What Is a Cranked Beam? Cranked Steel Beams Explained

A cranked beam is a steel beam that changes direction partway along its length. Instead of running straight from one support to the other, it has a bend, or crank, so it can follow a shape a straight beam can't.

Where cranked beams are used

They turn up wherever the structure has to follow something that isn't straight or level:

• Loft conversions and dormers, where a beam has to follow the line of the roof.
• Steps in floor or ceiling level, where one beam has to carry across two heights.
• Around obstructions such as a chimney, a drainage run or an existing beam that can't be moved.
• Staircase openings, to keep headroom where a straight beam would be too low.
• Canopies and feature structures that follow a profile or pitch.

How they're made

Cranked beams are fabricated, not bought off the shelf. There are two usual ways:

• Cut and weld. The beam is cut at the angle of the crank, and the two parts are welded back together. Stiffener plates are normally added at the crank to carry the forces through the change of direction.
• Bending. A section bender bends the beam to the angle or curve needed. This suits gentler angles and curved work.

Either way, the crank is a structural detail. The weld is designed by the engineer, made by qualified welders to an approved procedure, and inspected. That's part of what fabricating to EN 1090 covers.

What the engineer has to check

The crank is the point where the forces in the beam change direction, so it needs more thought than a straight beam. Your structural engineer will specify:

• the angle and position of the crank
• the size of the beam, which may need to be larger than a straight beam over the same span
• the weld and any stiffeners at the crank
• how the beam is supported at each end, because a cranked beam can push sideways on its supports as well as down

That last point catches people out. A wall that would happily carry a straight beam may not be able to resist the sideways push, so the supports may need posts, ties or a fixed connection.

One piece or two?

A single welded cranked beam is neat and stiff, but it's one long, awkward shape to get into a house. The alternative is two straight beams joined at the change of direction with a bolted connection, or a cranked beam made in two halves with a bolted splice. Two pieces are easier to carry in and fit, at the cost of a bolted joint. Which is right depends on access, and it's worth raising with the engineer before the design is fixed.

What it costs

More than a straight beam of the same size, because of the extra cutting, welding, stiffeners and inspection. How much more depends on the section size and the detail at the crank. Our guide to steel beam costs covers what drives the price.

Frequently asked questions

Do I need a structural engineer for a cranked beam? Yes. A cranked beam has to be designed and calculated like any structural beam, with the crank detail and supports checked as well. Building control will expect the calculations. See do I need a structural engineer for a steel beam.

Can a cranked beam be galvanised or painted? Yes. It's finished the same way as any other beam once the fabrication is complete.

Can you make one from my engineer's drawing? Yes. We fabricate steel beams and frames to engineers' drawings, cranked beams included. Send us the drawing and we'll quote.