Bridge efficiency score explained
How the load-to-mass efficiency score works, with worked examples, why grams matter, and what changes when a contest caps the load.
Updated · 2 min read
Most balsa bridge contests rank by efficiency, sometimes called the strength-to-weight ratio:
efficiency = load held ÷ bridge mass
Use the same units for both. If the load is in grams and the mass is in grams, efficiency is a plain number: how many times its own weight the bridge held.
Worked examples
| Bridge | Mass | Load held | Efficiency |
|---|---|---|---|
| A | 20 g | 15 kg (15,000 g) | 750 |
| B | 10 g | 10 kg (10,000 g) | 1,000 |
| C | 10 g | 12 kg (12,000 g) | 1,200 |
Bridge B holds less than A but wins, because it is half the weight. Bridge C shows the other lever: the same mass, more load.
Public examples show how high good bridges go. A Science Olympiad coach's blog describes a 9.47 g bridge on a 35 cm span that held 15 kg, an efficiency of about 1,584. A university outreach contest that caps the load at 100 lb (about 45 kg) on bridges of roughly 200 to 227 g has a top possible efficiency of about 200 to 230, because the cap limits the numerator. These are single reported results, not averages.
Why grams matter
Adding 1 g to a 10 g bridge is a 10% increase in mass. To keep the same efficiency it would have to hold 10% more load. Glue is the usual culprit. A coach's rule of thumb is that glue should be under about 5% of the structure's mass. A thin dab of glue is a few milligrams, but a bead is much more.
When the load is capped
If a contest stops loading at a maximum, every bridge that reaches the cap ties on strength, so the lightest bridge that survives the cap wins. Then the goal changes: don't build for the strongest bridge, build for the lightest one that is still safely above the cap. Read the rulebook to see which kind of contest you are in.
What to aim for
- Don't chase the highest possible load. Chase the best ratio.
- Weigh every stick and every finished bridge. A kitchen scale that reads to 0.1 g is not good enough for a 10 g bridge, so use one that reads to 0.01 g if you can.
- Know the mass limit and stay a safe margin under it, since scales differ.
Next: why balsa bridges fail, and how to choose lighter, stronger sticks.