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Improvement in the Coiisiruction of Bridges, ^c. 



Description of the Plates, with Definitions of 9- s«h 

 tech?iical terms, 6)''c. 



Numbers 1, 2, and 3 exhibit all the parts of a 

 bridge on the last improvement, with suspension 

 posts, &c., suited to a span of 170 to 220 feet, 

 according to the size and number of its parts, 

 which may be so varied as to suit the width of 

 span required. 



Definitions, Explanations and General Re- 

 marks. — 1st. A truss is that combination (No. 2) 

 of materials, which, with one or more other sim- 

 ilar ones, constitute the whole vertical support of 

 the bridge. They are placed in a vertical posi- 

 tion on the piers, and are kept so by the addition 

 of bottom beams, top beams, and horizontal and 

 side braces, as shown in No. 1. 



These few parts may be termed the- skeleton 

 of the bridge, and constitute the whole strength 

 and support of the superstructure ; to which a 

 floor is added, for the conveyance of travel, pro- 

 duce, goods, (fcc, and generally a roof and side 

 covering, to save the skeleton and floor from that 

 decay which is incident to its exposure to the 

 weather. 



2d. Truss-braces, or diagonal braces, are those 

 crossing each other, and forming the truss into 

 diamonds. There are two series or sets of such 

 truss-braces, separated by middle string-pieces, 

 six or seven inches thick. (See figs. 1 and 2.) 

 Each series of truss-braces may be termed a lat- 

 tice ; hence it is, by some, termed the double 

 lattice bridge. The diamond-truss bridge, double 

 diamond-truss, or double diamond bridge, would a:;: 

 be more appropriate and better understood. 



3d. String-pieces are all those horizontal parts 

 of a truss, at top and bottom, or intermediate, 

 which run the whole length, and are secured to 

 the truss-braces by three or four tree-nails, at 

 each intersection of them, over which the string- 

 pieces pass. 



4th. Intermediate string-pieces are those which 

 pass over the lines of intersections of the truss-bra- 

 ces, between the extreme top and bottom string- 

 pieces. Middle ones are here added, and the oth- 

 ers may be added when more strength is required. 



5th. Centre, or middle string-pieces, are all 

 those which run between the two series of truss- 

 braces, whether at top, at bottom, or intermedi- 

 ate ; intermediate middle ones only, are here 

 represented. i3':H:::1::i 



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