Nonlinear transient wind-induced dynamic response of bridges

Climate change is increasing the frequency of extreme, non-synoptic winds. Rapid variations in wind speed and wind angle of attack (W-AoA) can induce transient, potentially nonlinear effects on long-span bridges. Meanwhile, bridges with a main span longer than 1000 m have more than tripled in the last 25 years, increasing exposure and risk. Although full-scale characterization and load modelling of non-synoptic winds is expanding, transient variations in aeroelastic parameters remain less explored. We use a 2D Rational Function Approximation (Barni et al., 2021) to model nonlinear self-excited forces modulated by both W-AoA and mean wind speed, assuming these variations are slow compared with bridge natural frequencies. A time-variant aerodynamic admittance function is also adopted for buffeting forces. We apply this framework to the Halsafjorden Bridge (Norway; E39 project; 2000 m span) using a numerical reconstruction of the non-synoptic storm Thor (January 29-30, 2016) recorded at the Hardanger Bridge site (Norway). The event features a well-correlated gust front measured by on-site anemometers, with mean wind speed up to about 45 m/s. The nonstationary buffeting response is cast in state space, enabling efficient simulations and direct tracking of instantaneous modal frequencies and damping from the system state matrix, supporting interpretation of transient loads and aeroelasticity. Comparison with an equivalent synoptic wind clarifies how gust-front transients modify the bridge dynamic response and highlights the role of nonlinearities under these conditions.