Bending-Induced Stresses in Parallel Seven-Wire Strand Stay Cables: Fid-Bau Portal

Bending-Induced Stresses in Parallel Seven-Wire Strand Stay Cables

Lin, Qian / Tabatabai, Habib

The bundled parallel strand stay cables are important structural components of modern cable-stayed bridges. Strands commonly used in modern stay cables are seven-wire greased-and-sheathed strands. Stay cables primarily resist tension loads imposed by various loads, but they also resist bending stresses due to wind loads, vibration, and any traverse load applied on the cable. The movements of the deck and towers (at termination points of stay cables) also impose rotations at the cable ends, which will induce bending stresses in the strand bundles. Calculating and analyzing a stay cable’s bending-induced stresses is an important consideration, especially regarding fatigue resistance. Traditionally, designers have assumed a noncomposite moment of inertia for the bundle of parallel strands because of the presumed lack of sufficient interstrand bond along the length of cables. Therefore, the composite effect is generally not considered, and the moment of inertia is typically assumed to be equal to the number of strands times the moment of inertia of an individual strand. This assumption neglects the fact that unrestrained relative slippage between strands cannot occur because the anchorage plates enforce displacement compatibility at cable ends. Also, deviation clamps and periodic cable bands (tying of the strand bundle together) along the free length of the cable may provide some degree of composite action. This study examines the global bending-induced stresses developed near the termination points of stay cables due to the application of transverse loads on the cable. Experiments and computational analyses are conducted to assess these stresses. The computational models are verified using laboratory experiments performed on a five-strand cable. Detailed and simplified finite-element models are developed. Bending-induced stress calculation procedures are developed using a simplified stay cable model.

This study proposes calculation procedures to address the important issue of bending-induced stresses in the design of the most crucial components of cable-stayed bridges—stay cables. At present, there are no reliable equations to estimate the maximum strand stresses due to bending action of stay cables under transverse loading. Therefore, qualification tests of relatively short stay cable specimens are sometimes utilized to evaluate fatigue resistance under bending action. For bridge design engineers, the proposed analysis approach and equations can be useful in assessing the long-term fatigue resistance of strands. This study establishes that bundles of greased-and-sheathed seven-wire strands do not act independently within the transition zone of stay cables near anchorages, and the angle change imposed due to transverse loading is not the only factor affecting bending-induced stresses. The size (number of strands) and the length of the cable are the other influential factors that must be considered.