The Chartership Forum

Structural Analysis Fundamentals / module 5

Cables

Structures that carry load in pure tension, and why their shape depends on how the load is applied.

What you will understand

  • Explain why a cable can only carry tension, and what that means for its shape
  • Show that the horizontal component of tension is constant along a cable
  • Analyse a cable carrying concentrated loads
  • Derive the parabolic cable profile and the relationship between sag, span and thrust
  • Distinguish the parabola from the catenary and know when each applies

Lessons

  1. Cables carrying point loads

    Why the horizontal pull is constant, and how to analyse a loaded cable.

  2. Distributed loads: parabola and catenary

    Deriving the cable shape under a uniform load, and when the catenary matters.

Module summary
  • A cable has no bending stiffness, so it takes up the shape that carries the load in pure tension
  • The horizontal component H is the same everywhere along the cable
  • For a load uniform along the horizontal projection, the cable is a parabola with H = wL²/8h
  • For self-weight (uniform along the cable) the true shape is a catenary
  • Tension is greatest at the supports and least at the lowest point
  • Flatter cables need much larger tensions and pull much harder on their anchorages
Teaching basis and review status

These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.

Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.

Read the lesson register and outstanding review notes

Check your understanding

Practice

A cable spans 50.0 m between level supports carrying 6.0 kN/m along the span, with a sag of 5.0 m. What is the horizontal thrust in kN?

Check yourself

For a smooth symmetric cable under a downward uniform load, where is the tension smallest?

Check yourself

A cable's sag is halved while the span and load stay the same. What happens to the horizontal thrust?