Konstantinos Nikolaos Bakis

Doctor of Philosophy, New College, University of Oxford, Hilary Term 2016

Active and Passive Aeroelastic control of Long-Span Suspension Bridges

Long-span bridges constitute landmark projects, whose iconic impact blends together aesthetic creativity and structural competence. The simplicity of their form is contrasted by the magnitude of their scale, and the span length, in particular, relates to the structure’s technical efficiency as well as to its visual impression. Simple extrapolation of rather conventional concepts, however, does not usually lead to great progress in technological development and this might prove to be the case for the aeroelastic performance of such structures as the trend to increase their slenderness continues.
The aim of this work is to establish a framework for implementing control devices, primarily in the form of aerodynamic appendices, for suppressing aeroelastic instabilities and mitigating wind induced vibrations in long-span bridges. First, a simplified, sectional structural bridge model is created, and its interaction with a constant velocity airstream is analysed using thin-aerofoil theory. Two different passive control strategies are then considered, separately and in combination. The first makes use of trailing and leading-edge flaps adjacent to the bridge deck. The rotating motion of the flaps is triggered by the deck’s movement through a combination of connecting springs, dampers and the newly introduced inerter device. The second approach combines the aerodynamic stabilizing effect of the flaps with a driving force provided by a suspended mass, placed inside the box girder. For both strategies, special attention is given to ensuring that the proposed passive control system attains optimum robustness margins, that is, maximum tolerance to the uncertainties which accompany any physical system.
The analysis is then generalized by extending it to a discretized bridge aeroelastic model, which considers full multimodal interaction. The structural component of the modelling makes use of a reduced-size finite element formulation, in which the contribution of both the girder and the main cables is combined into single structural elements, thus reducing computation effort. The introduction of aerodynamic forces follows both thin-aerofoil theory and the flutter derivatives approach and the fluid-structure interaction is cast in a state space form in the Laplace domain. This framework is particularly convenient for control analysis and design. Two control approaches are considered: an active approach, which demands an external power source and digital control system, and a purely passive mechanical network approach, building on the earlier sectional investigation. The passive control configuration proposed has the advantages of: simultaneously increasing flutter and torsional divergence limits, being easily implementable while avoiding the use of external linkages and finally dispensing the need to be anchored to a ground reference point. Implementation of the proposed feedback mechanism to the bridge aeroelastic model proves its effectiveness during the early construction stages of a suspension bridge as well as in its completed stage.

Thesis (20.4Mb, pdf)