Ophaalbruggen over delflandsluis Vlaardingen

A double-beam drawbridge, seesaw or folding bridge is a movable bridge . It opens by rotation about a horizontal

axis An axis (plural ''axes'') is an imaginary line around which an object rotates or is symmetrical. Axis may also refer to: Mathematics * Axis of rotation: see rotation around a fixed axis * Axis (mathematics), a designator for a Cartesian-coordinat ...

parallel to the water. Historically, the double-beam drawbridge has emerged from the

drawbridge A drawbridge or draw-bridge is a type of moveable bridge typically at the entrance to a castle or tower surrounded by a moat. In some forms of English, including American English, the word ''drawbridge'' commonly refers to all types of moveable ...

. Unlike a drawbridge, a double-beam drawbridge has

counterweight A counterweight is a weight that, by applying an opposite force, provides balance and stability of a mechanical system. The purpose of a counterweight is to make lifting the load faster and more efficient, which saves energy and causes less wear ...

s, so that opening requires much less energy. Unlike the bascule bridge, the simple drawbridge and the "oorgat" (earhole) bridge, the double-beam drawbridge has two

hinge A hinge is a mechanical bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a fixed axis of rotation: all other ...

s. The road surface is connected to the bottom hinge. Above the hinge is a portal, the hamei gate. A rotating arm, the balance, is attached to this hamei gate. On one side of the balance hangs the counterweight, the balance box, on the other side, the arm is connected to the tip of the bridge deck. So when the bridge goes up, the balance and the bridge deck turn parallel. The drawback of a traditional double-beam drawbridge is that it has a limited clearance height. That is why the balance box sometimes does not hang between the two arms, but on the arms themselves, without the arms being connected to each other. So there are two balance boxes. Many variants have been made on the basic design of the double-beam drawbridge. They are both double-rotating (with two separate bridge decks) and single-rotating and with a diagonal bridge deck. In the latter case, there is only one tower or Hamei style, which is next to the road. The pivot point of the balance with the balance box can either fall behind the hamei gate (as in the photo), or directly above it. The latter system is referred to as an "Amsterdam type". The advantage of this is that the tower is only loaded under pressure. When the pivot point falls behind the column, the tower is loaded eccentrically; this gives both pressure and flexion. When double-beam drawbridges are opened and closed electrically, an electric motor with a gear is often located at about half height in the Hamei styles. This engages in a rack that is hinged to the

bridge deck A deck is the surface of a bridge. A structural element of its superstructure, it may be constructed of concrete, steel, open grating, or wood. Sometimes the deck is covered by a railroad bed and track, asphalt concrete, or other form of ...

. This construction can also be seen in the photo. The rack is often not straight, but with a curvature at the end, like a hockey stick—this is to give the electric motor time to rev up and gradually raise (and lower) the bridge.

Sources

{{Bridge footer Moveable bridges Pages translated from Dutch Wikipedia

See also

Sources