What is a span?
A span is the distance a structural member bridges freely between two supports. A joist across a room, a beam over a garage opening, a truss over a shed, a bridge deck between two piers: the span is the length covered without an intermediate support.
An important detail: design is normally based on the effective span, the distance between the centres of the bearings, not on the clear opening between the walls. Over short spans the difference is small, but it is the dimension the codes work with.
Why span dominates everything
For a uniformly loaded beam, the bending moment increases with the square of the span and the deflection with the fourth power. Making a beam 20 per cent longer therefore means roughly 44 per cent more moment and more than twice the deflection for the same section. Span is by far the most sensitive variable in a design — more so than the load itself.
What you can do about it, in order of effect:
- Reduce the span with an intermediate support or a beam.
- Increase the depth of the member. Depth enters the stiffness to the third power; width only once. Adding 50 mm of depth helps far more than adding 50 mm of width.
- Reduce the spacing between joists, so each one carries less load.
- Choose a stronger material or section, such as glulam or a steel beam.
Deflection usually governs
In timber floors the beam rarely fails — the deflection becomes unacceptable first. In practice, limits in the order of span divided by 250 to 360 are used for deflection under imposed load, depending on the construction and the finishes above. A tiled floor or a plastered ceiling is considerably more sensitive to deflection than a timber floor with a timber ceiling.
Have any span over about 3.5 metres, and any span carrying a wall or floor above, calculated by a structural engineer.
Related terms
- Beam
- Joists
- Lintel
- Deflection
- Structural engineer
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