There are two forms of the Strauss bascule, one with the counterweight below the floor of the bridge in a shallow pit ; the other with the counterweight elevated upon a tower—in which case no pit is necessary. A distinct variation of the latter type is the "Heel-trunnion)) bas cule, in which the counterweight is carried by one end of a trussed frame rocking on the secondary or counterweight trunnion at the top of a stationary tower.
The Page bascule makes use of a movable approach span as a counterweight. This ap proach span is pivoted at the shore end, and its outer end is weighted and arranged to travel down a curved track Axed upon the short arm of the bascule cantilever. As the leaf lifts, it is accurately counterbalanced by the varying gravity pressure exerted by the weighted shore span upon the curved track member — the curves being calculated to govern the pressure utilized.
In the Chicago City bascule the trunnions supporting the trusses are in the line of the lower chord, and on the shoreward side of the centre of gravity of the moving span. The counterweights are fixed rigidly to the short arm of the cantilever, and as the bridge is opened they sink into a pit.
In the Waddell & Harrington bascule the trunnions are high above the roadway, in the line of the upper chord of the truss, and the bearings are on tower-like structures. The counterweight is rigid with the truss and over head, and is so placed relative to the centre of face of the quadrant-shaped shore end of the bridge has large steel cogs which engage in massive steel racks fixed upon the foundation piers. As the centre of gravity of the bridge is at the centre of the quadrant, the span is perfectly balanced in any position, and requires only power sufficient to rock it on the foun dation. The counterweight is generally fixed, either forming a part of the quadrant — in which case no pit is required; or carried on an extension of the shore end of the quadrant — in which case a shallow pit is provided to re ceive it. In some instances, where the ground will not afford a substantial support for the foundation, it becomes necessary to spread the strains over a large surface, and the counter weight may be suspended from towers, relieving the rack from so much of the weight. This is a common form of bridge for electric railways and in such instances is very cheaply operated by electricity Larger bridges are operated by steam or hydraulic power. The operating mech anism is of the rack and pillion type and acts upon the top chord of the truss.
In the Rall bascule the trunnions are large and the bridge rolls on them instead of swing ing or rocking. When the bridge is closed it rotation that the moving span is always in equilibrium. The bridge is swung on its pivots, instead of being rocked, as in the other bas cules, and the operating is done with a system of cables instead of with the usual rack and pinion mechanism.
Rolling Lift— It is claimed for the roll ing lift type of bascule bridges that besides being more scientific in supporting the weight directly on a foundation instead of on pivots, they are much more economical, saving .12 per cent in length, 24 per cent in the amount of steel used, 45 per cent in the amount of coun terweight and over 50 per cent in the weight of the machinery necessary to operate them. The only considerable objection which has been against the rolling lift type is that the strains upon the foundation are shifting, being on the channel end when the bridge is closed, and on the shore end when it is open. That this difficulty is overcome in practice is evidenced by the fact that the type is a favorite one with engineers, and that more of the roll ing lift bridges are being installed than of all other forms of bascules put together.
In the Scherzer bascule bridge the outer sur VOL rests upon a pivot, which is connected by a swinging strut with a second pivot. As the bridge opens it turns upon the first pivot. The connecting strut comes into play to stop that rotation, the trunnions roll backward upon the track girders. and the channel end of the bridge is pulled upward, the second pivot acting as a fulcrum.
Roller Bearing.— The two extant types of the roller-bearing bascules are so nearly alike that charges of infringement of the Waddell patent have been urged against the Cowing pat ent granted a year later. The general idea is the same — the counterweight member of circular outline, having its centre at the centre of gravity of the moving leaf, rolls on its cir cumference through an arc of 90 degrees in a nest of rollers held in a socket-shaped bearing. In the Waddell bridge the structure turns upon seven compound rollers, each made up of a large roller surrounded by 11 small rollers. In the Cowing bridge the structure turns upon 29 solid simple rollers. There is also a differ ence in the distribution of the counterweight: in the Waddell bridge it is overhead; in the Cowing bridge it is divided, part being below the deck.