Mitigation of Wind-Induced Vibration of Stay Cables: Numerical Simulations and Evaluations

CHAPTER 1: INTRODUCTION

Cable-stayed bridges, with their high cost efficiency and unique aesthetic features, have firmly established their position for use in medium-to-long-span bridges. The engineering principles of stay cables were originally borrowed from the suspension cables and post-tensioning technology. However, recent advances in materials engineering, construction technology, and analytical capabilities further accelerated the adoption of cable-stayed bridges as the desired structural form. The range of span length for cable-stayed bridges has been expanded in either direction, being increasingly shorter and increasingly longer.

Stay cables are laterally flexible members with low fundamental frequency and limited inherent damping. Without additional damping from external sources, stay cables are susceptible to large amplitude oscillations due to excitations from wind and rain/wind combined action as well as during construction.⁽²⁾ Cumulative fatigue damage to the cable assemblies resulting from such vibrations has become an important issue and has led to the incorporation of some mitigation measures such as surface modifications, cable crossties, and external dampers into the design of stay cables.

A substantial amount of research on this subject had been conducted by researchers from academia, consulting firms, and cable suppliers in the United States and abroad. A recent research study under the coordination of the Federal Highway Administration investigated the wind-induced vibration of stay cables.⁽³⁾ The objective of the study was to develop a set of uniform design guidelines for vibration mitigation of stay cables.

A series of wind tunnel tests and analytical studies were conducted, and relevant databases were generated. The wind tunnel tests were conducted to study different mechanisms of wind-induced vibration of stay cables, and two databases covered the reference materials retrieved from available literature and the inventory of U.S. cable-stayed bridges, respectively. Researchers developed theories and conducted an analysis of the behavior of cable crossties and external dampers in the context of vibration mitigation of stay cables.

Useful new theories were developed, and some existing theories were extended dealing with linear and nonlinear viscous dampers and cable crossties. The theories were validated, and the effectiveness of mitigation measures was demonstrated via comparison with field measurements on several U.S. cable-stayed bridges. Based on the findings and information from the study, some tentative design guidelines were proposed for mitigation of wind-induced stay cable vibrations.

However, some of the analytical procedures developed from this study are complicated and may not be suitable for routine use by engineers in designing mitigation measures. Also, the study did not include explicit simulations of the behavior of stay cable systems subjected to realistic wind events when the cable systems are equipped with different types of mitigation measures. The free-vibration analysis method developed from the study offers useful insight into the mode-frequency behavior of stay cable systems networked with crossties; however, an explicit time-history analysis would be necessary to verify the implications derived from such an analysis.

This lack of information led to the current follow-up research study to investigate the effectiveness of cable crossties and external dampers in mitigation of wind-induced stay cable vibrations. Explicit numerical simulations of the behavior of stay cable systems, augmented with different types of mitigation measures, were conducted using the finite element method (FEM). Particular emphasis was placed on investigating the effectiveness of different strategies of mitigation involving cable crossties, external dampers, and combinations of the two. Also, the dependence of mitigation effectiveness on input wind conditions was analyzed.

Some existing theories on the vibration of taut strings with different levels of complexity are reviewed in chapter 2 followed by preliminary numerical analysis of stay cable vibrations using simplified models in chapter 3. Also included in chapter 3 is an illustrative application to the Fred Hartman Bridge in Houston, TX, for which analysis conducted by other researchers is available for comparison and benchmarking. The free-vibration and time-history analysis of stay cable systems equipped with crossties are covered in chapters 4 and 5, respectively. Chapters 6 and 7 discuss the time-history analysis of stay cable systems with external dampers and external dampers combined with crossties, respectively.