What is Bird Caging in Cables?
Birdcaging in Cables

Definition:
Birdcaging in cables refers to the deformation of a cable's structure, where the strands of the conductor push outward and create a bulging or birdcage-like appearance. This typically occurs when the cable is subjected to excessive torsion, tensile stress, or sudden release of tension.

Causes:

• Improper tensioning during installation or operation.
• Excessive torque that twists the cable beyond its design limits.
• Longitudinal compression forcing strands to buckle outward.
• Overloading that exceeds the cable's rated tensile strength.

Calculation and Analysis:

Key Parameters:

• Axial Tensile Stress (σt):
  
  Code Select Expand
  σt = Ft / AcWhere:
  - Ft: Axial tensile force (N)
  - Ac: Cross-sectional area of the cable (m²)
• Torsional Stress (τ):
  
  Code Select Expand
  τ = T / JWhere:
  - T: Applied torque (Nm)
  - J: Polar moment of inertia (m⁴)
• Bending Stress (σb):
  
  Code Select Expand
  σb = (M * c) / IWhere:
  - M: Bending moment (Nm)
  - c: Distance from the neutral axis (m)
  - I: Moment of inertia (m⁴)
• Critical Strain for Birdcaging (εc):
  
  Code Select Expand
  εc = σy / EWhere:
  - σy: Yield stress of the material (Pa)
  - E: Modulus of elasticity (Pa)

Example Calculation:
Given:
- Cable Diameter (D) = 10 mm
- Tensile Force (Ft) = 1 kN
- Torque (T) = 5 Nm
- Yield Stress (σy) = 250 MPa
- Modulus of Elasticity (E) = 200 GPa

1. Cross-sectional Area:
Ac = π * D² / 4 = 7.85 × 10⁻⁵ m²

2. Axial Tensile Stress:
σt = Ft / Ac = 12.74 MPa

3. Torsional Stress:
J = π * D⁴ / 32 = 9.82 × 10⁻¹⁰ m⁴
τ = T / J = 5.09 MPa

4. Combined Stress:
σ_combined = σt + τ = 17.83 MPa
Since 17.83 MPa < 250 MPa, birdcaging is unlikely.

Mitigation Strategies:

• Use cables with higher tensile strength or better torsion resistance.
• Implement proper tensioning techniques during installation.
• Ensure the cable's design matches the application's load requirements.
• Utilize anti-birdcaging features like pre-tensioned strands or tighter lay lengths.