Ole Øiseth

Modal Analysis of a Floating Bridge Without Side-Mooring

The Norwegian Public Roads Administration is currently planning a ferry-free Coastal Highway Route E39. Floating bridges represent feasible options in this project with already two long span floating bridges in function, i.e. the Bergsoysund and Nordhordaland Bridges. In connection with this project, one of the main objectives is to quantify the accuracy of the numerical methods used to predict dynamic behaviour of floating bridges. An extensive monitoring system is installed to measure structural response as well as environmental actions from wind and waves on an existing floating bridge: the Bergsoysund Bridge. These measurements are used to estimate the modal system properties of the structure. The system identification is performed using a parametric time-domain Stochastic Subspace Identification method as well as the Frequency Domain Decomposition method. Challenges of system identification for highly damped structural systems, such as a floating bridge, are especially emphasized. The results are also compared with numerical predictions from a two part combined linear frequency-domain model set-up. The first part consists of a hydrodynamic model, including wave excitation as well as fluid-structure interaction, and relies on linearized potential theory. The results from this are thereafter introduced into a finite element model, for a complete structural dynamic analysis.

Measured Buffeting Response of a Long-Span Suspension Bridge Compared with Numerical Predictions Based on Design Wind Spectra

AbstractWind-induced vibrations of the Hardanger Bridge deck were studied with reference to turbulence characteristics at the bridge site to evaluate the performance of the state-of-the-art methods...

Long-term extreme response analysis of marine structures using inverse SORM

In this paper the first order reliability method (FORM) found in connection with structural reliability analysis is first used in an inverse manner to efficiently obtain an approximate solution of the full long-term extreme response of marine structures. A new method is then proposed where the second order reliability method (SORM) is used to improve the accuracy of the approximation. This method is compared with exact results obtained using full numerical integration. The new method is seen to achieve improved accuracy for large return periods, yet keep the number of required short-term response analyses within acceptable levels.Copyright

Full-Scale Measurements on the Hardanger Bridge During Strong Winds

Completed in 2013, the Hardanger Bridge is now the longest suspension bridge in Norway and it is among the most slender suspension bridges in the world. In the context of an extensive research project initiated by the Norwegian Public Roads Administration, the bridge was installed with a comprehensive monitoring system to investigate the wind characteristics at the site along with the dynamic response of the bridge. Wind velocities are measured using nine anemometers installed on different locations at the bridge span. A representative 10 min recording with high mean wind velocity and perpendicular wind direction is selected to present the preliminary results. Time series of turbulence components and one-point spectra are presented. Spatial properties of the wind field are studied by examining the coherences of turbulence components for several separation distances. Dynamic response of the bridge is measured using 20 accelerometers distributed along the main span. Time histories of the accelerations as well as the response spectra are presented. The relationship between bridge vibrations and wind measurements is discussed.

Covariance-Driven Stochastic Subspace Identification of an End-Supported Pontoon Bridge Under Varying Environmental Conditions

The Bergsoysund Bridge is currently being extensively monitored with accelerometers, anemometers, wave radars and GNSS sensors. By applying Covariance-driven Stochastic Subspace Identification (Cov-SSI), the modal parameters of the bridge are estimated. The results are interpreted in the context of the environment, represented by significant wave heights. The problem is characterized by the fact that modes are closely spaced in frequency and have high damping. Two weighting algorithms for the Cov-SSI are applied, to assess their performance for application on structures with these characteristics.

Monitoring Wind Velocities and Dynamic Response of the Hardanger Bridge

The Hardanger Bridge is the longest suspension Bridge in Norway and among the top 10 longest suspension bridges in the world. A comprehensive monitoring system was installed after it was completed in August 2013. The monitoring system is designed to provide data that can be used to verify the numerical methods used to predict wind induced dynamic response of slender bridges located in complex terrain. The monitoring system is outlined in this paper together with preliminary analysis of the accuracy of the model used to describe the self-excited forces acting on the bridge deck. Extensive wind tunnel testing was performed in the design of the Hardanger Bridge to achieve an excellent aerodynamic behaviour of the cross-section of the bridge deck. The experimental results of the aerodynamic derivatives that describe the self-excited forces have been combined with a finite element model of the bridge to predict the in-wind natural frequencies and damping ratios of the combined structure and flow system. The numerical predictions have been compared to results obtained from measured data using data-driven and covariance-driven stochastic subspace identification. It is concluded that the model for the self-excited forces provides in-wind frequencies and damping ratios that corresponds well to the observations from measured data.

Time Domain Modelling of Frequency Dependent Wind and Wave Forces on a Three-Span Suspension Bridge With Two Floating Pylons Using State Space Models

Floating suspension bridges, one of several new designs to make it possible to cross deep and wide fjords, consist of three spans and supported by two tension leg platforms and two fixed traditional concrete pylons. Geometric nonlinearities, nonlinear aerodynamic and hydrodynamic forces and nonlinear mooring systems can become of high importance. Time domain methods are commonly applied when nonlinearities need to be considered. The main challenge in time domain simulation of the floating suspension bridge is the modelling of frequencydependent aerodynamic self-excited forces and hydrodynamic radiation forces. This paper shows how rational functions fitted to aerodynamic derivatives and hydrodynamic added mass and potential damping can be converted to state space models to transform the frequency-dependent forces to time-domain. A user element is implemented in the software ABAQUS to be able to include the self-excited forces in the dynamic analysis. The element is developed as a one node element that is included in the nodes along the girder and the tension leg platforms. The responses of the floating suspension bridge under turbulent wind forces and first-order wave excitation forces are calculated in a comprehensive case study and compared with results obtained using a multi-mode frequency domain approach to illustrate the performance of the presented time-domain methodology.

Model-Based Estimation of Hydrodynamic Forces on the Bergsoysund Bridge

Knowledge of excitation loads on bridges are important for reliable design. Load models are however prone to uncertainties. Force identification using dynamic response measured on full-scale structures can be used to reduce the uncertainty. In this contribution, numerical simulations are performed to examine the feasibility of force identification on the floating pontoon Bergsoysund Bridge. We present a practical case study in which wave excitation forces and motion induced forces are estimated using only acceleration output. The sensor network considered represents the monitoring system currently installed on the bridge. A reduced order model with 26 modes is used to represent the structure in the identification. Wave force time series are generated by Monte Carlo simulations, and the acceleration response is obtained from a frequency domain solution of the equations of motion. The generated acceleration data is polluted with noise and subsequently used for identification. The results show that a joint input-state estimation algorithm is able to adequately identify a subset of hydrodynamic forces acting on the pontoons in the presence of both measurement and model errors. The translational forces are identified with a larger accuracy than the moments. Lastly, considerations and improvements for an analysis with experimental field data are presented.

Identification of Rational Functions with a Forced Vibration Technique Using Random Motion Histories

Rational Functions are used to describe the self-excited forces acting on the bridge deck in the time domain. They can be identified indirectly based on aerodynamic derivatives or directly with the free (E2RFC method) or forced vibration technique, which can significantly decrease the testing time. The approach presented herein enables the extraction of Rational Function Coefficients by testing the section model at only one wind speed. This aim is achieved by increased complexity of the forced motion compared to the previous tests, which made it possible to test a wider range of reduced velocities by adjusting the motion frequency. In this study, motion histories generated from the assumed flat spectra are used. Wind tunnel tests on a streamlined section model utilizing simultaneous vertical, horizontal and torsional vibrations were performed to extract Rational Function Coefficients associated with 3-degree-of-freedom motion. Restrictions and improvements arising from the proposed methodology are described.

Finite Element Model Updating of a Long Span Suspension Bridge

Errors and uncertainties in numerical models of structures affects the ability of these models to accurately predict the dynamic behaviour. However, model updating techniques can be used to calibrate the models based on experimental data. This paper presents a case study of sensitivity-based model updating applied to the Hardanger Bridge, a long span suspension bridge. Thirteen stiffness and mass parameters are chosen to represent the system uncertainties in a finite element (FE) model. Thirty vibration modes from system identification based on acceleration data is used to calibrate the FE model, using identified natural frequencies and mode shapes as objectives. In the updated model the average error in natural frequencies is reduced from 3.65% to 1.28%. The MAC numbers for the updated modes range from 0.678 to 0.999. The study indicates FE models of large suspension bridges can be significantly improved, but many uncertainties related to modelling simplifications are still present.