Which way should the grain run in a balsa bridge?
Balsa is much stronger along its grain than across it. Why every member should run along the grain, why joints are weak, and how to spot bad grain.
Updated · 1 min read
Short answer: along the length of every member. Balsa is far stronger along the grain than across it, so sticks should carry load along their length, and any load across the grain should be spread out or avoided.
How big is the difference?
Using our planning estimates for balsa at about 150 kg/m³:
| Direction | Typical strength |
|---|---|
| Compression along the grain | about 10 MPa |
| Tension along the grain | about 20 MPa |
| Compression across the grain | about 0.8 MPa |
Along the grain is more than ten times stronger. These are estimates, not guarantees. See how strong is balsa.
What it means
- Members: sticks come with the grain along their length, so a straight stick used as a truss member is already right.
- Joints: where a stick ends against the side of another, the force is across the grain of the receiving stick, which is the weak direction. That is why joints fail first. See gluing joints.
- Load points and supports: balsa crushes across the grain under a small contact, so spread the load over a wider area.
- Sheet balsa: the grain direction in a gusset matters too, and sheet is weakest across the grain.
Spot bad grain
Sight down each stick. Reject sticks where the grain runs off at a slope, or with knots, dark streaks or soft spots, because they break early. See choosing sticks.