As soon as the detail drawings have been pre pared. the engineer draws up a set of specifica tions stating the location, purpose, and main di mensions of the bridge, the character and strength of the material to be used in each part, the quality of workmanship reqnired in constructing and erecting these parts, and a variety of other stipulations tending to secure in the finished structure the embodiment of his intentions as regards the finished bridge. The detail drawings and the specifications are then submitted to bridge-builders with the request that they make bids for performing the required work. Gener ally the lowest bid is accepted, providing the engineer is satisfied of the reliability and com petency of the person making it to perform the work. After the contract has been let, the Midge ;raker receiving it begins the manufacture of the bridge. First wooden templates and models are made front the engineer's drawing of at least the most important members. These are used for marking the shape and dimensions to which the steel is to he cut, and in molding the castings. The steel, being marked. is cut to the required dimensions and forms by shears and planers. Each piece is then sent to the punches, drills, and boring-machines to have the rivet and pin holes bored. The next step is to assemble the pieces forming each member and fasten them together by temporary pins. The assembled member is then taken to the riveting-machines, which drive the permanent rivets. The next step is to finish the member. mark it with its dis tinctive number, and load it for shipment. The attempt is always made to ship the metal-work in as large sections as possible. since the con nections made at the bridge-shops equipped with special machinery for the purpose are more solid and cheaper than those made in the field by portable machinery. At best. however, the bridge material reaches the site of the bridge in a great number of pieces, and the work of COD fleeting these pieces to form the completed bridge is called bridge erection. .Nletal bridges are
usually erected on a supporting falsework of timber, but where such a falsework is impracti cable they are built out from one or both ends piece by piece, the type of structure per mits, or are erected on falsework standing on barges which are floated into the proper position to place the complete span on its piers by lower ing it onto its seats and withdrawing the harge. After erection, the bridge is painted and other wise finished, and finally is commonly tested by running a very heavy load over it. The testing finishes the bridge so far as the bridge engi neer is concerned. For pontoon and other forms of military bridges, see BRIDGES AND DOCKS, IIILIT Y.
BIBLIOGRAPHY. The literature on bridge-buildBibliography. The literature on bridge-build- ing is quite extensive and is widely scattered, much of the most valuable descriptive matter being found in the periodical literature of the engineering profession. Among engineering papers whose columns contain particularly full records of current bridge construction are: Le, Genie riril (Paris, 1SS0, current) ; Zeitsehrift des Vereins Deutscher Ingenicure (Berlin, 1857, current) : Engincering (London, IS66, weekly, current) : Et/pi/ter:ring :Yews ( New York, cur rent) ; and the Engineering Record (New York, current). The Proceedings of the Institution of Ciril Engineers (London, 183S. current) ; Anna/es des Ponts et rhaussees (Paris, 1831, current) ; and Transactions American Society of Ciril Engineers (New York, ISGS, current), also contain much important information relating to bridges. Among the more recent text-books and books of reference are: Johnson, Practice of Modern Framed Structures (New York, 1900) ; Du Bois, Stresses in Framed Structures (New York, 1896) : Merriman and Jacoby. Roofs and Bridges (New York, 1900) ; Waddell, De Ponti bus (New York, 189S) ; Wright, The Designing of Draw Spans (New York, 1897).