BRIDGES, Lift. Among movable bridges the so-called lift bridges claim a constantly increasing importance, especially in locations when the waterways are narrow, and the popu lation congested. In such circumstances they are in many cases supplanting the existing swing bridges. The chief advantages which lift bridges possess over swinging bridges are: First, they afford a wide channel free from the centre pier and its protective outwork; second, they are more economical of valuable docking space, occupying only their own width, whereas the swing bridge on a narrow waterway re quires more than its length; third, they are much more rapid in operation, in many in stances passing the river traffic when only partly opened.
The earliest lift bridges were of the hinge type, and were raised by chain or wire cables running from the loose end of the bridge over pulleys at the top of towers at the hinge joint and attached to the compensating weights. To obviate the difficulty of overbalance when the bridge neared the upright position, the weights were made to slide or roll down a cowing track which approached more and more the horizontal as the bridge structure approached a vertical position. An interesting variation of this early type was the or fold ing lift bridge, in which the floor of the bridge was jointed at the middle and the lifting cables attached on the inner side of the joint. The outer end of the bridge was supported by fixed cables. When the bridge was raised the centre was lifted, the outer end folding down ward as both halves came to an upright position against the towers.
Modern lift bridges are of two general classes, the bascule or balanced bridges; and the vertical lift or elevator bridges. In the former, the bridge structure is a cantilever with unequal arms, which is swung or rocked through a vertical arc of 90 degrees; in the latter, the movable span is lifted bodily in its normal hori zontal position by mechanism acting upon both end simultaneously.
Bascule The bascule bridges fol low one of three distinct types: (1) The trun nion type— in which the weight of the bridge together with its counterweight is supported while opening and closing on a trunnion or pivot shaft; (2) the rolling-lift type —in which the counterweight short arm of the cantilever is in the form of a quadrant of a circle, on the circumference of which the bridge is rocked backward and upward; (3) the roller-bearing type — in which the trunnion idea dominates, but the bearing is expanded to a circle of con siderable size which rolls in a socket filled with roller bearings. All of these types are repre
sented in single-leaf bridges, of which one shore end is raised; and also in double-leaf bridges in which the two midstream ends of the leaves are raised. In the trunnion and roller-bearing types the centre of rotation is a fixed point, and placed as nearly as possible at the centre of gravity of the moving weight. In the rolling lift type the centre of rotation changes, the centre of gravity moving in a horizontal line backward away from the chan nel. In the endeavor to overcome the inherent difficulties of the bascule type of bridge a num ber of different designs have been brought for ward by engineers; and as a consequence there are Strauss, Page, Brown, Chicago City and Waddell & Harrington bridges of the trunnion type; Scherzer and Rall bridges of the rolling lift type; and Cowing and Waddell bridges of the roller-bearing type.
In the Brown bascule bridge, the lifting is done by the old-time method of hoisting cables, which are attached to the span near the hinge joint, and thence guided around an iron form so curved that the pull of the counterweight shall always exactly balance the weight of the bridge.
In the Strauss bridge the specific peculiarity lies in the fact that the counterweight is not made a part of the short arm of the cantilever, hut moves upon a secondary pivot, acting through a parallelogram of links, or struts, which cause the counterweight to move always in parallel with its original position, thus main taining the leaf in equilibrium at all times.