Restrained Bending Fatigue Design of Bridge Cables S.W. Khan 1 , and M. Raoof 1 1 School of Civil and Building Engineering, Loughborough University, Ashby Road, Loughborough, Leics, United Kingdom, LE11 3TU; PH +44 (0)1509 222610 S.W.Khan@lboro.ac.uk, m.raoof@lboro.ac.uk ABSTRACT In previous work, certain shortcomings of traditional design approaches against restrained bending fatigue of axially preloaded spiral strands used in cable stayed structures (e.g. bridges) based on the extreme fibre direct stresses have been identified and subsequently resolved. Restrained bending fatigue life of such spiral strands clamped at the end(s), is shown to be governed by interlayer fretting fatigue. Interlayer fretting is found to be greatest near the so-called neutral axis and not the extreme fibre position, where, during tests, initial wire failures have invariably been found to occur. A new parameter has thus been developed for design against such rather common and often very costly fatigue failures. The reliability of any restrained bending design procedure is based on a reasonably accurate estimation of the minimum critical radii of curvature at the fixed end(s) with such values of curvature then used as an input into the fatigue design model. The present paper makes significant progress in resolving the longstanding debate in the literature as to whether the traditional method of assuming a constant effective bending stiffness for estimating such critical curvature(s) is a practically reasonable one. Keywords: steel cables, bending stiffness, structural integrity, fatigue, bridges INTRODUCTION This paper is concerned with certain aspects of restrained bending fatigue performance of realistic multilayered large diameter spiral strands Figure 1(a). Such type of bending takes place in the absence of sheaves or other formers, so that the radius of curvature of the strand is not predetermined. In the present terminology, the term restrained bending fatigue refers to those cases where spiral strand fatigue failures occur in the vicinity of partially or fully restrained terminations of various types caused by hydrodynamic or aerodynamic cyclic loading. Such spiral strand restrained bending fatigue failures are a source of concern in structures ranging from: floating offshore platforms, suspension and cable-stayed bridges, the stays for guyed masts, and electromechanical cables. In this context, it is worth mentioning that for example, inspection of a large number of cable stayed bridges around the world has revealed occurrence of corrosion and fatigue problems around the anchor joints of cables (Watson and Stafford, 1988). In quite a few reported cases, cables (spiral strands or locked coil ropes) of such bridges have had to be replaced. In the course of design against such restrained bending fatigue failures, it is common to introduce a mathematically convenient constant effective bending stiffness EI eff , for the cable. By using EI eff , the minimum and most critical radii of 113 FORENSIC ENGINEERING 2012 © ASCE 2013 Forensic Engineering 2012 Downloaded from ascelibrary.org by Loughborough University on 12/27/13. Copyright ASCE. For personal use only; all rights reserved.