What is a safety factor, and does it matter in a bridge contest?
Real bridges are designed to carry several times their expected load. A contest bridge is the opposite: built to the limit, with no margin on purpose.
Updated · 1 min read
Short answer: a safety factor is the ratio of what a structure can carry to what it is expected to carry. Real bridges have a margin of several times, to cover errors and unknowns. A contest bridge is different: its goal is to carry as much as possible, so there is no margin by design.
In real engineering
If a bridge is expected to carry a 10 tonne load and can carry 20 tonnes before it fails, its safety factor is 2. Engineers add margins for variation in materials, mistakes, wear and loads they can't predict.
In a contest
The score rewards the highest load for the lowest mass, so you want to use every bit of strength. That means:
- Don't over-build parts that never fail. It is wasted weight.
- Do build in a margin where something is uncertain, such as a joint you can't test, or sticks that vary a lot.
- If the load is capped, you need only a small margin over the cap, to allow for variation on the day.
Why variation matters
Balsa strength varies by 15 to 25% even at the same density, and glue joints vary more. A bridge that was only just strong enough in your test may fail the next one. See predicting capacity and practice bridges.