Should a balsa bridge be symmetrical?
Usually yes, left to right and across the middle, because the load is centered and the two trusses should share it equally. When asymmetry makes sense.
Updated · 1 min read
Short answer: usually yes. A symmetrical bridge shares the load equally between its two trusses and its two ends, which uses material best when the load is at the center.
Two kinds of symmetry
- Side to side. The two trusses should be identical mirror copies, with the load centered between them. If one truss is weaker or the load is off-center, that truss fails first and the bridge twists.
- End to end. If the load is at the middle of the span, the forces are mirror-symmetrical, so a mirror-symmetrical bridge puts material in the right places on both halves.
When asymmetry makes sense
- The load point is off-center by the rules.
- The supports differ, such as at different heights.
- The clear space or opening is on one side.
In those cases, design for the real force pattern, not an assumed one. See calculating forces and designing around the loading block.
Build tips
Build both trusses on the same plan, one on top of the other if you can, so they match. See drawing the plan.