Numerical predictions of the dynamic response of complex structures are often uncertain due to uncertainties inherited from the assumed load effects. Inverse methods can estimate the true dynamic response of a structure through system inversion, combining measured acceleration data with a system model. This article presents a case study of the full-field dynamic response estimation of a long-span floating bridge: the Bergøysund Bridge in Norway. This bridge is instrumented with a network of 14 triaxial accelerometers. The system model consists of 27 vibration modes with natural frequencies below 2 Hz, which is solved using a tuned finite element model that takes the fluid-structure interaction with the surrounding water into account. Two methods, a joint input-state estimation algorithm and a dual Kalman filter, are applied to estimate the full-field response of the bridge. The results demonstrate that the displacements and the accelerations can be estimated at unmeasured locations with reasonable accuracy when the wave loads are the dominant source of excitation.

4Numerical models of large civil engineering structures are prone to errors and uncertain system parameters, which inevitably affect the ability of such models to accurately predict dynamic behaviour. Finite element (FE) model updating can be used to calibrate the numerical models towards the observed behaviour. In this paper, a case study of the sensitivity method in FE model updating is presented. The methodology is applied to the Bergsøysund Bridge, which is a long-span floating pontoon bridge in Norway. A system identification is performed based on acceleration data and thirty vibration modes are identified. The FE model is calibrated by reducing the difference between the identified and numerical natural frequencies and mode shapes of the bridge. The model uncertainties are parametrized with a total of 27 parameters. We demonstrate how an analytical sensitivity matrix can be constructed for floating structures, where the system mass and damping matrices are functions of frequency due to fluid-structure interaction.After updating, the mean error in natural frequencies is decreased from 3.23% to 2.34%, and the average MAC number is increased from 0.87 to 0.94. Although the largest errors are significantly reduced, the updated parameters are believed to be affected by noise from the system identification. Challenges related to the presence of very closely spaced vibration modes are also shown, in which matching the identified modes to the modelled modes becomes difficult. This study indicates that models of large bridges can be significantly improved, but many practical issues still exist.

Identifying the modal parameters of structures located in ice-infested waters may be challenging due to the interaction between the ice and structure. In this study, both simulated data from a state-of-the-art ice–structure interaction model and measured data of ice–structure interaction were both used in conjunction with a covariance-driven stochastic subspace identification method to identify the modal parameters and their corresponding variances. The variances can be used to assign confidence to the identified eigenfrequencies, and effectively eliminate the eigenfrequencies with large variances. This enables a comparison between the identified eigenfrequencies for different ice conditions. Simulated data were used to assess the accuracy of the identified modal parameters during ice–structure interactions, and they were further used to guide the choice of parameters for the subspace identification when applied to measured data. The measured data consisted of 150 recordings of ice actions against the Norströmsgrund lighthouse in the Northern Baltic Sea. The results were sorted into groups defined by the observed ice conditions and governing ice failure mechanisms during the ice–structure interaction. The identified eigenfrequencies varied within each individual group and between the groups. Based on identified modal parameters, we suggested which eigenmodes play an active role in the interaction processes at the ice–structure interface and discussed the possible sources of errors. This article is part of the theme issue ‘Environmental loading of heritage structures’.

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