BRIDGE DAMS. 



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BRIDGE DAMS. 



The bridge dam invented by M. Tavemier and employed at Poses, Mericourt, 

 Meulan, and Port Mort, in France, while more expensive than those heretofore described, 

 yet may be advantageously used in many situations, particularly at points where a 

 railway or highway is to cross a stream. While the application of dams supported by 

 bridges is of late date, yet M. Frimot proposed such an arrangement in 1829, and a 

 bridge dam was constructed on the Upper Yonne as far back as 1836. These designs, 

 however, did not provide for navigation under the arches, except when the dam was 

 open and the water low. 



BRIDGE DAM ON THE YONNE, FRANCE. 1836. 



On the river Elbe, at Pretzien, in Prussia, a bridge dam was built in 1874-75. 

 It differs from those in France in that the closing is done with sliding gates instead of 

 hinged curtains. .It is built with nine bays, 41 feet wide, separated by piers on which 

 rests the bridge, which is too low to permit boats to pass underneath, even when the 

 dam is open. The floor of the dam is above low-water level. The pool level is 10 

 feet above the sill. 



Poses Dam. The following description of the dam at Poses on the Lower Seine 

 will give a general idea of the system. This dam, which was completed in 1885, and 

 has a lift of 13.7 feet, measures 771.5 feet between the abutments, and is divided into 

 seven passes, five deep and two shallow ones. 



The system adopted required the establishment of two upper bridges, first, the 

 down-stream bridge to hold the suspended frames, and second, the up-stream bridge 

 to hold the windlasses while the frames are being raised, and also to sustain a part 

 of the weight of the raised frames themselves. The first may be called the suspending, 

 and the second the hoisting, bridge. Two of the passes are arranged so the spans can 

 be moved in time of floods, to permit the passage of boats. 



The roadways are for two different purposes, and at different levels. The down- 

 stream longitudinal girder of the hoisting-bridge is omitted, and its supporting cross 

 girders are attached directly to the longitudinal up-stream girder of the suspending- 

 bridge, thus affording easy communication between the bridges, and adding to the 

 horizontal strength of both. The up-stream roadway has an opening 4.99 by 8.20 

 feet; large enough to admit of passing the curtain through it endwise. In the non- 

 navigable passes facility of communication is insured by putting a third roadway 

 above the beams of the down-stream roadway. 



The lattice girders supporting the roadway have their uprights 7.61 feet apart, 

 corresponding to the widths of the moving parts. The cross girders take the strain of the 

 hanging-frames by means of the brackets arranged under them. These girders, 3.80 



