bascule bridge

from The Century Dictionary.

• noun A drawbridge arranged with a counterpoise, so that, as the floor of the bridge is raised, the counterpoise descends into a pit prepared for it: the commonest form of medieval drawbridge. See balance-bridge.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A movable bridge consisting of one or two opening leafs equipped with balancing counterweights that ease and speedy up their operation.