474 



THE POPULAR SCIENCE MONTHLY. 



any great extent, upon the railways of this country. This bridge 

 is called the Post bridge (Fig. 24), taking its name from its in- 

 ventor. The characteristic features are that the compression 

 members are inclined at what is claimed to be the most econom- 

 ical angle that is, the most economical in regard to the amount 

 of strength obtained for the amount of material used. They are 



POST TRUSS 



Fig. 24 



so inclined as to cross one panel of the bridge, while the tie-rods, 

 running at an angle with the braces, cross two panels. This is 

 the only advantage that can be claimed for this form of truss, and 

 much of this so-called advantage is more than counterbalanced by 

 some of the difficulties encountered in the actual construction ; 

 and whether the bridge really in itself is a more economical bridge 

 than the Pratt, yet remains to be practically proved. 



We stated in the beginning that bridges consist of arches and 

 suspension bridges as well as framed trusses. The relation be- 



Fig. 25 



BOWSTRING GIRDER. 



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Fig. 27 



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tween the framed truss and the arch will be readily seen by an 

 examination of Fig. 25. Take a truss of the Pratt pattern ; then, 

 in place of having the top chord parallel with the lower chord, 

 let the compression members be increased in length, as shown in 

 the drawing, and the top chord take the form of an arch, and we 

 have the bow-string girder. The ends of the arch on each side 

 are simply held together by means of the lower chord, which acts 

 as a tie-rod upon them. 



