DRAWBRIDGES.
Under the generic term of drawbridges are included all forms of bridges designed to be opened and closed so as to give passage alter nately to vessels and to the road or railway traffic passing over the bridge. Drawbridges may be divided into the following types: Swing bridges, in which the opening span swings hori zontally on a turntable or vertical pivot carried by the pier supporting the span; bascule bridges, which consist of one or two draws hinged at the abutment on horizontal shafts so that they are opened by raising the bridge to a vertical posi tion, leaving the channel clear; rolling bridges, which have the span mounted on rollers, so that it may be hauled inshore and leave the channel (dear; lift bridges, in which the span is placed be tween two tall towers and so arranged that it is lifted clear of passing vessels, like an ordinary passenger elevator. There are several modifica tions of each of these types, some of these being patented in part, and for these the special treatises on bridge construction, which are men tioned later, should be consulted.
The most commonly used form of a draw bridge is the swing bridge. Generally the sup porting pier, called the pivot pier, is placed under the centre of the span, which is always either a plate girder or a truss span. Sometimes, how ever, the pivot pier is located nearer to one end of the span than the other, in which ease the shorter arm has to be counterweighted to balance the longer arm. When a swing bridge is open. the axis of the span is parallel to the axis of the stream; there are two open channels on each side of the centre pier and the ends of the span are swung clear of any support. When the bridge is closed the ends of the span are supporled on piers. The longest swinging span ever con structed is that for the Interstate Bridge at Omaha, Neb., which has a total length of 520 feet. The Thames River Drawbridge, New Lon don, Conn.. has a swing span of 503 feet, and the Arthur Kill Drawbridge between Staten Island, N. V., and the New Jersey mainland, has a swing span of 4961/4 feet. The manner of supporting a swing span on the pivot pier so that movement is possible is quite simple. A casting at the centre of the pier earries a vertical pivot. Around this pivot as a centre is bolted to the pier a circular, track having gear-teeth on the outer circumference. To the bottom of the span is attached a circular drum of the same diameter as the track on the pier. and at the centre of this drum is a socket which fits over the pivot on the pier. Between the bottom of the drum
and the pier-track is placed a chain of rollers kept in position by radial arms running to the centre pivot. The span swings on these rollers, the centre pivot serving only to hold it in posi tion or true to its centre. Movement is brought about by a vertical shaft running in bearings attached to the outside of the drum and having a gear-wheel at its bottom end which meshes into the gear-teeth on the outside of the fixed pier-track; the rotation of this shaft causes the drum to turn on the rollers and swing the span. In large spans two shafts are employed. The motive powers employed to operate the shafts are steam-engines, gas-engines, electric motors, and hydraulic motors. One modification of the cen tre pier swing span is the form already men tioned as having the pivot pier nearer to one end than the other; another is where the pivot is at the extreme shore end of the span while the other end is supported by a floating pontoon which swings with the span; another consists of two unequal arm spans with their pivot piers in the opposite banks, and the ends of the long arms meeting and locking over the centre of the channel.
During recent years there has been a notable development in the use of bascule spans, where the waterways to be crossed are so restricted in width as to prevent further restriction by placing a pivot pier in the centre or where the land along the banks is too valuable to permit. space to be taken up hy a swing space with a shore pivot pier. Notable bridges of this type have been built in Chicago, llilwaukee, Buffalo. and Bos ton in the United States, and also in various foreign cities. The Tower Bridge. crossing the river Thames, near the Tower, in London, has a bascule span of 200 feet, formed of two leaves hinged at the opposite abutments, and meeting and locking at the centre.
Lift bridges are seldom used. Perhaps the most important one ever constructed crosses the Chicago River at North Halsted Street, in Chicago. On each side of the river are erected two steel framework towers feet high, be tween which is suspended a truss span 130 feet long. When the bridge is closed this span rests on abutments like any simple-truss span. To open it, steel cables passing from each end of each span up and over sheaves at the tops of the towers, and thence to suitable winding drums at the surface, are wound up, raising the span to near the tops of the towers so that vessels may pass beneath. The clear lift, of this bridge is 155 feet above low water.