How balsa bridge contests work
The common shape of a balsa bridge contest: a span, a load, a mass limit and an efficiency score, and what organizers usually check before you test.
Updated · 2 min read
Balsa bridge contests differ in the details, but most share one idea: build the lightest bridge that carries the most load. A judge weighs your bridge, loads it until it breaks (or until a cap is reached), and scores how much it held for how little it weighed.
What every contest specifies
Read your rulebook for these items first. They decide what a legal design looks like.
- The span. The gap between the two supports your bridge must cross. Some contests fix it, some give a range.
- Size limits. Maximum length, width and height, and often a clear space the bridge must leave open, such as a minimum opening a toy car or a loading rod must pass through.
- The load and where it goes. Usually weight hung from a point at the middle of the span, or a block or rod resting on the deck. Where the load attaches can force a particular design.
- A mass limit. A maximum weight for the bridge, often a few tens of grams.
- Materials. Which balsa sticks (for example 1/8 in square), whether sheet balsa, gussets or other woods are allowed, and which glues.
- Joint rules. Some contests ban certain joints or limit how much glue you may use.
How the score works
Most contests rank bridges by efficiency: the load held divided by the bridge's own mass. A 10 g bridge that holds 10 kg beats a 20 g bridge that holds 15 kg. See how efficiency is scored for the arithmetic and why a few grams matter so much.
Some contests add other prizes or requirements, such as a design report, an interview about how you built it, or a prediction of the failure load. Some cap the load, so every strong bridge ties on strength and the lightest wins.
What happens on test day
- Check-in: the judge measures your bridge against the size limits and weighs it.
- Loading: weight is added gradually, by hanging a bucket and adding sand or weights.
- Failure or cap: the bridge breaks, bends past a limit, or reaches the maximum load.
- Score: the judge records the load and computes the result.
A bridge that fails the size or mass check may be disqualified before it is ever loaded, so check every limit with a ruler and a scale before you pack it.
Where to start
- Read your rulebook carefully.
- Learn how trusses carry load.
- Pick good sticks and practice your joints.
Rules change from year to year and between regions. On our contest pages we put the rules in plain words, but the organizer's rulebook is always the final word.