Evaluation of bending fatigue life for bridge stay cables subjected to vibration

  • Gang Shen

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Large-amplitude cable vibrations are remarkably common in cable-stayed bridges subjected to wind and mechanical loading. This is due to low inherent damping of these structures. Although the high slenderness ratio of cables means that bending moments are not normally significant over their free length, local effects near the cable anchorage induce non-negligible bending stress variation under dynamic loading, which could lead to fatigue failure of stay cables in practical bridges.

An analytical bending fatigue framework is proposed to estimate the fatigue life of low-sag cables subjected to harmonic loading. This work demonstrates that the use of a guide deviator (i.e. a device to relieve the bending moment at the anchorage) can significantly extend the cable’s fatigue life. The fatigue life is greatly reduced if the cable jumps to a multi-modal dynamic response, so it is important to limit the cable response to within the single-mode zone. This can be achieved by using external damping.

This framework also reveals that the dynamic response of cables under external loading is crucial for evaluating cable fatigue life more reliably. When the natural frequencies of a global vibrational mode and a cable mode are close in a cable-stayed bridge, there can be significant interactions between the cable and deck. This affects the cable’s dynamic response and hence vulnerability to cable fatigue. A nonlinear cable-supported model is introduced, representing a single cable attached to the top of a deck section. Analytical solutions in the form of non-dimensional polynomial equations of the motion of the deck and cable are derived for the nonlinear system under external loading. The feedback of the cable vibration on the deck and cable response is non-negligible even for a relatively large mass ratio of the deck to cable. Therefore, for practical cable-stayed bridges the interaction between cable and deck needs to be considered to obtain an accurate dynamic response in some cases.

In modern cable-stayed structures, the fatigue performance of cables depends on the fatigue properties of the individual seven-wire monostrands which together make up the cable. A new bending fatigue testing device for seven-wire monostrand was designed and manufactured. The key feature of this device is that it can control the fatigue loading in the axial and transverse directions of the cable independently. The fatigue endurance of monostrand with guide deviators is significantly longer than without guide deviators present, especially for smaller bending fatigue amplitudes. This is the intrinsic cause for the corresponding conclusion given by the dynamic framework. The bending fatigue life of monostrand is also reduced by increased constant or dynamic axial stress. Furthermore, experimental tests demonstrated that fatigue occurs due to the bending stress variation caused by the combination of bending moments and axial forces at the extreme edges of the monostrand cross-section, rather than due to relative movement and fretting between adjacent wires within the monostrand.

The understanding of cable dynamics and the mechanisms of cable bending fatigue developed in this work could help designers build more efficient and more durable cable-stayed structures.
Date of Award20 Jun 2023
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorHarry Coules (Supervisor) & Flavia De Luca (Supervisor)

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