BRIDGES: CONSTRUCTION. The design and construc tion of a bridge involve a combination of science and practical knowledge to an extent not required in perhaps any other engi neering work. Lavish outlay of money and great accumulations of material in the structure itself will not necessarily result in a A new type of bridge recently developed is the Wichert continu ous truss or girder, distinguished by the open quadrilateral over the intermediate piers. The first application was the Pittsburgh Homestead bridge, over the Monongahela river, completed 1937, with i o semi-continuous arched trusses, including two spans of 5331 feet.
Since then, 7 more Wichert type bridges have been built, in cluding one over the Susquehanna river at Havre de Grace, Md., and one over the Potomac river at Dahlgren, Virginia.
The longest movable bridge spans in the world are in the U.S.
strong and enduring bridge; the latter may, on the contrary, weaken it by the imposition of material in the wrong place so that the added weight does positive harm. This is particularly the case in bridges of large span, which cannot be built without a thorough knowledge of the materials that can be used in their construction.