What is specific strength in a bridge contest?
Specific strength is the load a bridge carries divided by its own weight. How it's calculated, its units and how it relates to efficiency.
Updated · 1 min read
Short answer: specific strength is the maximum load a bridge held divided by the bridge's own weight. It is the same idea as the efficiency score in most balsa contests, just under a different name.
specific strength = maximum load ÷ self-weight
Use the same unit on top and bottom. If a bridge weighing 300 g holds 6 kg (6,000 g), its specific strength is 20: it held 20 times its own weight.
Why contests use it
Raw strength rewards heavy bridges. Dividing by weight rewards clever use of material, which is what engineers want to teach.
Which numbers are different?
- Balsa contests often quote efficiency in the hundreds or thousands, because balsa bridges are so light and hold so much.
- Contests built from heavier sticks give smaller numbers.
- Always check what counts as the load (does it include the bucket or hook?) and as the weight (is glue included, and are the loading pieces?).