How does span length affect how much a bridge can hold?
Longer spans cut capacity quickly: chord forces grow with span, long members buckle sooner, and bridges get heavier. A simple explanation with numbers.
Updated · 1 min read
Short answer: a longer span makes the bridge much weaker for the same depth and wood. As a rough guide, doubling the span roughly doubles the forces in the top and bottom chords for the same load, and makes buckling more likely.
Why
For a load P at mid-span, the bending effect on a simple truss grows in proportion to the span L (it is PL/4). The top and bottom chords resist it as a pair separated by the truss depth h, so each carries about PL/(4h).
- Double L, same h: the chord forces double.
- Double L and h: the chord forces stay about the same, but the members are longer, so buckling is worse and the bridge is heavier.
You can see the effect of depth in the worked example.
In practice
- Longer spans need more panels, to keep pushed members short.
- They need deeper trusses, within the height limit.
- They weigh more, which lowers efficiency.
- Joints are loaded harder.
This is why contests with long spans have lower scores, and why you shouldn't compare results across different spans. See what a good efficiency score is and why deeper trusses are stronger.
The formula is a simplification, so treat it as a guide for comparing designs.