Parallel Mono-strand Stay Cable Bending Fatigue

Students: Jan Winkler
Supervisor(s): Christos T. Georgakis; Gregor Fischer

Industry partner: ATKINS A/S

Description: Large-amplitude vibrations of bridge cables have been frequently reported in the last decade. Despite extensive research on the mechanisms of dynamic excitation, it remains unclear what effect these vibrations have on the internal stresses and the fatigue life of the cable.  Due to lack of published high-strength steel cable bending fatigue spectra, the fatigue resistance of most stayed structures subjected to cable vibrations has never properly been evaluated.  

The objective of the proposed research is firstly, the extension of mono-strand bending fatigue tests undertaken at DTU Byg, to enhance the reliability and improve the resolution of the existing spectrum. Secondly, a localised bending fatigue behavior of high-strength steel mono-strands will be studied in detail to investigate fatigue induced failure modes of a mono-strand cables. Thirdly, a relationship between the fatigue resistance of a mono-strand and a full parallel mono-strand stay cable would be made, through mathematical modelling and full-scale static testing. The proposed research should lead to a simplified model that can be used for the assessment of stay cable fatigue.

 With this research, one of the most basic oversights in the lifetime assessment of cable-supported structures, namely the bending fatigue resistance of parallel mono-strand cables, will be addressed.

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Understanding of Bridge Cable Vibration Mechanisms under varying Meteorological Conditions

Students: Giulia Matteoni
Supervisor(s): Christos T. Georgakis; Holger H. Koss
Industry partner: IPU

Description: The project's main objective is the comprehensive understanding of the mechanics behind bridge cable vibrations under the most prevalent meteorological conditions in Scandinavia. To achieve this several deliverables have been defined.

First, an in-depth literature review of the state-of-the-art in the field, with a database of significant works, will be compiled. Then, a characterization of existing cable vibration mechanisms will be made; the mechanisms are currently grouped as:

• vortex-induced oscillations;
• rain-wind vibrations;
• wake galloping of groups of cables;
• inclined cable dry galloping;
• iced cable galloping;
• motion-induced and paramtereic excitation;

motions due wind buffeting.

Subsequently a wind tunnel programme will be established where a full series of tests will be outlined. The tests will be separated into categories, i.e. dry, ice or sleet and will be performed for both laminar and turbolent flow. The turbolence shall be modelled after the prevalent meteorological conditions. Different wind directions and intensities will also be investigated. Wind tunnel charcaterisation and testing will take place at the new DTU/FORCE climatic facility.

A detailed interpretation of the wind-tunnel test data will help create a clearer understanding of several of the more dominant excitation mechanisms. The ultimate goal of this will be the creation of appropriate wind loading models, to be used for the calculation of cable response to wind. Verification of the loading models shall be made by comparing theoretically predicted cable vibrations to the measured responses of actual cables.

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Cable Aerodynamic Control

Students: Kenneth Kleissl
Supervisor(s): Christos T. Georgakis; Holger H. Koss

Collaboration partner: FORCE Technology, Department of Hydro and Aerodynamic

Description: Through improved bridge monitoring and a greater openness between bridge owners and engineers, it has become increasingly apparent that a large number of the world’s long-span cable-supported bridges suffer some form of cable vibration. These vibrations have the potential to lead to long-term fatigue damage and economic loss, through a reduction of consumer confidence.

Attempts to eliminate or dampen these vibrations have been met with varying degree of success. To date no ultimately successful cable vibration control system has been devised for all types of cable under all conditions. This is most probably due to the fact that the observed vibrations are a result of varying excitation mechanisms, several of which may need differing control strategies to combat.

Nevertheless, recent work on aerodynamic control of cables through cross-sectional shape modification has shown great promise. Aerodynamic control refers to means of eliminating undesirable vibrations of a structure through careful modification of the structural shape and surface, either passively or actively. Kleissl recently showed that circular cylinders that exhibit galloping instability, at specific wind angles of attack, can be modified passively, so that these instabilities are eliminated altogether.

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Risk Assessment of Stay Cable Fatigue

Students: Joan Hee Roldsgaard

Supervisor(s): Christos T. Georgakis;

 Background
Bridges often form indispensable components of transportation links. The preservation of the robustness and residual lifetime of these structures is thus of great interest for bridge operators and society as a whole.

The number of long span cable-supported bridges has increased significantly since the 1970’s and is still increasing. With this, the number of reports of unanticipated bridge cable vibrations has also increased, despite the fact that in many cases cable vibration countermeasures have been installed [1].

Very limited research has focused on the probabilistic assessment of fatigue damage of bridge cables due to wind-induced vibrations. The mechanisms, including those linked to rain, ice or sleet, often cause large amplitude vibrations under varying combinations of meteorological conditions. The often unanticipated cable vibrations in combination with the effects of traffic loading cause concern, as regards the lifetime and risk of failure due to fatigue damage of the cables.

The damage assessment of the cables is not straightforward though, as the mechanisms causing the large amplitude vibrations and fatigue failures are not yet fully understood. It can be understood that the wind-induced vibration mechanisms mainly cause bending stresses around the anchorages of the cables and that the traffic loading mainly causes axial stresses in the cables. Depending on the characteristics of the loading and the material, the stresses may lead to fatigue failure of the cables.

Project
The objective of the research is to establish a probabilistic risk assessment model that is able to include both axial and bending fatigue of stayed bridge cables.

The research will include a classification of the different vibration mechanisms, which are inducing fatigue failure of the cables. The risk assessment model will be based on the Probabilistic Model Code (PMC) of the Joint Committee on Structural Safety (JCSS). Finally a verification of the probabilistic risk assessment model of stayed cable fatigue will be performed on a real-life structure.

Perspective
The project will lead to enhance of the engineering best practices concerning the assessment of fatigue performance of cable supported structure by a probabilistic model. The enhance will be obtained by a deeper understanding of the governing vibration mechanisms, which induces fatigue failure, probabilistic representation of the both axial and bending fatigue spectra and relevant probabilistic techniques suitable for this type of risk assessment.

References:
[1] Sun, L. & Dong, X. 2011. Researches and applications on vibration control of long stay cables in China. Key-note lecture at the 9th International Symposium of Cable Dynamics, 18-20th Oct. Shanghai, China.

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