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📘 The roadway becomes a lever

You are at the riverbank, watching the roadway rise like a seesaw. Its hidden rear tail carries the counterweight.

4
lessons
~20 min
to learn
Adults
level
Start the course →

What you’ll learn

  1. A bridge is a seesawIdentify the leaf, trunnion, counterweight, and turning moments in a bascule bridge.A drawbridge is a rotating lever whose hidden tail helps balance the roadway around its pivot.
  2. The hidden tailExplain how counterweight material, placement, and double-leaf layouts keep a moving span balanced.The counterweight is dense, attached, and geometrically placed so its opposing moment follows the leaf's motion.
  3. The drive controls the balanceDescribe how motors, gears, controls, and safety interlocks operate a counterweighted leaf.Drive machinery controls a balanced structure while locks, signals, and sensors make opening safe.
  4. The balance changesExplain why changing geometry, wind, and structural loads mean balance is not complete safety.Torque changes through the swing, so engineers combine counterweights with structure, brakes, locks, and inspection.

Questions this course answers

What does the counterweight primarily reduce?

The counterweight creates an opposing moment around the trunnion, so the drive does not fight the full gravitational imbalance.

What does the Trunnion do?

A bascule is a system: pivot, balance, drive, and restraint solve different problems.

Put the safe opening sequence in order.

The bridge must be isolated and checked before it moves, then secured after movement.

Why can a counterweighted bridge still need powerful machinery and brakes?

Balance makes operation manageable; it does not make the structure weightless or automatically safe.

Grounded in trusted sources

  • Transport for NSW — Movable Span Bridge in Action: Volume 2 — Bascule and Swing Span Bridges: https://www.transport.nsw.gov.au/system/files/media/documents/2023/moveable-span-bridge-study-volume-2-bascule-and-swing-span-bridges-part-1.pdf
  • Maine Department of Transportation — Operations and Maintenance Manual: Movable Span Bridge: https://www.maine.gov/dot/sites/maine.gov.dot/files/dot-project-documents/bhom022202.00a_0.pdf
  • International Journal for the History of Engineering & Technology — The Bélidor Bascule Bridge Design: https://www.tandfonline.com/doi/full/10.1179/1758120612Z.00000000010
  • Caltrans — Movable Bridge Manual: https://dot.ca.gov/-/media/dot-media/programs/engineering/documents/structureconstruction/movablebridges/sc-mbm-a11y.pdf
  • Heavy Movable Structures — A Century of Motion: The History and Engineering of the Mystic River Bascule Bridge: https://heavymovablestructures.org/wp-content/uploads/2017/12/37120909-Mystic-Paper-HMSSUBMITTED2.pdf

Every Wunder lesson is built from real, reputable sources — never invented.

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