8 Tensile force of self-propelled optical cable

The tensile force in a self-supporting figure-8 optical cable depends on the cable's weight, span length, sag, and environmental loading, and can be calculated using standard catenary or parabolic approximations.

Cable Structure and Tensile Considerations

Self-supporting figure-8 optical cables integrate a steel messenger strand with the optical fiber core in a single jacket, eliminating the need for a separate support wire. The messenger strand carries the axial load, while the optical fibers are protected from stress by the cable design . The tensile force applied to the messenger is influenced by:

• Cable weight per unit length
• Span length between poles
• Sag (vertical displacement at midspan)
• Environmental loads such as wind, ice, and temperature variations . The objective is to maintain low stress on the fibers while keeping sag within safe limits.

Calculating Tensile Force

For a parabolic approximation of a sagged cable, the horizontal tensile force at midspan can be estimated as: R_x = (q × L²) / (8 × h) Where:

• R_x = horizontal tensile force (N)
• q = uniform load per unit length (N/m)
• L = span length (m)
• h = sag at midspan (m) . The vertical support force at the ends is: R_y = (q × L) / 2 The resultant tensile force at the support is: R = √(R_x² + R_y²) This accounts for both the cable weight and sag-induced tension. For example, a 30 m span with 10 m sag and a uniform load of 4 kN/m results in a horizontal force of 45 kN and a resultant support force of approximately 45 kN .

Practical Considerations

• Proof testing: Optical fibers are typically proof-tested to 1% strain (~0.69 GPa) to remove weak flaws, ensuring the cable can withstand installation tension without fiber breakage .
• Sag management: Proper tensioning of the messenger strand is critical to resist expected loads while minimizing sag .
• Environmental factors: Ice, wind, and temperature changes can increase tensile forces; design calculations must include these loads .

Summary

The tensile force of a self-supporting figure-8 optical cable is primarily carried by the integrated messenger strand and is determined by span, sag, cable weight, and environmental loads. Using the parabolic approximation, engineers can calculate horizontal, vertical, and resultant forces to ensure safe installation and long-term reliability of the aerial optical network .