Bending-Induced Stresses in Parallel Seven-Wire Strand Stay Cables
Publication: Journal of Bridge Engineering
Volume 29, Issue 11
Abstract
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.
Practical Applications
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.
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Data Availability Statement
Experimental data generated and used during the study are available from the corresponding author upon request.
Acknowledgments
This research was funded by the Department of Civil and Environmental Engineering at the University of Wisconsin–Milwaukee. The authors are grateful to Mr. David Goodyear for his review and feedback on this paper. The authors express their gratitude to Mr. John Crigler and Dr. Zuming Xia of Structural Technologies for donating the seven-wire strands and wedges used in the tests.
Notation
The following symbols are used in this paper:
- As
- cross-sectional area of a single strand (mm2/in2);
- d
- vertical distance between the centroid of a strand and the center of the cable (mm/in);
- di1 and di2
- vertical distances between the ith strand and the centroid of the cable cross section at the anchor head and the tension ring, respectively (mm/in);
- ds1
- distance between strand s and the centroid of the cable at the anchor head (mm/in);
- ds2
- distance between strand s and the centroid of the cable at the tension ring (mm/in);
- E
- young’s modulus of the stay cable (GPa/ksi);
- fUTS
- ultimate tensile strength of the cable material (MPa/ksi);
- I
- moment of inertia of the cable (mm4/in4);
- Is
- moment of inertia of a single strand (mm4/in4);
- L
- cable length (m/ft);
- LT
- length of the transition zone (m/ft);
- MFE
- fixed-end moment (kN·m/kips·ft);
- MG
- bending moment at the guide deviator (kN·m/kips·ft);
- Mmax
- maximum bending moment in the cable (kN·m/kips·ft);
- Msi
- bending moments in the ith strand (kN·m/kips·ft);
- M1 and M2
- in-plane bending moments developed at the anchor head and at the tension ring, respectively (kN·m/kips·ft);
- n
- number of strands in the cable;
- P
- concentrated force (kN/kips);
- r
- distance between the extreme fiber and the neutral axis (mm/in);
- rg
- radius of gyration of the section (mm/in);
- Ts
- axial cable force (kN/kips);
- Tsi
- tension force in the ith strand (kN/kips);
- W
- distributed load (kN/m or kips/ft);
- ΔLsb1
- change of length of strand s at the anchor head (m/ft);
- ΔLsb2
- change of length of strand s at the tension ring (m/ft);
- α
- rotation angle of the cable (rad);
- βG
- coefficient such that MG = βGMmax;
- γ
- inclination angle of the stay cable;
- δ
- maximum displacement at the cable midlength (mm/in);
- εs
- total strain in strand s due to bending action of the cable (transverse loads);
- εsa
- axial force-induced strain in strand s due to bending action of the cable (transverse load);
- εsb
- moment-induced strain in strand s due to bending action of the cable (transverse loads);
- maximum bending strain;
- θ
- angle change at the transition zone (rad);
- λ
- axial loading parameter (fraction of ultimate strength);
- maximum bending stress of the cable (MPa/ksi);
- σs
- bending stress (MPa/ksi);
- σs·max
- maximum bending stress (MPa/ksi); and
- ϕ
- total angular deviation (rad).
References
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© 2024 American Society of Civil Engineers.
History
Received: Oct 29, 2023
Accepted: Jul 9, 2024
Published online: Sep 5, 2024
Published in print: Nov 1, 2024
Discussion open until: Feb 5, 2025
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