What is a built-up beam?
A built-up beam (also called a composite or compound beam) is a beam made up of two or more parts, connected in such a way that they act together as a single unit. You make one when a single piece of timber or steel is not large, long or strong enough, or is not available. Examples are two or three glued or bolted boards side by side, a dowelled beam of beams laid on top of each other, a timber I-joist and a box beam.
Why the connection determines everything
If you lay two loose boards on top of each other and load them, they slide along each other and each board works on its own. If you connect them fully against slip, for example by gluing, they act as one deep beam. The difference is large: with two equal parts on top of each other, the beam with full composite action becomes roughly twice as strong and four times as stiff as the two loose parts together. This is because stiffness increases with the cube of the depth.
Parts placed side by side (for example two beams screwed together) do not each become stronger through the connection, but they do work together and share the load.
| Two loose boards on top of each other | They slide along each other and each board works on its own. |
|---|---|
| Two equal boards on top of each other, fully connected against slip, for example glued | Roughly twice as strong and four times as stiff as the two loose parts together. |
| Parts side by side, for example two beams screwed together | Each part does not become stronger through the connection, but they work together and share the load. |
Applications
- Strengthening an existing beam: during a renovation, a sagging floor joist is sometimes reinforced with an extra beam alongside it (sistering).
- Lintels and supporting beams made of several parts screwed or bolted side by side.
- Long spans: glued laminated beams and timber I-joists are factory-made built-up beams.
- Historic structures: dowelled beams with timber dowels or keyed joints between the layers.
Points to note
- Always have a built-up beam calculated by a structural engineer if it carries anything: the number and position of screws, bolts or glue determine the actual strength.
- Loose parts without a proper connection do not add up the way you would expect.
- Stagger butt joints if you need to join parts lengthwise, and never let a butt joint fall in the middle of the span.
Related terms
- Girder
- Glued laminated timber
- Wooden I-joist
- Box girder
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