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📘 How drawbridge counterweights work

Picture the road deck cut at the edge of a waterway. Instead of lifting the whole bridge straight up, engineers let one section, called a leaf, rotate around a horizontal pivot. That makes a bascule bridge a giant seesaw. In traffic positio

5
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~25 min
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Adults
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What you’ll learn

  1. The bridge as a seesawIdentify the leaf, pivot, backspan, and the difference between single- and double-leaf bascules.A bascule leaf rotates around a horizontal pivot, while its hidden rear structure makes the bridge behave like a controlled seesaw.
  2. Torque does the heavy thinkingExplain how torque and moving linkages let a counterweight reduce the drive force across the opening arc.Balance depends on force and distance from the pivot. Linkages keep the counterweight useful as the bridge changes angle.
  3. Where the weight can liveCompare under-deck, overhead, fixed-pivot, and rolling-lift counterweight arrangements.The counterweight's location responds to water level, foundations, clearance, wind, and the geometry of the chosen bascule design.
  4. Opening is a whole sequenceDescribe the safety and control sequence that turns a counterweighted span into a reliable traffic bridge.Signals, barriers, motors, brakes, sensors, seats, and locks work with the counterweight so boats pass and traffic can safely resume.
  5. The design in one pictureApply the complete torque-and-control model to a documented Strauss drawbridge and identify a next observation.The Saugus example shows how counterweight geometry, machinery, site constraints, maintenance, and human operation meet in one working bridge.

Questions this course answers

What does a counterweight mainly reduce?

A counterweight offsets much of the leaf's gravitational torque, reducing the drive force needed to move and hold it.

Why can a smaller counterweight balance a larger leaf?

A smaller force can create an equal turning effect if it acts farther from the pivot.

What is a trunnion in a fixed-pivot bascule?

The trunnion is the rotating support or pivot for the movable leaf.

Why might engineers choose an overhead counterweight?

An overhead arrangement can avoid the deep chamber needed for an under-deck weight.

What must happen before traffic returns after closing?

Seating and locks secure the closed bridge and transfer traffic loads safely into the supports.

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