The mechanical stiffness and damping coefficients of a spring-suspended deck section model in still air are conventionally assumed to be constants and can be extracted through free decay vibrations. The extracted stiffness and damping coefficients, indeed, include both mechanical and aerodynamic components. In this study, the vertical and torsional single-degree-of-freedom (SDOF) free decay vibration tests of two typical bridge deck section models were carried out in still air to study the nonlinear vibration characteristics of the sections. SDOF motion equations of the section models were developed on the basis of the equivalent linearization theory. A system identification procedure was developed to extract the amplitude-dependent frequency and damping ratio from the measured acceleration responses. The efficacy of the equivalent linearization and the identification accuracies of the mechanical and aerodynamic parameters were verified by an example. The mechanical parameters (stiffness and damping ratio) and aerodynamic parameters [added mass (or moment of inertia), and damping ratio] of the spring-suspended systems were extracted separately. The aerodynamic damping was found to be the major damping source for the spring-suspended system. The influences of the initial condition, mass (or moment of inertia), vibration frequency, and amplitude on the mechanical and aerodynamic parameters and nonlinearities were thoroughly investigated. The aerodynamic parameters were more sensitive to the vibration amplitude than were the mechanical parameters. The conclusions advance a better understanding of the nonlinear vibration characteristics of bridge deck section models in still air.
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