Abstract: This study numerically investigates the nonlinear aerodynamic behavior of an H-shaped cross-section (with a width-to-depth ratio of 5:1) under large-amplitude vibration, representative of the old Tacoma Narrows Bridge deck. Using CFD simulations, the amplitude-dependent characteristics of flutter derivatives and higher-harmonic responses are analyzed. The physical mechanism behind the amplitude dependence of flutter derivatives is interpreted through Spectral Proper Orthogonal Decomposition (SPOD) in conjunction with surface pressure-derived flutter derivatives. Furthermore, the emergence of high-order harmonics is explained via spatiotemporal power spectral analysis. By applying Travelling Wave Mode Decomposition (TWMD), the aerodynamic forces are decomposed into coherent components, enabling a reinterpretation of the Tacoma Bridge collapse from both vortical and energy-based perspectives. Key findings indicate that the 3rd and 5th harmonic components grow significantly with vibration amplitude. The flutter derivative A_2^* shifts from negative to positive in the reduced wind speed range of 3–4, with its magnitude and slope decreasing as amplitude increases. A_3^* shows an initial increase followed by a decrease at moderate amplitudes (3° and 9°), while exhibiting monotonic increase at larger amplitudes. At U/fB = 4 (flutter initiation), the flow is dominated by travelling waves featuring a base-frequency vortex and harmonic pressure waves propagating at a constant speed of approximately 0.268U. The n-th harmonic wavelength scales as 1/n of the fundamental mode. In contrast, at U/fB = 8 (collapse point), the aerodynamic response is characterized by a hysteresis mode with large-scale flow separation and linearized pressure distribution. The failure process is delineated into two distinct stages: an initial vortex-induced vibration stage dominated by travelling waves, where the surface flutter derivative a₂ is negative near the leading edge—consistent with negative A_2^* and energy dissipation—and a final flutter-driven collapse stage governed by hysteresis effects, featuring positive a₂ over the front half and negative behind, resulting in positive aerodynamic work. A transitional regime exhibits competing mechanisms with reversed hysteresis loops. The evolution of both travelling-wave and hysteresis components plays a critical role throughout the failure process.