Why every bridge is made of triangles: a square frame folds, a triangle can't
30seconds
Push on the corner of a square frame and it folds flat. Push on a triangle made of the same sticks and nothing moves. The reason is simple: three side lengths fix a triangle's shape, but four don't fix a square's.
Drag the joints of a pin-jointed frame, add or remove bars, and watch a live count of how many ways it can still fold. You'll see why bridges, cranes and roof frames are nets of triangles.
Deep dive · 4 min
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