What is a Cranked Beam? Structural Design Explained (UK Guide)

A cranked beam is a type of steel beam that changes direction along its length. It is commonly used in UK construction where a straight beam cannot follow the required building layout, such as loft conversions, extensions, vaulted hipped roofs and split-level floors.

Because of its geometry, a cranked beam behaves differently from a standard straight beam and requires careful engineering analysis.

Why Cranked Beams Are Used

Cranked beams are typically used when:

  • Floor levels change within a structure
  • Roof designs require stepped steelwork
  • Architectural layouts prevent straight beam installation
  • Load paths need to be redirected

They allow engineers to achieve structural solutions where standard beams are not practical.

How Cranked Beams Work Structurally

Unlike straight beams, cranked beams do not transfer loads in a single linear path. Instead, forces change direction at the crank point.

This means the beam must resist:

  • Bending forces
  • Shear forces
  • Axial forces created by the change in geometry

The crank point becomes the most highly stressed area of the beam.

What Happens at the Crank Joint

At the point where the beam changes direction, forces are transferred through welded or bolted connections.

This area experiences:

  • High shear forces
  • Localised tension and compression
  • Stress concentration in plates and welds

If not properly designed, this can lead to excessive stress in the connection area.

Do Cranked Beams Need Structural Calculations?

Yes. Structural calculations are required for all cranked beams in the UK.

This is because Building Control must verify that the beam is safe and complies with:

Without calculations, there is no evidence the beam is structurally adequate.

What Engineers Check in Cranked Beam Design

When designing a cranked beam, engineers assess:

  • Load combinations
  • Beam bending resistance
  • Shear capacity
  • Deflection limits
  • Connection forces at the crank

The crank connection is often the governing factor in the design.

Web Stiffeners in Cranked Beams

In some designs, web stiffeners are added at the crank location.

These help to:

  • Prevent local buckling of the beam web
  • Improve load transfer through the joint
  • Strengthen the connection area

Whether stiffeners are required depends on the structural calculations.

Building Regulations Requirements (UK)

Cranked beams must comply with:

  • Approved Document A
  • BS EN 1990
  • BS EN 1991
  • BS EN 1993 (Eurocode 3)

Building Control will require structural calculations before approving installation.

When to Speak to a Structural Engineer

You should consult a structural engineer if:

  • Your beam changes direction
  • You are planning a loft conversion or extension
  • Your architect specifies a cranked steel member
  • You need Building Control approval

Need Structural Calculations for a Cranked Beam?

We provide full Structural Calculations for Cranked Beams across the UK, including connection design, stiffener checks, and Building Control submission packages.