How do you strengthen a bridge where it sits on the supports?
Balsa crushes across the grain, so the support points need wider bearing, vertical members with the grain running up and down, and sometimes a small pad.
Updated · 1 min read
Short answer: give the end a wide bearing area, and let the force reach the support through sticks whose grain runs the same way as the force. Balsa is much weaker across the grain than along it.
Why the ends are a weak point
All the load passes through the supports. If the end of a member sits on the support with its side, so that the push is across the grain, the balsa crushes. Our planning estimate for crushing across the grain is only about 0.8 MPa, compared with about 10 MPa along it. See grain direction.
Fixes
- Use vertical or steeply sloped end members so the force runs along the grain, down into the support.
- Widen the bearing. Let the end sit on a bigger foot, such as two sticks side by side or a short cross-piece, so the force is spread.
- Add a small pad or plate if the rules allow it, of balsa sheet, glued over a wide area.
- Brace the end so it can't spread or kick out.
- Check the rules on where the bridge may touch the supports, and how long the bearing may be. See reading your rulebook.
Test it
Look at the ends after a test. Dents or crushed fibres at the bearing point mean it needs more area. See why bridges fail.