Duke street bridge isn't the bridge a Bascule type bridge.
Quite right. It's a 'rolling bascule' type, meaning it doesn't hinge about one point but is pulled back so that the curved portion is rolling along the big teeth you see on the ground each side. There's a balance weight high overhead at the back and the effect is to even out the power required as it moves. The mechanics of it are simpler and above ground. This photo shows the arrangement, where the horizontal beams are drawn into and through the building by a sort of rack and pinion system driven electrically.
Simple, and they've lasted well!