Is an arch or a truss better for a balsa bridge?

Short answer: for most balsa contests a truss is easier to build and design, and is the usual choice. An arch can be efficient, but it pushes outward on its supports and needs curved members that are hard to make from straight balsa.

How they differ

• Truss: straight members in triangles. Each is pushed or pulled. The supports only hold it up.
• Arch: a curve that carries load mostly in compression, and pushes outward at the feet, so something has to stop the feet spreading, such as a tie along the bottom or strong abutments.

The challenges with an arch

1. Curved members. Balsa is easiest to use straight. Curves need bending or many short straight pieces.
2. Spreading at the feet. Contest supports often just rest the bridge on blocks, so the bridge has to resist the outward push itself, with a bottom tie. Some contests say the bridge must carry its own thrust, and others require no fixings at the supports.
3. Buckling. An arch is compression, so it can buckle.

When to consider an arch

If the rules allow it, you have the time to make accurate curves, and a bottom tie keeps the shape. Otherwise, a truss is a safer bet. Check your rulebook first, since some contests specify the form.