A drawbridge is a movable bridge in which the moving deck — the leaf — is pulled up by hanger rods or chains from a balance beam mounted above the deck. The counterweight in that beam keeps the bridge in balance, so very little force is needed to open it.
It is the bridge everyone recognises from Dutch and Flemish canal scenes: two raking posts, a cross beam over the top, and a balance beam that works like a giant see-saw.
The parts
- Leaf — the moving deck that tilts upwards.
- A-frame or gantry — the above-deck frame of two posts and a cross beam, carrying the pivot of the balance.
- Balance beam — the see-saw beam above the bridge, with the counterweight at the rear.
- Hanger rods or chains — the link between the tip of the balance beam and the end of the leaf.
- Counterweight — a box of stone, sand or concrete, or a solid block. Well balanced, opening takes almost no effort.
Drawbridge, bascule or simple leaf bridge?
| Type | Counterweight | Recognised by |
|---|---|---|
| Drawbridge | In a balance beam above the deck | Gantry with visible balance beam and hangers |
| Bascule bridge | Below the deck, in a bascule chamber | No superstructure; the leaf pivots just behind the abutment |
| Simple leaf bridge | Small or absent | Plain hinged leaf, usually short spans |
A drawbridge is in effect a leaf bridge with the counterweight placed above ground.
Why they are still built
- No deep chamber needed. A bascule bridge requires a large watertight chamber below ground. Where the water table is high, that saving is substantial.
- Townscape value. In conservation areas the drawbridge is often the form that fits.
- Simple mechanics. Hand operation remains feasible on smaller bridges.
Maintenance points
- Timber leaves are light but need regular attention to the deck surface and the fixings. Steel leaves last longer but weigh more.
- Pivots and hangers are the critical items: this is where wear and corrosion start.
- Keep the balance right. If the deck gets heavier after refurbishment, the counterweight must be adjusted to match.
- Clear navigation width and headroom are set by the waterway class and the navigation authority.
Related terms
- Bascule bridge
- Beam bridge
- Bridge abutment
- Listed building
Learn more about building terms in our knowledge base at fredsdiyplans.com.