, of order 1 2 (that is, ω e = 2ω i ) and 1 3 (ω e = 3ω i ). For the perfect system ω 1 = ω 2 , the fundamental and principal resonances correspond 1074 DIEGO ORLANDO, PAULO BATISTA GONÇALVES, GIUSEPPE REGA AND STEFANO LENCI to the nondimensional forcing frequency values = 1 3 , = 2 3 , and = 1, respectively, whereas for the imperfect system the two natural frequencies differ from each other [Orlando 2010] and the same occurs for the relevant resonant conditions. Two cases are considered in Figure 7: the uncoupled case, when perturbations only in θ 1 andθ 1 are considered and only these coordinates are excited, and the coupled case, when very small perturbations in θ 2 andθ 2 are also considered after each load step (θ 2 =θ 2 = 0.001), causing the coupling of the two modes.…”