Aeroelastic and Non-linear Structural Dynamic Interactions of Slender Structures

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As more slender and more adventurous structures, such as cable-stayed bridges, are constructed, they become increasingly susceptible to large amplitude vibrations, particularly due to aerodynamic loading. Wind-induced vibrations of bridge decks, cables, towers, lamp columns and overhead electricity cables are indeed very common. This can lead to unacceptably large movements, direct structural failure, or dangerous long-term fatigue damage of structural components. Complex interactions between the wind and the structure and also between different components of the structure (e.g. cables and bridge deck) can lead to vibration problems, so for proper understanding of the behaviour, both aerodynamic and structural effects need to be considered.Whilst some of the mechanisms of wind loading of structures are reasonably well understood, others are not, and many instances of vibrations, particularly of cables, are not well explained. Recent work has developed a generalised method for analysing 'galloping' vibrations. These are caused by changes in wind forces on a structure when it starts to move, which actually tend to increase the motion. For typical bridge cables (or other similar size structures) in moderately strong winds, a particular change in the wind flow around the cable occurs, known as the drag crisis. This changes the forces on the cable and causes a special case of galloping-type vibrations, which the new method of analysis is able to predict, for the first time. Comparisons of these calculations with wind tunnel test results on inclined cylinders have confirmed that the basic method does work, but there is a need to consider additional effects, such as wind turbulence, torsional motion of the structure and more accurate account of the changes in the aerodynamic forces as the structure moves. It is proposed to develop the approach to include these effects, using further wind tunnel data, to eventually create a unified framework for wind loading analysis of any real structure for galloping, together with the other aerodynamic mechanisms buffeting (due to wind turbulence) and flutter.Meanwhile, interactions between vibrations of structural components can cause serious effects. For example, very small vibrations of a bridge deck can cause very large vibrations of the cables supporting it, through the mechanism of 'parametric excitation'. Even more surprisingly, in other instances, localised cable vibrations can lead to vibrations of the whole structure. Research under another grant is already considering these effects for very simplified structures, but it is proposed to extend the analysis to realistic full structures. Also, often cables are tied together to try to prevent vibrations of individual cables, but they can then all vibrate together as a network. This project therefore aims to analyse full cable networks, to understand how their vibrations can be limited.Finally, it is proposed to bring together the above two main areas, to include both aerodynamic and structural dynamic interactions in the analysis of slender structures. For example, because of the interactions, the wind loads on relatively small elements, such as cables, can have surprisingly large effects on the overall dynamic response of large structures. At present this is generally ignored, but the joint approach will address this issue. Also, in some instances, only a combined view of the phenomena may be able to explain the behaviour observed on full-scale structures in practice. The holistic view of the wind loading and structural behaviour should provide tools to help avoid undesirable and potentially dangerous effects of vibrations of slender structures in the future. Based on the analysis, this could be achieved by modifying the shape of the elements to change the wind loads, or introducing dampers to absorb enough vibration energy.