1911 Encyclopædia Britannica/Bridges
BRIDGES. 1. Definitions and General Considerations.—Bridges (old forms, brig, brygge, brudge; Dutch, brug; German, Brucke; a common Teutonic word) are structures carrying roadways, waterways or railways across streams, valleys or other roads or railways, leaving a passage way below. Long bridges of several spans are often termed “viaducts,” and bridges carrying canals are termed “aqueducts,” though this term is sometimes used for waterways which have no bridge structure. A “culvert” is a bridge of small span giving passage to drainage. In railway work an “over bridge” is a bridge over the railway, and an “under bridge” is a bridge carrying the railway. In all countries there are legal regulations fixing the minimum span and height of such bridges and the width of roadway to be provided. Ordinarily bridges are fixed bridges, but there are also movable bridges with machinery for opening a clear and unobstructed passage way for navigation. Most commonly these are “swing” or “turning” bridges. “Floating” bridges are roadways carried on pontoons moored in a stream.