A girder is a heavy beam that runs beneath the floor joists and supports them. Because the joists gain an intermediate bearing, their span is halved and deflection drops dramatically. Today a girder is usually a steel I-section or H-section; in older buildings it is often a substantial timber beam. The Dutch term is onderslagbalk.
Why use a girder
Deflection increases very sharply with span. Halve the span and deflection reduces many times over. A girder is therefore the standard answer when:
- the joists would have to span too far for the available joist depth
- a wall running at right angles to the joists has to be carried
- an existing load-bearing wall is removed during alterations and the floor above still needs support
- the floor bears on pads, piers or brick columns rather than continuous walls
The bearing is the critical detail
The beam itself is rarely the problem — its bearings usually are:
- Adequate bearing length. On masonry, 100-150 mm is a common minimum, and more for heavier loads; the engineer sets the exact figure
- Spread the point load. A bearing plate, a concrete padstone or a masonry pier belongs under each beam end; bearing straight onto a single brick leads to cracking or crushing
- Prop during installation. When removing a load-bearing wall, the structure above must be propped before anything is cut
- Separate steel from moisture. A steel beam in an external wall needs galvanising or a protective coating, plus a damp-proof separation
- Consider fire resistance. Unprotected steel loses strength quickly in a fire; depending on the requirement the section is boxed in with fire-rated board or given an intumescent coating
> A girder is always structural work. The section, the bearings and the foundations beneath the support points should be designed by a structural engineer — and for most alterations this is a building control requirement in any case.
Related terms
- Floor joist
- Padstone
- Span
- Deflection
- Propping
- Lintel
- Bearing
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