Wind Induced Dynamic Response of Concrete Box Girders During a Balanced Cantilevered Construction Stage

Abstract

In this thesis a theoretical study of wind induced dynamic response of line-like structures has been performed, followed up by calculations of the dynamic response of Dolmsundet Bridge in its critical construction stage. Dolmsundet Bridge is constructed by the balanced cantilever method, which implies that until the cantilevered girders are connected to an adjacent cantilever or supports on shore all forces on the girder must be obtained by the pillar as moment and torsional stress. Thus is the construction particularly vulnerable against wind.The theoretical study focuses on a frequency domain approach of the calculation of buffeting and vortex shedding induced dynamic response of line-like structures. Some statistical principles and a stochastic description of the turbulent wind must be presented before expressions of the loads and thereafter the response due to vortex shedding and buffeting wind can be derived. A section showing how the cross sectional forces can be divided into a resonant and a background part in order to reduce inaccuracy is also included.A MATLAB program is composed to perform the calculations of the wind induced dynamic response of Dolmsundet Bridge in the construction stage. The response due to the first two modes is calculated, and time domain simulations of the displacements at the tip of the longest cantilever are created in order to find the maximum values. The calculations show that vortex shedding would be the most important phenomenon for the mode in the vertical direction.Several assumptions and simplifications are made, possibly introducing inaccuracy. It is discussed possible sources of errors and to what extent selected parameters will affect the result. It was found that the structural damping had less influence on the calculated response. The vortex shedding parameters, and especially the nondimensional root mean square lift coefficient, could represent a source of error, and must be chosen with care. It was also discovered that galloping might be an issue due to negative lift derivatives. There should be performed investigations towards ensuring that the aerodynamic damping does not significantly reduce the total damping.