The longest span of a stone arch bridge in America is that which carries the Washington aqueduct and a highway over Cabin John Creek, near Washington, D. C. The span is 220 feet long in the clear.
Of movable bridges the largest swing span existing was erected in 1908 over the Willa mette River at Saint Johns, near Portland, Ore. Its length is 521 feet between centres of end supports, and is located on the Northern Pa cific Railway. The Illinois Central' Railroad Bridge over the Missouri River at East Omaha, Neb., has a span one foot shorter and was completed in 1893. Both of them are double track bridges.
In a direct-lift bridge a simple truss span is lifted vertically to permit boats to pass. The largest span is 220 feet and is in the Oregon Washington Railroad and Navigation Com pany's bridge over the Willamette River at Portland, Ore., completed in 1912. This is a double-deck structure with the highway on the upper and the railroad tracks on the lower deck. In the lift span the lower deck is sus pended from die trusses and may be lifted 46 feet independently of the truss span to give a clear height above low water of 72 feet. When more clearance is required, the truss span is also lifted. The upper deck has a lift of 93 feet, thus giving a total clearance of 165 feet above low water or 144 feet above high water. In the Kansas City bridge referred to as hav ing the longest simple trusses with joints riv eted throughout, there is one span, 428 feet long, which has only the suspended railroad deck movable. In 1916, 33 direct-lift bridges had been built in the United States and Canada.
In cities where dock room is very valuable, there are serious objections to the swing bridge, which requires so much room in which to turn. In narrow streams like the Chicago River and its branches the centre or pivot pier also in terferes seriously with the waterway and forms an obstruction to navigation. The bascule bridge was introduced to obviate these diffi culties. It acts somewhat like a trap-door hinged at one end. One type of bascule, known as a rolling lift bridge, was first built over the Chicago River at Van Buren street in 1895. Another type is known as a trunnion lift bridge. The longer spans have two leaves, while the shorter ones have only one leaf. The largest double-leaf bascule bridge has a span of 336 feet, built in 1913, over the Sault Sainte Marie Ship canal on the Canadian Pacific Rail way. In closing, the leaves of this span come
together at the centre and interlock so as to form a simple-truss bridge when closed. The lower chords are joined like railroad cars but with a double coupler that can transmit a high tensile stress.
The longest span with a single leaf is 235 feet, built in 1913. It is at the crossing of the Calumet River, South Chicago, by the Balti more and Ohio Railroad and is a double-track bridge. The Canadian Pacific Railway Bridge over the Kaministiquia River has two decks, accommodating a double-track railroad on the lower deck and a highway on the upper deck. Its span is 186 feet long and was built in 1913. The Lake Shore and Michigan Southern Rail way Bridge over the Portage River at Port Clinton, Ohio, is a four-track bridge, two tracks being supported between and two outside of the trusses. Its span is 118 feet and was com pleted in 1914.
Several other improved forms of hinged lift bridges have also been designed and built in Chicago and elsewhere. In a small bridge erected in 1896 on the Erie Railroad in the Hackensack Meadows there is only a single leaf hinged at one end and lifted by a cable attached to the other end. The counterweight rolls on a curved track so designed as to make' the counterbalance equally effective in all stages of opening and closing the bridge.
A novel bridge was built in 1902 over the ship canal at Duluth, which is different from any other type in this country. The general scheme is similar to that of a design made by a French engineer who built three of the struc tures in different countries. It consists of a simple truss bridge 393 feet 9 inches long, sup ported on towers at a clear height of 135 feet above high water. Instead of supporting the usual floor of a highway bridge, it supports the track of a suspended car which is properly stif fened against wind pressure and lateral vibra tion, the floor of the car being on a level with the docks. This ferry is operated by electricity. The loaded car, its haters, trucks and ma chinery weigh 120 tons. In the French design a suspension bridge was used instead of the simple truss bridge.