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Why does my balsa bridge twist or lean sideways?

Sideways collapse is a common failure of tall narrow trusses. Why it happens and how cross-bracing, a wider base and a centered load fix it.

Updated · 1 min read

Short answer: a tall, narrow truss has nothing to hold its top in line, so under load it can tilt or twist before any stick breaks. Cross-bracing between the two trusses, a wider base and a centered load fix most of it.

Why it happens

The top chord of a truss is pushed. Like any pushed stick, it can bow sideways. A flat truss is stiff in its own plane but flexible out of it, and a small push sideways grows quickly.

Fixes

  1. Cross-brace between the trusses, at the top and at the bottom, with diagonals or short cross-pieces.
  2. Make the base wider if the rules allow. A wider bridge resists tipping.
  3. Center the load. An off-center load twists the bridge.
  4. Build square. Check that both trusses are the same size and stand upright.
  5. Don't skimp on the top. In a through truss, with the road at the bottom, the top is the free edge that needs the bracing.

Test for it

Load a practice bridge slowly and watch from the end. A top chord that starts to lean is a warning. Then see why bridges fail and deck or through truss.