The travelling cage is supported over the truss through movable wheels and is transported bodily from one bank to the other when the cage carries the traffic as in ferry, service. An overhead truss spanning the channel to be crossed by the traffic is supported by two towers erected on either bank of the channel. 18.12) consists of a travelling cage through which the traffic is transported. A buffer is used to stop over-rolling of the rollers/wheels so as to prevent accidental fall of the bridge truss. When moves forward, the nose of the bridge acts like a cantilever for which the bridge truss has to be designed. The bridge traverses over rollers or wheels.

18.11) built over a small channel can be drawn towards the shore on one of the approaches when riverine traffic has to cross the bridge. Traverser Bridge :Īnother type of movable bridge is traverser bridge or draw bridge (Fig. There is a bascule chamber into which the counter weight of the bascule is lowered. The rack is fixed to the bascule near or at the center of gravity of the bascule which is counter-weighted at the ends for the facility of easy lifting and keeping safely in the lifted position. 18.10b) is constructed since single bascule would be too heavy to operate.įormerly, a cable was used to lift the bascule but now a day the bascule is rotated by a motor through a rack and pinion arrangement. 18.10a) is used but for large channels, double bascule bridge (Fig. For small channels, single bascule bridge (Fig. The bridges, when in use, remain in a horizontal position but having small free-board or the clearance over the water level, boats and other riverine transport cannot cross the bridge unless these are lifted vertically. The stability of the trusses with central bearing, while swinging, is maintained by few balance wheels fixed to the truss or the floor system in such a way that they roll on a circular path over the piers. This bearing is normally a phosphor bronze disc between two hardened steel discs.Īn alternative arrangement of support is by a circular girder resting on rollers, i.e., a turn-table (Fig. Both the truss of the bridge is supported at the centre over a common cross girder which rests on a central bearing or a pivot (Fig.

Movable bridge