Art. 123. 



MODERN BRIDGE TRUSSES. 



223 



are made of two sticks of timber between which the counter 

 braces pass. The chords are made of uniform section throughout 

 the length of the bridge. The panels are short and the loaded 

 chord is made deeper than the other chord, because it carries the 

 floor and acts as a beam in addition to taking the direct stress. 

 Counter braces are used in each panel. 



The Howe truss bridge is sometimes called a combination 

 bridge, but this designation belongs more properly to those 

 bridges in which all tension members are made of iron or steel. 1 



The Pratt truss, which is shown with inclined end posts in 

 Fig. 161 and with vertical end posts in Fig. 154, was patented 



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 / \ 



PRATT TRUSS 

 Fig. 161. 



in 1844. It differs from the Howe truss in that the main diagonal 

 members have an opposite inclination and are, therefore, in ten- 

 sion while the vertical members (except the suspenders) are in 

 compression. At first, only the ties were made of iron, and it 

 was not popular on account of requiring more of this expensive 

 material than the Howe truss. The compression members are 

 shorter than in the Howe truss and, therefore, require less mater- 

 ial, but this was unimportant until iron bridges were introduced. 

 On account of its simplicity, the Pratt truss has become by far 

 the commonest type of bridge truss in this country. Its charac- 

 teristic is diagonal tension and vertical compression members. 



As the stress in the top chord increases toward the middle, 

 and since it is made of the same outer dimensions in each panel, 

 it becomes difficult, in long spans, for several practical reasons, to 

 avoid an excess of material in the end panels, unless the depth of 

 the truss varies, so as to make the stresses in the panels more 

 nearly equal. Fig. 162 shows a form often used and is commonly 

 called a camel-back truss. The appearance is much enhanced if 

 all the joints of the top chord are put on a regular curve. 



*For details of modern 1 Howe truss bridges, see Johnson's Modern 

 Jfyamed Structures, Chapter XXIII. 



