How do you prevent buckling in a balsa bridge?
Why long thin sticks buckle, how the length and size of a stick change its buckling load, and four ways to stop it: shorten, brace, enlarge, stiffen.
Updated · 1 min read
Short answer: make compression members shorter, brace them sideways, give them a bigger cross-section, or use stiffer wood. Length matters most.
Why it buckles
A long thin stick that is pushed from both ends doesn't crush. It bows sideways and suddenly can't carry more. This is buckling. A short, thick stick is far more likely to crush, which takes far more load.
How length and size change it
- Length: the load a stick can carry before buckling falls with the square of its length. Doubling the free length cuts the buckling load to about a quarter.
- Size: for a square stick, the stiffness against bending rises with the fourth power of its width. A stick twice as wide is 16 times stiffer in bending, though only four times heavier.
- Stiffness of the wood: a stiffer, denser stick buckles later. See choosing sticks.
Four fixes
- Shorten the free length. Add a cross-brace, or add a panel to the truss, so each pushed stick is shorter.
- Brace it sideways. Tie it to the other truss with cross-members so it can't bow out of plane.
- Enlarge it where the rules allow, or laminate two sticks together. Watch the weight.
- Use your stiffest sticks for the members being pushed.
Where to look
In a simple truss under a downward load, the top chord and some diagonals are being pushed. See which members are pushed or pulled.